Electrical Testing Measurement Handbook Vol 7

Electrical Testing & Measurement

Handbook Volume 7

Electrical Testing and Measurement Handbook Vol. 7 Published by The Electricity Forum

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ELECTRICAL TESTING AND MEASUREMENT HANDBOOK VOLUME 7 Randolph W. Hurst Publisher & Executive Editor Khaled Nigim Editor Cover Design Don Horne Layout Ann Dunbar Handbook Sales Lisa Kassmann Advertising Sales Carol Gardner Tammy Williams

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The Electricity Forum A Division of the Hurst Communications Group Inc. All rights reserved. No part of this book may be reproduced without the written permission of the publisher. ISBN-978-0-9782763-2-4 The Electricity Forum 215 - 1885 Clements Road, Pickering, ON L1W 3V4

© The Electricity Forum 2007

Electrical Testing and Measurement Handbook – Vol. 7

TABLE OF CONTENTS ELECTRICAL MEASUREMENT AND TESTING CONTACT-LESS SENSING AND THE AUTO-DETECT INFRASTRUCTURE Forward - Khaled Nigim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 DON’T RISK IT: USE CORRECT ELECTRICAL MEASUREMENT TOOLS AND PROCEDURES TO MINIMIZE RISK AND LIABILITY Larry Eccleston . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7 ISOLATION TECHNOLOGIES FOR RELIABLE INDUSTRIAL MEASUREMENTS National Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 RESISTANCE MEASUREMENTS, THREE- AND FOUR-POINT METHOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15 CLAMP-ON GROUND RESISTANCE TESTER, MODELS 3711 & 3731 STEP-BY-STEP USAGE Chauvin Arnoux, Inc. and AEMC® Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21 MEASURING MAGNETIC FIELDS, ELECTRIC AND |MAGNETIC FIELDS Australian Radiation Protection and Nuclear Agency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23 ELECTRIC AND MAGNETIC FIELDS, MEASUREMENTS AND POSSIBLE EFFECT ON HUMAN HEALTH, WHAT WE KNOW AND WHAT WE DON’T KNOW IN 2000 California Department of Health Services and the Public Health Institute California Electric and Magnetic Fields Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25 A NEW APPROACH TO QUICK, ACCURATE, AFFORDABLE FLOATING MEASUREMENTS Tektronix IsolatedChannel Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31 HIGH-VOLTAGE MEASUREMENTS AND ISOLATION -GENERAL ANALOG CONCEPTS NI Analog Resource Center. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35 STANDARD MEASUREMENTS: ELECTRIC FIELDS DUE TO HIGH VOLTAGE EQUIPMENT Ralf Müller and Hans-Joachim Förster . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .39 IDENTIFICATION OF CLOSED LOOP SYSTEMS NI Analog Resource Center . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43 SELECTING AND USING TRANSDUCERS FOR TRANSFORMERS FOR ELECTRICAL MEASUREMENTS William D. Walden . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45 HOW TO TROUBLESHOOT LIKE AN EXPERT, A SYSTEMATIC APPROACH Warren Rhude, Simutech Multimedia Inc. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .53 ELECTRICAL INDUSTRIAL TROUBLESHOOTING Larry Bush . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .55 THE ART OF MEASURING, LOW RESISTANCE Tee Sheffer and Paul Lantz, Signametrics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .59 STANDARDS FOR SUPERCONDUCTOR AND MAGNETIC MEASUREMENTS National Institute of Standards and Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .63 MULTI CHANNEL CURRENT TRANSDUCER SYSTEMS DANFYSIK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .67 FALL-OF-POTENTIAL GROUND TESTING, CLAMP-ON GROUND TESTING COMPARISON Chauvin Arnoux, Inc.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .69 AN INTRODUCTION TO ANTENNA TEST RANGES, MEASUREMENTS AND INSTRUMENTATION Jeffrey A. Fordham Microwave Instrumentation Technologies, LLC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .71

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DERIVING MODEL PARAMETERS FROM FIELD TEST MEASUREMENTS J.W. Feltes, S. Orero, B. Fardanesh,E. Uzunovic, S. Zelingher, N. Abi-Samra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .79 TESTING ELECTRIC STREETLIGHT COMPONENTS WITH LABVIEW-CONTROLLED VIRTUAL INSTRUMENTATION Ahmad Sultan, Computer Solutions, Inc. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .85 ASSET MANAGEMENT, THE PATH TO MAINTENANCE EXCELLENCE Mike Sondalini, Feed Forward UP-TIME Publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .87 THINK SYNCHRONIZATION FIRST TO OPTIMIZE AUTOMATED TEST ni.com . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .89 USING NATIONAL INSTRUMENTS SYSTEM IDENTIFICATION, CONTROL DESIGN AND SIMULATION PRODUCTS FOR DESIGNING AND TESTING A CONTROLLER FOR AN UNIDENTIFIED SYSTEM ni.com . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .93 MAGNETO-MECHANICAL MEASUREMENTS FOR HIGH CURRENT APPLICATIONS Jack Ekin, NIST- Electromagnetic Division . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .101 A BASIC GUIDE TO THERMOGRAPHY Land Instruments International Infrared Temperature Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .105


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ELECTRICAL MEASUREMENT AND TESTING CONTACT-LESS SENSING AND THE AUTO-DETECT INFRASTRUCTURE Forward by Khaled Nigim Maintaining a highly functional electric system is dependent on the operational and maintenance level of the integrated components that are geared together to serve the customer. An effective preventive maintenance setup is dependent on the reliability of the sensing devices and relaying instrumentation as well as on the operator’s understanding of the process functionality. Early measuring devices were designed and based on electromechanical indicating instrumentation. Their solo operability necessitated around the clock operator attention. Such devices were accurate but provided limited adaptability for interfacing with today’s centralized centers. As the semi-conducting integrated circuits devices start to invade the market, many instruments are now inter-actable with each other and some can be used to sense and record data from various sensing elements in a sequential manner and generate their own diagnostic reports within a very brief time. Today’s sensors are built around plug-and-play infrastructure which is based on the IEEE 1451.4 standard that brings plug-and-play capabilities to the world of transducers. With plug-and-play technology, the operator stores a Transducer Electronic Datasheet (TEDS) directly on a sensor. The sensor identifies itself with all needed information once and is hooked to a data bus. TEDScompatible measurement systems can auto-detect and automatically configure these “smart sensors” for measurement, reducing setup time and eliminating transcription errors that commonly occur during sensor configuration. This enables the operator to focus on overall system operation rather than on individual component operation. Furthermore, measuring relaying units and associated sensing elements technologies has advanced rapidly over the past 20 years. A particular advancement is noted in the contactless measuring sensors and measured data handling capability. This progression in the testing and measurement field provides a wider scope of applications and shorter time for interrupting early failure signals. As an example, the cases where infra-red imaging techniques are used are now part of the routine maintenance of distribution transformers. The infrared image indicates the hottest spot and temperature distribution inside a large distribution transformer without the need of embedding sensors. Earlier techniques for measuring temperature were based on collecting data from various temperature sensors entrenched inside the transformer windings. If one or more sensors were faulty, the gathered data would be incomplete and the transformer has to be taken out of service. Replacing the sensors is a timely and costly procedure. Today’s data handling and processors that either control the data flow from one or more sensors or part of the human machine interface supervisory system, have the capability to run self-diagnostics routines to alert the operator to any abnormal behavior from the various sensing elements, and generate a check list to help figure out any culprits. This edition of the Electrical Testing and Measurement

Handbook introduces the fundamental applications of electrical testing and instrumentation and guidelines on the correct procedures, and how to interpret and diagnose measured reports that enable the operator to maintain a high degree of functionality of the system with minimum interruption. This handbook addresses various practical aspects of today’s electrical engineering infrastructure through selected articles available for scientific sharing. The articles are grouped into 4 sections. Section 1 addresses the basics and fundamentals of electric testing techniques using various measuring sensors normally incorporated in many of today measuring instruments. Section 2 addresses safe operation, procedures and handling of instruments. Section 3 introduces various sensing and measuring devices that can be used in a wide area of application. And finally, section 4 showcases field applications of instrumentation in various parts of the electrical engineering industry. The Electricity Forum endeavors to provide correct and timely information for their readers in their handbook series. We welcome readers’ suggestions and constructive feedback, and contributions. Please submit your technical articles that show case your experience in testing and measurement tools and systems directly to the handbook editor’s desk ([email protected]).

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DON’T RISK IT: USE CORRECT ELECTRICAL MEASUREMENT TOOLS AND PROCEDURES TO MINIMIZE RISK AND LIABILITY Larry Eccleston, Product Testing Manager, Fluke Corporation, Member, IEC Standards Committee

Between five and ten times on any given day, arc flash explosions sufficient to send a burn victim to a special burn center take place in the U.S. These incidents and other less serious electrical accidents result in injury – sometimes death – lost work time, medical costs and insurance claims, downtime, the list goes on. The cost to both the victim, the victim’s family and the company involved, are high. Yet many of these accidents can be prevented. The combination of training, good measurement technique, and the use of proper tools can significantly reduce the chance of an accident occurring.

electrical accidents: disruption of operations, higher insurance costs, litigation and, most importantly, human suffering. In today’s society, where medical costs are escalating and lawsuits are common, wise managers will take every step to reduce the level of risk, help increase employee safety and minimize the organization’s operational and financial exposure. This means that management must ensure that employees use appropriate personal protective equipment, including new-generation test tools independently tested to help ensure that they perform up to specification. And employees must use that equipment correctly, and receive training in safe electrical measurement procedures.

IS YOUR COMPANY AT RISK? HOW WOULD YOU ANSWER THE FOLLOWING QUESTIONS?

2. INTRODUCTION: MANAGING HAZARDS IN THE ELECTRICAL ENVIRONMENT

1. EXECUTIVE SUMMARY

1. Do you have a documented electrical measurement safety program? 2. Do you regularly inspect your electrical measurement equipment for damage that could imperil safety? 3. Do your workers involved in taking electrical measurements receive annual, intensive training on how to work safely? 4. Does your organization insure that only properly rated test instruments are used in your facility? If you answered yes to three of the questions above, congratulations – you’re doing a better job than most employers to reduce the chance of accidents associated with taking electrical measurements. But there’s still room to do more. This resource kit was designed to help you develop an electrical measurement safety program that significantly reduces your risk. The high-energy electrical systems common in today’s workplace bring not only increased efficiency, but increased levels of hazard and risk for electrical workers and their employers. Workers taking electrical measurements on high-energy systems frequently work close to potentially lethal electrical currents. This danger can significantly increase due to the presence of transient voltage spikes. Transient spikes riding on these powerful industrial currents can produce the conditions that cause the extremely hazardous phenomenon of arc flash. To help manage the risks inherent in high-energy electrical systems, national and international standards bodies have developed rules that categorize electrical environments according to their potential danger. Personal protective equipment, including test instruments, is categorized according to the NFPA-70E Standard for Electrical Safety Requirements for Employee Workplaces, related to the incident energy levels and arc flash boundary distances. To help ensure safety in today’s high-energy, high-hazard environments, leading manufacturers have re-engineered their test instruments to enhance both reliability and safety. Such tools can help companies avoid the many perils caused by high-energy

Today’s industrial and business electrical supply systems deliver high levels of electrical energy – up to 480 volts in the United States, and up to 600 volts in Canada. Such high-energy circuits can create significant hazard and risk. Another characteristic of most high-energy electrical supply systems is the presence of short-duration voltage kickback spikes, called transients. When such spikes occur while measurements are being made, they can cause a plasma arc to form – inside the measurement tool, or outside. The high fault current available in 480-volt and 600volt systems can make the resulting arc flash extremely hazardous. Mitigating such risks requires the use of Personal Protective Equipment (PPE) including test instruments engineered and tested to meet appropriate standards, adherence to safe measurement procedures, and proper inspection and maintenance of test instruments. In this paper we will cover: • Understanding the High-Energy Environment • Voltage Transients • The Danger of Arc Flash • Measurement Categories CAT I, CAT II, CAT III and CAT IV • Measurement Tools as Part of Personal Protective Equipment • Safety Requirements for Measurement Tools • Test Tool Inspection and Maintenance • Safe Measurement Processes and Procedures • Conclusions and Recommendations

3. UNDERSTANDING THE HIGH-ENERGY ENVIRONMENT Businesses simply could not survive without large amounts of electrical power. Manufacturing operations and office heating, ventilation and air conditioning systems require large amounts of power, and computer systems have now become major power users. The need to supply large amounts of power in the most cost-effective way has led firms to choose higher-energy, highervoltage supply systems, which cost less to install.

8 As a result of these trends, industrial and business operations today incorporate higher levels of electrical energy, which can lead to increased hazard and risk for those who build and maintain these systems. It is common for industrial and commercial maintenance workers and electricians to work with high levels of energy. In the U.S., 480-volt, three-phase electrical supply systems are commonplace. In Canada, systems use up to 600 volts. Although classified as “low voltage”, both 480-volt and 600-volt systems can easily deliver potentially lethal amounts of current sufficient to fuel an arc flash – an extremely hazardous occurrence.

4. VOLTAGE TRANSIENTS: DANGER IN A MICROSECOND The presence of voltage kickback spikes, called transients, is another characteristic of electrical supply systems that adds to the potential danger encountered when taking electrical measurements. Transients are present in almost every electrical supply system. In industrial settings, they may be caused by the switching of inductive loads, and by lightning strikes. Though such transients may last only tens of microseconds, they may carry thousands of amps of energy from the installation. For anyone taking measurements on electrical equipment, the consequences can be devastating. When such spikes occur while measurements are being made, they can cause a plasma arc to form – inside the measurement tool, or outside. The high fault current available in 480-volt and 600-volt systems can generate an extremely hazardous condition called arc flash.

5. UNDERSTANDING ARC FLASH How can such a problem develop? A transient of sufficient magnitude can cause an arc to form between conductors within an instrument, or across test leads. Once an arc occurs, the total available fault current similar to the bolted current can feed the arc and cause an explosion. The result may be an arc flash, which can cause a plasma fireball fueled by the energy in the electrical system. Temperatures can reach about 6,000 degrees Celsius, or 10,000 degrees Fahrenheit. Transients are not the only source of arc-flash hazard. A very common misuse of handheld multimeter can trigger a similar chain of events. If the multimeter user leaves the test leads in the amps input terminals and connects the meter leads across a voltage source, that user has just created a short through the meter. While the voltage terminals have a high impedance, the amps terminals have a very low impedance. This is why a meter’s amps circuit must be protected with fuses. Another common and dangerous misuse of test equipment is measuring ohms or continuity on a live circuit. These measurements should be made only on circuits that are not energized.

6. ARC FLASH AS A SAFETY ISSUE Detailed information on the frequency and cost of arc flash accidents is difficult to find. Accident reports may not distinguish arc flash from electric shock. In addition, employers may be reluctant to discuss or report incidents that can be so dangerous and costly. Dr. Mary Capelli-Schellpfeffer of the University of Chicago provides the most authoritative estimates of arc flash frequency. Her firm, CapSchell, Inc., a Chicago-based research and consulting firm, estimates that between five and ten times a day, arc flash explosions sufficient to send a burn victim to a special

Electrical Testing and Measurement Handbook – Vol. 7 burn center take place in the U.S.

7. MEASUREMENT CATEGORIES: CAT I, CAT II, CAT III AND CAT IV To provide improved protection for users, industry standards organizations have taken steps to clarify the hazards present in electrical supply environments. The American National Standards Institute (ANSI), the Canadian Standards Association (CSA), and the International Electro-Technical Commission (IEC) have created more stringent standards for voltage test equipment used in environments of up to 1000 volts. The pertinent standards include ANSI S82.02, CSA 22.21010.1 and IEC 61010. These standards cover systems of 1000 volts or less, including 480-volt and 600-volt, three-phase circuits. For the first time, these standards differentiate the transient hazard by location and potential for harm, as well as the voltage level. ANSI, CSA and IEC define four measurement categories of over-voltage transient impulses. The rule of thumb is that the closer the technician is working to the power source, the greater the danger and the higher the measurement category number. Lower category installations usually have greater impedance, which dampens transients and helps limit the fault current that can feed an arc. • CAT (Category) IV is associated with the origin of installation. This refers to power lines at the utility connection, but also includes any overhead and underground outside cable runs, since both may be affected by lightning. • CAT III covers distribution level wiring. This includes 480-volt and 600-volt circuits such as 3-phase bus and feeder circuits, motor control centers, load centers and distribution panels. Permanently installed loads are also classed as CAT III. CAT III includes large loads that can generate their own transients. At this level, the trend to using higher voltage levels in modern buildings has changed the picture and increased the potential hazards. • CAT II covers the receptacle circuit level and plug-in loads. • CAT I refers to protected electronic circuits. Some installed equipment may include multiple categories. A motor drive panel, for example, may be CAT III on the 480-volt power side, and CAT I on the control side.

8. MEASUREMENT TOOLS PART OF PERSONAL PROTECTIVE EQUIPMENT Another organization playing an important role in establishing safety standards for electrical workers is the National Fire Protection Association (NFPA). NFPA establishes guidelines for electrical measurement tools in its standard 70E, “Standard for Electrical Safety Requirements for Employee Workplaces, 2004 Edition”. Standard 70E also includes important requirements regarding the use of other Personal Protective Equipment (PPE) in various environments and installation/maintenance activities. The NFPA standard makes it clear that test instruments and accessories must be matched to the environment where they will be used. These are the pertinent sections: • “Test instruments, equipment, and their accessories shall be rated for circuits and equipment to which they will be connected.” (Part II, Chapter 3, Paragraph 3-4.10.1) • “Test instruments, equipment, and their accessories shall

Electrical Testing and Measurement Handbook – Vol. 7 be designed for the environment to which they will be exposed, and for the manner in which they will be used.” (Part II, Chapter 3, Paragraph 3-4.10.2) A table included in NFPA Standard 70E, Table 3-3.9, “Hazard Risk Category Classifications,” provides additional guidance regarding the personal protective equipment recommended for use in work on a variety of equipment types at various voltage levels.i

9. SAFETY REQUIREMENTS FOR MEASUREMENT TOOLS Management must ensure that, in compliance with NFPA 70E, test tools meet the standards for the environment where they are used. The entire testing ‘system’, including the meter and its internal fusing system, as well as the test leads and attachments, must comply with regulations for measurement environment and hazard level. In addition, tools must be included as an integral part of the Personal Protective Equipment that technicians are required to use when working on high-energy systems. Beyond these requirements, however, management must ensure that the measurement tools in use are designed, certified and maintained so that they will meet the more advanced and stringent safety requirements of today. Management must account for three factors when assessing test tool safety: Category rating (older, unrated tools were not made for today’s electrical environment), independent testing and certification, and regular inspection and maintenance. It is important to note that the category rating for personnel protective equipment has no relationship to the CAT ratings identified as part of the markings of test and measurement equipment. Category rating for PPE – Testers should be rated for the electrical environment in which they will be used. For example, a 220-volt, three-phase system requires a tester rated CAT III or IV. Old, unrated test instruments do not meet IEC guidelines for required PPE. While they may be perfectly accurate and appear to perform well, even the best meters of yesterday were designed for a world where working conditions and safety standards were far different. Such test tools may not meet contemporary standards. Independent Testing and Certification – Even in the vital area of safety, some tools may not perform as promised by the manufacturer. Measuring devices rated for a high-energy environment may not actually deliver the safety protections, such as adequate fusing, claimed on their specification sheets.

THE CRUCIAL DIFFERENCE BETWEEN ‘DESIGNED’ AND ‘TESTED’ It is important to understand that standards bodies such as ANSI, CSA and IEC are not responsible for enforcing their standards. This means that a meter designed to a standard may not actually have been tested and proven to meet that standard. It is not uncommon for meters under test to fail before achieving the performance their manufacturers claim for them. The best assurance for users and their employers is to select test instruments that have been tested and certified to perform up to specification by independent testing laboratories. To provide an extra measure of confidence, select test tools labeled to show that they have been certified to meet the appropriate contemporary standards by two or more independent labs. This ensures that test instruments have passed the most rigorous tests and meet every applicable standard. Such independent testing labs include Underwriters Laboratories (UL) in the United States, Canadian Standards Association (CSA) in Canada and TUV Product Service in Europe.ii

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10. TEST TOOL INSPECTION AND MAINTENANCE Regular Inspection and Maintenance – To perform accurately and safely, test tools must be regularly inspected and maintained. The need for inspection is clearly recognized by the National Fire Protection Association. NFPA Standard 70E lays out the requirement that test tools must be visually inspected frequently to help detect damage and ensure proper operation. Part II, Chapter 4, Paragraph 4-1.1 provides the details: Visual Inspection. Test instruments and equipment and all associated test leads, cables, power cords, probes, and connectors shall be visually inspected for external defects and damage before the equipment is used on any shift. If there is a defect or evidence of damage that might expose an employee to injury, the defective or damaged item shall be removed from service, and no employee shall use it until repairs and tests necessary to render the equipment safe have been made.iii Visual inspection alone, however, may not detect all possible test instrument problems. To help ensure the highest level of safety and performance, additional inspection and testing should be conducted: Additional Visual Inspection – Test tools should be checked for the following points: • Look for the 1000-volt, CAT III or 600-volt, CAT IV rating on the front of meters and testers, and a “double insulated” symbol on the back. • Look for approval symbols from two or more independent testing agencies, such as UL, CSA, CE, TUV or CTICK. • Make sure that the amperage and voltage of meter fuses is correct. Fuse voltage must be as high or higher than the meter’s voltage rating. The second edition of IEC/ANSI/CSA standards states that test equipment must perform properly in the presence of impulses on volts and amps measurement functions. Ohms and continuity functions are required to handle the full meter voltage rating without becoming a hazard. • Check the instrument’s manual to determine whether the ohms and continuity circuits are protected to the same level as the voltage test circuit. If the manual does not indicate, your supplier should be able to determine whether the meter passed the second edition of IEC61010 or ANSI S82.02. • Check the overall condition of the meter or tester, looking for such problems as a broken case, worn test leads or a faded display. • Use the meter’s own test capability to determine whether fuses are in place and functioning properly. Step 1: Plug test lead in V/ Ω input. Select Ω. Step 2: Insert probe tip into mA input. Read value. Step 3: Insert probe tip into A input. Read value. Typically a fuse in good condition should show mvalue of close to zero, but you should always check your meter owner’s manual for the specified reading. Inspecting Test Leads and Probes – As integral components of the test tool system, test leads, probes and attachments must meet the requirements of the testing environment and be designed to minimize hazard. Test leads must be certified to a category that equals or exceeds that of the meter or tester. • Examine test leads for such features as shrouded connectors, finger guards, CAT ratings that equal or exceed those of the meter, and double insulation.

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Electrical Testing and Measurement Handbook – Vol. 7 • Visually inspect for frayed or broken wires. The length of exposed metal on test probe tips should be minimal. • Test leads can fail internally, creating a hazard that cannot be detected through visual inspection. But it is possible to use the meter’s own continuity testing function to check for internal breaks. Step 1: Insert leads in V/ Ω and COM inputs Step 2: Select Ω, touch probe tips. Good leads are 0.1 – 0.3 Ω.

11. SAFE MEASUREMENT PROCESSES AND PROCEDURES In addition to the consistent use of safe, correctly rated and inspected test tools discussed in the preceding sections, safe electrical measurement requires adherence to correct measurement procedures. Safety training programs should incorporate both elements of safe measurement – equipment and procedures. In addition to equipment inspection (detailed in Section 10 above), safe measurement procedures include: • Lockout/Tagout procedures – NFPA provides extensive information and guidance on lockout/tagout practices and devices in Part II, Chapter 5 of NFPA 70E.iv • Three-step test procedure – Before making the determination that a measured circuit is dead, it is important to verify that test instruments are operating correctly. To do so, the technician should use a three-step test procedure. First, check for correct test tool operation by using the tool to test a circuit known to be live. Then, test the target circuit. Finally, as a double check on test tool operation, test the original known circuit once again. This procedure provides the user a strong measure of confidence that the test tool is operating correctly, and that the target circuit is performing as measured. • Neutral first and last – The user should attach the test lead to a neutral contact first, then attach a lead to a hot contact to conduct the test. In detaching test leads, first remove the hot contact, then remove the neutral test lead. • One hand only – When possible, it is good practice to follow the old electrician’s advice and keep one hand in a pocket when testing. But common sense must rule. Conditions at the test location may make it impractical to use this technique.

12. CONCLUSIONS AND RECOMMENDATIONS Unlike some other important safety initiatives, the measures required to bolster the safety of electrical measurement tools and procedures are not difficult or costly. Yet these steps can provide important benefits by improving worker safety, avoiding the disruption of business operations, reducing risk and avoiding possible increases in insurance costs. Employers should begin by ensuring that technicians are fully trained in correct use of all personal protective equipment, including test instruments. As a companion measure, make sure the required PPE is readily available, meets today’s standards, and is inspected to ensure it is in optimum condition. Test instruments are an essential component of PPE. Employers should inspect all test instruments to ensure they are rated, tested and certified by independent testing agencies to meet safety requirements for the environments where they are used. Replace test instruments that do not meet current standards, because they may create extra hazard, risk and liability. Finally, personnel should be trained in the correct procedures for taking safe measurements, including methods for personally inspecting and testing their instruments to ensure they are in good condition and function correctly. NFPA 70E Standard for Electrical Safety Requirements for Employee Workplaces, 2000 Edition, pages 55 through 58. © 2000 NFPA

i

For more information on these testing organizations, visit their websites: http://www.ul.com/ http://www.csa.ca/Default.asp?language=English http://www.tuvamerica.com/services/electrical/lowvolt.cfm

ii

NFPA 70E Standard for Electrical Safety Requirements for Employee Workplaces, 2000 Edition, page 63. © 2000 NFPA iii

iv

Ibid, pp 64-66.


11

ISOLATION TECHNOLOGIES FOR RELIABLE INDUSTRIAL MEASUREMENTS National Instruments OVERVIEW

NEED FOR ISOLATION

Voltage, current, temperature, pressure, strain, and flow measurements are an integral part of industrial and process control applications. Often these applications involve environments with hazardous voltages, transient signals, common-mode voltages, and fluctuating ground potentials capable of damaging measurement systems and ruining measurement accuracy. To overcome these challenges, measurement systems designed for industrial applications make use of electrical isolation. This white paper focuses on isolation for analog measurements, provides answers to common isolation questions, and includes information on different isolation implementation technologies.

Consider isolation for measurement systems that involve any of the following: • Vicinity to hazardous voltages • Industrial environments with possibility of transient voltages • Environments with common mode voltage or fluctuating ground potentials • Electrically noisy environments such as those with industrial motors • Transient sensitive applications where it is imperative to prevent voltage spikes from being transmitted through the measurement system Industrial measurement, process control, and automotive test are examples of applications where common-mode voltages, high-voltage transients, and electrical noise are common. Measurement equipment with isolation can offer reliable measurements in these harsh environments. For medical equipment in direct contact with patients, isolation is useful in preventing power line transients from being transmitted through the equipment. Based on your voltage and data rate requirements, you have several options for making isolated measurements. You can use plug-in boards for laptops, desktop PCs, industrial PCs, PXI, Panel PCs, and Compact PCI with the option of built-in isolation or external signal conditioning. Isolated measurements can also be made using programmable automation controllers (PACs) and measurement systems for USB.

UNDERSTANDING ISOLATION Isolation electrically separates the sensor signals, which can be exposed to hazardous voltages1, from the measurement system’s low-voltage backplane. Isolation offers many benefits including: • Protection for expensive equipment, the user, and data from transient voltages • Improved noise immunity • Ground loop removal • Increased common-mode voltage rejection Isolated measurement systems provide separate ground planes for the analog front end and the system backplane to separate the sensor measurements from the rest of the system. The ground connection of the isolated front end is a floating pin that can operate at a different potential than the earth ground. Figure 1 represents an analog voltage measurement device. Any commonmode voltage that exists between the sensor ground and the measurement system ground is rejected. This prevents ground loops from forming and removes any noise on the sensor lines.

Figure 1. Bank Isolated Analog Input Circuitry Hazardous Voltages are greater than 30 Vrms, 42.4 Vpk or 60 VDC

Figure 2. Isolated Data Acquisition Systems

12

METHODS OF IMPLEMENTING ISOLATION Isolation requires signals to be transmitted across an isolation barrier without any direct electrical contact. Light emitting diodes (LEDs), capacitors, and inductors are three commonly available components that allow electrical signal transmission without any direct contact. The principles on which these devices are based form the core of the three most common technologies for isolation – optical, capacitive, and inductive coupling.

Electrical Testing and Measurement Handbook – Vol. 7 second coil by placing it in close vicinity of the changing magnetic field from the first coil. The voltage and current induced in the second coil depend on the rate of current change through the first. This principle is called mutual induction and forms the basis of inductive isolation.

OPTICAL COUPLING LEDs produce light when a voltage is applied across them. Optical isolation uses an LED along with a photo-detector device to transmit signals across an isolation barrier using light as the method of data translation. A photo-detector receives the light transmitted by the LED and converts it back to the original signal.

Figure 3. Optical Coupling

Figure 5. Inductive Coupling

Inductive isolation uses a pair of coils separated by a layer of insulation. Insulation prevents any physical signal transmission. Signals can be transmitted by varying current flowing through one of the coils, which causes a similar current to be induced in the second coil across the insulation barrier. Inductive isolation can provide high-speed transmission similar to capacitive techniques. Because inductive coupling involves the use of magnetic fields for data transmission, it can be susceptible to interference from external magnetic fields.

ANALOG ISOLATION AND DIGITAL ISOLATION Optical isolation is one of the most commonly used methods for isolation. One benefit of using optical isolation is its immunity to electrical and magnetic noise. Some of the disadvantages include transmission speed, which is restricted by the LED switching speed, high-power dissipation, and LED wear.

CAPACITIVE COUPLING Capacitive isolation is based on an electric field that changes based on the level of charge on a capacitor plate. This charge is detected across an isolation barrier and is proportional to the level of the measured signal. One advantage of capacitive isolation is its immunity to magnetic noise. Compared to optical isolation, capacitive isolation can support faster data transmission rates because there are no LEDs that need to be switched. Since capacitive coupling involves the use of electric fields for data transmission, it can be susceptible to interference from external electric fields.

Several commercial off-the-shelf (COTS) components are available today, many of which incorporate one of the above technologies to provide isolation. For analog input/output channels, isolation can be implemented either in the analog section of the board, before the analog-to-digital converter (ADC) has digitized the signal (analog isolation) or after the ADC has digitized the signal (digital isolation). Different circuitry needs to be designed around one of these techniques based on the location in the circuit where isolation is being implementing. You can choose analog or digital isolation based on your data acquisition system performance, cost, and physical requirements. Figure 6 shows the different stages of implementing isolation.

Figure 6a. Analog Isolation

Figure 4. Capacitive Isolation

INDUCTIVE COUPLING In the early 1800s, Hans Oersted, a Danish physicist, discovered that current through a coil of wire produces a magnetic field. It was later discovered that current can be induced in a

Figure 6b. Digital Isolation


13

The following sections cover analog and digital isolation in more detail and explore the different techniques for implementing each.

ANALOG ISOLATION The isolation amplifier is generally used to provide isolation in the analog front end of data acquisition devices. “ISO Amp” in Figure 6a represents an isolation amplifier. The isolation amplifier in most circuits is one of the first components of the analog circuitry. The analog signal from a sensor is passed to the isolation amplifier which provides isolation and passes the signal to the analog-to-digital conversion circuitry. Figure 7 represents the general layout of an isolation amplifier. Figure 8. Use of Isolation Amplifiers in Flexible Signal Conditioning Hardware

DIGITAL ISOLATION

Figure 7. Isolation Amplifier

In an ideal isolation amplifier, the analog output signal is the same as the analog input signal. The section labeled “isolation” in Figure 7 uses one of the techniques discussed in the previous section (optical, capacitive, or inductive coupling) to pass the signal across the isolation barrier. The modulator circuit prepares the signal for the isolation circuitry. For optical methods, this signal needs to be digitized or translated into varying light intensities. For capacitive and inductive methods, the signal is translated into varying electric or magnetic fields. The demodulator circuit then reads the isolation circuit output and converts it back into the original analog signal. Because analog isolation is performed before the signal is digitized, it is the best method to apply when designing external signal conditioning for use with existing non-isolated data acquisition devices. In this case, the data acquisition device performs the analog-to-digital conversion and the external circuitry provides isolation. With the data acquisition device and external signal conditioning combination, measurement system vendors can develop general-purpose data acquisition devices and sensor-specific signal conditioning. Figure 8 shows analog isolation being implemented with flexible signal conditioning that uses isolation amplifiers. Another benefit to isolation in the analog front end is protection for the ADC and other analog circuitry from voltage spikes. There are several options available on the market for measurement products that use a general-purpose data acquisition device and external signal conditioning. For example, the National Instruments M Series includes several non-isolated, general-purpose multifunction data acquisition devices that provide high-performance analog I/O and digital I/O. For applications that need isolation, you can use the NI M Series devices with external signal conditioning, such as the National Instruments SCXI or SCC modules. These signal conditioning platforms deliver the isolation and specialized signal conditioning needed for direct connection to industrial sensors such as load cells, strain gages, pH sensors, and others.

Analog-to-digital converters are one of the key components of any analog input data acquisition device. For best performance, the input signal to the analog-to-digital converter should be as close to the original analog signal as possible. Analog isolation can add errors such as gain, non-linearity and offset before the signal reaches the ADC. Placing the ADC closer to the signal source can lead to better performance. Analog isolation components are also costly and can suffer from long settling times. Despite better performance of digital isolation, one of the reasons for using analog isolation in the past was to provide protection for the expensive analog-to-digital converters. As the ADCs prices have significantly declined, measurement equipment vendors are choosing to trade ADC protection for better performance and lower cost offered by digital isolators (see Figure 9).

Figure 9. Declining Price of 16-Bit Analog-to-Digital Converters Graph Source: National Instruments and a Leading ADC Supplier

Compared to isolation amplifiers, digital isolation components are lower in cost and offer higher data transfer speeds. Digital isolation techniques also give analog designers more flexibility to choose components and develop optimal analog front ends for measurement devices. Products with digital isolation use currentand voltage-limiting circuits to provide ADC protection. Digital isolation components follow the same fundamental principles of optical, capacitive, and inductive coupling that form the basis of analog isolation.

14 Leading digital isolation component vendors such as Avago Technologies (www.avagotech.com), Texas Instruments (www.ti.com), and Analog Devices (www.analog.com) have developed their isolation technologies around one of these basic principles. Avago Technologies offers digital isolators based on optical coupling, Texas instruments bases its isolators on capacitive coupling, and Analog Devices isolators use inductive coupling.

OPTOCOUPLERS Optocouplers, digital isolators based on the optical coupling principles, are one of the oldest and most commonly used methods for digital isolation. They can withstand high voltages and offer high immunity to electrical and magnetic noise. Optocouplers are often used on industrial digital I/O products, such as the National Instruments PXI-6514 isolated digital input/output board (see Figure 10) and National Instruments PCI-7390 industrial motion controller.

Electrical Testing and Measurement Handbook – Vol. 7 encoding and converts rising and falling edges on the digital lines to 1 ns pulses. These pulses are transmitted across the isolation barrier using the transformer and decoded on the other side by the receiver circuitry (see Figure 11). The small size of the transformers, about three-tenths of a millimeter, makes them practically impervious to external magnetic noise. iCouplers can also lower measurement hardware cost by integrating up to four isolated channels per integrated circuit (IC) and, compared to optocouplers, they require fewer external components.

Figure 11. Introduction Coupling-Based iCoupler Technology from Analog Devices Source: Analog Devices (www.analog.com/iCoupler)

Measurement hardware vendors are using iCouplers to offer high-performance data acquisition systems at lower costs. National Instruments industrial data acquisition devices intended for high-speed measurements, such as the isolated M Series multifunction data acquisition devices, use iCoupler digital isolators (see Figure 12). These devices provide 60 VDC continuous isolation and 1,400 Vrms/1,900 VDC channel-to-bus isolation withstand for 5 s on multiple analog and digital channels and support sampling rates up to 250 kS/s. National Instruments C Series modules used in the NI PAC platform, NI CompactRIO, NI CompactDAQ, and other high-speed NI USB devices also use the iCoupler technology. Figure 10. Industrial Digital I/O Products Optpcouplers

For high-speed analog measurements, optocouplers, however, suffer from speed, power dissipation, and LED ware limitations associated with optical coupling. Digital isolators based on capacitive and inductive coupling can alleviate many optocoupler limitations.

CAPACITIVE ISOLATION Texas Instruments offers digital isolation components based on capacitive coupling. These isolators provide high data transfer rates and high transient immunity. Compared to capacitive and optical isolation methods inductive isolation offers lower power consumption.

INDUCTIVE ISOLATION iCoupler® technology, introduced by Analog Devices in 2001 (www.analog.com/iCoupler), uses inductive coupling to offer digital isolation for high-speed and high-channel-count applications. iCouplers can provide 100 Mb/s data transfer rates with 2,500 V isolation withstand; for a 16-bit analog measurement system that implies sampling rates in the mega hertz range. Compared to optocouplers, iCouplers offer other benefits such as reduced power consumption, high operating temperature range up to 125 °C, and high transient immunity up to 25 kV/ms. iCoupler technology is based on small, chip-scale transformers. An iCoupler has three main parts – a transmitter, transformers, and a receiver. The transmitter circuit uses edge trigger

Figure 12. National Instruments Isolated M Series Multifuntion DAQ Uses

SUMMARY Isolated data acquisition systems can provide reliable measurements for harsh industrial environments with hazardous voltages and transients. Your need for isolation is based on your measurement application and surrounding environments. Applications that require connectivity to different specialty sensors using a single, general-purpose data acquisition device can benefit from external signal conditioning with analog isolation. Where as applications needing lower-cost, high-performance analog inputs benefit from measurement systems with digital isolation technologies.


15

RESISTANCE MEASUREMENTS THREE- AND FOUR-POINT METHOD FOUR-POINT RESISTANCE MEASUREMENTS Ohmmeter measurements are normally made with just a two-point measurement method. However, when measuring very low values of ohms, in the milli- or micro-ohm range, the two-point method is not satisfactory because test lead resistance becomes a significant factor. A similar problem occurs when making ground mat resistance tests, because long lead lengths of up to 1000 feet are used. Here also, the lead resistance, due to long lead length, will affect the measurement results. The four-point resistance measurement method eliminates lead resistance. Instruments based on the four-point measurement work on the following principle: • Two current leads, C1 and C2, comprise a two-wire current source that circulates current through the resistance under test. • Two potential leads, P1 and P2, provide a two-wire voltage measurement circuit that measures the voltage drop across the resistance under test. • The instrument computes the value of resistance from the measured values of current and voltage.

only three test terminals. The three-point method for ground system testing is considered adequate by most individuals in the electrical industry and is employed on the TPI MFT5010 and the TPI ERT1500. The four-point method is required to measure soil resistivity. This process requires a soil cup of specific dimensions into which a representative sample of earth is placed. This process is not often employed in testing electrical ground systems although it may be part of an initial engineering study.

PURPOSE/TPI INSTRUMENT FEATURES PURPOSE The purpose of electrical ground testing is to determine the effectiveness of the grounding medium with respect to true earth. Most electrical systems do not rely on the earth to carry load current (this is done by the system conductors) but the earth may provide the return path for fault currents, and for safety, all electrical equipment frames are connected to ground. The resistivity of the earth is usually negligible because there so much of it available to carry current. The limiting factor in electrical grounding systems is how well the grounding electrodes contact the earth, which is known as the soil/ground rod interface. This interface resistance component, along with the resistance of the grounding conductors and the connections, must be measured by the ground test. In general, the lower the ground resistance, the safer the system is considered to be. There are different regulations which set forth the maximum allowable ground resistance, for example: the National Electrical Code specifies 25 ohms or less; MSHA is more stringent, requiring the ground to be 4 ohms or better; electric utilities construct their ground systems so that the resistance at a large station will be no more than a few tenths of one ohm.

Figure 1

TPI GROUND TEST INSTRUMENT CHARACTERISTICS

THREE-POINT RESISTANCE MEASUREMENTS The three-point method, a variation of the four-point method, is usually used when making ground (earth) resistance measurements. With the three-point method, the C1 and P1 terminals are tied together at the instrument and connected with a short lead to the ground system being tested. This simplifies the test in that only three leads are required instead of four. Because this common lead is kept short, when compared to the length of the C2 and P2 leads, its effect is negligible. Some ground testers are only capable of the three-point method, so are equipped with

• To avoid errors due to galvanic currents in the earth, TPI ground test instruments use an AC current source. • A frequency other than 60 hertz is used to eliminate the possibility of interference with stray 60 hertz currents flowing through the earth. • The three-point measurement technique is utilized to eliminate the effect of lead length. • The test procedure, known as the Fall-of-Potential Method, is described on the following page.

16

Figure 2

THREE-POINT FALL-OF-POTENTIAL TEST PROCEDURE GROUND TEST PROCEDURE In the Fall-of-Potential Method, two small ground rods – often referred to as ground spikes or probes – about 12" long are utilized. These probes are pushed or driven into the earth far enough to make good contact with the earth (8" – 10" is usually adequate). One of these probes, referred to as the remote current probe, is used to inject the test current into the earth and is placed some distance (often 100') away from the grounding medium being tested . The second probe, known as the potential probe, is inserted at intervals within the current path and measures the voltage drop produced by the test current flowing through the resistance of the earth. In the example shown on the following page, the remote current probe C2 is located at a distance of 100 feet from the ground system being tested. The P2 potential probe is taken out toward the remote current probe C2 and driven into the earth at ten-foot increments. Based on empirical data (data determined by experiment and observation rather than being scientifically derived), the ohmic value measured at 62% of the distance from the ground-under-test to the remote current probe, is taken as the system ground resistance. The remote current probe must be placed out of the influence of the field of the ground system under test. With all but the largest ground systems, a spacing of 100 feet between the groundunder-test and the remote current electrode is adequate. When adequate spacing between electrodes exists, a plateau will be developed on the test graph. Note: A remote current probe distance of less than 100 feet may be adequate on small ground systems.

Electrical Testing and Measurement Handbook – Vol. 7 When making a test where sufficient spacing exists, the instrument will read zero or very near zero when the P2 potential probe is placed near the ground-under-test. As the electrode is moved out toward the remote electrode, a plateau will be reached where a number of readings is approximately the same value (the actual ground resistance is that which is measured at 62% of the distance between the ground mat being tested and the remote current electrode). Finally, as the potential probe approaches the remote current electrode, the resistance reading will rise dramatically. It is not absolutely necessary to make a number of measurements as described above and to construct a graph of the readings. However, we recommend this as it provides valuable data for future reference and, once you are setup, it takes only a few minutes to take a series of readings. The electrical fields associated with the ground grid and the remote electrodes are illustrated on AN0009-5. An actual ground test is detailed on AN0009-6, and a sample Ground Test Form is provided on AN0009-7. See AN0009-8 for a simple shop-built wire reel assembly for testing large ground systems.

SHORT-CUT METHOD The short cut method described here determines the ground resistance value and verifies sufficient electrode spacing – and it does save time. This procedure uses the 65' leads supplied with the TPI instruments. • Connect the T1 instrument jack with the 15' green lead to the ground system being tested. • Connect the T3 instrument jack with the red lead to the remote current electrode (spike) placed at distance of 65' (full length of conductor) from the ground grid being tested. • Connect the T2 instrument jack with the black lead to the potential probe placed at 40 feet (62% of the 65' distance) from the ground grid being tested and measure the ground resistance. • Move the P2 potential probe 6' (10% of the total distance) to either side of the 40' point and take readings at each of these points. If the readings at these two points are essentially the same as that taken at the 40' point, a measurement plateau exists and the 40' reading is valid. A substantial variation between readings indicates insufficient spacing.

THREE-POINT FALL-OF-POTENTIAL METHOD INSTRUMENT SET-UP

Figure 3


17

A NOTE ON INSTRUMENT LABELING CONVENTIONS The TPI MFT5010 and TPI ERT1500 use the terminal designations T1 (C1/P1), T2 (P2), and T3 (C2). The corresponding lead designations on the MFT5010 are E (Earth), S & H. The corresponding lead designations on the ERT1500 are E (Earth), P (Potential), C (Current).

TEST CURRENT PATH • Test Current (AC ) flows from instrument T3 to remote current probe C2 on the red lead. • Test Current flows from remote current probe C2 back through the earth to the ground being tested as shown by dashed blue line. • Test current flows out of ground grid back to instrument T1 on the short green lead. • Black potential lead P1 is connected to instrument T2 and is taken out at 10' increments. It measures voltage drop produced by the test current flowing through the earth. (P1 to P2 potential)

Figure 4

EQUAL-POTENTIAL PLANES THE EXISTENCE OF EQUAL-POTENTIAL PLANES • When current flows through the earth from a remote test electrode (in the case of a ground test) or remote fault, the voltage drop which results from the flow of current through the resistance of the earth can be illustrated by equal-potential planes. The equal-potential planes are represented in the dashed lines in drawings below where the spacing between concentric lines represents some fixed value of voltage. • The concentration of the voltage surrounding a grounding element is greatest immediately adjacent to that ground. This is shown by the close proximity of lines at the point where the current enters the earth and again at the point where the current leaves the earth and returns to the station ground mat.

Figure 5

• In order to achieve a proper test using the Fall-of-Potential Ground Test Method, sufficient spacing must exist between the station ground mat being tested and the remote current electrode such that the equal-potential lines do not overlap. As shown by the black line in the Sample Plot, adequate electrode spacing will result in the occurrence of a plateau on the resistance plot. This plateau must exist at 62% of the distance between the ground mat and the remote electrode for the test to be valid. Insufficient spacing results in an overlap of these equal-potential planes, as illustrated at the bottom of this page and by the red line on the Sample Plot. • See the Safety Note on AN0009-6 for information on the hazards of Step and Touch-Potentials.

18


Figure 6

ACTUAL FIELD TEST This actual ground test was conducted on a pad-mount transformer in a rural mountain area. The single-phase transformer is supplied by a 12470/7200 volt grounded wye primary and the transformer is grounded by its own ground rod as well as being tied to the system neutral which is grounded at multiple points along the line. The distribution line is overhead with just the “dip” to the transformer being underground.

Ground Test Data Remote Current Probe C2 @ 100 Feet P2 Distance from Transformer in Feet

Instrument Reading in Ohms

10

1.83

20

3.59

30

3.85

40

3.95

50

4.0

60

4.25

62*

4.3

70

4.5

80

5.4

90

7.3

100

25.02

* Actual Ground resistance.

TEST PROCEDURE Terminal T1 of the TPI MFT5010 tester was connected to the transformer case ground with the short green lead. The remote Current Probe C2 was driven in the ground at a location 100 feet from the transformer and connected to Terminal T3 of the instrument with the red test lead.

Terminal T2 of the tester was connected, using the 100' black lead, to the P2 potential probe. This ground stake was inserted into the ground at 10' intervals and a resistance measurement was made at each location and recorded in the table above. The relatively constant readings in the 4 ohm range between 40 and 70 feet are a definite plateau that indicates sufficient lead

Electrical Testing and Measurement Handbook – Vol. 7 spacing. The initial readings close to the transformer are lower, and there is a pronounced “tip-up” as the P2 probe approaches the remote current electrode C2.

19 The measured ground resistance at 62 feet (62% of the distance) was 4.3 ohms and is taken as the system ground resistance. This is an excellent value for this type of an installation.

20

SAFETY NOTE – POSSIBLE EXISTENCE OF HAZARDOUS STEP AND TOUCH POTENTIALS It is recommended that rubber gloves be worn when driving the ground rods and connecting the instrument leads. The possibility of a system fault occurring at the time the ground test is being conducted is extremely remote. However, such a fault could result in enough current flow through the earth to cause a possible hazardous step potential between a probe and where the electrician is standing, or hazardous touch potential between the probes and the system ground. The larger the system, in terms of available fault current, the greater the possible risk.

REEL ASSEMBLY A SHOP-BUILT GROUND TEST WIRE REEL ASSEMBLY This simple, low-cost, and easy-to-build wire reel assembly is handy for making Ground (Earth) Resistance measurements on large ground systems. The unit shown below has 500 feet of wire for testing medium-to-large ground fields typical of those found in industrial plants and substations. For testing even larger systems, such as those installed for power generating plants, wire lengths of 1000 feet can be used. Wrap-on wire markers are installed every ten feet on the current lead to simplify placement of the remote current and potential probes. Your electrical distributor will probably have empty surplus reels available for the asking – the ones shown below are about 12 inches in diameter. The conductor is standard #12 THHN. Even though the TPI ERT1500 and the MFT5010 use an AC test signal, the test results are unaffected by the inductance of any wire left on the reels.



21

CLAMP-ON GROUND RESISTANCE TESTER MODELS 3711 & 3731 STEP-BY-STEP USAGE

Chauvin Arnoux, Inc. AEMC Instruments 4. Observe instrument reading – the reading should be within 1.0W of gauge specification (25W). If reading is correct, proceed to step 5. If not, clean instrument and repeat steps 3 and 4. If you are not able to get the instrument to read within 1.0W after cleaning instrument, do not proceed. Have the instrument repaired. 5. Remove instrument from gauge. Observe instrument reading with nothing in the clamps. The reading should be greater than 1000W OR read. If either of these conditions is observed, continue to step 6. If not, clean instrument (see instructions below) and repeat steps 3 through 5. If, after cleaning instrument, you are still unable to get the instrument to perform as described in steps 4 and 5, open the jaws approximately 1/2 inch and let them snap shut. Make sure that the jaws close properly. If the unit still does not perform properly, do not proceed. Have the instrument repaired. 1. Turn instrument on by pressing the green “ON/OFF” button (far right). Continue holding the green button down until the battery life indicator comes on. 2. Check battery life indicator – make sure at least 20 percent remains.

6. Switch instrument to Current Mode. (Press button labeled “A” for Amps) 7. Clamp instrument around the ground wire or rod. 8. Observe reading – if less than 1.0A, proceed to step 9. If between 1.0 and 5.0A, make note of reading and continue to step 9. If greater than 5A, terminate test and remove instrument from the ground wire or rod and correct the problem before re testing. 9. Switch instrument to Resistance (W) Mode. (Press button labeled with Ohm (W) symbol)

step 2

3. Check calibration – locate the 25W calibration gauge supplied with the tester and clamp the meter around any leg of the gauge.

10. Wait for reading to stabilize and record reading. Lock reading by pressing “HOLD”. 11. Remove instrument from ground wire or rod and reclamp to gauge. 12. Observe reading – the reading should be within 1.0W of gauge value. If reading is OK – measurement is valid. If reading is wrong, clean instrument (see instructions below) and repeat from step 4.

CLEANING THE HEADS

step 3

To ensure optimum performance, it is important to keep the probe jaw mating surfaces clean at all times. Failure to do so may result in erroneous readings. To clean the probe jaws, use a very fine sandpaper (600 grit) to avoid scratching the surface, then gently clean with a soft cloth. Make sure that the instru-

22 ment is oriented such that no debris or filings will fall into the unit while cleaning. Check with your finger afterwards to be sure that no foreign material remains on the jaw surfaces (both top and bottom).

CLAMP-ON GROUND RESISTANCE TESTING The clamp-on ground resistance testing technique offers the ability to measure the resistance without disconnecting the ground. This type of measurement also offers the advantage of including the bonding to ground and the overall grounding connection resistances.

PRINCIPLES OF OPERATION Usually, a common distribution line grounded system can be simulated as a simple basic circuit as shown in Figure A or an equivalent circuit, shown in Figure B. If voltage E is applied to any measured grounding system. Rx through a special transformer (used in Models 3711 and 3731), current I flows through

the circuit, thereby establishing the following equation. Therefore, E/I = Rx is established. If it is detected with E kept constant, measured grounding resistance can be obtained. Refer again to Figures A and B. Current is fed to a special transformer via a power amplifier from a 2.3 kHz constant voltage oscillator. This current is detected by a detection CT. Only the 2.3 kHz signal frequency is amplified by a filter amplifier. This occurs before the A/D conversion and after synchronous rectification. It is then displayed on the LCD of the Model 3711/3731 meter. The filter amplifier is used to cut off both earth current at commercial frequency and high-frequency noise. Voltage is detected by coils wound around the injection CT, which is then amplified, rectified, and compared by a level comparator. If the clamp is not closed properly, an “open jaw” annunciator appears

Electrical Testing and Measurement Handbook – Vol. 7 on the LCD. The important points to consider for proper use of the clamp-on ground tester are: 1. There is a series-parallel resistance path down stream from the measurement point that is lower in resistance than the point being measured. 2. That the earth is the return path to the point where the clamp-on meter is connected and not wire or other metal structures (see Figure C). 3. If the measurement point is not connected to a seriesparallel low resistance network (such as the case with a single rod), a temporary path may be created by connecting a jumper cable from the measurement point to a low resistance like a pole ground (see Figure D).


23

MEASURING MAGNETIC FIELDS ELECTRIC AND MAGNETIC FIELDS Australian Radiation Protection and Nuclear Agency Every thing electrical from a toaster to a high-voltage power line produces electric and magnetic fields. Both the electric and magnetic fields are strong close to an operating source. The strength of the electric field depends on the voltage and is present in any live wire whether an electrical appliance is being used or not. Magnetic fields, on the other hand, are produced by electric currents and are only present when an appliance is operating i.e. there is no magnetic field when an electrical appliance is turned off.

HEALTH EFFECTS Currently there is no evidence that exposure to electric fields is a health hazard (excluding electric shock). Whether exposure to magnetic fields is equally harmless remains an open question. A large number of scientific studies performed on animals and cells have not found a health risk. Some epidemiological studies, however, have suggested a weak link between intense and prolonged exposure to magnetic fields and childhood leukaemia.

MAGNETIC FIELD UNITS The strength of the magnetic field is expressed in units of Tesla (T) or microtesla (µT). Another unit, which is commonly used is the Gauss (G) or milligauss (mG), where 1 G is equivalent to 10-4 T (or 1 mG = 0.1 µT).

THE GAUSS METER There is a range of different instruments that can measure the magnetic field strength. The gauss meter is a hand-held device that provides a simple way of performing such measurements. ARPANSA has two different gauss meter models available for hire, which are a Teslatronics Model 70 and a Sypris Model 4080. Both these instruments operate in a similar manner and they are shown in the figure below.

Both gauss meters measure alternating fields from 25 Hz to 1000 Hz in units of mG. They do not measure and will give false readings from mobile phones. Readings taken very close (a few cm) to other electronic devices (as distinct from electrical devices such as heaters, washing machines etc) may also give false readings. Shaking or vibrating either unit may also give false readings. Since the meters only measure varying magnetic fields, they will not measure the earth’s magnetic field which is static and has a value of approximately 500 mG. When either meter is turned on, it will perform an initial self-diagnostic test by showing all available readouts on its digital display. Following the initial test, the meter will display the magnetic field intensity at the location where it is held or placed and the intensity will change if moved accordingly. If the negative sign is still showing after the initial test, that indicates that the meter is running low on power and the battery needs to be replaced.

PERFORMING MEASUREMENTS Measurements of the magnetic field in the home are generally taken in the middle of the room at about one metre from the ground or in locations where people spend a significant amount of time, for example, the bed. Measurements should also be performed several times over the course of a day. This is to allow for possible variations to electricity demand which presumably would peak during the evening at about 7.00 pm. Measurements can also be made at any other locations of interest. It is important to remember that, as mentioned earlier, research suggests that if any health effects exist, they are associated with prolonged magnetic field exposure. Measurements taken with the gauss meter are instantaneous (i.e. measured at one point in time) and do not accurately reflect prolonged exposure levels.

TYPICAL MAGNETIC FIELD STRENGTHS Magnetic fields within homes can vary at different locations and also over time. The actual strength of the field at a given location depends upon the number and kinds of sources and their distance from the location of measurement. Typical values measured in areas away from electrical appliances are of the order of 2 mG. Magnetic fields from individual appliances can vary considerably as well, depending on the way they were designed and manufactured. One brand of hair dryer, for example, may generate a stronger magnetic field than another. In general, appliances, which use a high current (such as those which have an electric motor) will lead to relatively high readings. It should also be noted that different body parts will be exposed to different magnetic field levels from the same appliance, depending on how far that part of the body is from the appliance when in use. Typical values of magnetic fields measured at normal user distance from some common domestic electrical appliances are listed in the following table.

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HOMES NEAR POWER LINES The power lines that are present in typical neighbourhoods are called “distribution” lines and they usually carry less voltage than “transmission” lines, which carry very high voltages. As stated earlier, however, it is the current and not the voltage that is associated with the strength of the magnetic field. Therefore, proximity to high voltage lines will not necessarily give a high reading unless those lines are also carrying a large current. Typical values of magnetic fields measured near power lines and substations are listed in the table below.


Electricity Testing and Measurement Handbook – Vol. 7

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ELECTRIC AND MAGNETIC FIELDS

MEASUREMENTS AND POSSIBLE EFFECT ON HUMAN HEALTH — WHAT WE KNOW AND WHAT WE DON’T KNOW California Department of Health Services and the Public Health Institute California Electric and Magnetic Fields Program INTRODUCTION

EMF: INVISIBLE LINES OF FORCE

Our daily use of electricity is taken for granted, yet scientific and public concern has arisen about possible health effects from electric and magnetic fields (EMF) that are created by the use of electricity. Because of this concern, the California Public Utilities Commission authorized a statewide research, education and technical assistance program on the health aspects of exposure to magnetic fields and asked the Department of Health Services to manage it. Even though both electric and magnetic fields are present with the use of electrical power, interest and research have focused on the effects of 50 and 60 Hertz (Hz) magnetic fields, called “power frequency” fields, from sources such as power lines, appliances and wiring in buildings. This is because it is known that magnetic fields are difficult to shield and because early scientific studies showed a possible relationship between human exposure to certain magnetic field sources and increased rates of cancer. Even now, scientists are not sure if there are health risks from exposure to 50 and 60 Hz magnetic fields, or if so what is a “safe” or “unsafe” level of exposure. People frequently ask about EMF risk when they are choosing where to live. This choice should include consideration of proven risks of the location, such as the possibility of earthquake, flooding, or fire, or the presence of traffic, radon, or air pollution. To some people even limited evidence for a possible EMF risk weighs heavily in their decisions. For others, different considerations take precedence. There really is no one right answer to these questions because each situation is unique. The California EMF Program developed this fact sheet to give an overview of the present state of knowledge and provide a basis for understanding the current limitations on the ability of science to resolve questions about the possible health risks of magnetic field exposure. This paper describes electric and magnetic fields, high field sources and how to interpret field measurements once they are made. It includes discussions of the controversy about possible health effects, as well as current California state policy and what the government is doing to address public concern.

Wherever there is electricity, there are also electric and magnetic fields, invisible lines of force created by the electric charges. Electric fields result from the strength of the charge while magnetic fields result from the motion of the charge, or the current. Electric fields are easily shielded: they may be weakened, distorted or blocked by conducting objects such as earth, trees, and buildings, but magnetic fields are not as readily blocked. Electric charges with opposite signs (positive and negative) attract each other, while charges with the same sign repel each other. The forces of attraction and repulsion create electric fields whose strength is related to “voltage” (electrical pressure). These forces of attraction or repulsion are carried through space from charge to charge by the electric field. The electric field is measured in volts per meter (V/m) or in kilovolts per meter (kV/m). A group of charges moving in the same direction is called an “electric current.” When charges move they create additional forces known as a “magnetic field.” The strength of a magnetic field is measured in “gauss” (G) or “tesla” (T), while the electric current is measured in “amperes” (amps). The strength of both electric and magnetic fields decrease as one moves away from the source of these fields.

WHAT ARE ELECTRIC AND MAGNETIC FIELDS OR “EMF” Before man-made electricity, humans were exposed only to the magnetic field of the earth, electric fields caused by charges in the clouds or by the static electricity of two objects rubbing together, or the sudden electric and magnetic fields caused by lightning. Since the advent of commercial electricity in the last century we have been increasingly surrounded by man-made EMF generated by our power grid (composed of powerlines, other electrical equipment, electrical wiring in buildings, power tools, and appliances) as well as by higher frequency sources such as radio and television waves and, more recently, cellular telephone antennas.

FIELDS VARY IN TIME An important feature of electric and magnetic fields is the way they vary in time. Fields that are steady with respect to direction, rate of flow, and strength are called “direct current” (DC) fields. Others, called “alternating current” (AC) fields, change their direction, rate of flow, and strength regularly over time. The magnetic field of the earth is DC because it changes so little in one year that it can be considered constant. However, the most commonly used type of electricity found in power lines and in our homes and work places is the AC field. AC current does not flow steadily in one direction, but moves back and forth. In the U.S. electrical distribution system it reverses direction 120 times per second or “cycles” 60 times per second (the direction reverses twice in one complete cycle). The rate at which the AC current flow changes direction is expressed in “cycles per second” or “Hertz” (Hz). The power systems in the Untied States operate at 60 Hz, while 50 Hz is commonplace elsewhere. This fact sheet focuses on “power frequency” 60 Hz fields and not the higher frequency fields generated by sources such as cellular phone antennas.

DESCRIBING MAGNETIC FIELDS The concentration of a chemical in water can be described by citing a single number. Unlike chemicals, alternating electric and magnetic fields have wave-like properties and can be described in several different ways, like sound. A sound can be loud or soft (strength), high or low-pitched (frequency), have

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periods of sudden loudness or a constant tone, and can be pure or jarring. Similarly, magnetic fields can be strong or weak, be of high frequency (radio waves) or low frequency (powerline waves), have sudden increases (“transients”) or a constant strength, consist of one pure frequency or a single dominant frequency with some distortion of other higher frequencies (“harmonics”). It is also important to describe the direction of magnetic fields in relation to the flow of current. For instance, if a magnetic field oscillates back and forth in a line it is “linearly polarized.” It may also be important to describe how a field’s direction relates to other physical conditions such as the earth’s static magnetic fields.

Table 1. Examples of magnetic field strengths at particular distances from appliance surfaces.

MEASURING MAGNETIC FIELDS AND IDENTIFYING THE SOURCES OF ELEVATED FIELDS MEASURING MAGNETIC FIELD STRENGTH The strength or intensity of magnetic fields is commonly measured in a unit called a Gauss or Tesla by magnetic field meters called “gaussmeters.” A milligauss (mG) is a thousandth of a gauss, and a microtesla (uT) is a millionth of a tesla (one milligauss is the same as 0.1 microtesla). The magnetic field strength in the middle of a typical living room measures about 0.7 milligauss or 0.07 microtesla. As noted above, the strength of the magnetic field is only one component of the mixture that characterizes the field in a particular area. Measuring only magnetic field strength may not capture all the relevant information any more than the decibel volume of the music you are playing captures the music’s full impact. The main health studies to date have only measured magnetic field strength directly or indirectly and assessed its association with disease. Some scientists wonder if the weak association between measured magnetic fields and cancer in these studies might appear stronger if we knew which aspect of the EMF mixture to measure. Other scientists wonder if any such aspect exists.

WHERE ARE WE EXPOSED TO 60 HZ EMF? There are “power frequency” electric and magnetic fields almost everywhere we go because 60 Hz electric power is so widely used. Exposure to magnetic fields comes from many sources, like high voltage “transmission” lines (usually on metal towers) carrying electricity from generating plants to communities and “distribution” lines (usually on wooden poles) bringing electricity to our homes, schools, and work places. Other sources of exposure are internal wiring in buildings, currents in grounding paths (where low voltage electricity returns to the system in plumbing pipes), and electric appliances such as TV monitors, radios, hair dryers and electric blankets. Sources with high voltage produce strong electric fields, while sources with strong currents produce strong magnetic fields. The strength of both electric and magnetic fields weakens with increasing distance from the source (table 1). Magnetic field strength falls off more rapidly with distance from “point” sources such as appliances than from “line” sources (power lines). The magnetic field is down to “background” level (supposed to be no greater than that found in nature) 3-4 feet from an appliance, while it reaches background level around 60-200 feet from a distribution line and 300-1000 feet from a transmission line. Fields and currents that occur at the same place can interact to strengthen or weaken the total effect. Hence, the strength of the fields depends not only on the distance of the source but also the distance and location of other nearby sources.

IDENTIFYING SOURCES OF ELEVATED MAGNETIC FIELDS Sometimes fairly simple measurements can identify the external or internal sources creating elevated magnetic fields. For example, turning off the main power switch of the house can rule out sources from use of power indoors. Magnetic field measurements made at different distances from power lines can help pinpoint them as sources of elevated residential magnetic fields. Often, however, it takes some detective work to find the major sources of elevated magnetic fields in or near a home. Currents in grounding paths (where low voltage electricity returns to the system in plumbing pipes) and some common wiring errors can lead to situations in which source identification is difficult and requires a trained technician. It is almost always possible to find and correct the sources of elevated magnetic fields when they are due to faulty electrical wiring, grounding problems, or appliances such as lighting fixtures.

60 HZ MAGNETIC FIELD EXPOSURE DURING A TYPICAL DAY Exposure assessment studies of adults who wore measurement meters for a 24- to 48-hour period suggest that the average magnetic field level encountered during a typical 24 hours is about 1 mG. About 40% of magnetic field exposures found in homes come from nearby power lines, while 60% come from other sources such as stray currents running back to the electrical system through the grounding on plumbing and cables, current “loops” due to incorrect internal wiring in the home, and brief exposure to appliances and electrical tools.


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MAGNETIC FIELD SURVEY OF HOMES IN THE SAN FRANCISCO BAY AREA

DOSE-RESPONSE RELATIONSHIP

The California Department of Health Services surveyed homes in the San Francisco Bay Area in the mid-1990s. In this study, magnetic field measurements were taken in the middle of the bedroom, family room and kitchen and at the front door of these homes under normal power conditions (any appliances or electrical devices turned on at the onset of the measurement period were left on). As shown in table 2, about half the houses in the Bay Area had an average level below 0.71 mG and 90 percent had average levels below 1.58 mG.

A special problem in the study of health effects of environmental factors is how to measure exposure in a way that adequately reflects the true amount of the person’s exposure to the substance being studied. This true amount is called the “dose.” With cigarette smoke and toxic chemicals, there is a positive relationship between the size (or strength) of the dose and the adverse health effect it produces: the higher the dose, the greater the effect. With magnetic fields, however, some laboratory evidence suggests that this is not always the case, and very confusing relationships have been seen. Biological effects or changes appear at strengths of certain levels, disappear at higher levels, only to appear again at still higher levels. Varying the frequency (speed of alternation), for example from 60 Hz to 120 Hz, shows similar “effect windows” of magnetic fields. To complicate things further, some laboratory experiments have shown an effect with intermittent (“pulsed”) exposures, others with “spikes” or transients, and still others with continuous exposure. There is some evidence that the orientation of alternating fields in relation to the direction of the earth’s static magnetic field is also important in making a biological effect. Generally, the effects observed are only biological changes that may or may not translate into true health effects.

Table 2. Distribution of average magnetic field strength of San Francisco Bay Area homes.

LIMITATIONS OF DIRECT MAGNETIC FIELD MEASUREMENTS MAGNETIC FIELDS GENERATED BY CURRENT FLOWING THROUGH WIRES CAN BE REDUCED Two wires with current flowing in opposite directions create magnetic fields going in opposite directions. If the wires are placed close together and have currents of similar magnitude the magnetic fields cancel each other. This principle is often used to lower magnetic fields. For example, an underground distribution cable has a “hot” line (carrying current to the user) and a “neutral” line (carrying it away) that generate low magnetic fields when they are placed close together. The underground cables can be placed close together because it is possible to insulate them heavily to prevent arcing. Overhead power lines cannot be placed this close together because of the weight of the needed insulation and the need for worker safety. For most distribution and transmission lines, however, California utilities use three-wire or four-wire systems. The current in these lines alternates in strength and direction in slightly different phases (not alternating completely together). It is sometimes possible to optimize these phase differences so that the magnetic fields from the wires cancel each other.

WHAT CAN WE SAY ABOUT A MEASUREMENT ONCE WE HAVE IT? A concerned person would like to know if the measurements found in his or her home are “safe” or “unsafe.” Right now, most scientists do not feel that the data are solid enough to make predictions about the health risks of magnetic field strength. When magnetic field exposure (or its estimate) increases there is no evident orderly increase of a health risk. The highest level of magnetic field strength measured in homes is below the intensity found in almost all the cellular and animal experiments that have produced subtle biological effects. This makes scientists and policy makers reluctant to set health-based standards for magnetic field exposures. However, it is possible to find out how measurements in your home compare to other homes and if these measurements are “typical” or not. The information in tables 1 and 2 may be helpful in deciding if your home is typical.

Those human health studies investigating the relationship of magnetic field exposure and cancer measured magnetic fields using one-time, short-term measures (i.e., for 24 hours) of one area such as the bedroom, or one-time spot measurements (i.e., for one minute) in several different rooms of the participants’ homes. It was assumed that these home measurements adequately estimate a person’s total exposure. However, these measures can not be used to assess the biological importance of the length of exposure, the number of times there are high exposures, or the presence of other components of the field such as harmonics. Also, field intensity (strength) varies at different times of day and different seasons, depending on electricity use. Dinnertime readings are often higher than readings in the middle of the night. In addition, an area measure may not reflect a personal exposure that is dependent on the amount of time a person spends in the area measured.

CONTROVERSY ABOUT POSSIBLE HEALTH EFFECTS The controversy about EMF health effects derives from: 1) the fact that many scientists believe power line magnetic fields emit little energy and are therefore too weak to have any effect on cells; 2) the inconclusive nature of laboratory experiments; and 3) the fact that epidemiological studies of people exposed to high EMF are inconclusive.

1. WEAK FIELDS MAY HAVE TOO LITTLE ENERGY TO CAUSE BIOLOGICAL EFFECTS The electromagnetic spectrum covers a large range of frequencies (expressed in cycles per second or Hertz). The higher the frequency, the greater the amount of energy in the field. Xrays have very high frequencies, and are able to ionize molecules and break chemical bonds, which damages genetic material and can eventually result in cancer and other health disorders. High frequency microwave fields have less energy than x-rays, but still enough to be absorbed by water in body tissues, heating them and possibly resulting in burns. Radio frequency fields from radio and TV transmitters are another step weaker than microwaves. Although they alternate millions of times per second, they

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can’t ionize molecules and can only heat tissues close to the transmitter. Electric power fields (50 and 60 Hz) have much lower frequencies than even radio waves and hence emit very low energy levels that do not cause heating or breakage of bonds. They do create electrical currents in the body, but in most cases these currents are much weaker than those normally existing in living organisms. For these reasons, many scientists argue that it is unlikely that 60 Hz power frequency magnetic fields at the strengths commonly found in the environment have any physical or biological effects on the body.

leukemia and residential proximity to this type of power lines.1 For these studies, a child’s exposure to magnetic fields was estimated three ways. First, the type and proximity of power lines (“wire codes”) near the child’s home was assessed. Those houses with lines nearby with the potential to carry high current were classified as “high current configuration” and were assumed to have higher magnetic field levels (due to higher current) than houses near lower current configuration power lines (figure 1). Second, exposure was estimated by measurements of magnetic fields taken in the child’s home at the time of the study – often many years after diagnosis of their cancer. And third, exposure was approximated by estimating what the home magnetic field levels were right after the children were diagnosed, using line distance from the house and past utility records of current flow in the lines during the appropriate time period. The NRC made a statistical summary and comparison of these eleven studies. They concluded that children living in high current configuration houses are 1.5 times as likely to develop childhood leukemia than children in other homes. Despite this conclusion, the NRC was a unable to explain this elevated risk and recommended that more research be done to help clarify the issue. One reason for this uncertainty is that wire-code classification assumes that houses with high wire-codes have higher magnetic field levels than low wire-code houses, but high wirecodes may also be a proxy for some type of exposure besides magnetic fields that is not yet understood. For example, high wire-code houses tend to have higher traffic density nearby, resulting in higher air pollution levels. However, traffic density seems to be an unlikely explanation for the wire-code association found in these studies. In 1997, the NRC statement seemed to be contradicted by the findings of Dr. M. S. Linet of the National Cancer Institute in a large epidemiological study1i. Her researchers estimated exposure to magnetic fields in two ways, wire-codes as defined above (based on distance of different types of power lines near the home) and home area measurements. The study found no association between living in high wire-code houses and childhood leukemia. On the other hand, the study found that children living in houses with high average magnetic field levels did have higher rates of cancer in general.

2. INCONSISTENT LABORATORY RESULTS As stated above, 60 Hz power frequency magnetic fields do create weak electric currents in the bodies of people and animals. In the mid-1970s a variety of laboratory studies in cell cultures and whole animals demonstrated that these fields produce biological changes when applied in intensities of hundreds or thousands of milligauss. Some scientists observed effects at lower strengths, but average daily personal exposure is only about 1 mG. Biological effects that seem to be attributable to magnetic fields are subtle and difficult to reproduce. These studies are continuing in an effort to understand how magnetic fields affect living tissue. Some laboratory scientists have found that magnetic fields can produce changes in the levels of specific chemicals the human body makes (such as the hormone melatonin), as well as changes in the functioning of nerve cells and nervous systems of other animals. However, the jury is still out as to whether this type of change can lead to any increased risk to human health. In the mid-1990s, scientists conducted a series of EMf animal studies. Most of these studies showed little or no association between EMF and cancer or adverse reproductive effects. This convinced some scientists that EMF’s were harmless. However, others pointed out that the animals’ EMF exposures in these studies might not adequately capture some aspect of EMF exposure that could have biological effects on humans.

3. INCONCLUSIVE EPIDEMIOLOGICAL STUDIES Epidemiology examines the health of groups of people, and epidemiological studies make statistical comparisons about how often diseases occur in “exposed” and “nonexposed” groups. Studies in which the disease rate is higher for the exposed group than nonexposed (said to have “positive” results) do not necessarily show a direct cause for disease, but rather indicate that there is some sort of relationship between exposure and disease. Most epidemiological studies of magnetic fields have been of two types. One kind focused on children with cancer to see whether their home magnetic field measurements were higher or if they were more likely to live in homes with overhead powerlines carrying high current than a comparable group of children without cancer. The other type of study looked at rates of death and disease of adults assumed to be heavily exposed to magnetic fields at work, with exposure often indirectly assessed by using job titles, to determine if their rates were higher than adults assumed to be working in low magnetic field environments.

CHILDHOOD CANCER STUDIES Public concern has arisen because of media reports about epidemiological studies that showed an association between childhood cancer and proximity to high current-carrying overhead power lines. In 1996, a special committee of the National Research Council (NRC) made a careful review of 11 epidemiological studies examining the relationship between childhood

Figure 1. Summary of results of power line distance(“wire code”) and childhood leukemia studies.


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THE EMF RAPID PROGRAM WORKING GROUP STATEMENT ON CHILDHOOD LEUKEMIA

magnetic fields? First, no magnetic field epidemiological study has found an association with disease that is as strong as that implicating a two-pack-a-day smoking habit. The strength of the association found for leukemia in electric train engineers, who are exposed to magnetic fields of hundreds of milligauss all day long, is no stronger than the strength of the association relating residential magnetic field levels (generally less than 10 mG) to childhood leukemia. Second, there is no laboratory evidence about magnetic field exposure that is as convincing as that for lung cancer and smoking— magnetic field animal studies have been inconsistent. These differences make scientists much more cautious about interpreting the magnetic field epidemiology as dangerous than the environmental tobacco smoke epidemiology.

In 1998, a working group of experts gathered by the federal EMF RAPID program (see “Governmental Regulation,” below) reviewed the research on the possible health risks associated with EMF. A majority felt that the epidemiology studies of childhood leukemia provide enough evidence to classify EMF as a “possible human carcinogen,” meaning they think it might cause cancer. This does not mean that it definitely causes cancer, however. The working group’s findings are published in a report posted on the program’s Web site (see address below).

IF REAL, HOW IMPORTANT WOULD THIS RISK OF CHILDHOOD LEUKEMIA BE? Each year an average of six cases of leukemia are diagnosed per 100,000 children. Six percent of American houses are near high-current-carrying power lines.2 If the epidemiological association is correct that means that in such houses there would be three additional cases of leukemia among 100,000 children due to the effects of EMF from the nearby power lines. (This is almost the increased risk of lung cancer of an adult nonsmoker who lives in a smoking household.) Among the 500,000 children in California who live nearest high-current-carrying power lines there could be a theoretical 15 extra cases of leukemia each year compared to the number of cases if they lived further away. In California, we regulate chemicals whose typical exposures generate a theoretical life-time risk of one per 100,000. An added risk of three sick children per 100,000 per year is larger than this. From an individual’s point of view, this risk, if real, would be small: 99,991 out of 100,000 children would not get leukemia each year.

OCCUPATIONAL STUDIES The occupational studies looking at magnetic field exposure and various health outcomes show mixed results. Occupations assumed to have higher than normal magnetic field levels included electricians, telephone linemen, electric welders, electronic technicians, utility workers, electrical engineers and sewing machine operators. In general, but not always, workers of these occupations were more likely to have higher rates of brain tumors, leukemia, testicular tumors and male breast cancer than expected. A particular brain tumor (astrocytoma) occurred more often among men who worked for many years in jobs with high estimated exposure levels such as electricians, linemen, and electrical engineers.3 A large study of Canadian and French utility workers found an association between estimated high magnetic field exposures based on area measures of certain occupations and myeloid leukemia, a rare type of blood cancer.4 On the other hand, another large study found no increase in mortality from brain tumors, leukemia or other cancers among electrical workers with estimated high magnetic field exposure over many years.5 Differences among study results may exist simply because the studies used different study populations and methods for estimating high occupational magnetic field exposure. Also, these surrogate measures estimating high occupational magnetic field levels could be proxies for other types of exposure at work besides magnetic fields.

COMPARING THE SCIENTIFIC EVIDENCE ON MAGNETIC FIELDS TO THAT OF ENVIRONMENTAL TOBACCO SMOKE There are regulations in place protecting us from environmental tobacco smoke. They are based on the strength of its association with disease and the consistent epidemiological evidence for it. What’s the difference between this evidence and that for

GOVERNMENTAL REGULATION STATE REGULATIONS Lack of understanding has kept scientists from recommending any health-based regulations. Despite this, several states have adopted regulations governing transmission line-generated magnetic fields at the edge of the “right-of-way” (“ROW,” the area immediately surrounding power lines left clear for access for maintenance and repairs) because of concern about the risk of electric shock from strong electric fields present in these areas (table 3). All current regulations relate to transmission lines; none govern distribution lines, substations, appliances or other sources of electric and magnetic fields. The California Department of Education requires minimum distances between new schools and the edge of transmission line rights-of-way. The setback guidelines are: 100 feet for 50-133 kV lines, 150 feet for 220-230 kV lines, and 350 feet for 500-550 kV lines. Once again, these were not based on specific biological evidence, but on the rationale that the electric field drops to background levels at the specified distances. Table 3. Transmission line EMF standards and guide-lines adopted by certain states for utilities’ rights-of-way (ROW).

The California Public Utilities Commission (CPUC), upon the recommendation of a Consensus Group composed of citizens, utility representatives, union representatives, and public officials, recommended that the state’s investor-owned utilities carry out “no and low cost EMF avoidance measures” in construction of new and upgraded utility projects. This means that 4% of the total project cost is allocated to mitigation measures if these measures will reduce magnetic field strength by at least 15%. The strategy is to address public concern and cope with

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potential but uncertain risks until a policy based on scientific fact can be developed. The CPUC also followed the Consensus Group’s recommendation to establish the research, education and technical assistance programs of the California EMF Program under the guidance of the California Department of Health Services. It is expected to provide information that will be useful to those responsible for making public policy in the future.

computer monitor. Table 1 shows how quickly fields fall off as one moves away from appliances – they virtually disappear at 35 feet. You might stop using an electric appliance you do not really need. You may also consider home testing, which can identify faulty electrical wiring that can produce shock hazards and current code violations as well as elevated magnetic fields. In California, the investor-owned utilities are required by the CPUC to provide magnetic field measurement at no charge to their customers. So far, in the absence of conclusive scientific evidence, there is no sufficient basis for enacting laws or regulations to limit people’s exposure to EMF, so it is up to individuals to decide what avoidance measures to take, based on the information available.

FEDERAL EFFORTS At the Federal level, the Federal Energy Policy Act of 1992 included a five-year program of electric and magnetic field (EMF) Research and Public Information Dissemination (EMF-RAPID). The EMF-RAPID Program asked these questions: Does exposure to EMF produced by power generation, transmission, and use of electric energy pose a risk to human health? If so, how significant is the risk, who is at risk, and how can the risk be reduced? In 1998, a working group of experts gathered by the EMF-RAPID Program met to review the research that has been done on the possible health risks associated with EMF. This group reviewed all of the studies that have been done on the subject, and then voted on whether they believed that exposure to EMF might be a health risk. They then published a report describing their findings. A majority of the scientists on this working group voted that the epidemiology studies of childhood leukemia and residential EMF exposures provide enough evidence to classify EMF as a “possible human carcinogen.”6 This means that, based on the evidence, these researchers believe that it is possible that EMF causes childhood leukemia, but they are not sure. About half of the group’s members thought that there is also some evidence that workplace exposure to EMF is associated with chronic lymphocytic leukemia in adults. The group also concluded that there was not enough evidence to determine whether EMF exposure might cause other diseases.6 The EMF-RAPID Program released its final report to Congress in 1999. This report explains the program’s findings, including the results of its working group and many research projects. The final report states that “the NIEHS believes that there is weak evidence for possible health effects from [power frequency] ELF-EMF exposures, and until stronger evidence changes this opinion, inexpensive and safe reductions should be encouraged.”7 (page 38) The report specifically suggests educating power companies and individuals about ways to reduce EMF exposure, and encouraging companies to reduce the fields created by appliances that they make, when they can do so inexpensively7 (page 38). For more information on the EMF-RAPID program or to look at these reports, contact the EMF-RAPID Program, National Institute of Environmental Health Sciences, National Institutes of Health, P.O. Box 12233, Research Triangle Park, North Carolina27709, or visit their Web site at http://www.niehs.nih.gov/ emfrapid. When ordering a copy of the final report, refer to NIH publication number 99-4493.

CONCLUSION Public concern about possible health hazards from the delivery and use of electric power is based on data that give cause for concern, but which are still incomplete and inconclusive and in some cases contradictory. A good deal of research is underway to resolve these questions and uncertainties. Until we have more information, you can use “no and low cost avoidance” by limiting exposure when this can be done at reasonable cost and with reasonable effort, like moving an electric clock a few feet away from a bedside table or sitting further away from the

REFERENCES 1. a) Wertheimer N et al. Electrical wiring configurations and childhood cancer. American Journal of Epidemiology. 1979; 109:273-84. b) Fulton JP et al. Electrical wiring configurations and childhood leukemia in Rhode Island. American Journal of Epidemiology. 1979; 111:292-96. c) Savitz DA et al. Case control study of childhood cancer and exposure to 60-Hz magnetic fields. American Journal of Epidemiology. 1988; 128:21-38. d) Coleman M et al. Leukaemia and residence near electricity transmission equipment: A case-control study. British Journal of Cancer. 1989; 60:793-98. e) London SJ et al. Exposure to residential electric and magnetic fields and risk of childhood leukemia. American Journal of Epidemiology. 1991; 134:923-37. f) Feychting M. et al. Magnetic fields and cancer in children residing near Swedish high-voltage power lines. American Journal of Epidemiology. 1993; 138:467-81. g) Fajardo-Gutierrez AJ et al. Residence close to high-tension electric power lines and its association with leukemia in children (Spanish). Biol Med Hosp Infant Mex. 1993; 50:32-38. h) Petridou ED et al. Age of exposure to infections and risk of childhood leukaemia. British Medical Journal. 1993; 307:774. i) Linet MS et al. Residential exposure to magnetic fields and acute lymphoblastic leukemia in children. New England Journal of Medicine. 1997; 337:1-7. 2. Zaffanella L. Survey of residential magnetic sources. EPRI Final Report. 1993; No. TR 102759-v1. No. TR 102759-v2. 3. Savitz DA et al. Magnetic field exposure in relation to leukemiaand brain cancer mortality and electric utility workers. American Journal of Epidemiology. 1995; 141: 1-12. 4. Theriault G et al. Cancer risk associated with occupationalexposure to magnetic fields among utility workers in Ontario and Quebec, Canada and France. American Journal of Epidemiology. 1994; 139: 550-572. 5. Sahl JD et al. Cohort and nested case-control studies of hematopoietic cancers and brain cancer among electric utility workers. Epidemiology. 1993; 4: 104-114. 6. National Institute of Environmental Health Sciences. Assessment of health effects from exposure to power-line frequency electric and magnetic fields. NIEH Working Group Report. 1998. 7. National Institute of Environmental Health Sciences. Health effects from exposure to power-line frequency electric and magnetic fields. NIEH Final Report ot Congress. 1998.


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A NEW APPROACH TO QUICK, ACCURATE, AFFORDABLE FLOATING MEASUREMENTS Tektronix IsolatedChannel Technology Engineers and technicians often need to make “floating” measurements where neither point of the measurement is at ground (earth) potential. This measurement is often referred to as a differential measurement. “Signal common” may be elevated to hundreds of volts from earth. In addition, many of these differential measurements require the rejection of high common-mode signals*1 in order to evaluate low-level differential signals. Unwanted ground currents can also add bothersome hum and ground loops. Too often, users resort to the use of potentially dangerous measurement techniques to overcome these problems. The TPS2000 Series oscilloscopes use innovative Isolated Channel technology to deliver the world’s first 4-isolated-channel, battery-operated oscilloscope to allow engineers and technicians to make multi-channel isolated measurements quickly, accurately and affordably – all designed with your safety in mind.

FLOATING AN OSCILLOSCOPE: A DEFINITION “Floating” a ground-referenced oscilloscope is the technique of defeating the oscilloscope’s protective grounding system – disconnecting “signal common” from earth, by either defeating the grounding system or using an isolation transformer. This technique allows accessible parts of the instrument such as chassis, cabinet, and connectors to assume the potential of the probe ground lead connection point. This technique is dangerous, not only from the standpoint of elevated voltages present on the oscilloscope (a shock hazard to the operator), but also due to cumulative stresses on the oscilloscope’s power transformer insulation. This stress may not cause immediate failure, but may lead to future dangerous failures (a shock and fire hazard), even after returning the oscilloscope to properly grounded operation. Not only is floating a ground-referenced oscilloscope dangerous, but the measurements are often inaccurate. This potential inaccuracy results from the total capacitance of the oscilloscope chassis being directly connected to the circuit-under-test at the point where the ground lead is connected. *1 A “common-mode signal” is defined as a signal that is present at both points in a circuit. Typically referenced to ground, it is identical in amplitude, frequency, and phase. Making a floating measurement between two points requires rejecting the “common-mode signal” so the difference signal can be displayed.

A GUIDE TO MAKING QUICK, ACCURATE AND AFFORDABLE FLOATING MEASUREMENTS There are several products that enable you to make floating measurements, but they may lack the versatility, accuracy or affordability that you need. In addition, there are four key measurement considerations that a user needs to take into account when selecting the right product to make an accurate floating or differential measurement:

Management and Safety in the Workplace While the subject of this technical note is floating measurements, some definitions of terms and general precautions must be understood before proceeding. Historically, floating measurements have been made by knowingly defeating the built-in safety ground features of oscilloscopes or measurement instruments in various manners.

THIS IS AN UNSAFE AND DANGEROUS PRACTICE AND SHOULD NEVER BE DONE! Instead, this technical note describes instruments, accessories, and practices that can make these measurements safely as long as standard safety practices and precautions are observed. When making measurements on instruments or circuits that are capable of delivering dangerously high-voltage, high-current power, measurement technicians should always treat exposed circuits, bus-bars, etc., as being potentially “live,” even when circuits have been shut off or disconnected. This is particularly true when connecting or disconnecting probes or test leads.

1 – What is the differential measurement range? 2 – What is the common mode measurement range? 3 – What are the loading characteristics of the probe? Are they balanced or unbalanced? 4 – What is the Common Mode Rejection Ratio (CMRR) over the measurement frequency range?

TRADITIONAL OSCILLOSCOPES Traditional oscilloscopes are limited to making groundreferenced measurements. Let’s examine why: Most oscilloscopes have their “signal common” terminal connected to the protective grounding system, commonly referred to as “earth” ground or just “ground”. This is done so that all signals applied to, or supplied from, the oscilloscope have a common connection point. This common connection point is usually the oscilloscope chassis and is held at (or very near to) zero volts by virtue of the third-wire ground in the power cord for AC-powered equipment. It also means that, with few exceptions, all measurements must be made with respect to earth ground. This constrains the typical oscilloscope (at least in a single measurement) from being used to measure potential differences between two points where neither point is at earth ground. A common, but risky, practice is to disconnect the oscilloscope’s AC main power cord ground and attach the probe ground lead to one of the test points. Tektronix strongly recommends against this unsafe measurement practice. Unfortunately, this practice puts the instrument chassis, which is no longer grounded to earth, at the same voltage as the test point that the probe ground lead is connected to. The user touching the instrument

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becomes the shortest path to earth ground. Figure 1 illustrates this dangerous situation. V1 is the “offset” voltage above true ground, and VMeas is the voltage to be measured. Depending upon the unit-under-test (UUT), V1 may be hundreds of volts, while VMeas might be a fraction of a volt.

complexity to the measurement apparatus. They may require an independent power supply, and their gain and offset characteristics must be factored into every measurement. Differential probeequipped oscilloscopes emphasize performance and safety (bandwidth, isolation), trading off form-factor benefits such as portability and cost.

SIGNAL FIDELITY BEGINS AT THE PROBE TIP

Figure 1: A floating measurement in which dangerous voltages occur on the oscilloscope chassis. V1 may be hundreds of volts.

Floating the chassis ground in this manner threatens the user, the UUT, and the instrument. In addition, it violates industrial health and safety regulations, and yields poor measurement results. Moreover, line-powered instruments exhibit a large parasitic capacitance when floated above earth ground. As a result, floating measurements will be corrupted by ringing, as shown in Figure 2.

An oscilloscope is actually a measurement system consisting of preamplifiers, acquisition/measurement circuits, displays, and probes. The role of the probe is sometimes overlooked. Nevertheless, improper probes or probing techniques can affect the measurement outcome. Obviously, it’s essential to use compatible probes that match the instrument’s bandwidth and impedance. Less understood is the effect of ground-lead inductance. As lead length increases, parasitic inductance increases (Lparasitic in Figure A). Lparasitic is in the signal path and forms a resonant LC circuit with the inherent parasitic capacitance of the oscilloscope (Cparasitic). As Lparasitic increases, the resonant frequency decreases, causing “ringing” (see Figure 2) that visibly interferes with the measured signal. Simply stated, the common lead must be as short as physical constraints of the circuit-under-test will allow.

Figure 2: Parasitic inductance and capacitance can affect measurement quality

Figure 2: Ringing caused by parasitic inductance and capacitance distorts the signal and invalidates measurements

Battery-operated oscilloscopes, such as the TDS3000B Series oscilloscopes, when operated from AC line power using a standard power cord, exhibit the same limitations as traditional oscilloscopes. However, AC power is not always available where you want to make oscilloscope measurements. In the case of the TDS3000B Series oscilloscopes, the optional battery pack (TDS3BATB) allows you to operate the oscilloscope without the need for AC power. However, it can only make safe floating measurements up to 30 VRMS. Traditional oscilloscopes emphasize performance (bandwidth, versatility), trading off the ability to make floating measurements.

DIFFERENTIAL OR ISOLATED PROBES Differential or isolated probes offer a safe and reliable way to adapt a grounded oscilloscope to make floating measurements. Neither of the two probe contacts need be at earth ground and the probe system as a whole is isolated from the oscilloscope’s chassis ground. Differential probes offer a balanced impedance load to the device-under-test (DUT). However, they add a layer of cost and

In regard to capacitance, even isolated, battery-powered oscilloscopes exhibit capacitance with respect to earth ground. In Figure A, Cparasitic describes the oscilloscope’s parasitic capacitance from its ground reference (through the isolated housing) to earth ground. Like parasitic inductance, Cparasitic must be kept to a minimum in order to force the resonant frequency of the LC circuit as high as possible. If Cparasitic is large, ringing may occur within the test frequency range, hampering the measurement. An instrument’s parasitic capacitance to ground is dictated by its internal design. The physical environment can also prompt ringing. Holding the instrument or placing it on a large conductive surface during measurements can actually increase Cparasitic and lead to ringing. For extremely sensitive measurements, it might even be necessary to suspend the oscilloscope in mid-air!

A NEW APPROACH TO QUICK, ACCURATE, AFFORDABLE FLOATING MEASUREMENTS The most common method of isolation in a wide bandwidth oscilloscope system in use today is a two-path approach in which the input signal is broken up into two signals: low frequency and high frequency. This approach requires expensive optocouplers and wideband linear transformers for each input channel. The TPS2000 Series uses an innovative approach, Isolated Channel technology, which eliminates the two-path method and uses only one wideband signal path for each input channel – from DC to the bandwidth of the oscilloscope. This


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patent-pending technology enables Tektronix to offer the world’s first four-input Isolated Channel, low-cost, battery-operated oscilloscope, featuring eight hours of continuous battery operation. The TPS2000 Series oscilloscopes are ideal for engineers and technicians who need to make four-channel isolated measurements and need the performance and ease-of- use of a low-cost, battery operated oscilloscope. The TPS2000 Series’ four Isolated Channel input architecture provides true and complete channel-to-channel isolation for both the “positive” input and the “negative reference” leads, including the external trigger input. Figure 3 illustrates the Isolated Channel concept. The most demanding floating measurement requirements are found in power control circuits, such as motor controllers and uninterruptible power supplies, and industrial equipment. In such application areas, voltages and currents may be large enough to present a threat to users and test equipment. Isolated Channel technology is the preferred solution for measurement quality and is designed with your safety in mind.*2 The TPS2000 oscilloscopes offer an ideal solution when a large common mode signal is present. True channel-to-channel isolation minimizes parasitic effects; the smaller mass of the measurement system is less prone to interaction with the environment.

quency information, such as glitches and edge anomalies that eludes other oscilloscopes in its class, so that you can be sure to get a complete view of your signal to speed debug and characterization.

Figure 3: TPS2000 Series oscilloscope’s Isolated Channel architecture provides complete isolation from dangerous voltages

A properly isolated battery-powered instrument doesn’t concern itself with earth ground. Each of its probes has a “Negative Reference” lead that is isolated from the instrument’s chassis, rather than a fixed ground lead. Moreover, the “Negative Reference” lead of each input channel is isolated from that of all other channels. This is the best insurance against dangerous short circuits. It also minimizes the signal degrading impedance that hampers measurement quality in single-point grounded instruments. The TPS2000 Series oscilloscope inputs are always floating whether operated from battery power or connected to AC power through an AC power adapter. Thus, these oscilloscopes do not exhibit the same limitations as traditional oscilloscopes.

SPEED DEBUG AND CHARACTERIZATION WITH DRT SAMPLING TECHNOLOGY (TIP) The TPS2000 Series oscilloscopes offer digital real-time (DRT) acquisition technology that allows you to characterize a wide range of signal types on up to four channels simultaneously. Up to 2 GS/s real-time sample rate is the key to the extraordinary bandwidth – 200 MHz in the TPS2024. This bandwidth/ sample rate combination makes it easy to capture the high-fre-

MAKING QUICK, ACCURATE FLOATING MEASUREMENTS WITH TPS2000 SERIES OSCILLOSCOPES POWER CONTROL CIRCUITS: Power control technologies use both high-power silicon components and low-power logic circuits. The switching transistors at the heart of most power control circuits require measurements not referenced to ground. Moreover, the power circuit may have a different ground point (and therefore a different ground level) than the logic circuit, yet the two often must be measured simultaneously. *2 Do not float the P2220 probe common lead to > 30 VRMS. Use the P5120 probe (floatable to 600 VRMS CAT II or 300 VRMS CAT III) or a similarly rated passive high-voltage probe, or an appropriately rated high-voltage differential probe when floating the common lead above 30 VRMS, subject to the ratings of such high-voltage probe.

The channel-to-channel isolation of the TPS2000 Series provides a real-world measurement advantage in addition to its obvious safety benefits. Figure 4 is a screen image depicting waveforms taken at two different points in a power control circuit. Notice that the lower waveforms are about 200 A p-p, while the upper trace is about 5 V p-p. Because each of the TPS channels is fully isolated from the other (including the negative reference leads), and equipped with its own uncompromised Digital Real Time digitizer, there’s no cross-talk between the two signals. Were the oscilloscope channels not adequately isolated, there might be misleading artifacts coupled from the 200 A signal to the smaller waveform; these might be misinterpreted as a circuit problem when in reality it’s an instrument problem. The ability of the TPS Series to discretely capture two waveforms of vastly differing amplitudes reduces guesswork and improves productivity.

Figure 4: The 4-channel TPS2024oscilloscope’s channel-to-channel isolation eliminates cross-talk effects when large and small signals are captured simultaneously

HARMONICS MEASUREMENTS REVEAL UNSEEN POWER PROBLEMS An understanding of the harmonics within a power grid is essential to the safe and cost-effective use of electrical power. Line harmonics are a growing problem in a world moving increasingly toward nonlinear power supplies for most types of electronic equipment. Nonlinear loads, such as switching power supplies, tend to draw non-sinusoidal currents. Their impedance varies over the course of each cycle, creating sharp positive and negative current peaks rather than the steady curve of a sine

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wave. The rapid changes in impedance and current in turn affect the voltage waveform on the power grid. As a result, the line voltage is corrupted by harmonics; the normally sinusoidal shape of the voltage waveform may be flattened or distorted. There’s a limit to the amount of harmonic distortion that equipment can tolerate. Load-induced harmonics can cause motor and transformer overheating, mechanical resonances, and dangerously high currents in the neutral wires of three phase equipment. In addition, line distortions may violate regulatory standards in some countries. The TPS2024’s comprehensive, four-channel capability, along with its optional power analysis software, enables connection to all three conductors of a three-phase system to measure and analyze line harmonics. Its “Harmonics” mode – invoked with a single button–captures the fundamental frequency plus harmonics 2 through 50. Using only the oscilloscope’s standard voltage probe, it’s possible to execute a harmonic voltage measurement. An optional current probe acquires current harmonics with the same ease. Figure 5 illustrates a current harmonic measurement. The amplitudes are computed by the instrument’s internal DFT (Discrete Fourier Transform) algorithm. In this case the bar graph reveals a very strong fifth harmonic level. Excessive fifth harmonic levels (along with certain other odd harmonics) are a classic cause of neutral-wire currents in three-phase systems.

require a current probe (or its equivalent) and a voltage probe working in tandem. All of these measurements employ the instrument’s one-button application function. Figure 6: TPS Series’ instantaneous power analysis

Figure 7: TPS Series’ waveform analysis Figure 8: TPS Series’ dv/dt and di/dt cursors (dv/dt cursors shown)

MEASURING SWITCHING LOSS TO IMPROVE PRODUCT EFFICIENCY Today’s power designers face increasing pressure to improve the efficiency of their power designs. A major factor affecting the efficiency is the power loss occurring in the switching section of the design. Optimizing this factor can prove complex. The TPS Series allows the designer to look at switching losses in their design through the instrument’s one-button application function. The switching loss will be characterized as turnon loss, turn-off loss, conduction loss and total device loss. Figure 9 is a TPS Series screen image showing the switching loss measurements.

Figure 5: Harmonic distortion measurements

POWER READINGS – MORE THAN JUST WATTS Voltage and current measurements are by nature straightforward and absolute. A test point has only one voltage and one current value at a given instant in time. In contrast, power measurements are voltage-, current-, time-, and phase-dependent. Terms like “reactive power” and “power factor,” which were devised to characterize this complex interaction, are not so much measurements as computations. The power factor is of particular interest in these computations. This is because many electrical power providers charge a premium to users whose power factor is not sufficiently close to 1.0, the ideal value. At a power factor of 1.0, voltage and current are in phase. Inductive loads – especially large electric motors and transformers – cause voltage and current to shift phase relative to each other, reducing the power factor. Some utility companies apply a surcharge in such cases because the inefficiency causes energy loss in the form of heat in the power lines. There are procedures to remedy power factor problems, but first the power characteristics must be quantified. The TPS Series embraces a full suite of power measurements. Among these are true power, reactive power, crest factor, phase relationships, di/dt and dv/dt, and of course power factor. Figures 6, 7 and 8 show TPS Series screen images summarizing these and other power measurements. All of the measurements, with the exception of waveform analysis and phase relationships,

Figure 9: TPS Series’ switching loss display showing turn-on, turn-off and conduction losses

CONCLUSION Engineers and technicians confront high voltages and currents and must often make potentially hazardous floating measurements. Where other alternatives may lack the versatility, accuracy or affordability to make floating measurements, the TPS2000 Series employs unique IsolatedChannel technology to allow engineers and technicians to make these measurements quickly, accurately and affordably.


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HIGH-VOLTAGE MEASUREMENTS AND ISOLATION – GENERAL ANALOG CONCEPTS NI Analog Resource Center OVERVIEW This tutorial is part of the NI Analog Resource Center. Each tutorial will teach you a specific topic by explaining the theory and giving practical examples. There are many issues to consider when measuring high voltage. When specifying a data acquisition system, the first question you should ask is whether or not the system will be safe. Making high-voltage measurements can be hazardous to the equipment, to the unit under test, and to you and your colleagues. To ensure that the system is safe, you should provide an insulation barrier, using isolated measurement devices, between the user and hazardous voltages.

WHAT IS ISOLATION? Isolation is a means of physically and electrically separating two parts of a measurement device, and can be categorized into electrical and safety isolation. Electrical isolation pertains to eliminating ground paths between two electrical systems. By providing electrical isolation, you can break ground loops, increase the common-mode range of the data acquisition system, and level shift the signal ground reference to a single system ground. Safety isolation references standards have specific requirements for isolating humans from contact with hazardous voltages. It also characterizes the ability of an electrical system to prevent high voltages and transient voltages from transmitting across its boundary to other electrical systems with which you can come in contact. Incorporating isolation into a data acquisition system has three primary functions: preventing ground loops, rejecting common-mode voltage, and providing safety.

GROUND LOOPS Ground loops are the most common source of noise in data acquisition applications. They occur when two connected terminals in a circuit are at different ground potentials, causing current to flow between the two points. The local ground of the system can be several volts above or below the ground of the nearest building, and nearby lightning strikes can cause the difference to rise to several hundreds or thousands of volts. This additional voltage itself can cause significant error in the measurement, but the current that causes it can couple voltages in nearby wires as well. These errors can appear as transients or periodic signals. For example, if a ground loop is formed with 60 Hz AC power lines, the unwanted AC signal appears as a periodic voltage error in the measurement. When a ground loop exists, the measured voltage, Vm, is the sum of the signal voltage, Vs, and the potential difference, Vg, which exists between the signal source ground and the measurement system ground, as shown in Figure 1. This potential is generally not a DC level; therefore, the result is a noisy measurement system, often showing power-line frequency (60 Hz) components in the readings.

Figure 1. A Grounded Signal Source

GROUND-REFERENCED SYSTEM INTRODUCES GROUND LOOP To avoid ground loops, ensure that there is only one ground reference in the measurement system, or use isolated measurement hardware. Using isolated hardware eliminates the path between the ground of the signal source and the measurement device, therefore preventing any current from flowing between multiple ground points.

COMMON-MODE VOLTAGE An ideal differential measurement system responds only to the potential difference between its two terminals, the (+) and (-) inputs. The differential voltage across the circuit pair is the desired signal, yet an unwanted signal can exist that is common to both sides of a differential circuit pair. This voltage is known as common-mode voltage. An ideal differential measurement system completely rejects, rather than measures, the commonmode voltage. Practical devices however, have several limitations, described by parameters such as common-mode voltage range and common-mode rejection ratio (CMRR), which limit this ability to reject the common-mode voltage. The common-mode voltage range is defined as the maximum allowable voltage swing on each input with respect to the measurement system ground. Violating this constraint results not only in measurement error, but also in possible damage to components on the board. Common-mode rejection ratio describes the ability of a measurement system to reject common-mode voltages. Amplifiers with higher common-mode rejection ratios are more effective at rejecting common-mode voltages. The CMRR is defined as the logarithmic ratio of differential gain to common-mode gain. CMRR (dB) = 20 log (Differential Gain/Common-Mode Gain). (Equation 1)

Common-mode voltage is shown graphically in Figure 2. In this circuit, CMRR in dB is measured as 20 log Vcm/Vout where V-= Vcm.

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Electricity Testing and Measurement Handbook – Vol. 7 isolation barrier is typically an air gap. The light intensity is proportional to the measured signal. The light signal is transmitted across the isolation barrier and detected by a photoconductive element on the opposite side of the isolation barrier.

Figure 2. CMRR Measurement Circuit

In a non-isolated differential measurement system, an electrical path still exists in the circuit between input and output. Therefore, electrical characteristics of the amplifier limit the common-mode signal level that can be applied to the input. With the use of isolation amplifiers, the conductive electrical path is eliminated and the common-mode rejection ratio is dramatically increased.

ISOLATION CONSIDERATIONS There are several terms with which to be familiar when configuring an isolated system: Installation Category: A grouping of operating parameters that describe the maximum transients that an electrical system can safely withstand. Installation categories are discussed in more detail later. Working Voltage: The maximum operating voltage at which the system can be guaranteed to continuously safely operate without compromising the insulation barrier.

Figure 3. Optical Isolation

ELECTROMAGNETIC ISOLATION Electromagnetic isolation uses a transformer to couple a signal across an isolation barrier by generating an electromagnetic field proportional to the electrical signal. The field is created and detected by a pair of conductive coils. The physical barrier can be air or some other form of non-conductive barrier.

Test Voltage: The level of voltage to which the product is subjected during testing to ensure conformance. Transient Voltage (Over-voltage): A brief electrical pulse or spike that can be seen in addition to the expected voltage level being measured. Breakdown Voltage: The voltage at which the isolation barrier of a component breaks down. This voltage is much higher than the working voltage, and often times is higher than the transient voltage. A device cannot operate safely near this voltage for an extended period of time.

Figure 4. Transformer

CAPACITIVE ISOLATION Capacitive coupling is another form of isolation. An electromagnetic field changes the level of charge on the capacitor. This charge is detected across the barrier and is proportional to the level of the measured signal.

ISOLATION TYPES Physical isolation is the most basic form of isolation, meaning that there is a physical barrier between two electrical systems. This can be in the form of insulation, an air gap, or any non-conductive path between two electrical systems. With pure physical isolation however, we imply that no signal transfer exists between electrical systems. When dealing with isolated measurement systems, you must have a transfer, or coupling, of energy across the isolation barrier. There are three basic types of isolation that can be used in a data acquisition system:

OPTICAL ISOLATION Optical isolation is common in digital isolation systems. The media for transmitting the signal is light and the physical

Figure 5. Capacitor

ISOLATION TOPOLOGIES It is important to understand the isolation topology of a device when configuring a measurement system. Different topologies have several associated cost and speed considerations.


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CHANNEL-TO-CHANNEL

SAFETY AND ENVIRONMENTAL STANDARDS

The most robust isolation topology is channel-to-channel isolation. In this topology, each channel is individually isolated from one another and from other non-isolated system components. In addition, each channel has its own isolated power supply. In terms of speed, there are several architectures from which to choose. Using an isolation amplifier with an analog to digital converter (ADC) per channel is typically faster because you can access all of the channels in parallel. A more cost-effective, but slower architecture, involves multiplexing each isolated input channel into a single ADC. Another method of providing channel-to-channel isolation is to use a common isolated power supply for all of the channels. In this case, the common-mode range of the amplifiers is limited to the supply rails of that power supply, unless front-end attenuators are used.

When configuring a data acquisition system, you must take the following steps to ensure that the product meets applicable safety standards: • consider the operational environment, which includes the working isolation voltage and installation category. • choose the method of isolation in the design based on these operational and safety parameters. • choose the type of isolation based on the accuracy needed, the desired frequency range, the working isolation voltage, and the ability of the isolating components to withstand transient voltages. Not all isolation barriers are suitable for safety isolation. Even though measurement products may have components rated with high-voltage isolation barriers, the overall product design, not just the components, dictates whether or not the device meets high-voltage safety standards. Safety standards have specific requirements for isolating humans from contact with hazardous voltages. These requirements vary among different applications and working voltage levels, but often specify two layers of protection between hazardous voltages and human-accessible circuits or parts. In addition, the standards for test and measurement equipment are not only concerned with dangerous voltage levels and shock hazards, but also with environmental conditions, accessibility, fire hazards, and valid documentation for explaining the use of equipment in preventing these hazards. They maintain specific construction requirements of isolation equipment to ensure that the integrity of the isolation barrier is maintained with changes in temperature, humidity, aging, and variations in manufacturing processes. When dealing with safety standards, the European Commission and Underwriters Laboratories, Inc. (UL) have outlined the standards that cover the design of high-voltage instruments. There are approximately 200 individual safety standards harmonized (approved for use to demonstrate compliance) to the Low Voltage Directive, which was the initial document that outlined the specifications for the voltage levels that require safety consideration. The relevant standard for instrument manufacturers is EN 61010 – Safety Requirements for Electrical Equipment for Measurement, Control, and Laboratory Use. EN 61010 states that 30 Vrms or 60 VDC are dangerous voltages. In addition to high-voltage design requirements, EN 61010 also includes other safety design constraints (such as flammability and heat). Instrument manufacturers must meet all the specifications in EN 61010 to receive the CE label. There are two other standards very similar to EN 61010 –IEC 1010 and UL 3111. IEC 1010, which was established by the International Electrotechnical Commission, is the precursor to EN 61010. The European Commission adopted it and renamed it EN 61010. UL 3111 is also a child of IEC 1010. UL took IEC 1010, made some modifications and adopted it as UL 3111. This new, strict UL standard replaces the older, more lenient UL 1244 standard for measurement, control, and laboratory instruments. For new designs, instrument manufacturers must meet all of the specifications in UL 3111 to receive a UL listing.

Figure 6. Channel-to-Channel Multiplexed Topology

BANK Another isolation topology involves banking, or grouping, several channels together to share a single isolation amplifier. In this topology, the common-mode voltage difference between channels is limited, but the common-mode voltage between the bank of channels and the non-isolated part of the measurement system can be large. Individual channels are not isolated, but banks of channels are isolated from other banks and from ground. This topology is a lower-cost isolation solution because this design shares a single isolation amplifier and power supply.

INSTALLATION CATEGORIES Figure 7. Bank Topology

The IEC defined the term Installation Category (sometimes referred to as Over-voltage Category) to address transient voltages. When working with transient voltages, there is a level

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of damping that applies to each category. This damping reduces the transient voltages (over-voltages) that are present in the system. As you move closer to power outlets and away from high-voltage transmission lines, the amount of damping in the system increases. The IEC has created four categories to partition circuits with different levels of over-voltage transient conditions. • Installation Category IV – Distribution Level (transmission lines) • Installation Category III – Fixed Installation (fuse panels) • Installation Category II – Equipment consuming energy from a Category III fixed installation system. (wall outlets) • Installation Category I – Equipment for connection to circuits where transient over-voltages are limited to a sufficiently low level by design.

FUEL CELL MEASUREMENT Fuel cell test systems make a variety of measurements that require signal conditioning before the raw signal is digitized by the data acquisition system. An important feature for the testing of fuel cell stacks is isolation. Each individual cell can generate about 1 V, and a stack of cells can produce several kV. To accurately measure the voltage of a single 1 V cell in a large fuel cell stack requires a large common-mode range and high common-mode rejection ratio. Because adjacent cells have a similar common-mode voltage, bank isolation is sometimes acceptable.

HIGH COMMON-MODE THERMOCOUPLE MEASUREMENT Some thermocouple measurements involve high commonmode voltages. Typical applications include measuring temperature while a thermocouple is attached to a motor, or measuring the temperature dissipation capabilities in a conductive coil. In these cases, you are trying to measure small, millivolt changes with several volts of common-mode voltage. It is therefore important to use an isolated measurement system with good common-mode rejection specifications.

SERIAL COMMUNICATION

Figure 8. Installation Categories

TYPICAL APPLICATIONS REQUIRING ISOLATION SINGLE-PHASE AC MONITORING To measure power consumption with 120/240 VAC power measurements, you record instantaneous voltage and current values. The final measurement, however, may not be instantaneous power, but average power over a period of time or cost information for the energy consumed. By making voltage and current measurements, software can make power measurements or do other analyses. To make high-voltage measurements you need some type of voltage attenuator to adjust the range of the signal to the input range of the measurement device. Current measurements require a precision resistor. The voltage drop across the resistor is measured, and Ohm’s Law (I = V/R) produces a current value.

Reliability is a number one concern when designing equipment to be resistant to the interference inherent in a harsh environment. Commercial and industrial applications such as POS networks, ATMs, bank teller stations, and CNC-based production lines are susceptible to voltage spikes and noise. Isolation reduces the possibility of damaging control systems and ensure that systems can remain operational. Other applications that may require isolation are industrial process control, factory automation, serial networking devices, high speed modems, monitoring equipment, long distance communication devices, printers and remote serial device control.


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STANDARD MEASUREMENTS: ELECTRIC FIELDS DUE TO HIGH VOLTAGE EQUIPMENT Ralf Müller and Hans-Joachim Förster The standards for personal safety in electric and magnetic fields have been tightened. Three-dimensional measurement of the fields and the combination of these components into the equivalent field strength is now required. Is this extra effort justified? As part of a study project at the Fachhochschule Reutlingen, high voltage lines, transformer stations and the working environment were investigated. The results show that three-dimensional measurement is indeed necessary.

MEASUREMENT METHODS An E field sensor basically consists of a pair of condenser plates placed side by side, across which the dielectric current is measured. The disadvantage of this simple arrangement is its directional characteristic. To measure accurately, the direction of the field lines has to be known and the sensor positioned accordingly. This is seldom possible in practice. As a result, the trade association [1] requires the measurement to be made in each of the three orthogonal spatial axes and the so-called equivalent field strength calculated by summing the squares of the three field components. This is theoretically possible with a simple probe by making three consecutive measurements in the three directions, assuming that the field remains constant over time. The practical answer is to use a sensor that has a three dimensional structure. Modern measuring equipment uses sensors made up from three plate condensers arranged at right angles to each other, and calculate the equivalent field strength automatically. The isotropy, i.e. the actual non-directionality of the sensor, is important in this context. This can be assessed by rotating the sensor in an homogeneous field; the indicated field strength must remain constant [3]. This is the only way to ensure that dangerous field strengths are not present.

SIMPLEST CASE: THE HIGH VOLTAGE LINE Our first example is a high voltage line running across open land. If the field is measured at the lowest point of the cable sag, i.e. as far as possible from the masts, it can even be assumed that the field lines are vertical. As expected, the measurement results of a three dimensional (isotropic) and a one dimensional (so-called y only measurement) differ only slightly from one another. The maximum difference is below 5%. The slight unsymmetry in the measurement curve is due to the terrain which showed a slight upward slope from left to right. The phase relationships between the conductors are of no consequence in this case, as the measurement distance from the conducting cables is too large.

Figure 1: High-voltage line. Results of electric field measurements in one and three dimensions

MEASUREMENT CONDITIONS Several factors must be observed if measurements are to conform to relevant standards [1]: • No person should be present in the immediate vicinity of the measurement. • Objects in the vicinity that distort the field, such as trees, bushes, machinery, etc., must be noted. • Environmental effects such as air humidity, temperature, type of terrain, etc., must also be noted. • No condensation may be present on the sensor or its supporting tripod as this will lead to measurement errors. • The persons operating the measuring instrument must ensure that they do not stand between the field source and the probe during the measurement. These measures are required in order that comparable and reproducible results can be obtained under varying operating conditions.

MORE INTERESTING: LINE CROSSING The second example shows the field profile in the area where two lines cross. The measurement conditions were: • Voltages 110 kV and 220 kV • Three-phase conductors • Line 1 (top to bottom): 220 kV, christmas tree masts approx. 40 m high • Line 2 (left to right): 110 kV single layer masts approx. 26 m high The ambient conditions at the time of the measurement were: Temperature 16°C, average air humidity, very damp ground. Figure 2 shows the basic measurement path. Some trees and a number of small bushes were located in the immediate vicinity of the measurement. The distorting effects of these objects on the field profile are discussed in the evaluation.

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Figure 2: Measurement path beneath two high voltage lines that cross. Green areas indicate bushes and trees

Figure 3 shows the field strength profile that was measured. The starting point of the measurement in the diagram is at the position of the mast. The last measurement was made at a distance of 60 m from this point. The effect of the mast can be clearly seen up to the area where the lines cross. The crossing begins 30 m from the starting point. A field strength maximum occurs at the 28 m point. This is due to the addition of the field strengths of the two lines. In the area of the crossing, the field components of the upper line are compensated, resulting in a minimum at this point. The field strength increases again rapidly after the crossing area, at 52 m. This is due to the fact that the screening effect of the mast is now reduced and the area of the crossing has been left.

Figure 3: Electric field profile where two high voltage lines cross.

Figure 4 shows the relative difference between the one-dimensional and three-dimensional measurements. The maxima are found at the entry and exit of the crossing. The difference is up to 13%. The lower conductors in the crossing area compensate out the field components of the upper conductors. The variation in the field within the area of the crossing in figure 3 is due to the uneven terrain. This section is therefore shown in more detail in figure 5.

Figure 4: Relative difference between one-dimensional and three-dimensional measurements


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Figure 5: Zoomed representation of the crossing area from figure 3

Figure 5 clearly shows the effect of the terrain on the measurement result. A threedimensional measurement is clearly to be preferred where the terrain is very uneven. Accurate results are not given by a one-dimensional measurement or by a computer simulation.

COMPLEX: TRANSFORMER STATION The Neckarwerke Esslingen AG kindly allowed us to make measurements in a transformer station. A measurement path was selected that included several conductor arrangements, insulators and carriers. It is depicted in figure 6. The ambient conditions at the time of the measurement were: Temperature 5°C, average air humidity, very damp ground. The measurement results clearly show that significant differences in the results of one-dimensional and three-dimensional measurements occur in the vicinity of crossing conductors, switching equipment, current busbars and the like. The relative error is very dependent on the measurement position. Directly beneath the conductors, it is small, but it can be as much as 60% at points between the conductors. This difference cannot be accepted when measurements are made for personal safety, especially where legal settlements are involved. The difference clearly shows that the indicated field strength is lower than the actual field strength and hence the assumed safety is not given. This exposes a weakness in IEC standard 833 [4] which exclusively defines measurement in the vicinity of high voltage lines and is therefore not applicable in cases where labor laws are involved.

Figure 7: Direct comparison of onedimensional and three-dimensional measurement results.

View from above:

View from side:

Figure 6: Measurement path in a transformer station

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Figure 8: Relative error between onedimensional and three-dimensional measurements

UNCLEAR: MOST WORKING ENVIRONMENTS

REFERENCES

The conditions of most working environments in industry are far removed from the simple case of a high-voltage line; switching equipment, transformer stations, induction heaters and machinery may all play a part in the field profile. It is thus not possible to predict the spatial field profile or its variation with time. Further uncertainty results from the frequency spectrum. Several standards specify different limit values for different frequencies. Broadband measurement equipment cannot, therefore, be used if the frequency of the field is unknown or if several fields are superimposed. As an example of this, an induction heater emits radiation at the AC. line frequency of 50 or 60 Hz and its harmonics and also at the frequency of the heating current. The latest test equipment copes with this situation by employing built-in filters to detect the main radiation components and evaluate their frequencies. The use of three-dimensional measurement techniques coupled with filters is an absolute must if personal safety measurements are to be made that are reproducible and which conform to the relevant standards.

[1] Precision Engineering and Electrical Engineering Trade Association: Rules for health and safety at work involving exposure to electric, magnetic or electromagnetic fields (in German) [2] Electric and Magnetic Fields Everyday Electricity (in German) Electricity Industry Information Center (Informationszentrale der Elektrizitätswirtschaft e.V.) 60596 Frankfurt [3] Progress Report VDI Series 8: Measurement, Control and Regulation Dipl.-Ing. Georg Bahmeier, Untermeitingen Field probes for calibration and for determining the magnitude and direction of electric field strength (in German) [4] International Standard IEC 833: Measurement of power frequency electric fields [5] German Standard VDE 0848 Part 1: Endangerment due to electromagnetic fields. Measurement and calculation methods (in German) [6] German Standard VDE 0848 Part 4: Safety in electromagnetic fields. Field strength limit values for personal safety in the frequency range from 0 Hz to 30 kHz.


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IDENTIFICATION OF CLOSED LOOP SYSTEMS NI Analog Resource Center

Often it is necessary to identify a system that must operate in a closed-loop fashion under some type of feedback control. This may be due to safety reasons, an unstable plant that requires control, or the expense required to take a plant offline for test. In these cases, it is necessary to perform closed-loop identification. There are three basic approaches to closed-loop identification. These approaches are direct, indirect, and joint input-output. In this article we outline each approach and the system identification techniques that may be used to implement them.

INDIRECT The second method of interest in closed-loop identification is the Indirect Approach as shown in Figure 2. In this method we identify the closed loop system (Gcl) using measurements of the reference input r(t) and the output y(t) and retrieve the plant model making use of a known regulator structure. The transfer function for the open loop plant G, with regulator H, can be retrieved from

DIRECT The first method of interest is the Direct Approach. In this method, we measure the output of the system y(t) and the input to the plant u(t), ignoring any feedback and the reference signal, to obtain the model. This is illustrated in Figure 1. This has the advantage of requiring no knowledge about the feedback in the system and becomes an open-loop identification problem. The suggested system identification model structures when using this method are ARX, ARMAX and state-space models. Optimal accuracy occurs if the chosen model structure contains the true system (including the noise properties) and the main drawback to the method is that a poor noise model can introduce bias into the model. This bias will be small when any or all of the following hold • The noise model is representative of the actual noise • The feedback contribution to the input spectrum is small • The signal to noise ratio is high Spectral analysis will not provide correct results in the closed-loop case when using the direct approach so avoid nonparametric methods of identification such as impulse response and bode response estimation.

The advantages in using the indirect approach are that any method will work in determining the closed-loop transfer function Gcl and the need for an accurate representation of the noise model is alleviated. The main disadvantage is that any error in H (including deviations due to saturations or anti-windup logic) will be imposed directly into G resulting in bias errors.

Figure 2 Indirect Approach to Closed-Loop System Identification.

JOINT INPUT-OUTPUT The last method is the Joint Input-Output Approach. As shown in Figure 3, we consider the plant input u(t) and the system output y(t) as outputs of the system. The inputs to the system are the reference signal r(t) and the noise signal v(t).

Figure 1 Direct Approach to Closed-Loop System Identification.

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CONCLUSION It is often necessary to perform identification under closed-loop conditions to increase safety or reduce the costs of the modeling. The three approaches outlined in this article provide accurate estimations of plant dynamics under feedback control using simple measurements. Using the LabVIEW System Identification Toolkit provides the necessary identification algorithms to aid in these closed-loop identification problems.

Figure 3 Joint Input-Output Approach to Closed-Loop System Identification.

This identification method results in a multidimensional system of the form

Where the system matrix A is comprised of two models, the closed-loop model Gcl and the model relating u(t) to r(t), Gru. The plant model, G, is then estimated from the relation.

This approach is advantageous because the regulator structure is not needed nor is an accurate noise model necessary. It suffers from the disadvantages of requiring additional acquisition hardware (sensors) and requires acquiring a greater quantity of data. When using the indirect and joint input-output methods, the reference signal r(t) should be as informative as possible. This means it should provide good spectral coverage of the domain of interest. This may be done by adjustments to the system set points (or adjustments to the regulator) as much as allowed by the system being identified.


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SELECTING AND USING TRANSDUCERS FOR TRANSFORMERS FOR ELECTRICAL MEASUREMENTS William D. Walden Transducers for electrical measurement are an essential part of any monitoring, measuring, or controlling system where electrical quantities are involved. In order to use these transducers, it is important to know what they do, what kind of signal they provide, and how to connect them. Part I provides an introduction in using voltage, current, and power (watt) transducers along with using potential and current transformers.

POTENTIAL TRANSFORMERS Most manufactures’ transducers accept up to a maximum of 600 volts AC direct. For AC voltages greater than 600 volts, potential transformers are required. Potential transformers are precision transformers that step the voltage down to 120 volts AC, a standard transducer input. These transformers, particularly when used with power or watt transducers, must be instrument grade transformers. They must not only be precise in stepping down the voltage but in maintaining the phase or time relationship of the voltage. This is very important. Do not attempt to save money by using control class transformers. Transducer and meter loads are connected in parallel to the potential transformer. Take care not to exceed the transformer burden rating. This burden is expressed in VA for volt-amperes (the product of volts and amps).

These transformers are most often the ‘donut’ type. The current carrying conductor is passed through the opening or window of the ‘donut’. The secondary winding of the current transformer is wound by the manufacturer on the toroidal iron core which makes the ‘donut’ shape. On most North American manufactured current transformers, the secondary is wound to produce 5 amperes when rated current is passed through the window. The turns ratio is expressed as 100:5 or 3500:5 (read as 100 to 5 and 3500 to 5). The first number represents the rated full-scale primary current. The primary winding consists of the single pass of the current carrying conductor through the window. The second number represents the full-scale secondary current in amperes. A 100:5 ratio current transformer steps the current from 100 amperes down to 5 amperes. The 3500:5 ratio current transformer steps the current from 3500 amperes down to 5 amperes. As with potential transformers, only use instrument grade current transformers with power measuring transducers. Connect the loads on current transformers in series being careful not to exceed the burden rating. The phase angle shift introduced by current transformers is sensitive to the loading. Therefore, keep the burden to a minimum by using adequate size secondary leads and keeping secondary leads as short as possible.

CURRENT TRANSFORMERS For AC applications, most manufacturers’ transducers will not accept direct current input over 20 amperes. For higher amperages, current transformers are utilized.

CAUTION: Current transformers can and will develop a lethal voltage and possibly self destruct if the secondary is open when primary current is present! People have been hurt and equipment damaged when the secondary winding of a current transformer was opened. Never disconnect the secondary or leave it open when there is the possibility of primary current. It is essential that experienced persons install current transformers. If you must make a connection to the current transformer while it is in use, SHORT THE SECONDARY WINDING before doing anything. Some current transformers have a shorting block for this purpose. Auxiliary shorting blocks are available for this purpose too.

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Current transformers are rated for the voltage class for which they are to be used. These classes are: 600 volts, 5000 volts, 8700 volts, 15 kilovolts, 25 kilovolts, and 34.5 kilovolts. Make certain that the current transformers are rated for the voltage with which they are working or that the conductor is insulated for the class voltage. Current transformers being used on conductors with voltages greater than 600 volts must have the secondary grounded to an earth ground.

• True RMS (Root Mean Square) measuring. These transducers calculate the RMS value of the voltage input and provide a DC output directly proportional to the effective value of the voltage input. This type should be used whenever the voltage is distorted. Transducer models are available for nominal input voltages of 69, 120, 240, and 480 volts. These typically have a measuring range of 0 to 125% of the nominal input rating. Thus, a 120-volt model has a range of 0 to 150 volts. For voltage input higher than 600 volts, one should use a potential transformer.

VOLTAGE TRANSDUCERS Voltage transducers provide a DC current or voltage output directly proportional to the AC input voltage. AC voltage transducers typically have a transformer input to isolate the transducer from the voltage input. Following the transformer are the electronics.

There are two types of AC voltage transducers. • Absolute average measuring, rms than 600 volts, one should use a calibrated (or mean value measuring, potential transformer. rms calibrated). These inexpensive transducers simply convert the AC input to DC Current transducers provide a DC current or and have the output calibrated to voltage output directly proportional to the represent the root mean square AC input current. AC current transducers (RMS) value for sine wave input. Typically have a transformer input to isolate. This type is very adequate for the transducer from the current input. This type is very adequate for situations in which the voltage wave shape is not distorted. Any odd harmonic or discontinuity will introduce large error. Use the true RMS measuring type when distortion of a sine wave is present. • True RMS (root mean square) measuring. These transducers calculate the RMS value of the current input and provide a DC output directly proportional to the effective value of the current input. This type should be used whenever the current is distorted. Ohio Semitronics, Inc. has a wide range of models available for various situations. Models are available with or without current transformers, with current transformers built in, and with split core current transformers.

CURRENT TRANSDUCERS Current transducers provide a DC current or voltage output directly proportional to the AC input current. AC current transducers typically have a transformer input to isolate the transducer from the current input. Following the transformer are the electronics.

There are two types of AC current transducers. • Absolute average measuring, rms calibrated (or mean value measuring, rms calibrated). These inexpensive transducers simply convert the AC input to DC and have the output calibrated to represent the root mean square (RMS) value for sine wave input. This type is very adequate for situations in which the current wave shape is not distorted. Any odd harmonic or discontinuity will introduce large error. Use the true RMS measuring type when distortion of a sine wave is present. • True RMS (root mean square) measuring. These transducers calculate the RMS value of the current input and provide a DC output directly proportional to the effective value of the current input. This type should be used whenever the current is distorted. Ohio Semitronics, Inc. has a wide range of models available for various situations. Models are available with or without current transformers, with current transformers built in, and with split core current transformers.


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POWER OR WATT TRANSDUCERS

SINGLE-PHASE WATT TRANSDUCERS

A watt or power transducer measures true electrical power delivered to a load and converts that measurement to a DC voltage or current signal proportional to the power measured. To measure power, the watt transducer must monitor both the voltage and current in a circuit. Further, it must be able to accurately determine the phase relationship between the voltage and current. This is the angle by which the current leads or lags the voltage. This measurement is very important to accurately determine true power.

The most common application for a watt transducer is monitoring a single-phase load such as a heater element or small motor. This requires a single element watt transducer connected directly between the power line and the load as illustrated below.

The watt transducer must also measure the power in each of the branches of the circuit. Your house, apartment, or small office is wired in what is often referred to as the Edison system. This is a three-wire, single phase system with two power lines and a neutral. The watt transducer must measure the power in each of the power lines or mains. This circuit requires a two-element watt transducer. A two-element watt transducer has two-watt transducers in the same case. The outputs of the two transducers or multipliers are summed so that the output signal of the entire watt transducer represents total power. One, two, and three element watt transducers are discussed in Part II.

The single-phase watt transducer shown above has a single multiplier or element inside the electronics package. Often the combined loads of an entire house, apartment, or office are monitored with a watt transducer. This requires a two-element model with current transformers. The two-element, single-phase watt transducer is connected as shown below.

What type of watt transducer to use? • Analog watt transducers including Hall effect provide good accuracy even with distorted wave shapes, discontinuity, or where there is poor frequency regulation. The two-element watt transducer shown above has two multipliers inside the electronics package. The output of these two multipliers is summed to obtain the total power. The output signal of this watt transducer thus represents the total power being used.

THREE-PHASE WATT TRANSDUCERS

• Electronic watt transducers with sampling or pulsewidth, pulse-height type multipliers provide excellent accuracy but may have problems with discontinuity or where there is poor frequency regulation. Before ordering watt transducers, it is to your advantage to assess your specific needs and conditions.

Most motors in industry are three-phase, three-wire motors. These require two-element watt transducers. Do not attempt to save money and use a single element transducer – it will not provide correct or useful information. Smaller threephase motors may be connected directly to the watt transducer. Larger three-phase motors will require the use of current and/or potential transformers. All three cases are shown in the three diagrams that follow.

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Electricity Testing and Measurement Handbook – Vol. 7 In special cases where a three-phase, four-wire load is known to be balanced in load and voltage, a single element watt transducer may be used to give an indication of total power by multiplying the value represented by the transducer output by three.

THE 2 1/2 ELEMENT WATT TRANSDUCER Monitoring three-phase, four-wire systems frequently involves using potential transformers. These transformers can cost much more than the watt transducer. To reduce cost, two potential transformers instead of three are used. The watt transducer from the two voltages can derive the third voltage.

Factories and large stores are typically supplied with three-phase, four-wire power. Heavy loads such as motors are connected line-to-line in a three-phase, three-wire configuration and lighter loads are connected line to neutral. Three element watt transducers are required to monitor the entire facility. This requires the use of current transformers. The connections are shown below.

OUTPUT SIGNALS FROM TRANSDUCERS The voltage, current, and watt transducers discussed above are available with DC current or voltage output. The least expensive and simplest voltage and current transducers are available only with a current output. How are these outputs used? The most common is for metering. The transducer output is driving either an analog or digital meter. The use of either is simple.

ANALOG METER If you are using an analog meter, buy transducers that are supplied with a 0 to 1 mADC output and a 0 to 1 mADC meter movement. The meter supplier can scale the meter face to match the transducer range. Some examples are shown on the next page.


49 In the first example the rated output of the transducer is 1000 watts. We would like the digital meter to read 1000. If we supply 1 volt to the meter, it will read 1.000. Digital meter manufacturers build their meters so that the decimal point can be moved. This is done using wire jumpers on the connection strip of the meter, by DIP switches on the meter, or by wire jumpers or foil jumpers that the user cuts. Follow the meter manufacturer’s instructions. In our example, set the meter to display 1000 or 1000.0 when 1 volt is applied.

DIGITAL METER Some digital meters allow the user to scale the meter to display to the transducer range. If you use one of these meters with a watt transducer that has a 4 to 20 mADC output representing 0 to 960 kilowatts, simply adjust the meter to read 0 at 4 mADC and 960 at 20 mADC. If you are using a 0 to 2 volt DC input meter that does not allow scaling, use a scaling resistor. Some examples are shown below.

How did we get the 1-volt from the transducer? Use Ohm’s law. The value of the resistor equals the desired voltage divided by the current. Or for our example, R=1/0.001 or 1000. Use of a 1000 Ω resistor will provide 1 volt at full scale of 1 mADC. Our meter will read 1000 for 1000 watts. In the second example we used a twenty ampere current transducer with a digital meter. We want the meter to read 19.99 at full scale to take advantage of the four digits. (A 3-_ digit meter will read to 1.999 volts. Above this it will flash at you to let you know that the meter is over ranging.) How do we get 2 volts? Again use Ohm’s Law. The value of the resistor will be R=2/0.001 or 2000 Ω. Set the decimal point so that the meter will read 19.99 at 1.999 volts. Your meter is now scaled to match the transducer.

OTHER OUTPUT SIGNALS AVAILABLE Most transducers manufacturers have transducers available with 0 to 1 mADC, 0 to 5 volts DC, 0 to 10 volts DC, or 4 to 20 mADC outputs. The 0 to 10 volt and 0 to 5 volt outputs are typically (but not exclusively) used with data acquisition equipment, strip chart recorders, analog input cards for computers, or control interface devices. The 4 to 20 mADC output is used with process control equipment, for long (over 200 feet) transmission of the signal, and frequently as a ‘fail safe’ monitoring of the signal. If the watt transducer output is 4 mADC, then one knows that the power being monitored is zero. However, if the output signal is zero, something is wrong – the transducer may have failed or it may have lost instrument power. The user can take corrective action.

CABLES FOR ANALOG SIGNALS

In both examples a 2-volt DC digital meter is being used. By applying Ohm’s Law (R=E/I, the value of the resistor equals the voltage divided by the current), one can determine the value of the resistor required. Remember that the output of the transducer is in milliamperes, 1/1000 of an ampere. How did we figure the value of the resistor? Always base the resistor on the rated output of the transducer, the rated output is the wattage level or current value that is represented by 1 mADC.

Ohio Semitronics, Inc. recommends using a shielded twisted pair of 22 gauge or larger wire to conduct an analog voltage or current signal from the transducer to the meter or instrument. If you are using a 1-mADC-output transducer and a load resistor, we recommend putting the load resistor on the meter or instrumentation package. Ground the shield at the receiving end only. Do not ground at both ends. Doing so can cause severe problems. I have known the shields to melt when a lightening strike has occurred nearby.

EXAMPLES OF POWER, VOLTAGE, AND CURENT MONITORING Monitoring voltage, current, and power delivered to a test load. In this application a refrigerator is being examined. Transducers used: • CT5-010A current transducer is wired in series with the load.

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• VT-120A voltage transducer is wired in parallel with the load. • PC5-010A is wired in series with the load for monitoring the current and in parallel with the load for monitoring the voltage. These are used as examples. Other transducers that may be used include the multifunction board level transducer PTB. This board provides analog outputs proportional to each phase of true RMS Current, each phase of true RMS voltage, and total power. This example uses digital meter which are scaled using precision load resistors.


Load resistors are selected as follows: • Remember Ohm’s Law: R=V/I where R is the resistance in ohms (Ω ), V is the voltage that we want to apply to the digital meter, and I is the current from the transducer. • The CT5-010A provides an output of 1 mADC at 10 amperes AC through terminals 3 and 4. 1 mADC represents 10 amperes AC. Adjust the decimal point of the digital meter so that it displays 10.00 with 1 mADC through a 1000Ω load resistor. • The VT-120A provides an output of 1 mADC at 150 volts AC applied to terminals 3 and 4. 1 mADC represents 150 volts. Adjust the decimal point of the digital meter so that it displays 150.0 with 1 mADC through a 1500Ω load resistor. • The PC5-010A provides an output of 1 mADC at 1000 watts. 1 mADC represents 1000 watts. Adjust the decimal point on the digital meter so that it displays 1000 with 1 mADC through the 1000Ω resistor. Now all three meters are scaled correctly and may be labeled amperes, volts, and watts. Note that the power does not equal volts times amperes. This is because the refrigerator has a power factor of 0.866 that is normal for older refrigerators. For the single-phase situation, power factor may be determined by dividing the power reading by the product of volts and amperes.

POWER, VOLTAGE, CURRENT, AND POWER FACTOR In one example, we are monitoring only one phase for current, between two lines for voltage, and using a two-element watt transducer that measures two lines of current and two lines of voltage with respect to the third.

Electricity Testing and Measurement Handbook – Vol. 7 We are assuming a balanced condition to compute power factor given one current reading, one voltage reading, and total power. Power Factor: PF= watts ÷ (apparent power in VA). Apparent power for a three-phase, three-wire load may be calculated from the product of voltage, current, and the square root of 3 (1.732) or

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COMMENTS A watt transducer monitoring a three-phase, three-wire load must be a two-element watt transducer because the voltage, as measured and the current are out of phase by ± 30º at unity power factor, +30º on one leg and -30º on the other leg. Total power measured by the watt transducer is as follows: Ptotal = [Ia * Vac * Cos (_+ 30°) +Ib * Vbc * Cos (_ - 30°)]

PF = watts ÷ (V*I*1.732) = 81,000 ÷ (479*231*1.732) = 0.423

This is a low power factor and is very typical of some lightly loaded induction motors. Where does the 1.732 come from? It is the square root of 3 rounded to three decimal places. The square root of three comes from the ratio of line to line voltage to line to neutral voltage in the three-phase system. Please refer to POWER MONITORING IN PART TWO of this brochure.

PITFALLS Monitoring AC voltage and AC current is simple enough, but in monitoring power, one must follow the connection diagrams exactly. • Watt transducers are polarity sensitive. They sense not only the power but also the direction in which it is flowing. Should a current transformer be installed backwards, the watt transducer will sense this as reverse power flow and provide an output reversed in polarity, a negative output • Watt transducers are also phase sensitive. If a current transformer is installed on the wrong phase line, the watt transducer will interpret this as a 120-degree phase angle shift and give the wrong result. The most frequent complaint I receive on three-phase watt transducers is “I am not getting the correct output.” Conservatively stated, 90% of the time, the watt transducer is not correctly connected – a current transformer may be installed backwards or on the wrong line, voltage connections may be cross phased, or voltage connections may reference the wrong line. The other 9.5% of the time, the following gives the user trouble. The electrical quantity – WATT – is a measure of the rate at which work is being done. If an electric motor is not doing any work or is doing very little work, it will not consume very much power in watts even though the electric current is relatively high. The power factor will be low and a watt transducer monitoring this motor will have a low output. This is to be expected! The output from a watt transducer reflects the rate at which the motor is doing work. If you encounter incorrect readings from a watt transducer, double check your connections against the connection diagram on the transducer case or connection sheet.

Where: • Ia is the current in leg A • Ib is the current in leg B • Vac is the voltage between leg A and C • Vbc is the voltage between leg B and C • θ is the phase angle shift between the voltage and current – the power factor angle. At a power factor of 0.867 one reading between two of the legs will be double that between the other two legs. The sum of the two is the correct total power. At a power factor of 0.500 one reading between two of the legs will be greater than 0 and the other will be 0. The total of the two is the correct total power. At a power factor of 0 the readings between the two sets of legs will be the same but opposite in sign. Again, the total of the two is the correct total power – zero!

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HOW TO TROUBLESHOOT LIKE AN EXPERT A SYSTEMATIC APPROACH Warren Rhude, Simutech Multimedia Inc. To expertly troubleshoot electrical equipment, problems must be solved by replacing only defective equipment or components in the least amount of time. One of the most important factors in doing this, is the approach used. An expert troubleshooter uses a system or approach that allows them to logically and systematically analyze a circuit and determine exactly what is wrong. The approach described here is a logical, systematic approach called the 5 Step Troubleshooting Approach. It is a proven process that is highly effective and reliable in helping to solve electrical problems. This approach differs from troubleshooting procedures in that it does not tell you step by step how to troubleshoot a particular kind of circuit. It is more of a thinking process that is used to analyze a circuit’s behavior and determine what component or components are responsible for the faulty operation. This approach is general in nature allowing it to be used on any type of electrical circuit. In fact, the principles covered in this approach can be applied to many other types of problem solving scenarios, not just electrical circuits. The 5 Step Troubleshooting Approach consists of the following: Preparation Step 1 Observation Step 2 Define Problem Area Step 3 Identify Possible Causes Step 4 Determine Most Probable Cause Step 5 Test and Repair Follow-up Let’s take a look at these in more detail.

PREPARATION Before you begin to troubleshoot any piece of equipment, you must be familiar with your organization’s safety rules and procedures for working on electrical equipment. These rules and procedures govern the methods you can use to troubleshoot electrical equipment (including your lockout/tagout procedures, testing procedures etc.) and must be followed while troubleshooting. Next, you need to gather information regarding the equipment and the problem. Be sure you understand how the equipment is designed to operate. It is much easier to analyze faulty operation when you know how it should operate. Operation or equipment manuals and drawings are great sources of information and are helpful to have available. If there are equipment history records, you should review them to see if there are any recurring problems. You should also have on-hand any documentation describing the problem. (i.e., a work order, trouble report, or even your notes taken from a discussion with a customer.)

STEP 1 – OBSERVE Most faults provide obvious clues as to their cause. Through careful observation and a little bit of reasoning, most faults can be identified as to the actual component with very little testing. When observing malfunctioning equipment, look for visual signs of mechanical damage such as indications of impact, chafed wires, loose components or parts laying in the bottom of the cabinet. Look for signs of overheating, especially on wiring, relay coils, and printed circuit boards. Don’t forget to use your other senses when inspecting equipment. The smell of burnt insulation is something you won’t miss. Listening to the sound of the equipment operating may give you a clue to where the problem is located. Checking the temperature of components can also help find problems but be careful while doing this, some components may be alive or hot enough to burn you. Pay particular attention to areas that were identified either by past history or by the person that reported the problem. A note of caution here! Do not let these mislead you, past problems are just that – past problems, they are not necessarily the problem you are looking for now. Also, do not take reported problems as fact, always check for yourself if possible. The person reporting the problem may not have described it properly or may have made their own incorrect assumptions. When faced with equipment which is not functioning properly you should: • Be sure you understand how the equipment is designed to operate. It makes it much easier to analyze faulty operation when you know how it should operate; • Note the condition of the equipment as found. You should look at the state of the relays (energized or not), which lamps are lit, which auxiliary equipment is energized or running etc. This is the best time to give the equipment a thorough inspection (using all your senses). Look for signs of mechanical damage, overheating, unusual sounds, smells etc.; • Test the operation of the equipment including all of its features. Make note of any feature that is not operating properly. Make sure you observe these operations very carefully. This can give you a lot of valuable information regarding all parts of the equipment.

STEP 2 – DEFINE PROBLEM AREA It is at this stage that you apply logic and reasoning to your observations to determine the problem area of the malfunctioning equipment. Often times when equipment malfunctions, certain parts of the equipment will work properly while others not. The key is to use your observations (from step 1) to rule out parts of the equipment or circuitry that are operating properly and not contributing to the cause of the malfunction. You should

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continue to do this until you are left with only the part(s) that, if faulty, could cause the symptoms that the equipment is experiencing. To help you define the problem area you should have a schematic diagram of the circuit in addition to your noted observations. Starting with the whole circuit as the problem area, take each noted observation and ask yourself "what does this tell me about the circuit operation?" If an observation indicates that a section of the circuit appears to be operating properly, you can then eliminate it from the problem area. As you eliminate each part of the circuit from the problem area, make sure to identify them on your schematic. This will help you keep track of all your information.

cases the fault will be such that you cannot identify the problem component by observation and analysis alone. In these circumstances, test instruments can be used to help narrow the problem area and identify the problem component. There are many types of test instruments used for troubleshooting. Some are specialized instruments designed to measure various behaviors of specific equipment, while others, like the multimeters, are more general in nature and can be used on most electrical equipment. A typical multimeter can measure AC and DC Voltages, Resistance, and Current. A very important rule when taking meter readings is to predict what the meter will read before taking the reading. Use the circuit schematic to determine what the meter will read if the circuit is operating normally. If the reading is anything other than your predicted value, you know that this part of the circuit is being affected by the fault. Depending on the circuit and type of fault, the problem area as defined by your observations, can include a large area of the circuit creating a very large list of possible and probable causes. Under such circumstances, you could use a “divide and eliminate” testing approach to eliminate parts of the circuit from the problem area. The results of each test provides information to help you reduce the size of the problem area until the defective component is identified. Once you have determined the cause of the faulty operation of the circuit you can proceed to replace the defective component. Be sure the circuit is locked out and you follow all safety procedures before disconnecting the component or any wires. After replacing the component, you must test operate all features of the circuit to be sure you have replaced the proper component and that there are no other faults in the circuit. It can be very embarrassing to tell the customer that you have repaired the problem only to have him find another problem with the equipment just after you leave. Please note, Testing is a large topic and this article has only touched on the highlights.

STEP 3 – IDENTIFY POSSIBLE CAUSES Once the problem area(s) have been defined, it is necessary to identify all the possible causes of the malfunction. This typically involves every component in the problem area(s). It is necessary to list (actually write down) every fault which could cause the problem no matter how remote the possibility of it occurring. Use your initial observations to help you do this. During the next step you will eliminate those which are not likely to happen.

STEP 4 – DETERMINE MOST PROBABLE CAUSE Once the list of possible causes has been made, it is then necessary to prioritize each item as to the probability of it being the cause of the malfunction. The following are some rules of thumb when prioritizing possible causes. Although it could be possible for two components to fail at the same time, it is not very likely. Start by looking for one faulty component as the culprit. The following list shows the order in which you should check components based on the probability of them being defective: • First look for components which burn out or have a tendency to wear out, i.e. mechanical switches, fuses , relay contacts, or light bulbs. (Remember, that in the case of fuses, they burn out for a reason. You should find out why before replacing them.) • The next most likely cause of failure are coils, motors, transformers and other devices with windings. These usually generate heat and, with time, can malfunction. • Connections should be your third choice, especially screw type or bolted type. Over time these can loosen and cause a high resistance. In some cases this resistance will cause overheating and eventually will burn open. Connections on equipment that is subject to vibration are especially prone to coming loose. • Finally, you should look for is defective wiring. Pay particular attention to areas where the wire insulation could be damaged causing short circuits. Don’t rule out incorrect wiring, especially on a new piece of equipment.

STEP 5 – TEST AND REPAIR Testing electrical equipment can be hazardous. The electrical energy contained in many circuits can be enough to injure or kill. Make sure you follow all your companies safety precautions, rules and procedures while troubleshooting. Once you have determined the most probable cause, you must either prove it to be the problem or rule it out. This can sometimes be done by careful inspection. However, in many

FOLLOW UP Although this is not an official step of the troubleshooting process, it nevertheless should be done once the equipment has been repaired and put back in service. You should try to determine the reason for the malfunction. • Did the component fail due to age? • Did the environment the equipment operates in cause excessive corrosion? • Are there wear points that caused the wiring to short out? • Did it fail due to improper use? • Is there a design flaw that causes the same component to fail repeatedly? Through this process, further failures can be minimized. Many organizations have their own follow-up documentation and processes. Make sure you check your organization’s procedures. Adopting a logical and systematic approach such as the 5Step Troubleshooting Approach can help you to troubleshoot like an expert!


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ELECTRICAL INDUSTRIAL TROUBLESHOOTING Larry Bush

TROUBLESHOOTING IN THE FIELD – MOTOR TESTING – MOTOR CONTROLLER – PROGRAMMABLE LOGIC CONTROLLERS (PLC) A laptop computer with PLC programming, communication, and operating programs is a necessary tool in today’s modern plant. Engineers, production supervisors, maintenance supervisors, maintenance technicians, electricians, instrument technicians, and maintenance mechanics all need to have PLC and computer knowledge, training and skills in troubleshooting. On-the-job training on PLCs is usually not very effective until the person being trained has reached a certain level of expertise in several areas. Knowledge and skills in electricity, troubleshooting, and computer operation are necessary prerequisites to effectively assimilate basic PLC training. The author found that long-term retention of material studied was higher from a vocational course taken at a local junior college than from a fast-paced, cram-course through a manufacturer. The manufacturer’s course covered essentially the same material as a course at the junior college (JC). The major differences were the amount of study time and shop time. The JC course was four hours of class time per week for 15 weeks. There were three hours of shop time doing actual hands-on work relating to the problems and material covered in the first hour. Additional time was spent at home studying the manual and writing programs. Also, the JC was open at night for extra shop time on PLCs and computers. In contrast, the manufacturer’s course was five, eight hour days. Class work was extremely fast and condensed in order to cover the amount of material involved. The instructor was very knowledgeable and covered the course material as we tried to input the programs into desktop training equipment in order to see how it worked. By the end of each day, our minds were jammed with information. By the end of the week, we all passed the course, but I had a hard time remembering what we had studied on the first day. Basic troubleshooting techniques apply to every situation and occupation. Positive identification of the problem(s) is absolutely essential to solving the problems. Many times, the inexperienced troubleshooter will mistake one or more of the symptoms for the problems. Solving the symptom(s) will normally just postpone the problems to a later date, by which time, the problems may have grown to mountainous proportions. An example is when a person experiences a headache and takes a mild pain reliever, such as aspirin. The actual problem might be any number of things: eyes need to be checked, medication or lack of medication, muscle strain, stress, tumor, blood vessel blockage, or old war injury. The same thing occurs in industry, a fuse in a circuit blows and the maintenance person gets the replacement fuse and inserts it into the fuse holder. There are many things that could have caused the fuse to blow,

depending on the complexity of the circuit. Excess current caused the fuse to open (blow). Excess current could have been caused by: overload on the load; short circuit between the wires, grounded wires, short circuit in the load, ground in the load, voltage spike, voltage droop, etc. If the maintenance person does not troubleshoot the circuit prior to replacing the fuse and restoring power, negative consequences could arise. It is not uncommon for a process to develop a number of small problems and continue to function at a degraded level of operational capability. Then, one more small problem occurs and the whole process breaks down. Finding and correcting the last problem will not necessarily restore the operational capability of the process. The process continued operations with the small problems, but the small problems may not allow the process to restart from a dead stop. All the other small problems must be identified and corrected before the process is restored to full operational capability. This situation arises in industry as well as a person. The person can continue to function with a number of small problems, such as fatigue, blood pressure problems, hardening of the arteries, artery blockage, but one more small blood clot in the wrong place could easily cause the death of the person. Clearing the blood clot does no good to the person. He/she will not be restored to full operational capability.

TROUBLESHOOTING IN THE FIELD: Unless prior experience dictates otherwise, always begin at the beginning. Ask questions of the Operator of the faulty equipment: • Was equipment running when the problem occurred? • Does the Operator know what caused the problem, and if so, what, in their opinion, caused the problem? • Is the equipment out of sequence? • Check to ensure there is power • Turn on circuit breaker, ensure motor disconnect switch is on, and operate start button/switch Use voltmeter to check the following at incoming and load side of circuit breaker(s) and/or fuses, ensure that voltages are normal on all legs and read voltage to ground from each leg: • main power, usually 460 VAC between phases and 272 to ground • control & power, 208/240 between phases and 120 to ground and 120 VAC to neutral on a grounded system • low voltage control power, usually 24 to 30 VAC and/or VDC between phases and possibly to ground, usually negative is connected to ground Check controlling sensors in area of problem, then make complete check of all sensors, limit switches to ensure they are in correct position, have power, are programmed, set, and are functioning correctly.

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If and when a problem is found, whether electrical or mechanical, the problem should be corrected and the fault-finding begun anew, a seemingly unrelated fault or defect could be the cause of the problem. When there is more than one fault, the troubleshooting is exponentially more difficult. Do not assume that all problems are solved after completing one. Always test the circuit and operation prior to returning the equipment to service. If available, check wiring diagrams and PLC programs to isolate problem. Variable Frequency Drive (VFD) can be reset by turning power off. Wait until screen is blank and restore power; on some VFDs, press Stop/Reset – then press Start. Check that wiring is complete and that wires and connections are tight with no copper strands crossing from one terminal to another or to ground. Ensure that the neutral reading is good and that the neutral is complete and not open.

MOTOR TESTING IN SHOP: Prior to connecting a motor: • Move motor to electric shop motor test and repair station • Connect motor leads for 460 volt operation and wrap connections with black electrical tape • Check motor windings with an ohmmeter, each reading between phases should be within one or two ohms of each other; A to B, B to C, A to C • Use megohmmeter to check insulation resistance to ground of motor windings on 500 volt scale; minimum reading is 1000 ohms of resistance per volt of incoming power that motor will be connected to • Connect motor to power test leads and safety ground after checking that test lead power is shut off; secure motor to table to prevent motor from jumping when started; turn disconnect on; press start button; check “T” leads for motor amperage; check for abnormal sounds and heat in bearings or windings; clean motor shaft; shut down and disconnect

MOTOR TESTING IN FIELD: When a motor overload or circuit breaker trips and/or blows fuses, certain procedures and tests should be carried out: • Lockout and tagout main circuit breaker; • Test insulation resistance of motor wires and windings by using megohmmeter between T1, T2, & T3 leads and ground, then; • Test “T” leads to motor with ohmmeter for continuity and ohmage of windings between A to B, B to C, A to C; each resistance should be within 1 or 2 ohms of each other; if the ohms readings are significantly different, or, if there is no continuity; go to the motor disconnect box, turn it off, perform the continuity and resistance test on the “T” leads, again; if the readings are good, the problem is in the wires from the motor controller to the disconnect switch. • Check the three wires by disconnecting all three wires from switch and twist together; go to controller and check for continuity between A to C, B to C, A to C; one or more wires will be open or grounded. • Correct solution is to pull all new wires in from controller to motor disconnect switch, whatever caused the problem may have damaged the other wires, also,

replace all wires. • If problem is on motor side of disconnect switch, open motor connection box and disconnect motor; • Check motor for resistance to ground with megohmmeter. If reading is below 500,000 ohms, motor is grounded and must be replaced. • Test motor windings for ohms between phases with ohmmeter A to B, B to C, A to C. Readings should be within 1 or 2 ohms of each other. If readings indicate open or a significant ohmage difference, replace motor; • If motor test readings are good, test the motor leads between the disconnect switch and the motor connection box for continuity and ground resistance. If readings are not good, replace wires. • If all readings are OK, reconnect motor, remove lockout, and restore to service. The problem could have been mechanical in nature; an overload on motor caused by the chain, belt, bad bearings, faulty gearbox, or power glitch.

MOTOR CONTROLLER: • Check motor Full Load Amps (FLA) at motor and check setting on controller overload (OL) device; most newer OL devices are adjustable between certain ranges, some older OL devices use heaters for a given amperage. • If circuit disconnecting means in controller is a circuit breaker, it should be sized correctly. • If the disconnecting means is a Motor Circuit Protector (MCP), the MCP must be correctly sized for the motor it is protecting and the MCP has a trip setting unit which has to be correctly set based on the Full Load Amperage of the motor. Using a small screwdriver, push in on the screw head of the device and move to a multiple of thirteen of the FLA. Example: a motor FLA of 10 amps would require that the MCP trip device be set to an instantaneous trip point of 130 amps. • Fuses protecting the motor should be the dual element or current limiting type and based on the motor FLA.

PROGRAMMABLE LOGIC CONTROLLERS (PLC): • Check to ensure main power is on( 120 VAC. • Check 24V power available. • Identify problem area. • Check sensor operation in problem area. • Check sensor Inputs to PLC. • Check on PLC that a change in sensor state causes the corresponding Input LED on the PLC to go on or off. • Identify Output controlled by Input on PLC ladder diagram. • Ensure that Output LED is cycling on/off with Input. • Check that Output voltage is correct and cycling on/off with Input. • Locate Output device and ensure that voltage is reaching device and cycling with Input. • Ensure that Output device is working correctly (solenoid coil, relay coil, contactor coil, etc.) • An Input or Output module can be defective in one area or circuit and work correctly in all other circuits • If each field circuit is not fuse protected, the modular internal circuit becomes a fuse and can be destroyed by a field short circuit or any other over-current condition • Check modular circuit. If bad, module must be replaced

Electricity Testing and Measurement Handbook – Vol. 7 after correcting field fault. • Shut down PLC prior to changing any module – main power and 24V power. • Locate fault in field circuit by disconnecting wires at module and field device. Check between wires for short circuit and to ground for short circuit. Replace wire is short circuit found • Check device for ground, short circuit, mechanical and electrical operation, even when problem found in wires. Always check device for another fault. Problem in wires can cause problem in device or vice versa. If device defective, replace device and then check total circuit before placing in operation and after restoring circuit, check again to ensure circuit and module are operating correctly. • Check power supply module. If no output, shut down power and replace supply module. • Back plane can go bad, some of the modules with power and others with no power. Replace backplane. • Sometimes, the PLC can be reset using the Reset key switch. Ensure that turning the PLC off won’t interrupt other running sub-set programs, turn key switch to far right. After 15 seconds, turn to far left, wait, then return to middle position. This operation should reset program and enable a restart. • The PLC program can have a latch relay with no reset under certain conditions. The key switch reset may have no effect on the latch. Try turning the power to the PLC off and back on. This operation may reset the latch and allow the program to be restarted. • The PLC is usually part of a control circuit supplied with 120VAC through a 460V/120V transformer as part of a system with motors, controllers, safety circuits, and other controls. Occasionally, cycling the main 480V power off/on will be necessary to try to reset all the safety and control circuits. • Possession and use of an up-to-date ladder diagram, elementary wiring diagram, manufacturer’s manuals & diagrams, troubleshooting skills, operator’s knowledge, and time are all required to solve issues involved in maintaining a modern manufacturing production line.

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THE ART OF MEASURING LOW RESISTANCE Tee Sheffer and Paul Lantz, Signametrics

Don’t heap all the blame for a wrong measurement on the DMM (Delayed Neutron Monitor). There can be several less obvious sources of the errors. Testing assemblies and components usually includes checking the continuity of connectors, wires, traces, and lowvalue resistive elements. Such applications typically require both a DMM and a switching system. Many users select a DMM and switching cards based only on the specifications of the DMM and later are surprised to find that their measurements are an order of magnitude less accurate than expected. Many users don’t recognize the error as a system problem and conclude that the DMM is not meeting its specification. Making accurate, stable, and repeatable resistance measurements is an art. There is plenty of technology involved, but the art is an important part, especially when you are measuring low resistance values. To achieve your accuracy goal, you need to understand the error sources in your application. It is important to start with a good DMM. But, there are significant error sources outside the DMM, some of which may not be obvious. Things may be more complex than they seem, and some types of errors may be misinterpreted.

LIMITATIONS AND ERROR SOURCES Not all materials are created equal. Most connectors and test probes are made of beryllium-copper or phosphor-bronze materials that closely match the electromotive force (EMF) of copper. For this reason, they do not cause significant thermally induced voltage errors. However, relays and some other devices use nickel-iron alloys that do not match the properties of copper. These can cause significant thermal EMF errors. Thermal voltages are generated when there is a mismatch of materials combined with a temperature difference. This is the same principle that makes thermocouples work as temperature sensors. If you expect readings that are accurate within a few milliohms, this is a big issue. This error source also affects higher value resistance measurements, but to a lesser degree. It is easy to overlook second-order specifications of a DMM, such as current drive levels used for resistance measurement. These specs may be in small print or missing, but they are important. For measuring low resistance, this spec tells what you can expect from the DMM. The accuracy specs of the DMM don’t tell the whole story. For example, the Signametrics SMX2064 PXI DMM uses a 10-mA current source, while most other DMMs are limited to 1 mA or less. Remember Ohm’s Law: V = I x R means that 10 times the current produces 10 times the voltage being sensed across the resistor. This larger signal is less susceptible to external errors and noise and provides more signal to measure.

The larger signal almost always produces a more accurate measurement. It is confusing to compare two DMMs having similar specifications in ohms if one has 10 times the current drive. The two are not the same. The one with the higher current will perform better, especially in a system. Good DMMs can measure signals down to a few microvolts. If you need to measure a resistance down to a few milliohms, a 1-mA test current only produces 1 µV of signal per 1 mΩ of resistance. In other words, you are operating right at the resolution limit of the DMM. With a 10-mA test current, there are 10 µV of signal per 1 mΩ of resistance. As a result, a DMM that uses 10x as much current for this test will give about 10x improvement in accuracy, stability, and repeatability for very low values. If your test has serious throughput requirements and you need to make hundreds of measurements per second, having a stronger signal combined with good noise performance in the DMM makes a huge difference. Remember that the DMM’s accuracy at higher speeds may be much more important than its best accuracy.

TWO-WIRE Everyone knows how easy it is to measure resistance using a two-wire connection. However, for low resistance, a two-

Figure 1. Two-Wire

wire connection has disadvantages (Figure 1). Test leads frequently add >1 Ω of resistance, and your test probe may add another 0.1 Ω of contact resistance to the measurement. These errors are significant if you are measuring 20 Ω. You can eliminate most of the test-lead errors from a twowire connection by shorting the leads and setting the RelativeOhms control. This enables the DMM to subtract the test-lead resistance from the readings that follow. This is a very handy tool when you are doing manual testing, but it is less useful in an automated test.

FOUR-WIRE Ω A four-wire connection is the standard method for measuring low resistance. It eliminates the resistance of the test leads

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from the measurement. One pair of leads carries the test current while the other pair of leads senses the voltage across the resistor under test (Figure 2).

resistance measurements? Two-wire resistance measurements certainly are attractive because you can fit twice as many two-wire measurements onto a card as you can four-wire measurements. The economics are attractive. Perhaps you can put a short circuit on one of the inputs to the switching card and measure that short to make a Relative-Ohms measurement? This line of reasoning also might lead you to select the highest density switching card possible. However, there are reasons to be careful. A typical switching card does not have the same resistance through all of its channels. Channel 0 may add 0.2 Ω to the reading while Channel 20 may add 0.8 Ω. Consequently, measuring a short on one does not give a good compensation for the other because they do not have the same resistance. Even if you could correct for the difference in channel-tochannel resistance, relays typically have about 50 mΩ of contact resistance that will shift around by 20 mΩ from one reading to the next. You might think that high-current relays will have lower contact resistance, but it doesn’t work that way. High-current relays usually have silver-plated contacts that give low resistance for currents above 100 mA. Unfortunately, silver-plated contacts have a high and unpredictable contact resistance for currents less than 50 mA. Relays are made of nickel-iron materials, and they all have problems with thermal EMFs. Frequently, this error source is not specified for high-density switching cards. If not, the thermal voltages probably are around 100 µV. If your DMM uses 10 mA to make this measurement, the switching card adds 10 mΩ of error to the measurement. If your DMM uses only 1 mA, the switch will add 100 mΩ of error to the measurement. Keep in mind that this error voltage is made up of all of the closed relay contacts connected to the sense lines of the DMM. The more complex the switching system is, the higher the error will be.

Figure 2. Four-Wire

The resistance of the current-carrying leads doesn’t matter because they are not in the measurement path. The resistance of the sensing leads doesn’t matter since they don’t carry any current. A four-wire connection is not immune to thermal EMF errors caused by mismatched materials. This usually is not important in manual testing situations, but it is a major issue in automated systems where a relay switch is used.

SIX-WIRE What if the resistor you want to measure is in a circuit with other components or resistors as in networks or on a loaded circuit board? Then you need a six-wire guarded connection. This method makes it possible to measure resistance in situations where it would be impossible otherwise. The SMX2064 DMM offers this capability (Figure 3).

FOUR-WIRE WITH A SWITCHING CARD Using a four-wire connection through your switching card takes care of the resistance issues associated with the switching card. This accuracy improvement happens at the expense of reduced channel density. However, it does not take care of the thermal EMF problems that come with some switching cards (Figure 4). Figure 3. Six-Wire Guarded In-Circuit Measurement

A guard amplifier drives the junction of parallel components to a voltage level that prevents any of the test current from leaking away from the resistor under test. This is a standard method used by large ATE in-circuit test systems. With the right DMM, you can implement it too.

MEASURING THROUGH A SWITCH MATRIX Many applications are for production test. In these cases, it is almost always necessary to perform multiple tests and measure multiple points. You usually do this by putting a switching card or a matrix ahead of the DMM. It is important to note that the switching card can be a major source of error, particularly when measuring low resistance.

TWO-WIRE WITH A SWITCHING CARD How does adding a switching card affect your two-wire

THERMAL VOLTAGE ERRORS, NOT CORRECTED Figure 4. Four-Wire Switching Card

One way to reduce this error is to use a DMM with the Offset-Ohms function. However, this method is very slow, it adds noise, and it is limited in its capability to reduce the error. For best results, start with a high-quality switching card that is specified for low thermal EMF error. How big a problem are thermal EMF voltages in relay switches? A high-quality switching card has about 10 µV while

Electricity Testing and Measurement Handbook – Vol. 7 a typical one has >100 µV of thermal voltages. There are a few instrumentation quality switches that exhibit 1µV or less. Take a look at Figure 5 to see the effect. The yellow plots depict the specs of two similar DMMs. One of the DMMs uses 1-mA excitation current while the other uses 10 mA. There are some things to note:

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SIX-WIRE WITH A SWITCHING CARD A six-wire resistance connection works just fine with a switching card as long as the card is organized to support it. Remember that a six-wire connection does not increase the accuracy of your measurement unless other resistors in the circuit need to be guarded. This is still the only way to guard-out parallel resistors that otherwise would make the measurement impossible.

EXAMPLES

Figure 5a. 1-mA Ohms Excitation

Figure 5b. 10-mA Ohms Excitation

• Both DMMs have very similar specifications as shown by the yellow lines. • As soon as you combine them with a relay card that has 10-µV offset, the system error is considerably greater than the DMM spec. For the DMM with 10-mA excitation, the system error is almost two times the DMM spec. For the DMM with 1-mA excitation, the system error is almost 10 times the DMM spec. • If you combine the DMMs with a relay card that has 100-µV offset, the error becomes huge. For the DMM with10-mA excitation, the system error is almost 10 times the DMM spec. For the DMM with 1-mA excitation, the system error is almost 100 times the DMM spec. • The effect of the relay offset voltages overwhelms the DMM specifications in both cases, but the DMM that uses 10-mA excitation current produces a system spec between five and 10 times better than the DMM that uses 1-mA excitation.

A manufacturer of semiconductor protection devices uses an SMX2064 on its low-resistance four-wire range to accurately measure resistances around 20 Ω before and after hitting the device with a high test voltage. Because the SMX2064 can take an accurate measurement in as little as 1 ms, test throughput is high. A manufacturer of hybrid circuits uses an SMX2064 to measure resistance values of less than 100 mΩ. In this case, speed is not an issue, but getting a useable measurement is. Other DMMs that use only 1-mA excitation current did not qualify to do the job.

CONCLUSION If you need to measure low resistance values, you benefit by using a DMM that has a 10-mA excitation current. A 1-mA source gives a much weaker signal to measure and presents system-level problems, particularly if there are switching cards involved. If you expect a stable, accurate result, you almost certainly need to use a four-wire connection. The accuracy spec of the DMM is important but not the whole story. Remember that everything in the measurement path affects the accuracy of the measurement, especially switching cards. Your best bet is to combine a DMM with good ohms specifications and high test current and a switching card with a low thermal EMF spec, preferably an instrumentation type.

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STANDARDS FOR SUPERCONDUCTOR AND MAGNETIC MEASUREMENTS National Institute of Standards and Technology GOALS This project develops standard measurement techniques for critical current, residual resistivity ratio, and magnetic hysteresis loss, and provides quality assurance and reference data for commercial high temperature and low-temperature superconductors. Applications supported include magnetic-resonance imaging, research magnets, magnets for fusion confinement, motors, generators, transformers, high-quality-factor resonant cavities for particle accelerators, and superconducting Probe for the measurement of the critical current of a bearings. Superconductor superconductor wire as a function of temperature. The applications specific to probe is inserted into the bore of a high field superthe electrical power conducting magnet. industry include transmission lines, synchronous condensers, magnetic energy storage, and fault-current limiters. Project members assist in the creation and management of international standards through the International Electrotechnical Commission for superconductor characterization covering all commercial applications, including electronics. The project is currently focusing on measurements of variable-temperature critical current, residual resistivity ratio, magnetic hysteresis loss, critical current of marginally stable superconductors, and the irreversible effects of changes in magnetic field and temperature on critical current.

CUSTOMER NEEDS This project serves the U.S. superconductor industry, which consists of many small companies, in the development of new metrology and standards, and in providing difficult and unique measurements. We participate in projects sponsored by other government agencies that involve industry, universities, and national laboratories. The potential impact of superconductivity on electric power systems, alternative energy sources, and research magnets makes this technology especially important. We focus on: (1) developing new metrology needed for evolving, large-scale superconductors, (2) providing unique databases of superconductor properties, (3) participating in interlaboratory comparisons needed to verify techniques and systems used by U.S. industry, and (4) developing international standards for superconductivity needed for fair and open competition and improved communication. Electric power grid stability, power quality, and urban power needs are pressing national problems. Superconductive applications can address many of them in ways and with efficiencies that conventional materials cannot. “Second-generation” Y-Ba- Cu-O (YBCO) superconductors are approaching the

targets established by the U.S. Department of Energy. The demonstration of a superconductor synchronous condenser for reactive power support was very successful and has drawn attention to the promise of this technology. Previous demonstration projects had involved first-generation materials, Bi-Sr-Ca-Cu-O (BSCCO). Variable-temperature measurements of critical current and magnetic hysteresis loss will become more important with these AC applications, and methods for reducing losses are expected to evolve as second-generation materials become commercial. Fusion energy is a potential, virtually inexhaustible energy source for the future. It does not produce CO2 and is environmentally cleaner than fission energy. Superconductors are used to generate the ultrahigh magnetic fields that confine the plasma in fusion energy research. We measure the magnetic hysteresis loss and critical current of marginally stable, high-current Nb3Sn superconductors for fusion and other research magnets. Because of the way superconductors are used in magnets, variable-temperature critical-current measurements are needed for more complete characterization. The focus of high-energy research is to probe and understand nature at the most basic level, including dark matter and dark energy. The particle accelerator and detector magnets needed for this fundamental science continue to push the limits of superconductor technology. The next generation of Nb3Sn and Nb-Ti wires is pushing towards higher current density, less stabilizer, larger wire diameter, and higher magnetic fields. The resulting higherectronics and Electrical Engineering Laboratory current required for critical-current measurements turns many minor measurement problems into significant engineering challenges. For example, heating of the specimen, from many sources, during the measurement can cause a wire to appear to be thermally unstable. Newer MgB2 wires may be used for specialty magnets that can safely operate at the higher temperatures caused by high heat loads. We need to make sure that our measurements and the measurements of other laboratories keep up with these challenges and provide accurate results for conductor development, evaluation, and application. Possible spin-off applications of particle accelerators are efficient, powerful light sources and free-electron lasers for biomedicine and nanoscale materials production. The heart of these applications is a linear accelerator that uses high-efficiency, pure Nb resonant cavities. We conduct research on a key materials property measurement for this application, the residual resistivity ratio (RRR) of the pure Nb. This measurement is difficult because it is performed on samples that have dimensions similar to those of the application. Precise variable-temperature measurements are needed for accurate extrapolations.

TECHNICAL STRATEGY International Standards – With each significant advance in superconductor technology, new procedures, interlaboratory

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comparisons, and standards are needed. International standards for superconductivity are created through the International Electrotechnical Commission (IEC), Technical Committee 90 (TC 90). Critical Current Measurements – One of the most important performance parameters for large-scale superconductor applications is the critical current. Critical current is difficult to measure correctly and accurately; thus these measurements are often subject to scrutiny and debate. The critical current is determined from a measurement of voltage versus current. Typical criteria are electric-field strength of 10 microvolts per meter and resistivity of 10–14 ohm-meters.

273 kelvins (0 degrees Celsius) and 4.2 kelvins (the boiling point of liquid helium). The value of RRR indicates the purity and the low-temperature thermal conductivity of a material, and is often used as a materials specification for superconductors. The low temperature resistivity of a sample that contains a superconductor is defined at a temperature just above the transition temperature or is defined as the normal-state value extrapolated to 4.2 kelvins. For a composite superconducting wire, RRR is an indicator of the quality of the stabilizer, which is usually copper or aluminum that provides electrical and thermal conduction during conditions where the local superconductor momentarily enters the normal state. For pure Nb used in radio-frequency cavities of linear accelerators, the low temperature resistivity is defined as the normal-state value extrapolated to 4.2 kelvins. This extrapolation requires precise measurements. We have studied some fundamental questions concerning the best measurement of RRR and the relative differences associated with different measurement methods, model equations for the extrapolation, and magnetic field orientations (when a field is used to drive the superconductor into the normal state). Magnetic Hysteresis Loss Measurements – As part of our program to characterize superconductors, we measure the magnetic hysteresis loss of marginally stable, high-current Nb3Sn superconductors for fusion and particle-accelerator magnets. We use a magnetometer based on a superconducting quantum interference device (SQUID) to measure the magnetic hysteresis loss of superconductors, which is the area of the magnetization-versusfield loop. In some cases, especially for marginally stable conductors, we use special techniques to obtain accurate results. Measurement techniques developed at NIST have been adopted by other laboratories.

Illustration of a superconductor’s voltage-current characteristic with two common criteria applied.

ACCOMPLISHMENTS

Electric field versus current at temperatures from 7.0 to 8.3 kelvins in steps of 0.1 kelvins for a Nb3Sn wire.These are typical curves for the determination of critical current.

Critical-current measurements at variable temperatures are needed to determine the temperature margin for magnet applications. The temperature margin is defined as the difference between the operating temperature and the temperature at which critical current Ic is equal to the operating current. When a magnet is operating, transient excursions in magnetic field H or current I are not expected; however, many events can cause transient excursions to higher temperatures T, such as wire motion, AC losses, and radiation. Hence the temperature margin of a wire is a key specification in the design of superconducting magnets. Variable-temperature critical-current measurements require data acquisition with the sample in a flowing gas environment rather than immersed in a liquid cryogen. Accurate high-current (above 100 amperes) measurements in a flowing gas environment are very difficult to perform. Residual Resistivity Ratio Measurements – The RRR is defined as the ratio of electrical resistivity at two temperatures:

Critical current versus temperature of a high-Tc Bi2Sr2CaCu2O8+x wire at various magnetic fields. Such curves are used to determine the safe operating current at different temperatures and fields.

• Superconductor Data Enables U.S. Company to Offer Product to Korean Project – New bismuth-based hightemperature superconductor wires are under active consideration for a 600 kilojoule superconducting magnetic energy storage (SMES) project lead by the Korea Electrotechnology Research Institute. The purpose of the SMES system is to stabilize the electric power grid. The magnet will be wound with 10-kiloampere superconducting cables composed of many round wires. It will be cooled to 20 kelvins by cryocoolers. A U.S. company turned to us for critical current measurements at 20 kelvins to

Electricity Testing and Measurement Handbook – Vol. 7 determine whether its conductor could meet the project’s specifications for critical current. Critical current, the largest current a superconducting wire can carry, is a key performance and design parameter. Critical current depends on temperature, magnetic field and, in many cases, the angle of the magnetic field with respect to the conductor. We made variable-temperature critical-current measurements on three wire specimens in magnetic fields up to 8 teslas, at various magnetic-field angles, and at temperatures from 4 to 30 kelvins. NIST has the only such multiparameter, high-current, variable-temperature measurement capability in the U.S. The largest current applied to the 0.81 millimeter diameter wire samples was 775 amperes. The results showed that the angle dependence of critical current for the wires was less than just 3 percent over the useful range of field and temperature, and that the round wires could be used at higher magnetic fields and temperatures than tape conductors. These data will be used to design the safe operating limits of the SMES magnet system. • Key Measurements for the International Thermonuclear Experimental Reactor – Superconducting magnets are used in fusion energy projects such as the International Thermonuclear Experimental Reactor (ITER), to confine and heat the plasma. The superconductors for ITER’s large magnet systems are all “cable-in-conduit conductors” (CICC), which provide both mechanical support for the large magnetic forces and a flow path for the liquid helium required to cool the cable. The superconducting magnet must be operated below the critical current of the cable, which is a function of magnetic field and temperature. Temperature is an important variable, and the local temperature of the conductor depends on the mass-flow rate of the coolant and the distribution of the heat load along the CICC. We designed and constructed a new variable temperature probe that allows us to make measurements in our 52-millimeter bore, 16-tesla magnet. This probe replaces one that was designed for our 86-millimeter bore, 12-tesla magnet. Fitting everything into the smaller bore was difficult, but the new probe performed as expected and allows us to make measurements at the ITER design field of 13 teslas. We made measurements up to 765 amperes with a Nb3Sn sample in flowing helium gas. Measurements were made at temperatures from 4 to 17 kelvins and magnetic fields from 0 to 14 teslas. Some measurements were made at 15 and 16 teslas for temperatures from 4 to 5 kelvins; however, these magnetic fields can be generated only when a sample is measured in liquid helium.

Critical current versus temperature at various magnetic fields for a Nb3Sn wire. These curves show the current carrying limits for various combinations of temperature and magnetic field.

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Electric field versus temperature at currents from 66 to 84 amperes in steps of 1.5 amperes for a Nb3Sn wire. These are typical curves for the determination of temperature margin.

The results of our unique variable-temperature measurements provide a comprehensive characterization and form a basis for evaluating CICC and magnet performance. We used these data to generate curves of electric field versus temperature at constant current and magnetic field. In turn, these give a direct indication of the temperature safety margin of the conductor. • International Standards on Superconductivity – Many of the 14 published IEC/TC 90 standards on superconductivity contain “precision” and “accuracy” statements rather than currently accepted statements of “uncertainty.” NIST has advocated that TC 90 adopt a more modern approach to uncertainty. In collaboration with the Information Technology Laboratory, we have developed a 50-page report on the possibility of changing statements of “accuracy” to statements of “uncertainty” in IEC/TC 90 measurement standards, which was presented at TC 90 meetings in June 2006. They included proposed change sheets for 13 of the 14 TC 90 document standards. Ultimately, all TC 90 delegates voted in favor of changing to uncertainty statements during the maintenance cycle of existing standards and during the development of new standards. • Current Ripple a Source of Measurement Errors – All high-current power supplies contain some current ripple and spikes. New high-performance conductors have high critical currents that require current supplies over 1000 amperes. High-current power supplies with the lowest level of current ripple and spikes are often more than a factor of ten times more expensive than conventional supplies. In addition, current ripple and spikes are a greater problem for short-sample critical current testing than for magnet operation because of the smaller load inductance. Therefore, we need to understand the effects of ripple and spikes on the measured critical current (Ic) and “n-value”, the index of the shape of the electric field-current curve. We focused on how ripple changes the n-value and showed that, in terms of percentage change, the effect of ripple on n-value was about 7 times that on Ic Interlaboratory comparisons often show variations in n-value much larger than the variations in Ic. We examined models and use the measurements on simulators to attempt to reproduce and understand the effects observed in measurements on superconductors. We believe that current ripple and spikes are sources of differences in n-values measured at different laboratories. • New Method to Evaluate the Relative Stability of Conductors – We recently started measuring voltage versus magnetic field (V-H) on Nb3Sn wires to assess their relative sta-

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bility. Voltage versus current (V-I) at constant field is usually measured to determine Ic. Low-noise V-H measurements were made at constant or ramping current with the same electronic instruments, apparatus, and sample mount as used in Ic measurements. High-performance Nb3Sn wires exhibit flux-jump instabilities at low magnetic fields, and low-noise V-H curves on these wires show indications of flux jumps. V-H measurements also reveal that less stable wires will quench (abruptly and irreversibly transition to the normal state) at currents much smaller than Ic at the lower magnetic fields. This new method needs to be further understood and may be standardized to ensure that it provides accurate and reliable data.

• IEC 61788-5 Superconductivity – Part 5: Matrix to Superconductor Volume Ratio Measurement – Copper to Superconductor Volume Ratio of Cu/Nb-Ti Composite Superconductors

STANDARDS COMMITTEES • Loren Goodrich is the Chairman of IEC/TC 90, the U.S. Technical Advisor to TC 90, the Convener of Working Group 2 (WG2) in TC 90, the primary U.S. Expert to WG4, WG5, WG6 and WG11, and the secondary U.S. Expert to WG1, WG3, and WG7. • Ted Stauffer is Administrator of the U.S. Technical Advisory Group to TC 90.

STANDARDS In recent years, we have led in the creation and revision of several IEC standards for superconductor characterization: • IEC 61788-1 Superconductivity – Part 1: Critical Current Measurement – DC Critical Current of Cu/Nb-Ti Composite Superconductors • IEC 61788-2 Superconductivity – Part 2: Critical Current Measurement – DC Critical Current of Nb3Sn Composite Superconductors • IEC 61788-3 Superconductivity – Part 3: Critical Current Measurement – DC Critical Current of Ag-sheathed Bi-2212 and Bi-2223 Oxide Superconductor • IEC 61788-4 Superconductivity – Part 4: Residual Resistance Ratio Measurement – Residual Resistance Ratio of Nb-Ti Composite Superconductors Critical current vs. temperature of a Bi-2212 tape at a magnetic field of 0.5 tesla and various magnetic field angles. Such curves are used to determine the safe operating current at various temperatures and field angles.

• IEC 61788-6 Superconductivity – Part 6: Mechanical Properties Measurement – Room Temperature Tensile Test of Cu/Nb-Ti Composite Superconductors • IEC 61788-7 Superconductivity – Part 7: Electronic Characteristic Measurements – Surface Resistance of Superconductors at Microwave Frequencies • IEC 61788-8 Superconductivity – Part 8: AC Loss Measurements – Total AC loss Measurement of Cu/Nb-Ti Composite Superconducting Wires Exposed to a Transverse Alternating Magnetic Field by a Pickup Coil Method • IEC 61788-10 Superconductivity – Part 10: Critical Temperature Measurement – Critical Temperature of Nb-Ti, Nb3Sn, and Bi-System Oxide Composite Superconductors by a Resistance Method • IEC 61788-11 Superconductivity – Part 11: Residual Resistance Ratio Measurement – Residual Resistance Ratio of Nb3Sn Composite Superconductors • IEC 61788-12 Superconductivity – Part 12: Matrix to Superconductor Volume Ratio Measurement – Copper to NonCopper Volume Ratio of Nb3Sn Composite Superconducting Wires • IEC 61788-13 Superconductivity – Part 13: AC Loss Measurements – Magnetometer Methods for Hysteresis Loss in Cu/Nb-Ti Multifilamentary Composites • IEC 60050-815 International Electrotechnical Vocabulary – Part 815: Superconductivity


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MULTI CHANNEL CURRENT TRANSDUCER SYSTEMS DANFYSIK

MULTI CHANNEL SYSTEM MCS FROM DANFYSIK The new multi channel systems MCS from DANFYSIK combine highest accuracy and bandwidth with lowest phase shift and common mode influence. The systems measure AC signals as well as DC signals with a linearity in the ppm range and work up to 1MHz. Different current transducer heads from 200Apk to 5000Apk are available as standard transducers. Optimised systems for the needs of power electronics and drives applications

Modular up to six channels • High linearity • Low offset • High bandwidth up to 1MHz • Extremely low phase shift • High CMR due to galvanic insulation • Transducer heads from 200A to 5000A • Current and voltage output • Optimised for power electronics needs

CURRENT ANALYSIS IN POWER ELECTRONICS APPLICATIONS To optimise power electronics components like inverters or drives, the electrical signals current and voltage must be measured with very high accuracy. The quality of the measurement results depends on linearity, offset, and width and phase shift of used instruments and connected current and voltage sensors. Standard current transformers have a limited bandwidth, fast impulse transducers and very low accuracy. In addition these transducers are not able to measure DC components in the signal or do not work at all at DC since the iron core gets saturated. Wideband-MHz-shunts are accurate and fast but do not allow a galvanic isolated connection of the instrument from the power electronics circuit. Also, high common mode signals result in disturbances and inaccuracy. The measurement of electrical power and the calculation of losses are even more problematic. At low power factor linearity, offset and phase shift of the transducers have a much higher influence on the resulting error than the instrument itself. Losses are normally calculated by the subtraction of the output power from the input power. The efficiencies of power electronics components are quite high, and therefore the actual losses compared to the measured input or output power are very small. Consequently, the error of measured input or output power can easily be as high as the loss itself if the sensor is not accurate enough.

The modularity of the MCS systems allows all types of analyses in the power electronics field to be connected. Many of the measurements are made on three-phase outputs of frequency inverters or three phase sinus inputs of electric motors. To analyse a complete frequency inverter, a six-phase system is necessary. Automotive applications normally measure the DC from the battery in addition to the three-phase output to the load. The MCS systems can be ordered from three to six channels. A threephase system can be updated with additional channels easily.

ZERO-FLUX-PRINCIPLE The transducer consists of a transducer head and an electronic module. In the transducer head there are three iron cores with a common secondary winding but with separate auxiliary windings. The primary current lp, via the winding Lp, produces a magnetic field in the three iron cores of the transducer. Thereby Lp mostly consists of the primary conductor, which is lead through the transducer. The compensation current lc compensates the magnetic field of the primary current and provides a steady zero-flux in the iron core. This compensation current is driven by an operation amplifier to which both inputs are connected with a signal which is proportional with the AC- and DC-component of the primary conductor current. The AC-component is thus induced into the auxiliary winding Lh1. The DC-component and the very lowfrequency component comes from the socalled Zero-FluxDetector (symmetry detector). Via an oscillator and the auxiliary windings Lh2 and Lh3, the other two iron cores are driven into saturation in different directions.

68 Both iron cores and the auxiliary windings Lh2 and Lh3 are built identically. The currents via Lh2 and Lh3 are thus identical. In this case, the main core flux is zero. A direct current via the primary conductor results in a flux via the core. Therefore, both Zero-Flux-Detector cores can no longer be driven into saturation identically, and the two currents via Lh2 and Lh3 are no longer equal. The difference between the currents is proportional with the DC component of the current lp. The Zero-Flux-Detector processes this signal and leads it to the DC-input of the operation amplifier which drives the compensation current. This way the DC-component of the primary current can also be compensated. The compensation current is an accurate reproduction of the primary current, and can be evaluated as a galvanic separated signal by all types of measuring instruments. The burden resistor is only to be used if the measuring instrument only has voltage inputs. The advantage of this technology is mostly the high accuracy of the transducer. The sensitivity of the Zero-FluxDetector and of the iron cores allows the best possible ppm-accuracy. A transducer bandwidth of a few hundred kHz can easily be obtained.



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FALL-OF-POTENTIAL GROUND TESTING, CLAMP-ON GROUND TESTING COMPARISON Chauvin Arnoux, Inc. On April 14, 2002, a ground resistance test was conducted to compare the results obtained from the Fall-of-Potential 3Point testing method to the clamp-on testing method. The grounding system consisted of four copper clad rods installed in an approximate 20 ft square. Three of the rods are 5/8" in diameter and 10 ft in length. The fourth rod is 1/2" in diameter and 8 ft in length. All rods were coupled together with 3-gauge aluminum wire. Figure 1 shows the schematic of the system.

conductor through the rod and clamp. Readings on rod number three ranged from 615 to 733mΩ at each bonding point, indicating that all connections were good. See Figure 2 for full results. Measurement Point

Resistance (µOhms)

A to B

713

C to B

615

A to C

733

Figure 2. Bonding resistance measurements

Figure 1. The Grounding System

The tests were conducted with the following equipment manufactured by AEMC instrument: • Model 4500, 4-Point Ground Resistance Tester • Model 4630, 4-Point Ground Resistance Tester • Model 3731, Clamp-On Ground Resistance Tester. Additionally, we used the AEMC Model 5600, a micro-ohmmeter to verify the bonding of the aluminum wire to the individual ground rods. The soil conditions in the test area were predominately loam with some gravel. Conditions on the day of the test were dry and sunny, some light rain had occurred the previous day to the test. Therefore, the soil was somewhat moist at the surface. The AEMC Model 5600 Micro-Ohmmeter was used to measure bonding resistance at each rod and was the first test completed. Measurements from each conductor to the rod were taken as well as measurements from conductor to

In the first test, the AEMC Model 4500 was used as 3Point ground tester. Rod number three was first disconnected from the other rods in the system so that its individual resistance could be measured. The X lead was attached to rod number three (see Figure 3). The Z lead was attached to an auxiliary electrode 100 feet away and the Y lead was initially connected to the auxiliary electrode 60 feet away. Readings were taken with the Y electrode at 90, 80, 70, 60, 50, 40, 30, 20 and 10 feet. Figure 3 shows the results of this test.

Figure 3. Three-Point test connection

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Figure 4. Model 4500 test results

The same test was repeated using the AEMC Model 4630 fall-of-potential ground tester. The results are shown in Figure 5.

Figure 5. Model 4630 test results

Finally, the AEMC Model 3731 was used to measure the resistance at rod number three with all other rods detached from it. A temporary cable was installed between rod number three and the municipal grounding system, thus setting up the required parallel paths necessary for accurate measurement using a clamp-on ground tester (see Figure 6). Under these conditions, the reading was 84.5Ω. The results of these tests showed that the clamp-on ground tester is indeed an effective tool in measuring ground resistance when used under the proper conditions. Readings between the clamp-on ground testing and the fall-of-potential ground testing method correlate. The advantages of using the clamp-on tester were the ability to test without disconnecting the rod from service and the ability to test without the need for auxiliary ground electrodes. These two points saved considerable amount of time in conducting the test

Figure 6. Single rod test using the Model 3731 clamp-on ground resistance tester


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AN INTRODUCTION TO ANTENNA TEST RANGES, MEASUREMENTS AND INSTRUMENTATION The basic principles of antenna test and measurement are discussed along with an introduction to various range geometries, and instrumentation Jeffrey A. Fordham, Microwave Instrumentation Technologies, LLC.

INTRODUCTION

DIRECTIVITY

By definition, all of today’s wireless communication systems contain one key element, an antenna of some form. This antenna serves as the transducer between the controlled energy residing within the system and the radiated energy existing in free space. In designing wireless systems, engineers must choose an antenna that meets the system’s requirements to firmly close the link between the remote points of the communications system. While the forms that antennas can take on to meet these system requirements for communications systems are nearly limitless, most antennas can be specified by a common set of performance metrics.

It is convenient to express the directive properties of an antenna in terms of the distribution in space of the power radiated by the antenna. The directivity is defined as 4p times the ratio of the maximum radiation intensity (power radiated per unit solid angle) to the total power radiated by the antenna. The directivity of an antenna is independent of its radiation efficiency and its impedance match to the connected transmission line.

ANTENNA PERFORMANCE METRICS In order to satisfy the system requirements and choose a suitable antenna, system engineers must evaluate an antenna’s performance. Typical metrics used in evaluating an antenna includes the input impedance, polarization, radiation efficiency, directivity, gain and radiation pattern.

INPUT IMPEDANCE Input impedance is the parameter which relates the antenna to its transmission line. It is of primary importance in determining the transfer of power from the transmission line to the antenna and vice versa. The impedance match between the antenna and the transmission line is usually expressed in terms of the standing wave ratio (SWR) or the reflection coefficient of the antenna when connected to a transmission line of a given impedance. The reflection coefficient expressed in decibels is called return loss.

POLARIZATION The polarization of an antenna is defined as the polarization of the electromagnetic wave radiated by the antenna along a vector originating at the antenna and pointed along the primary direction of propagation. The polarization state of the wave is described by the shape and orientation of an ellipse formed by tracing the extremity of the electromagnetic field vector versus time. Although all antennas are elliptically polarized, most antennas are specified by the ideal polarization conditions of circular or linear polarization. The ratio of the major axis to the minor axis of the polarization ellipse defines the magnitude of the axial ratio. The tilt angle describes the orientation of the ellipse in space. The sense of polarization is determined by observing the direction of rotation of the electric field vector from a point behind the source. Right-hand and left-hand polarizations correspond to clockwise and counterclockwise rotation respectively.

GAIN The gain, or power gain, is a measure of the ability to concentrate in a particular direction the net power accepted by the antenna from the connected transmitter. When the direction is not specified, the gain is usually taken to be its maximum value. Antenna gain is independent of reflection losses resulting from impedance mismatch.

RADIATION EFFICIENCY The radiation efficiency of an antenna is the ratio of the power radiated by the antenna to the net power accepted at its input terminals. It may also be expressed as the ratio of the maximum gain to the directivity.

RADIATION PATTERN Antenna radiation patterns are graphical representations of the distribution of radiated energy as a function of direction about an antenna. Radiation patterns can be plotted in terms of field strength, power density, or decibels. They can be absolute or relative to some reference level, with the peak of the beam often chosen as the reference. Radiation patterns can be displayed in rectangular or polar format as functions of the spherical coordinates q and f. A typical antenna pattern in a rectangular format is shown below1.

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ANTENNA RANGE SITING CONSIDERATIONS

aration between the transmitting antenna and the receiving antenna for a reasonable approximation of the far field gain and radiation patterns. At distances from a transmitting antenna, which are large compared with the antenna dimensions, the phase front of the emergent wave is nearly spherical in shape. For extreme separations, the radius of curvature is so large that for all practical purposes the phase front can be considered planar over the aperture of a practical antenna. As the antennas are brought closer together, a condition is reached in which, because of the short radius of curvature, there is an appreciable separation D between the wavefront and the edges of the antenna aperture.

The choice of an antenna test range is dependent on many factors, such as the directivity of the antenna under test, frequency range and desired test parameters. Often the mechanical features of the antenna (size, weight and volume) can have as much influence on the selection of an antenna range as do the electrical performance factors. In selecting an antenna range to evaluate antenna performance, care must be taken to ensure the performance metrics are measured with sufficient accuracy.

Rectangular Anechoic Chamber

Compact Antenna Test Range

Outdoor Elevated Range

Ground Reflection Range Spherical Phase Front Tangent to a Plane Antenna Aperture

A criterion that is commonly employed in determining the minimum permissible value of R is to hold D to a maximum of 1/16 wavelength (equivalent to 22.5° of phase variation). If this condition is met, the receiving antenna is said to be in the far field of the transmitting antenna. The mathematical expression for this minimum range: Planar Near-Field

Cylindrical Near-Field

A few of the more commonly used antenna test ranges are shown here. Regardless of the chosen test range, three key factors must be addressed and controlled to ensure a successful measurement. These factors are Spherical Near-Field the phase variations of the incident field, the amplitude variations of the incident field and the stray signals created by reflections within the test range.

VARIATIONS OF THE PHASE OF THE INCIDENT FIELD In order to accurately measure an antenna’s far zone performance, the deviation of the phase of the field across its aperture must be restricted. The criterion generally used is that the phase should be constant to within p/8 radian (22.5°). Under normal operating conditions, this criteria is easily achieved since there is usually a large separation between transmitting and receiving antennas. During antenna testing, it is desirable because of various practical considerations to make antenna measurements at as short a range as possible. Since the measurements must simulate the operating situation, it is necessary to determine the minimum sep-

The major effect of a small deviation D is to produce minor distortions of the sidelobe structure. Larger values of D will cause appreciable errors in the measured gain and lobe structure. Conversely, this condition can mask asymmetrical sidelobe structures which are actually present.

VARIATIONS OF THE AMPLITUDE OF THE INCIDENT FIELD A second and important siting consideration is the variation of the amplitude of the incident field over the aperture of the test antenna. Excessive variations in the field will cause significant errors in the measured maximum gain and sidelobe structure. This effect can be seen better from the viewpoint of reciprocity. Variations in the amplitude of the field over the aperture on receiving are analogous to the transmitting case of a modification of the aperture illumination by the primary feed. If the variation across the antenna under test is limited to about 0.5 dB, error in the measurements will be negligible for most applications. It is essential that the transmitting antenna be accurately directed so that the peak of its beam is centered on the aperture of the antenna under test. Improper alignment, which may not cause a noticeable loss

Electricity Testing and Measurement Handbook – Vol. 7 of signal level, results in an asymmetrical aperture illumination and error in the measurement of the sidelobe structure.

INTERFERENCE FROM REFLECTIONS The requirement of providing adequate separation between antennas to prevent excessive phase error makes it difficult to satisfy a further requirement that the site be free of large reflections from the ground or other sources of reflection. Addition of reflected fields at the test antenna can produce erroneous gain and pattern measurements. For instance, an interfering field which is 30 dB below the direct path signal can cause a variation of ±0.25 dB in the measured maximum gain and can seriously affect the measured sidelobe structure of the pattern. The usual method of minimizing the effects of fields caused by reflections are to (1) mount the transmitting antenna and test antenna sites on towers, (2) employ a directive transmitting antenna, (3) avoid smooth surfaces which are oriented so that they produce direct reflection into the test antenna, and (4) erect screens or baffles to intercept the reflected wave near the reflection point. An alternate procedure is to locate the transmitting and receiving antennas over a flat range and to take into account the specular reflection from the ground in making measurements. The heights of the antenna under test and the transmitting antenna are adjusted for a maximum of the interference pattern between the direct and ground reflected wave. Generally, it is more convenient to mount the test positioner and antenna on a fixed height tower or building and vary the height of the transmitting antenna. This can be accomplished with the transmitting antenna mounted to a motor driven elevator/carriage assembly that can travel up and down a tower. In cases where the antenna range length is reasonably short, the entire range can be housed indoors in an anechoic chamber. An indoor far-field anechoic chamber has the same basic design criteria as an outdoor range except that the surfaces of the room are covered with RF absorbing material. This absorber is designed to reduce reflected signal over its design frequency range. Testing indoors offers many advantages to conventional outdoor ranges including improved security, avoiding unwanted surveillance and improved productivity due to less time lost because of weather and other environmentally related factors. The advantages of testing indoors are primarily responsible for the trend toward more advanced test ranges such as the compact range and near-field ranges.

AMPLITUDE VARIATION – ELEVATED RANGES Variations in the amplitude of the field incident over a test aperture must also be restricted for accurate far-zone measurements. For range geometries employing comparatively large transmitting and test tower heights (i.e., elevated range geometries), it is advisable to restrict amplitude taper to the order of 1/4 dB or less by using the following criterion:

From the viewpoint of suppressing range surface reflections, it is also desirable to maintain the test height H, greater than or equal to 6D. If one must, for practical reasons, employ test heights less than approximately 4D, the ground-reflection technique should be considered.

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GROUND-REFLECTION ANTENNA TEST RANGES Ground-reflection antenna range geometries are often advantageous when the test situation involves low directivities and high accuracy requirements or when practical test heights are less than approximately four times the maximum vertical dimension of the test aperture. In this technique specular reflection from the range surface is caused to create constructive interference with the direct-path energy in the region of the test aperture, such that the peak of the first interference pattern lobe is centered on the test aperture. Four basic criteria are applicable to ground-reflection range geometries:

COMPACT ANTENNA TEST RANGES Compact ranges are an excellent alternative to traditional far-field ranges. Any testing that can be accomplished on a farfield range can be accomplished on a compact antenna test range. This method of testing allows an operator to employ an indoor anechoic test chamber at a reasonable cost and avoid the problems associated with weather and security often encountered when using an outdoor test range. In a research and development situation, the small size of a compact range allows it to be located convenient to the design engineers. In a manufacturing environment, the compact range can be located near to the final testing and integration facilities. By placing a compact range in a shielded chamber, one can eliminate interference from external sources. This last feature has become more important in the last several years as the proliferation of cell phone and wireless systems has created a background noise environment which has made antenna testing in a quiet electromagnetic environment more difficult The principle of operation of a compact range is based on the basic concepts of geometrical optics. Diverging spherical waves from a point source located at the focal point of a paraboloidal surface are collimated into a plane wave. This plane wave is incident on the test antenna. The resultant plane wave has a very flat phase front, but the reflector-feed combination introduces a small (but generally acceptable) amplitude taper across the test zone. In principle, the operation of a compact range is straightforward; however, its ultimate design, construction, and installation should be carefully considered.

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NEAR-FIELD ANTENNA TEST RANGES

integrated antenna assembly with its associated transmitting and control circuitry. Due to these issues, pulsed RF operation presents an additional set of test problems not often encountered in CW operation. As a result, instrumentation complexity increases and measurement system timing issues become critical. The basic pulsed antenna test parameters are identical to those encountered in CW measurements. Gain, sidelobe levels, pointing accuracy, beamwidth, null locations and depths, and polarization parameters are essential to fully characterizing an antenna. In addition to the traditional time invariant antenna performance parameters, some new time dependent parameters emerge when testing under pulsed conditions. These include transient effects such as beam formation and distortions as a function of time within a pulse or over an ensemble of pulses, power output (i.e. gain) as a function of time within a pulse or pulse burst, etc. Compounding these measurements is the additional burden of multi-channel, multi-frequency, and multi-state measurements as a function of pulse repetition frequency (PRF), duty factor (DF) and operating frequency. Due to the increasingly integrated nature of antennas with their transmitters, the measurement system must be responsive to external RF pulse generation and timing for both single and multiple pulse measurements.

Near-field ranges are used where large antennas are to be tested indoors in a relatively small space. This type of range uses a small RF probe antenna that is scanned over a surface surrounding the test antenna. Typically, separation between the probe and the antenna structure is on the order of 4 to 10 wavelengths. During the measurement, near-field phase and amplitude information is collected over a discrete matrix of points. This data is then transformed to the far-field using Fourier techniques. The resulting far-field data can then be displayed in the same formats as conventional far-field antenna measurements. In addition to obtaining far-field data, Fourier analysis techniques are used to back-transform the measured electromagnetic field to the antenna’s aperture to produce aperture field distribution information. This offers the ability to perform element diagnostics on multi-element phased arrays. In near-field testing, the test antenna is usually aligned to the scanner’s coordinate system and then either the probe or the test antenna is moved. In practice, it is easier and more cost effective to scan the RF probe over linear axes or the test antenna over angular axes. But this does not have to be the case. There are many scanning coordinate systems possible for collecting the near-field data. Three techniques are in common usage: Planar Near-Field Method – With planar near-field scanning, the probe usually is scanned in X and Y linear coordinates over the aperture of the test antenna. A large planar scanner is used to move the probe over a very accurate plane located in front of the test antenna’s aperture. Once aligned to the scan plane, the test antenna is not moved during the collection of the near-field data. Planar near-field provides limited angular coverage of the test antenna’s field due to the truncation caused by the scanner’s dimensions. Cylindrical Near-Field Method – For this method the probe typically is scanned in one linear dimension using a single axis linear positioner. The test antenna is stepped in angle on a rotary axis oriented parallel to the linear axis. The resulting scan describes a cylindrical surface around the test antenna. Cylindrical near-field scanning can provide complete angular coverage of the test antenna’s field in one plane. The orthogonal plane has limited angular coverage due to truncation caused by the finite length of the linear scanner. Spherical Near-Field Method – Spherical near-field scanning normally involves installing the test antenna on a spherical scanning positioner. The probe antenna is normally fixed in space. The test antenna is normally scanned in one angular axis and stepped in an orthogonal angular axis. The resulting data is collected over a spherical envelope surrounding the test antenna. Full or nearly-full coverage of the test antenna’s radiating field can be evaluated with this type of near-field system.

PULSED ANTENNA MEASUREMENTS Characterizing antennas under pulsed RF condition is becoming increasingly commonplace. Advanced radar and wireless systems and their enabling technologies such as monolithic microwave integrated circuits (MMICs) require testing methods to verify performance over a wide range of operating parameters. In addition to the pulse parameters, the major factors influencing pulsed RF testing include high transmit power levels, thermal management of the antenna under test (AUT) and its supporting equipment in the test environment, and interfacing to a highly

ANTENNA RANGE INSTRUMENTATION Regardless of the type of antenna range to be implemented, the complement of instruments to operate the range is very similar. Differences occur due to the location of the various instruments with respect to the source and test antennas, types of measurements to be performed and the degree of automation desired. A description of the basic instrumentation subsystems and typical applications of different types of antenna ranges, will be presented here. The instrumentation for measuring antenna patterns consists of four subsystems, which can be controlled from a central location. These subsystems are: 1. Positioning and Control 2. Receiving 3. Signal Source 4. Recording and Processing The test antenna is installed on a positioner and is usually, tested in the receive mode. The motion of the positioner (rotation of the test antenna) is controlled by a positioner control unit located in the control room. The positioner is equipped with synchro transmitters or high accuracy encoders to provide angle data for the position indicator and the recording/processing subsystem. To process the received signal for recording, the RF signal must be detected. In most cases, microwave receivers are employed on the antenna range to accept the very low-level signals from the test antenna and to downconvert these signals to lower frequencies for processing. Microwave receivers offer many advantages including improved dynamic range, better accuracy, and rejection of unwanted signals that may be present in the area. Also phase/amplitude receivers provide the ability to measure phase characteristics of the received signal. Phase information is required for many types of antenna measurements. A signal source provides the RF signal for the remote source antenna. The signal source can be permanently fixed on the ground or floor, or located on a tower near the source antenna, depending on the frequency of operation and mechanical considerations. The signal source is designed for remote operation. The source control unit is usually located in the control room with the measurement and control instrumentation.


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Often, a computer subsystem is added to the instrumentation to automate the entire measurement sequence. This computer subsystem employs a standard bus interface, like the IEEE-488, to setup and monitor the individual instruments. High-speed data busses are utilized for the measurement data to maximize data throughput and productivity. An automated antenna measurement system offers a high degree of repeatability, speed, accuracy, and efficiency with minimum operator interaction. Data storage is conveniently handled by a variety of media including a local hard drive, floppy disk, removable drives or bulk data storage on a local area network. After data acquisition is completed, an automated system supports analysis of the measured data such as gain and polarization plus a wide variety of data plotting formats such as rectangular, polar, three-dimensional, and contour plots.

The test positioner axes are controlled and read out by the positioner control and readout units. A typical control system consists of a control unit located in the operator’s console. It is interfaced to a power amplifier unit located near the test positioner. This configuration keeps the high power drive signals near the positioner and away from sensitive measurement instruments while providing remote control of positioner functions from the equipment console. The position readout unit is located in the equipment console to provide real time readout of position axes to the operator or, in the case of an automated system, to the computer. The source antenna is normally located at the opposite end of the range on a tower or other supporting structure. The signal source is installed near the source antenna to minimize signal loss. An outdoor enclosure protects the source from the elements. For some applications a multiplexer can be used between the signal source and a dual polarized source antenna. This configuration allows simultaneous co- and cross-polarization measurements to be performed. Motorized axes to position the source antenna’s polarization, height and boresight are controlled by a positioner control and indicator system. The signal source and positioner axes are remotely controlled from the operator’s console via serial digital link(s). Twisted pair cable, fiber optics or telephone lines can be used to interface the digital link from the source site to the control console. One or two positioner control systems may be used on an outdoor range depending upon the length of the range and the total number of axes to be controlled. On very long ranges, or in cases where the control room is not close to either positioner, it may be advantageous to use a separate control unit for each end of the range. Also, since outdoor ranges frequently have many axes due to the source tower axes, multiple controllers may be required to control all axes. A block diagram of a typical outdoor range is shown below.

TYPICAL APPLICATIONS OF ANTENNA RANGE INSTRUMENTATION OUTDOOR FAR-FIELD RANGE In an outdoor far-field range configuration, the test antenna is installed on the test positioner located on a tower, roof or platform outside the instrumentation control room. The receiver front end (Local Oscillator) is usually located at the base of the test positioner, with the mixer connected directly to the test antenna port. This configuration requires only a single RF path through the positioner, greatly simplifying system design. Use of the remote front end also minimizes local oscillator power loss to the mixer and maximum system sensitivity. An outdoor enclosure protects the local oscillator from the weather and temperature extremes. For multi-ported antennas, simultaneous measurements can be made on all ports through the use of multiplexers installed in front of the mixer. The receiver front end is remotely controlled from the control console through interfaces with the receiver.

Outdoor Range with Manual Control1

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INDOOR FAR-FIELD RANGE Anechoic Chambers are instrumented essentially the same way as outdoor ranges with range lengths the primary difference. The receiver front end is typically positioned near the test positioner with the mixer connected directly to the test antenna port. The source is located near the source antenna. The control room is generally centrally located and connected to both ends of the range via cables or digital links. Since these systems are located indoors, special enclosures for the receiver front end, positioner control, and signal source subsystems are not required. Usually, the source antenna requires only polarization control. This, as well as the short range length, usually allows a single positioner control unit to be used to control all the range axes. Anechoic chambers can be configured for either manual or automatic control.

Indoor Range with Automatic Control1

COMPACT RANGE In a point-source compact range, the feed is usually located just in front of and below the test antenna. In this configuration, the receiver local oscillator and signal source can be located very close together. Special care must be taken to guard against direct leakage of the signal source into the test antenna. High quality RF cables and special shielding are sometimes used to insure against this stray leakage. Otherwise, instrumentation for the compact range is very similar to an anechoic chamber.

Compact Range with Automatic Control Configured for Multiple Port Measurements1


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NEAR-FIELD RANGE

CONCLUSIONS

Near-field ranges usually are configured for automatic control. The large numbers of measurements required, and the need to transform the near-field data to the far-field, requires the use of a computer system both for data acquisition and for data reduction and display. The configuration of a near-field range is similar to a very short indoor range. The antenna may be tested in the transmit mode, receive mode, or both. Consequently, the design of the RF system and the location of the source and receiver front end must be considered for each application. The figure below is one example of a planar near-field application where the test antenna is to be tested in both transmit and receive modes.

As technology progresses, the requirements placed upon wireless communication systems and their associated antennas will continue to become more stringent. For example, the desire to increase network capacity will result in the requirement to reduce adjacent channel interference within the system, which will result in more stringent antenna sidelobe and cross-polarization requirements. The verification of the performance of antennas selected to meet these and other requirements will, in turn, require test ranges with higher accuracy measurement capability. Fortunately, the technologies used to advance the art of antenna design is also being used to advance the design of antenna test and measurement ranges and instrumentation. Many of the simulation tools available to antenna designers are also used to design antenna ranges. The increased use of commercial off-theshelf hardware and software, in conjunction with the increased use of automated test instrumentation networked into the local area network, will ensure that current state-of-the-art antenna measurement systems meet the needs of the advanced antennas and systems coming to the wireless marketplace.

REFERENCES: [1] Product Catalog, Microwave Measurements Systems and Product, Microwave Instrumentation Technologies, LLC. [2] R. Hartman and Jack Berlekamp, “Fundamentals of Antenna Test and Evaluation,” Microwave Systems New and Communications Tracking, June 1988. [3] J.S. Hollis, T.J. Lyon, and L. Clayton, eds., Microwave Antenna Measurements, Scientific-Atlanta, Inc., 1985. [4] R.C. Johnson and Doren Hess, “Conceptual Analysis of Measurements on Compact Ranges,” Antenna Applications Symposium, September 1979. [5] R.C. Johnson editor, Antenna Engineering Handbook, McGraw-Hill Inc., 3rd edition, 1993.

A Typical Planar Near-Field Application1

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DERIVING MODEL PARAMETERS FROM FIELD TEST MEASUREMENTS J.W. Feltes, S. Orero, Power Technologies, Inc., B. Fardanesh, E. Uzunovic, S. Zelingher, the New York Power Authority, N. Abi-Samra, EPRIsolutions, Inc. The purpose of DeriveAssist is to speed up the parameter derivation process and to allow engineers less versed in parameter matching and identification to get involved in the process. A major component of any power system simulation model is the generating plant which comprises three major subcomponents of interest: the generator, excitation system, and the turbine/governor. Accuracy of representation is dependent both on the structure of the component models and the parameter values used within those models. Since the accuracy of power system stability analysis depends on the accuracy of the models used to represent the generators, excitation control systems, and speed governing systems, the parameters used in those models could affect the calculated margin of system stability. Use of more accurate models could result in increases in overall power transfer capability and associated economic benefits. Alternately, inaccurate simulation models could result in the system being allowed to operate beyond safe margins. To assist in these efforts, Power Technologies, Inc. (PTI), the New York Power Authority (NYPA), and EPRI solutions, Inc. have developed an automated tool to assist engineering staff in the derivation of model parameters from the recorded results of staged tests.

NEED FOR BETTER PARAMETERS Modern power systems are highly dependent on the proper use of dynamic control. Special-purpose computer programs have been developed to simulate the dynamics of large complex interconnected power systems. At the same time, power systems have become more highly stressed through heavier system loading caused by transfer of low-cost energy and increased use of system controls to increase transfer limits with existing transmission. Inaccuracies in equipment modeling can be both due to inadequate model structures and, more often, due to lack of data on equipment model parameters. Model parameters currently used for stability analysis are usually provided by equipment manufacturers and calculated from design data and, in some cases, factory tests. Generally, they are not verified by field tests. Some pieces of equipment are tuned by field personnel with results of that initial tuning rarely incorporated into simulation models. Parameters may have changed from initial values due to retuning, aging, and equipment changes, such as generator rewinding. The extensive use of computer simulation requires a high degree of confidence in the computer models. The only way to assure that study assumptions are accurate is to field test equipment and validate simulation models by comparing model responses with those obtained from field tests. Thus, there is a major industry need to enhance equipment model development as well as model parameter identification and validation.

This need has been recognized by organizations responsible for system reliability. For example, the Western System Coordinating Council (WSCC) instituted a program requiring testing and model validation for all generating units greater that 10 MW. The North American Electric Reliability Council (NERC) is presently formulating its requirements in this area.

STAGED TESTS Staged field tests provide sufficient information to identify the values of the key parameters of the computer simulation models. Such tests are selected to minimize the effect on plant operation, allow ease of simulation of the staged tests, and, to the extent possible, reduce the complexity of the parameter derivation problem by having the response of an individual test significantly affected by only a few parameters. The test methodology described is just one testing methodology; other methods are used successfully also. However, the tools and procedures developed and used in this parameter derivation software could be adapted to these other variants in the testing process. The testing process is divided into two phases. One phase involves collecting steady-state measurements, which are used to establish base values of quantities and to identify values for parameters that are associated with steady-state operation. The second phase involves collecting the dynamic response of the generator, excitation system, and governor/turbine system to staged disturbances.

STEADY-STATE MEASUREMENTS The steady-state measurements are divided into two groups: the open circuit saturation curve measurements and online measurements. The open circuit saturation curve is measured with the unit operating offline at rated speed. The generator field excitation is varied, and measurements of terminal voltage, field voltage, and field current are taken. The online measurements (also sometimes called V-curve measurements) are performed with the unit connected to the electrical network and placed at a given load. At that load level, the generator field excitation is varied to change the reactive power output. Typical measurement points are given in graphical form in Figure 1.

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DYNAMIC TESTS

PROGRAM OVERVIEW

The gains and time constants of the models can be determined only from tests that excite the dynamic response of the equipment. The models of concern are those of the generator, excitation system, and governor. The purpose of the dynamic tests is to provide a simple and safe disturbance to excite the unit in order to record its dynamic response. The usual approach is a series of load rejection tests with the unit initially carrying partial load. Each of the tests has identification of certain parameters as its primary goal. The loading of the machine is selected to isolate those parameters as much as possible in order to reduce the complexity of the derivation process. The initial conditions for a typical set of load rejections are listed in Table 1.

DeriveAssist works through a graphical user interface (GUI) and is written to operate on a personal computer. The program uses the MATLAB/Simulink and the Optimization Toolbox as the calculation engine for the parameter derivation process. MATLAB is a high-performance language for technical computing. Simulink is a graphical tool for modeling, simulation, and analysis of dynamic systems. The Optimization Toolbox is a collection of routines that extend the capability of MATLAB for such problems as nonlinear minimization, equation solving, and curve fitting. By combining these tools with the experience gained through years of testing and parameter derivation, the process of parameter identification and derivation has been significantly advanced. The program is organized to facilitate the derivation of the parameters in a logical order, starting with the steady-state tests and then proceeding to the dynamic tests. The main entry point into the program is a window with several pull down menus. Each menu item has several submenu choices. Each of these submenu choices performs a particular task. The submenus were also designed to reflect a particular test. The five menus are: • File I/O • Steady-state tests • Dynamic tests • Control tuning • Help. The parameter derivation program is organized to facilitate the derivation of the parameters in a logical order, starting with the data from the steady-state tests and then proceeding to the dynamic tests for the generator, excitation system, and governor. Where a certain sequence of actions must be observed (i.e., read saturation curve first before trying to calculate the saturation parameters), the program performs a check and warns if the prerequisite tasks have not been performed.

TRADITIONAL GENERATOR PARAMETER DERIVATION PROCESS AND THE DERIVATION SOFTWARE The traditional method of determining the model parameters based on the recorded test results forms the basis for the approach used by the parameter derivation automation software. The traditional methodology has been highly dependant on skilled engineers applying their knowledge to select initial parameters, perform calculations using those parameters, and, based on the difference between measured and calculated values, adjust the parameters manually to improve the fit between model and realworld response. This iterative approach is quite time consuming and requires a skilled, experienced engineer to make the adjustments so as to accomplish a good match in a reasonable amount of time. The purpose of the software described here is to speed up that process and also allow engineers less versed in parameter matching and identification to get involved in the process.

DERIVATION OF STEADY-STATE PARAMETERS The steady-state parameters are derived first. They can be derived based on a series of measurements of steady-state quantities as described previously. The first task is to analyze the open circuit data (obtained from the offline open circuit test) to establish the base values for field current and field voltage. This has been traditionally accomplished by plotting terminal voltage versus field current and drawing the air gap line. The value of field current corresponding to rated terminal voltage on the air gap line is identified as the base value. Next, the saturation values S(1.0) and S(1.2), the parameters used to describe the shape of the saturation curve, are identified using the open circuit data. The program automates this task, calculating the base value and saturation parameters using a least squares fit. Figure 2 shows an example of the output of the program, showing the close match achievable between test and calculated values. Tabular output demonstrating the fit of the measured data and calculated results are also given. The online steady-state measurements are used to identify the values for Xd and Xq. Recordings of voltage, power, reactive power, field current, field voltage, and power angle are made at different power levels and reactive power outputs. The user can select which points to use in the calculation. The program calculates the reactances Xd and Xq that best fit the measured data, again using a least squares optimization. Each reactance can be calculated separately or both at the same time. If the user selects


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Xd only, then the error function to minimize uses only field current, Ifd. However, if the user selects Xq, the error function will be based on power angle. The last option is only possible when rotor angle measurements have been made during the field tests. Figure 3 shows the program output screen following the derivation of the generator steadystate reactances.

and measured quantities are quite different. For the generator, the information from the load rejection tests is used to calculate the time constants, the transient reactances and the subtransient reactances of the generator. For the excitation system, the AVR and exciter gains and time constants can be determined. The governor models vary significantly depending on the type of prime mover, that is, steam, gas, or hydroelectric turbine. However, the process determines the gains and time constants representing the governor and turbine dynamics. In all cases, a comparison was made between a measured signal and a simulated signal to define an error function. The program attempts to improve the model performance by adjusting the model parameters in the appropriate direction and repeating this simulation until the error signal is minimized. The process will be illustrated using the derivation of excitation system parameters, but the reader should keep in mind that the general process would be similar for the other equipment models.

EXCITATION SYSTEM PARAMETER DERIVATION

DERIVATION OF DYNAMIC MODEL PARAMETERS The traditional parameter derivation process for the parameters of the dynamic models required numerous simulations to see exactly what happens as one changes each of the model parameters. With each parameter change, a comparison is made between response of the model and that obtained from the actual tests. The determination of the parameters to adjust and the amount by which to adjust them to get a close match between the model response and the actual performance requires the skills of an experienced engineer. If there are nonlinear dynamic interactions among variables, it is usually very difficult to know how to set each of the parameter values to give the desired performance. The process of choosing the appropriate model parameters that provide the desired response can be automated by the use of optimization tools. MATLAB, Simulink, and the Optimization Toolbox provide a suitable advanced programming environment that can allow an optimization engine to interact with a dynamic simulation package. Thus the optimization phase of the model parameter derivation involves the automatic adjustment of the model parameters until the difference between the model response and the desired response (obtained from field tests) is minimized. The optimization process tries to find the combination of model parameters that most closely matches the measured response. A large task involved in the development of the DeriveAssist program was the building of generator, exciter, and governor models in Simulink, testing them against a widely used commercial power system simulation program (PTI’s PSS/E program) as a benchmark, and making the interface between all three components of the software: Simulink, MATLAB, and the optimization toolbox, as integrated as possible. Tests are performed to record the equipment response with different initial conditions and with disturbances designed to target the derivation of specific generator, excitation, and governor model parameters. The derivation process is similar for the generator, excitation system, and governor parameters, although the staged tests

The first step is, of course, to choose an appropriate model structure for the excitation system. The selection is usually guided by the manufacturer’s recommendation or from industry standards. In some cases, the schematics of the excitation system may need to be examined to make the proper model selection or, if a standard model structure is not appropriate, to create a new model. As noted above, the traditional strategy to identify the values for the excitation parameters involves an iterative hill-climbing technique by the engineer, who changes the value of one parameter at a time until a match between simulation results and recorded measurements are made. This process requires good familiarity with the specifics of how the equipment functions and of the effect that a change in a parameter or a set of parameters has on their dynamic response; unfortunately, such familiarity is quite rare. The parameter derivation program greatly simplifies the process. The user must read in the test data, which is easily performed using the program GUI. The GUI also facilities the selection of signals for the derivation process and manipulation of the test data as necessary, for example, conversion to per unit using the base values derived in the steady-state derivations. Figure 4 shows the connections of round rotor generator model (GENROE in PSS/E) with an IEEE type 1 (IEEET1) excitation system. Inputs to the generator model are field voltage, generator currents Id and Iq, and mechanical power Pm. Outputs are terminal voltage magnitude and angle and generator speed.

82 Inputs to the excitation model are terminal voltage (from the generator) and reference voltage, while the output is field voltage Efd, which is fed back as an input to the generator model. The inputs and outputs allow data to be passed between MATLAB and Simulink and between the models. The parameters are defined such that they can be changed and passed to Simulink in the optimization process. As an example, Figure 5 shows what lies under the excitation system block in Figure 4. The primary input is the voltage Ecomp and the output is the field voltage Efd. The reference Vref is calculated from the initial condition of the test. Auxiliary signals such as those from the power system stabilizer and under- or over-excitation limiters are present in the model structure but are not exercised purposely by the selection of the tests, concentrating on the excitation parameters. The optimization phase of the model parameter derivation involves the automatic adjustment of the model parameters until the difference between the Simulink model response and the desired response (measured response) is minimized. The optimization process tries to find the combination of model system parameters that best provide the desired response, that is, to find the values of the excitation system model parameters that will move the initial model response as close as possible to the measured response.


every few seconds as the model parameters are adjusted, a new simulation is performed, and the new output is displayed. The simulation output gradually shifts from the original response to very closely match the desired response. Figure 6 shows the shows a comparison of a simulation using an initial set of parameters and the measured response. Note that the original parameters are not a good approximation of the actual equipment. The simulation, in this case, is quicker and much better damped. The response gets progressively closer to the measured output following each pass of the derivation process until; at last, the two curves are essentially one on top of the other. The final plot is shown in Figure 7. The whole optimization process to determine the parameters takes only a minute or two on a typical PC. Generator and governor model parameters are derived in a manner quite similar to that described for the excitation systems. The tests performed on the unit are considerably different, of course, as the determination of the governor response characteristic requires a test resulting in a power imbalance and subsequent movement of generator speed while the tests for the generator parameters require isolation of the generator dynamics by placing the AVR on manual.

ONGOING EFFORTS

A comparison of the simulation output and the measured (desired) output is displayed for each successive pass of the optimization process. The user sees the simulation output change

The DeriveAssist parameter derivation software described so far allows the derivation of all the generator steady state parameters and includes most of the Simulink models for generators, exciters and governor systems. However, additional work is required to further develop the software. Some of the tasks for future work include: • Derivation of excitation system parameters from AVR reference step tests • Extension of the methodology to brushless excitation systems. • Develop algorithms designed to assist the user in the tuning of equipment such as exciters and stabilizers.

Electricity Testing and Measurement Handbook – Vol. 7 There are also a few technical issues that require further attention, including: • Additional investigations to overcome some problems in the automatic initialization of Simulink models • Improvements to the model library and the ease of selecting the model structures for the excitation system and governor • Improvements to the GUI and data passing between Simulink, MATLAB, and the Optimization Toolbox • Additional reporting routines. The ongoing further development work will improve its functionality and expand its capabilities.

ACKNOWLEDGMENTS This article describes research sponsored by EPRI and NYPA. The authors would also like to acknowledge F.P. de Mello for his contributions to the original ideas behind the parameter derivation process used in this project and Ricardo J. Galarza for his contributions in the development of this parameter derivation software. The idea of developing this MATLABbased tool was originally conceived by Bruce Fardanesh.

FOR FURTHER READING “Synchronous machine parameter derivation program,” EPRI, Palo Alto, CA, Rep. 10006653, 2001. F.P. de Mello and J.R. Ribeiro, “Derivation of synchronous machine parameters from tests,” IEEE Trans. Power App. Syst., vol. PAS-96, no. 4, pp. 1211-1218, Jul./Aug. 1977. F.P. de Mello and L.N. Hannett, “Determination of synchronous machine electrical characteristics by tests,” IEEE Trans. Power App. Syst., vol., PAS-102, no. 12, pp. 3810-3815, Dec. 1983. J.W. Feltes and L.N. Hannett, “Derivation of generator, excitation system and turbine governor parameters from tests,” presented at Int. Conf Large High Voltage Electric Systems, Colloquium on Power System Dynamic Performance, Florianopolis, Brazil, Sept. 1993. L.N. Hannett, J.W. Feltes, and B. Fardanesh, “Field tests to validate hydro turbine-governor model structure and parameters,” IEEE Trans. Power Syst., vol. 9, no. 4, pp. 1744-1751, Nov. 1994. L.N. Hannett, B. Fardanesh, G. Jee, “A governor/turbine model for a twin-shaft combustion turbine,” IEEE Trans. Power Syst., vol. 10, no. 1, pp. 133-140, Feb. 1995.

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TESTING ELECTRIC STREETLIGHT COMPONENTS WITH LABVIEW-CONTROLLED VIRTUAL INSTRUMENTATION Ahmad Sultan, Computer Solutions, Inc. The Challenge: Automated testing of magnetic ballasts used in electric streetlights. The Solution: Developing a PC-based virtual instrumentation system using a DAQ board controlled by LabVIEW.

INTRODUCTION Our task was to develop an automated test system for magnetic ballasts used in high-pressure sodium (HPS) streetlights. Our client, who manufactures ballasts for the North American and international markets, believes that product development and quality assurance require thorough and complete testing of prototypes and production samples to verify compliance with national and/or international standards. The test system needed to accommodate the following: • Different types of core and coil ballasts, such as reactor, autotransformer, constant wattage autotransformer (CWA), and constant wattage isolated transformer (CWI) • Operating voltages from 120 to 600 V and rated lamp wattage from 50 to 400 W • Capacitors for wattage control and/or power factor correction • Different lamp igniters • Open-circuit, short-circuit, lamp-starting, and lamp running tests At the ballast input and output ports, we needed to measure true rms values of current and voltage, true power, and the ratio of watts to volt-amperes (power factor, if the voltage and current waveforms are clean sinusoids). Because HPS lamps are nonlinear loads, we monitor current and voltage peak values and crest factors, along with total harmonic distortion.

Hardware Block Diagram of the Ballast

SYSTEM INTEGRATION APPROACH With the tight budget of a growing company, establishing a test bench with the functionality we required using conventional test equipment becomes difficult. We implemented a virtual instrumentation approach to achieve project objectives within budget while maintaining flexibility for future needs. Virtual instrumentation consists of using mainstream computers, off-the-shelf plug-in instrumentation boards, and software. Because the virtual instruments you create with these products are user-defined, not vendor-defined, you can tailor applications to meet your needs exactly. Some of the benefits of virtual instrumentation are ease of use, flexibility, and savings of time and money. We used LabVIEW software as the heart of the instrumentation system. BallastVIEW is the name of the LabVIEW application we wrote to acquire signals, process data, and present results to the user on the computer screen. The instrumentation system hardware consists of: • 486 DX2-66 PC (12 MB RAM, 340 MB hard drive) running Windows • Variac (manually adjustable transformer) supplying AC power to the ballast under test through the system test fixture • System test fixture containing switches and wiring required for the different test configurations • Transducers for sensing current and voltage signals (such as resistive dividers and current shunts) • Antialiasing RC filters, with components selected to avoid loading the board input amplifiers

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• 5B signal conditioning modules, to amplify and isolate the filtered signals • National Instruments Lab-PC+, installed in the PC, to digitize the conditioned signals The cut-off frequency of the antialiasing filters was set to half the sampling frequency; the RC filters also serve to protect the electronics items from the high-voltage spikes generated when the igniter starts the lamp. We configured the Lab-PC+ board for bipolar differential input (four channels). We set the sampling frequency to 7680 Hz/channel. Acquisition was software-triggered on the rising slope of the input voltage.

EXAMPLE RESULTS

BALLASTVIEW PRESENTATION The LabVIEW screen on the next page is the front panel of BallastVIEW. It illustrates a stack of VIs representing an input AC power analyzer, an output AC power analyzer, a waveform graph, and a harmonic analyzer. The controls at the top of the screen are switches for controlling acquisition, metering, harmonic analysis, and program execution. The user can capture a single shot or continuously acquire signals. For the power analyzers, the indicators (from left to right in each row) display the rms, maximum, minimum, peak average, and crest factor of each signal. The active and apparent power, and their ratios, are displayed in the right column. The waveform graph displays the signals acquired by the data acquisition (DAQ) board. Because both voltage current waveforms are displayed, the ordinate is labeled in relative units (PU). To find the true amplitude of a particular signal, multiply its measured value from the graph, in PU, by the respective base value from the PU Base table (to the right of the waveform). The line spectrum, shown in the bottom right corner, displays harmonic magnitude in either peak volts/amperes or per unit values normalized to the fundamental component of the respective signal. Magnitude of harmonics can be checked by flipping the cursors of the harmonic magnitude indicator (bottom center). The user can window signals before applying the Fast Fourier Transform.

The results presented in the BallastVIEW screen are test results for a 200 W CWI ballast. The output power analyzer indicates that the lamp is operating at rated lamp power. Lamp voltage and current are very close to the ANSI reference specifications (100 V and 2.4 A). Lamp current crest factor (CCF) is 1.6 (1.8 is the maximum permissible). The input power analyzer indicates that the ballast draws 2.037 A at rated input voltage. Ballast loss is approximately 39 W and the power factor is high (0.973 lagging). The waveform graph shows almost clean input voltage and current signals. Output (lamp) voltage is the square waveform of a typical arc in a high-intensity-discharge (HID) lamp, containing the full odd harmonics spectrum. The magnitude of the lamp voltage third-harmonic component is 39 percent of the fundamental. Total harmonic distortion (THD) of lamp voltage and lamp current are 33.84 percent and 3.73 percent, respectively. We verified the credibility of this system by obtaining agreement with test results from an independent test laboratory, electric utility companies, and customers of the ballast company. The result is a flexible, high-performance, easy-to-use, and cost-effective PC-based measurement system, which saved time in both product development and production testing.

CONCLUSION BallastVIEW measures and displays the electrical parameters required to test and develop ballasts and performs on-line waveform analysis. The result is a flexible, high-performance, easy-to-use, and cost-effective PC-based measurement system, which saved time in both product development and production testing. An advantage of using LabVIEW is our ability to increase BallastVIEW functionality in the future, for example, by monitoring the ballast-lamp characteristic curves and compiling results. The core of the BallastVIEW program constitutes the cornerstone for testing other electrical products, such as transformers, rectifiers, inverters, and UPSs, as well as for power line monitoring.

LabVIEW Front Panel showing BallastVIEW Test Results


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ASSET MANAGEMENT The Path to Maintenance Excellence Mike Sondalini, Managing Editor, Feed Forward UP-TIME Publications This article tell the Japanese way of doing asset management and maintenance. If you think you already have a good system then you will enjoy reading this month’s newsletter as you compare yours and theirs. If you have a poor system then you will get a totally different view of how great maintenance can be done.

I spent a week in Japan at the chemical plant of an internationally renowned chemical manufacturer. While there I asked them about how they do their maintenance. They told me about their maintenance philosophy. And I want to pass on to you what I learnt about the Japanese way of doing maintenance on that trip. You will read about how this Japanese company determines its equipment and component criticality. You will learn about a new, truly effective way, of making next year’s maintenance plan. We will cover condition monitoring the Japanese way. The Japanese are great maintenance investigators and you will be impressed when you learn how they do their failure analyses. We will also cover their psychology of maintenance – the way they think about maintenance and how they look at it. You will be astounded at their mind-set.

tion as the previously mentioned methods but they arrive at the rating and the response to it in a unique, quick four-step process. They used a simple flow chart that production and maintenance worked through together, equipment by equipment. Those failures that caused safety and environmental risks were not allowed to happen and either the parts were carried as spares and changed out before failure or the plant item was put on a condition monitoring program. Those failures that caused production loss or affected quality also were either not allowed to happen or put into a condition-monitoring program. And those failures that didn’t matter were treated as a breakdown. The flowchart let one arrive at a rating and a corrective action for each piece of equipment and component fast. No need to spend hours and days looking at failure modes and deciding what to do about them. If an equipment or component loss produced dangerous situations, or if the failure stopped production or affected quality, it was either changed out before the end of its working life or it was put on a monitoring program. The maintenance philosophy for every bit of plant could be arrived at in a four-step decision process. It was very easy to use and to decide what action to take.

A JAPANESE WAY TO DECIDE EQUIPMENT CRITICALITY

HOW TO TURN A MAINTENANCE PLAN INTO A STRATEGY

How do you decide what level and type of maintenance to use on an individual item of plant and its sub-assemblies? Not all equipment is equally important to your business. Some are critical to production and without them the process stops. Others are important and will eventually affect production if they cannot be returned to service in time. While other items of plant are not important at all and can fail and not affect production for a very long time. As a maintainer you want to know which equipment in your plant falls into each of those categories so you can determine your response. Furthermore you want to know which subassemblies in each item of equipment are critical to the operation of the machine. From this information you can decide which spares to hold on-site and which to leave as outside purchases. The equipment criticality also determines what level of preventive maintenance to use, what type and amount of condition monitoring to use and what type and amount of observation is required from the operators. You can also use it to justify on-line monitoring systems to protect against catastrophic failure. The western approach to determine criticality is often to use either Reliability Centered Maintenance or Risk Based Maintenance to determine consequences of failure and then address the appropriate response to prevent the failure. The Japanese chemical manufacturing company I visited had a novel way of determining their equipment criticality. They based the equipment and component criticality on the knock-on effect of a failure and the severity of the consequences. It is the same inten-

The maintenance plan my Japanese hosts showed me in August 2002 was on a big spreadsheet. It listed all the equipment in a plant by tag number covering the period 1994 through to 2003. The maintenance histories of problems on a piece of equipment for the past eight years were listed. A short note detailing the month of occurrence and the failure was made in the column of the year it happened. For this year, 2002, and the next, 2003, the spreadsheet listed what maintenance and modifications were going to be done on the equipment. It was a ten-year plan the like that I had never seen before! But now, as I write, it has become clear why it’s worthwhile doing it like that. What I saw was not a plan! What I saw was a strategy! It was a strategy to reduce the known production stoppages and to focus the maintenance effort. Can you see how something like that would work? You know what has gone wrong with the equipment over the last eight years, it’s listed right there in front of you. You can see how effective the past practices, methods and solutions have been. From that you can wisely decide what to do over the next two years to prevent the reoccurring problems. Instead of writing the usual ‘blue sky’ 5 or 10 year maintenance plan that no one believes anyway, you only plan for the believable two years ahead. You write down exactly what can really be done in the foreseeable future to reduce or prevent the real problems. The plan for the next two years would include proposed modifications, equipment replacements, new condition monitoring plans, etc. Now that is a great way to make next year’s maintenance

OVERVIEW

88 plan! It would be one that is totally defendable and fully justifiable to upper management because it is well thought out, rooted in getting the best return for your money and based on the important business requirements to continue in operation. My suggestion to cover the period beyond the next two or three years (and only if it is necessary in your company), is to use the spreadsheet to make forecasts. Project ahead based on what you plan to do in the coming two to three years to fix the current problems. If you aren’t going to fix the problems then don’t assume less maintenance in the future. Remember that a forecast is not a plan! A forecast is a best-guess suggestion, often known as ‘blue sky dreaming’. A plan is a set of action steps that over time will produce a desired result. They are totally different to each other.



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THINK SYNCHRONIZATION FIRST TO OPTIMIZE AUTOMATED TEST www.ni.com OVERVIEW Latencies and timing uncertainties involved in orchestrating the operation of multiple measurement components present a significant challenge in building automated test systems. These issues, often overlooked during the initial system design, limit the speed and accuracy of the system. However, with a good understanding of timing and synchronization technologies, you can address these issues from the onset and deploy a system optimized for throughput and performance. Before we proceed, first consider that most automated measurements for test fall into one of two categories. The first category, often called time-domain measurements, characterizes the variation of a device under test (DUTs) output over time. For these measurements, the accuracy of the measured response depends not only on the accuracy of its magnitude, but also on the time at which the signals are measured.

The second type or steady-state measurements occurs when one or more inputs of known value are applied to the DUT and its outputs can settle to their steady-state value before you measure the signals. In this case, the measurement process depends on the time of the measurement – if you measure the signals too early, accuracy suffers because the source output may not have fully settled. Although you can measure the signals accurately any time after the output has settled, you must minimize the delay to reduce test time. Many test developers insert an arbitrary delay in their test programs to ensure accurate results. While this is a simple fix, test time suffers. Analog electronic component evaluation and manufacturing test often involves measurements of both transient and steady-state parameters.

WHAT IS INVOLVED IN SYNCHRONIZATION? The main objective of synchronization of multiple measurement devices is correlated measurements and/or precise control of process execution. In most cases, you are interested in correlation in terms of time, but correlation can occasionally be in dif-

ferent terms, such as position. For temporal correlation, you must synchronize measurements to correlate with the sample clock of your measurement device. In other words, it is pointless to examine measurements synchronized to within nanoseconds if your sampling clocks are 1 MHz. The objective is a system with synchronized devices that are synchronized to sub-microsecond accuracy. Precise timing of measurements is a prerequisite of the measurement device. Let us take a digitizer as an example to elaborate on key timing technologies. The heart of a digitizer is an ADC, which samples your signal and converts it to digital data. The sample clock, which controls the timing of the ADC, is most often derived from an onboard crystal oscillator. Thus, the synchronization of measurements across multiple devices, such as a source or other digitizers, implies that you must synchronize all sample clocks to within the uncertainty of the period of the sampling clocks. Another important element to the measurement is collecting data. This is usually accomplished with trigger signals. External events or triggers are the main methodologies for initiating an acquisition. Triggers come in three forms – analog, digital, and software. Analog triggering refers to trigger generation when a monitored analog signal passes the imposed triggering condition. You can measure the analog signal itself or an auxiliary analog signal. Digital triggering refers to trigger generation when a digital signal, such as a TTL level signal, is received. Software trigger refers to trigger generation on software command. The software trigger can be as simple as hitting a ‘start’ button on the soft front panel or graphical user interface (GUI). Thus, synchronization of measurements requires not only synchronized sample clocks, but also the distribution of a trigger to all measurement devices to initiate operation at the same time. In synchronization applications, it is common to designate a ‘master’ measurement device to monitor the operation of the entire measurement system. When the system meets triggering conditions on this device, it distributes a common trigger signal to all other devices that are ‘slaved’ to the master. To achieve tight synchronization across multiple devices, you need to examine the distribution of clocks and triggers. There are three main schemes for synchronization:

1. START/STOP TRIGGERS CONTROL OPERATION ON ALL DEVICES This scheme for synchronization is the simplest. It involves a single start or stop trigger signal to all measurement devices involved. One device, designated as the ‘master’ device, monitors the operation. The master is set to look for an external trigger (analog or digital), or to generate a synchronizing trigger on a software command. When triggering conditions are met on the master device, or the software command is issued, the master distributes a trigger signal to all ‘slave’ devices to start operation as shown in Figure A.

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Some examples are: • Rotationally oriented measurements – A master digitizer or oscilloscope, monitoring defects found on rotating circular or cylindrical devices such as computer hard drives, industrial cylindrical tubes, and automotive wheel shafts, passes a digital trigger to a slave counter/timer device making quadrature encoder measurements (position measurements). The system can correlate defects and anomalies to angular and radial position rather than time. • High channel count measurements – Multiple digitizers acquire data on reception of an external digital trigger from an external triggering module or a master digitizer in the system. With the examples above, two issues arise: The trigger signal should arrive at each slave device with minimal delay and skew between each other. The delay and skew are separate issues and need equal consideration. With a significant delay from the master to the slaves, you lose synchronization. Minimal path length for signal propagation from master to slaves is crucial for tight synchronization. The other important, but subtle, consideration is the skew between slave devices. So that each slave triggers at precisely the same time, you need to minimize the device-to-device skew in time. At the least, the delay and skew should be identified to some uncertainty. Measurements that require relatively low sampling rates can tolerate a degree of slack in the specifications of a system set up. At high sampling rates, these issues can affect the measurement integrity.

tinually rewrites until it receives a trigger. After the device receives the trigger, the digitizer continues to acquire post-trigger samples if you specified a post-trigger sample count. The ability to correlate waveforms acquired on the various devices depends on the accuracy of the time-stamp of the trigger.

2. TRIGGERS AND A DIRECT SAMPLE CLOCK INITIATE AND CONTROL THE TIMING ON ALL DEVICES This scheme takes synchronization a step higher. It involves trigger signals and a sample clock to all the devices involved. One device, designated as the ‘master’ device, controls the operation of the entire measurement system. This device exports its sample clock to all slave devices. For example, a system comprised of multiple digitizers and analog output sources has a common sample clock from an appointed ‘master.’ As illustrated in Figure B, the master sample clock directly controls ADC and digital to analog conversion (DAC) timing on all devices. The master is set to look for an external trigger (analog or digital), or to generate the trigger on a software command. When triggering conditions are met on the master, the device distributes a trigger signal to all of the ‘slave’ devices to commence operation. The same issues that arose in the previous scenario are also present in this situation. The trigger and sample clock signals should arrive at each slave device with minimal delay and skew between each other. At the least, the delay and skew should be known to an uncertainty. The significant advantage of this scheme compared with the previous scheme is that you use a common sample clock to control all devices. With a common sample clock, all waveforms are precisely sampled at the same time. This resolves the central issue of synchronized measurements. With this technique, you benefit in another important way. If you employ the clock on each measurement device, you have to take the jitter and drift inherent in each clock into consideration. On each digitizer, different clock jitter and drift may give rise to sampling periods, which means you cannot correlate them with relative accuracy. The disadvantage of this scheme is that it is not optimal for high-speed sampling because of the propagation delay of the sample clock. The sample clock simply takes time to get to the slaves from the master. This issue does not arise if the sampling rate is slower than the propagation delay. For example, in a given system the propagation delay is measured to be 10 ns. If the sampling rate is 5 MSamples/s, the period between each rising edge of the clock is 20 ns. The sample clock reaches the slave devices before the delay time encumbers the measurements. Additionally, the path lengths from the master to each slave device have to be carefully matched so the skew time is shorter than the sampling clock period.

3. TRIGGERS AND A REFERENCE CLOCK TO INITIATE AND CONTROL THE TIMING ON ALL DEVICES

The second issue concerns the intrinsic accuracy of the measurement device – you should identify or calibrate the time that the device received the trigger signal to the first pre-trigger or post-trigger sampled point in each device. You can program many measurement devices, such as digitizers, to continuously acquire samples into a circular onboard memory buffer that con-

This scheme of synchronization is usually for high-speed synchronization. It involves start/stop trigger signals and a reference clock (typically 10 MHz) to all devices involved. The sampling clock of each measurement device is derived from the reference clock by dividing the reference clock to obtain higher speed sampling clocks. The master is set to look for an external trigger (analog or digital), or to start acquisition on a software command. When triggering conditions are met on the master, this device distributes a trigger signal to all slave devices to start operation.


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With the previous scheme, you could have a direct feed of the sample clock to each device. This is the ideal scenario, however, it is not easy to pass a high-speed sampling clock (such as 100 MHz clocks) across cables and/or trigger buses because of line integrity and propagation delays. So, this scheme shares a common reference clock for generation of all sample clocks. The method usually employed to synchronize and generate sampling clocks is phase lock looping (PLL). This method basically monitors the phase of the reference clock and produces a high-speed sampling clock that is phase locked to the reference clock, as shown in Figure C above. Third-party frequency sources, such as rubidium and oven-controlled crystal oscillator (OCXO)-based frequency sources, are ideal for synchronization applications because of their accuracy. These are frequency sources with accuracies of better than 100 parts per billion (ppb). Thus, an OCXO source with 100 ppb accuracy yields a 10 MHz clock with 1 Hz uncertainty. Another important property of your reference clock is multiple output capability for multiple instrument synchronization. The reference clock from either the master instrument or a precision frequency source should be capable of being driven to multiple destinations without any loss of signal integrity. An example of this would be a minimal phase offset between the reference clock outputs from the frequency source. The same issues that arose in the previous scenarios are also relevant in this scheme. The trigger and reference clock signals should arrive at each slave device with minimal delay and skew between each other. At the least, the delay and skew should be known to some uncertainty. The issue of minimal skew between each device is crucial for high-speed digitization. If the skew is large, the time stamp of the incoming trigger on each device will not be coincident in time, and you cannot accurately correlate events captured on separate devices.

are shared via a ribbon. You can serially chain two, three, four, or five boards together, thus achieving synchronization of several I/O channels. Another attractive feature of these trigger buses is built-in switching, so you can route signals to and from the bus on-the-fly through software programming. This eases the burden of having to manually configure your timing and triggering signal distribution on your boards. You can find examples of these features in National Instruments measurement products in the form of the RTSI bus. Connections Integrated with the Measurement Platform – Some of the computer-based measurement devices are implemented in form factors such as VME/VXI and CompactPCI/PXI. VME/VXI, an older industrial form factor, and PXI/CompactPCI, a newer industrial form factor, both address test and measurement, telecommunications, defense, industrial research, and many other markets. VXI and PXI extended VME and CompactPCI by adding timing and triggering buses to the form factors. This greatly simplifies synchronization of multiple devices.

SYNCHRONIZATION OPTIONS Measurement devices come with three main options for connecting synchronization signals – user-supplied cabling, proprietary vendor-defined cabling, and connections integrated with the measurement platform. User-Supplied Cabling – User-supplied cabling of signals for synchronization is available for both computer-based and stand-alone measurement devices. For example, you can often externally synchronize your function generator or digital storage oscilloscope (DSO) to a reference frequency source. When you decide to synchronize your instrumentation, you have to ensure that your cables from your frequency source to the other components of your measurement system are precisely matched in length in order to avoid skew. The same criteria need to apply in distribution of your trigger signal from master to all slave devices. As noted above, your frequency source should have the ability to distribute a common reference clock to multiple destinations. This is the only synchronization option for traditional stand-alone instruments. Proprietary Vendor-Defined Cabling – Some vendors of computer-based measurement devices, such as data acquisition boards, address synchronization by providing a proprietary bus, which may be external or internal to the computer. Sampling clocks, reference clocks, and triggers are distributed from master to slaves through the bus. These dedicated high-speed digital buses are designed to facilitate systems integration. The physical bus interface is a multipin connector on the board, and signals

MORE ON VXI AND PXI VXI and PXI are open standards and many companies make products for both variations. VXI is traditionally used in large test and measurement applications. Though relatively new to the market, PXI is gaining acceptance because of its relatively smaller footprint, portability, high throughput due to the PCI bus, and lower costs, made possible through use of standard commercial technologies spawned by the large PC Industry. Electrically, VXI and PXI add a trigger bus, a star trigger bus, a 10 MHz reference clock, and local buses. For synchronized measurements, the trigger bus, the 10 MHz reference clock, and STAR trigger bus are key features. The PXI features described below broadly apply to VXI as well. System Reference Clock – The PXI back-plane provides a built-in common reference clock for synchronization of multiple modules in a measurement or control system. Each peripheral slot features a 10 MHz TTL clock. Equal-length traces from the clock to each peripheral slot yield low skews of less than 1 ns between slots. The accuracy of the 10 MHz clock is usually 25 ppm (dependent on individual chassis vendors), making it a relatively reliable clock for synchronization applications that rely on PLL methods. If you need a more accurate reference clock, you can insert a PXI counter/timer device with an OCXObased clock source into the second slot of the chassis. The slot’s OCXO 10 MHz clock can be driven onto the PXI backplane clock lines in lieu of the PXI backplane clock. Then, the whole PXI chassis can inherit the OCXO clock stability. Trigger Bus – The PXI eight-line trigger bus provides intermodule synchronization and communication. Trigger or clock transmission can use the trigger bus lines. You can pass triggers from one module to any number of modules, so you can distribute digital trigger signals from master to slave measurement devices. With variable frequency sampling clock transmission, multiple modules can share a timebase that is not a derivative of the 10 MHz reference clock. For example, four data acquisition modules using a 44.1 kS/s CD audio sampling rate can share a clock that is a multiple of the 44.1 kHz or the direct 44.1 kS/s clock. For highspeed synchronization, the propagation delay and skew between slots can reach up to a maximum of 10 ns on a single PXI backplane. Star Trigger for Ultra High Speed Synchronization – The Star trigger bus has an independent trigger line for each slot

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that is oriented in a star configuration from a special Star trigger slot (defined as slot 2 in any PXI chassis). The trigger can provide an independent dedicated line for each of up to 13 peripheral slots on a single PXI backplane. The PXI Star line lengths are matched in propagation delay to within one nanosecond from the Star trigger slot. This feature addresses ultra high-speed synchronization where you can distribute start/stop trigger signals from the master measurement module in the Star trigger slot with low delay and skew.

CONCLUSIONS

Platform

Trigger Bus

Reference Clock

Star Bus

VXI

8 TTL, 2 ECL

10 MHz ECL

Yes

PXI

8 TTL

10 MHz TTL

Yes

Computer-based measurement components are transforming creation of synchronized measurement systems from integration of loosely coupled, and often incompatible instruments, into an orderly engineering process that results in tightly integrated, high-performance systems. For synchronized measurements, timing and triggering details are critical keys to your automated measurements. Precise synchronization requires proper distribution of clocks and triggers. The three main synchronization schemes and proper knowledge of the pros and cons of each and the capabilities of your measurement devices help you to make the right decision in choosing your solution.


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USING NATIONAL INSTRUMENTS SYSTEM IDENTIFICATION, CONTROL DESIGN AND SIMULATION PRODUCTS FOR DESIGNING AND TESTING A CONTROLLER FOR AN UNIDENTIFIED SYSTEM www.ni.com 1. INTRODUCTION This article describes the process of designing a closed loop control system, or plant, using the NI System Identification and Control Design Assistants. A DC Motor will be the plant (Figure 1).

tachometer as a function of motor velocity) and the set point (the command voltage you specify). The controller then determines the next voltage level to command to the motor to meet the specifications defined while designing the controller. Figure 2 shows the final closed-loop system.

Figure 2. The final closed loop system. The Plant Model is the QET (Figure 1).

Figure 1: The Quanser Engineering Trainer (QET) will be the plant for which we will design a closed loop controller.

The Quanser Engineering Trainer will be used in velocity mode. A voltage signal commands the motor to move and the tachometer output determines the velocity. The motor system is connected to a National Instruments Data Acquisition (DAQ) device, where Analog Input 0 (AI0) is connected to the tachometer and Analog Output 0 (AO0) is connected to the motor command input. For demonstration purposes, you can replace the DC motor with an RC circuit. This example uses the following LabVIEW add-ons: • NI LabVIEW System Identification Toolkit • NI LabVIEW Control Design Toolkit • NI LabVIEW Simulation Module You can purchase these products together in the Control Design and Simulation Bundle. To use these add-ons, you must install the following software: • NI LabVIEW 7.1 • NI Signal Express 1.0 The closed-loop system acts on the difference between two quantities: the process variable (the voltage output of the

This example describes the process of designing a simple PI controller for a system with unidentified dynamics. Note that all functionality described in the Express Workbench environment is also available in LabVIEW. All project scripts and LabVIEW VIs described in this document are available as attachments to this document.

2. IDENTIFYING THE SYSTEM To identify an open loop system we need to excite it with a signal that has voltage levels and frequency content that corresponds to its actual operating conditions. For more information about this process, refer to “Stimulus and Acquisition Considerations in the System Identification Process,” located at www.ni.com > NI Developer Zone > Development Library > Analysis and Signal Processing > PID Control/System Characterization/Stability. You can use many different signal types to identify a system, including chirp signals, square waves, square waves overlaid with white noise, and so on. For this example, the stimulus signal is a 3V p-p triangular wave. You create this signal using the “Create Signal” step available in the NI Express Workbench. Figure 3 shows how you create this signal.

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Electricity Testing and Measurement Handbook – Vol. 7 Next, you must have the DAQ device generate this signal as an analog output. Use the DAQmx Generate step to perform this function, shown in Figure 5.

Figure 3. Creating a 3 Volt p-p triangular wave.

Table 1 shows where to find the settings for this particular step and what values to use:

Figure 5. DAQmx Generate step to output the created signal as an analog signal on the DAQ card.

Table 1. Settings for creating the signal Step

Settings/Actions

Signal Input/Output-> Create Signal

Signal Type = Triangle Wave

Table 2 shows the settings for this step:

Frequency = 1 Hz Table 2. Settings for generating the signal on the appropriate “Device” and “Channel” (AO0 in this case).

Amplitude = 3 V Sample Rate = 1kS/s Block Size = 5000 samples

To display the created signal on the data viewer in Express Workbench, drag the “Calculated Signal” Output to the Data View (Figure 4)

Step Signal Input/Output -> Generate Signals -> NI DAQmx Generate

Settings/Actions Config Tab: Device: Make sure to select appropriate DAQ Device and Channel

NOTE: This example does not synchronize the AO and AI channels of the DAQ device. Typically you should synchronize these channels, which you can accomplish using the Advanced Timing page, because any delays caused by the difference in timing between AI and AO would be described by the transfer function of the open loop system, resulting in some error in the identification. In this example, the sample rate for AI and AO is 1 kHz, so the maximum jitter between the two channels is 0.5 ms. This amount of jitter is negligible compared to the plant dynamics. Use the DAQmx Acquire step to acquire the response of the plant to the stimulus signal. Figure 6 shows this acquisition.

Figure 4. The created signal in the data viewer in Express Workbench.


95 symbol are not dependent on the steps above. This symbol disappears after you use the Create and Acquire steps in the system identification process. When this dialog box appears, select the No button to create a new display for the signal. Next, run the project script once by clicking the green run arrow. This project generates and acquires 5000 data points at 1 kS/s for a total of five seconds of plant response data. This response data appears in the display you added in the previous step. Figure 8 shows the stimulus signal and the plant response data.

Figure 6. DAQmx Acquire step to acquire the response back from the DC motor plant as an analog signal to the DAQ card.

Table 3 shows the where to find and settings for this step: Table 3. Settings for acquiring the signal on the appropriate “Device” and “Channel” (AI0 in this case). Step

Settings/Actions

Signal Input/Output -> Acquire Signals -> NI DAQmx Acquire

Config Tab: Device: Make sure to select Device and Channel

Figure 8. The stimulus signal is in the upper display. The plant response to this signal is in the lower display.

Config Tab: Acq. Timing: 5000 samples to read Config Tab: Acq. Timing: 1 kHz sample Rate

Next, drag the output of the DAQmx Acquire step to the data viewing window. Express Workbench notifies you that the data from the current step appears unrelated to the data already on the display. A small disconnect symbol, circled in Figure 7, is also displayed between the DAQmx Generate step and DAQmx Acquire step, which indicates that the steps below the disconnect

You use the stimulus signal and the response data to define a transfer function for the open loop DC motor system. To define this model, you will use a parametric estimation of the motor model. For more information about parametric estimation, refer to “System Identification Model Structure Selection,” located at www.ni.com >> NI Developer Zone >> Development Library >> Analysis and Signal Processing >> PID Control / System Characterization / Stability. In this example, you use the default settings of the Parametric Estimation step, shown in Figure 9, to create a firstorder transfer function. The model order is based on the plant dynamics.

Figure 7. Create a new display to view the output from the DAQmx acquire step. The yellow circle locates the disconnect symbol displayed between the DAQmx Generate and DAQmx Acquire steps. This symbol indicates that the steps below the disconnect symbol are not dependent on the steps above.

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Figure 10: Saving the System Identification Model

Figure 9. Identifying a parametric estimation of the DC motor plant system.

Table 4 shows where to find and the settings for this step: Table 4. Settings for identifying a parametric estimation of the QET DC motor plant system. Step

Settings/Actions

System Identification -> Model Estimation > Parametric Estimation

Input Signals and Model Tab: Stimulus Signal: Calculated signal Response Signal: Device and Channel from NI DAQmx Acquire step.

At this step in the example, the transfer function is discrete. Although you can design a discrete proportional-integral (PI) controller in Express Workbench, this example converts the transfer function model to a continuous one because the motor is a continuous plant. To facilitate this design in the continuous domain, also known as the s-domain, this example transfers the model into a Control Design type function and then converts the model into continuous representation. Figures 11a and 11b show this process.

Add Display under “DAQmx Acq” Output Display Drag “Estimated Response” to new Display

Notice that the disconnect symbol, shown in Figure 7, no longer appears. NI Express Workbench removes this icon because you used the Create and Acquire steps in the system identification process. Also, notice that the largest prediction error typically occurs in the beginning of the signal. This error occurs for two reasons: the initiation effects of spinning up the system (which is typically not in perfect mechanical balance) and because the numerical algorithm used to identify the model requires several time steps to initialize itself. For example, the disc that this particular QET DC motor spins has 2 holes drilled through it, and depending on the location during startup the motor, might start slower or faster. Therefore, the coefficients of the resulting transfer function change slightly every time you run the final Express Workbench Project Script. After you identify a model, you must save the transfer function for further analysis. Select System Identification» Import-Export Model»Save System Identification Model, shown in Figure 10, to save this model.

Figure 11a: Converting the System ID Model to a Control Design Model Type (Transfer Function)

To display the step shown in Figure 11a, select System Identification»Import-Export Model»Convert to Control Design Model. To display the step shown in Figure 11b, select Control Design»Model Transformation»Discretize Model. On the Configuration page of this step, select Make Continuous from the Operation pull-down list.


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Figure 11b. Making the discrete model continuous Figure 13. Displaying the transfer function of the open-loop system plant in the s-domain.

Next, create a new display for the continuous transfer function. To create this display, right-click the DAQmx Acquire Result display in the data viewer window and select Add Display»Below from the shortcut menu. Figure 12 shows this creation.

3. DESIGNING THE CONTROLLER Now that the transfer function of the plant is available, the next step is to design a controller for this plan. This example describes how to design a controller that meets requirements for rise time, settling time, overshoot, and so on. This controller will complete the closed-loop system. This example designs a simple proportional-integral (PI) controller using the PID Synthesis step, shown in Figure 14. You also can perform a root locus or interactive bode design.

Figure 12. Adding a window to display the transfer function for the open-loop system.

Then, drag the output of the Discretize Model step to the new display, shown in Figure 13. The system has now been identified as a first order transfer function. Every time the Express Workbench project script is run, the coefficients of the transfer function will change slightly. This is due to the spinning wheel and other mechanics of the motor itself (and the tachometer, and the fact that AI and AO is not 100% synchronized etc.), as explained above during the Estimation of the Parametric Model.

Figure 14. Designing a PI controller for the DC motor.

Table 5 lists the settings for designing a PI controller. Table 5. Settings for designing a PI controller for the DC Motor Plant System. Step

Settings/Actions

Control Design -> Controller Design -> PID Synthesis

Controller Synthesis Tab: Check “Gain” and "Integral (s)" boxes Adjust P and I gains to obtain desired step response. Refer to Figure 14 for the recommended settings.

98 As you adjust the values of the P and I gains, the step response graph changes to show the resulting rise time, overshoot, ringing, settling time, and so on. Adjust the P and I gains so the step response looks similar to the step response shown in Figure 14. This step response has a rise time of approximately 25 ms and overshoot of less than 50% of the steady state value. Optionally, you can check these time domain specifications by adding a Time Domain Analysis step after the PID Synthesis step WARNING: Too much overshoot can causes the output of the controller to command a voltage much higher than the Analog Output board and the motor can handle. However, later on in this example, you will use the Simulation Module to enforce a limit on the valid range of the output. After you have properly adjusted the P and I gains, save the model by using the Save Control Design Model step, located at Control Design»Import-Export Model. Figure 15 shows this step.


Figure 16. Using the Simulation Module to simulate the behavior of the identified plant and the PI controller in a closed-loop configuration.

Figure 17 shows the response of the closed-loop system to a square wave input. Refer to Figure 14 to verify that this is the expected behavior.

Figure 15. Saving the PI controller.

4. SIMULATING THE CLOSED LOOP SYSTEM In this example, the previous sections provided information about identifying the plant model and designing a PI controller based on this plant model. Before you use this controller on the actual DC motor, you use the Simulation Module to verify the controller behaves as you expect. The Simulation Module includes several ordinary differential equation (ODE) solvers you use to integrate the continuous transfer function model over a period of time. For more information about the Simulation Module, refer to www.ni.com >> Products & Services >> RealTime Measurement and Control >> NI Real-Time Software >> Add-On Toolkits >> Simulation Module. Figure 16 shows the LabVIEW block diagram, including the Simulation Loop that defines the simulation diagram. Notice the pale yellow color of the simulation diagram to distinguish it from the LabVIEW block diagram. Also notice that the Simulation Module allows you to directly implement feedback, completing the closed-loop system. Notice LoadController.vi and LoadPlant.vi. These subVIs load the models that you created in Express Workbench and transfer the models into the appropriate Simulation functions. The LoadController subVI also converts the discrete transfer function into a continuous one. Recall that you implemented this step in the Express Workbench. However, all functionality available in the Express Workbench environment is available in LabVIEW.

[+] Enlarge Image Figure 17. Stimulating the closed-loop system with a square wave input and showing the response.

Notice the knobs on the front panel of Figure 17. You use these knobs to change the type, amplitude, and frequency of the stimulus signal while immediately viewing the response of the closed-loop system.

5. DRIVING THE MOTOR WITH THE CLOSED LOOP SYSTEM Now that you have verified the closed-loop response of the plant and controller models, the next step is to use this controller to drive the actual DC motor. First, this example demonstrates an open-loop system. NOTE: This example does not synchronize the input and output values of the system, because, the short jitter (≤0.5ms with 1000 kS/s analog input and output) ensures that the output does not display any significant difference.


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Figure 18 shows the LabVIEW block diagram for driving the DC motor in an open-loop configuration.

By closing the loop and adding the PI controller to the open-loop system, the response of the motor becomes faster and more accurate with respect to the RPM you specify. The controller compares the actual speed of the motor with the speed you specified and adjusts the motor speed accordingly. Figure 20 shows this increase in response time and accuracy.

Figure 18. Driving the plant (DC Motor) in an open loop configuration.

Figure 19 shows the front panel of this block diagram. Figure 20. Driving the plant (DC Motor) in a closed loop configuration. The response from the motor is fast, with overshoot, settling time characteristics as defined while designing the controller (Figure 14).

The integration term in the PI controller minimizes the steady-state error by taking the history of the error into account. Figure 21 shows the LabVIEW block diagram that corresponds to the front panel shown in Figure 20.

[+] Enlarge Image Figure 19. Driving the plant (DC Motor) in an open loop configuration. The response from the motor is slow.

Figure 19 shows how you specify the motor speed in rotations per minute (RPM). This example converts this value to the corresponding analog voltage as directed by the manufacturer of the DC motor. In this situation, the multiplier is 0.0015 volts/RPM. The Analog Output Channel 0 (AO0) of the DAQ device then sends this value to the DC motor. This example then uses Analog Input Channel 0 (AI0) of the DAQ device to acquire the data from the tachometer of the DC motor. This example then converts the tachometer value to RPM by using the manufacturer-supplied multiplier of 666.6 RPM/volts. After you press the Stop button, this example stops the motor by sending a value of 0 volts to AI0. Notice in Figure 19 that the motor is slow to respond to any change in specified RPM. This example also demonstrates steady-state error, which is a permanent difference between the specified and actual motor speeds. This error is due to the calibration uncertainty in the multiplication constants Figure 18 shows. The steady-state error is particularly noticeable at high speeds, because high speeds increase the relative error that results from not multiplying with the exact conversion factor.

Figure 21. Driving the plant (DC Motor) in a closed loop configuration.

The block diagram in Figure 21 converts the speed it is converted to and from corresponding Analog Voltage using the same multipliers described in Figure 18. The actual speed of the motor is compared with the speed you specify, or the Set Point. The controller is loaded from file as shown in Figure 15. Figure 21 shows how Saturation function limits the output voltage of the motor. This figure also shows how you can use the SIM Set Diagram Params VI to programmatically change the ODE solver and other parameters of the simulation. NOTE: In the real world, the Saturation function is not necessary, because the DAQ Analog Output Assistant Express VI has a control that sets a limit on the output voltage. However, this example demonstrates the capabilities of the Simulation Module and how you would place Saturation function in a closed

100 loop. Also, if you place a LabVIEW data probe before and after the Saturation function, changing the Set Point suddenly can cause the motor to overshoot.

6. CONCLUSIONS This example described how you can use LabVIEW and related software to identify, control, and simulate a real-world dynamic system. Although this example did not use any realtime (RT) hardware, you can use the LabVIEW Real-Time Module in conjunction with the Simulation Module to deploy a controller to any National Instruments RT Series hardware. Refer to Using CompactRIO, located at http://sine.ni.com /csol/cds/item/vw/p/id/538/nid/124200, for an example that demonstrates how to build a full-authority FPGA-based engine control system for a high-performance motorcycle engine. NOTE: You also can describe the simulation itself in the Express Workbench Project Script by adding a User-Defined Step. You also can translate an Express Workbench project script into LabVIEW code by launching LabVIEW and selecting Tools»Express Workbench»Convert Express Workbench Project from the pull-down menu. More complex systems, such as the high performance motorcycle engine described above, may have multiple inputs and multiple outputs. In these situations, you can use state-space model identification and control design methods to operate in the multiple-input multiple-output (MIMO) environment. The Control Design Toolkit, System Identification Toolkit and Simulation Module support these design methods.



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MAGNETO-MECHANICAL MEASUREMENTS FOR HIGH CURRENT APPLICATIONS Jack Ekin, NIST – Electromagnetics Division GOALS This project specializes in measurements of the effect of mechanical strain on superconductor properties such as criticalcurrent density for applications in magnetics, power transmission, and electronics. Recent research has produced the first electromechanical data for the new class of high-temperature coated conductors, one of the few new technologies expected to have an impact on the electric-power industry. The Strain Scaling Law, previously developed by the project for predicting the axial-strain response of low-temperature superconductors in high magnetic fields, is now being generalized to three-dimensional stresses, for use in finite-element design of magnet structures, and to high-temperature superconductors. Recent research includes extending the high-magnetic-field limits of electromechanical measurements for development of nuclear-magneticresonance (NMR) spectrometers operating at 23.5 teslas and 1 gigahertz, and the next generation of accelerators for high-energy physics. The project has diversified its research to include magnetoresistance studies on a new class of carbon nanostructures using our highfield superconducting magnet facility and a newly developed, variable-angle, variable-temperature measurement capability.

CUSTOMER NEEDS The project serves industry primarily in two areas. First is the need to develop a reliable measurement capability in the severe environment of superconductor applications: low temperature, high magnetic field, and high stress. The data are being used, for example, in the design of superconducting magnets for the magnetic-resonance-imaging (MRI) industry, which provides invaluable medical data for health care, and contributes 2 billion dollars per year to the U.S. economy. The second area is to provide data and feedback to industry for the development of high-performance superconductors. This is especially exciting because of the recent deregulation of the electric power utilities and the attendant large effort being devoted to develop superconductors for power conditioning and enhanced power-transmission capability. We receive numerous requests, from both industry and government agencies, for reliable electromechanical data to help guide their efforts in research and development in this critical growth period. The recent success of the second generation of high-temperature superconductors has brought with it new measurement problems in handling these brittle conductors. We have the expertise and equipment to address these problems. Stress and strain management is one of the key parameters needed to move the second-generation high-temperature coated conductors to the market place. The project utilizes the expertise and unique electromechanical measurement facilities at NIST to provide performance feedback and engineering data to companies and

national laboratories fabricating these conductors in order to guide their decisions at this critical phase of coated-conductor development.

TECHNICAL STRATEGY Our project has a long history of unique measurement service in the specialized area of electromechanical metrology. Significant emphasis is placed on an integrated approach. We provide industry with first measurements of new materials, specializing in cost-effective testing at currents less than 1000 amperes. Consultation is also provided to industry on developing its own measurements for routine testing. We also provide consultation on metrology to the magnet industry to predict and test the performance of very large cables with capacities on the order of 10 000 amperes, based on our tests at smaller scale. In short, our strategy has consistently been to sustain a small, well connected team approach with industry. Electromechanical Measurements of Superconductors – We have developed an array of specialized measurement systems to test the effects of mechanical stresses on the electrical performance of superconducting materials. The objective is to simulate the operating conditions to which a superconductor will be subjected in magnet applications. In particular, since most technologically important superconductors are brittle, we need to know the value of strain at which fractures occur in the superconductor. This value is referred to as the irreversible strain limit, since the damage caused by the formation of cracks is permanent. The effect of cracks is extrinsic. In contrast, below the irreversible strain, there exists an elastic strain regime where the effect of strain is intrinsic to the superconductor. In this elastic regime, the variation in the critical-current density (Jc) with strain, if any, is reversible and is primarily associated with changes in the superconductor’s fundamental properties, such as the critical temperature (Tc) and the upper critical field (Hc2), as well as changes in the superconductor’s microstructure due to the application of strain. Measurement Facilities – Extensive, advanced measurement facilities are available, including high-field (18.5 teslas) and split-pair magnets, servohydraulic mechanical testing systems, and state-of-the-art measurement probes. These probes are used for research on the effects of axial tensile strain and transverse compressive strain on critical current; measurement of cryogenic stress-strain characteristics; composite magnetic coil testing; and variable-temperature magnetoresistance measurements. Our electromechanical test capability for superconductors is one of the few of its kind in the world, and the only one providing specialized measurements for U.S. superconductor manufacturers. Collaboration with Other Government Agencies – These measurements are an important element of our ongoing work with the U.S. Department of Energy (DOE). The DOE Office of High Energy Physics sponsors our research on electro-

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mechanical properties of candidate superconductors for particleaccelerator magnets. These materials include low-temperature superconductors (Nb3Sn, Nb3Al, and MgB2), and high-temperature superconductors – Bi-Sr-Ca-Cu-O (BSCCO) and Y-BaCu-O (YBCO) – including conductors made on rolling-assisted, biaxially textured substrates (RABiTS) and conductors made by ion-beam-assisted deposition (IBAD). The purpose of the database produced from these measurements is to allow the magnet industry to design reliable superconducting magnet systems. Our research is also sponsored by the DOE Office of Electric Transmission and Distribution. Here, we focus on high-temperature superconductors for power applications, including power-conditioning systems, motors and generators, transformers, magnetic energy storage, and transmission lines. In all these applications, the electromechanical properties of these inherently brittle materials play an important role in determining their successful utilization. Scaling Laws for Magnet Design – In the area of lowtemperature superconductors, we have embarked on a fundamental program to generalize the Strain Scaling Law (SSL), a magnet design relationship we discovered two decades ago. Since then, the SSL has been used in the structural design of most large magnets based on superconductors with the A-15 crystal structure. However, this relationship is a one-dimensional law, whereas magnet design is three-dimensional. Current practice is to generalize the SSL by assuming that distortional strain, rather than hydrostatic strain, dominates the effect. Recent measurements in our laboratory suggest however that this assumption is invalid. We are now developing a measurement system to carefully determine the three-dimensional strain effects in A-15 superconductors. The importance of these measurements for very large accelerator magnets is considerable. The Strain Scaling Law is now also being developed for high-temperature superconductors since we recently discovered that practical high-temperature superconductors exhibit an intrinsic axialstrain effect.

The technique consists of measuring critical-current density (the maximum lossless current density that a superconductor can carry) versus axial strain for a number of copper-plated specimens of the same wire with different amounts of copper. We then deduced the strain properties of the virgin (noncopper-plated) wire by an extrapolation technique. Copper plating made the niobium-tin wires electrically stable enough to characterize, but the extra copper also influenced the value of the pre-compressive strain (εmax); hence the need for extrapolation. We confirmed that εmax indeed decreased linearly with increasing niobium fraction. However, we found that other parameters such as the matrix material and wire diameter also influence εmax. The pre-compressive strain for high-niobium-fraction wires can be reduced to about 0.1 percent, a very small strain window for magnet design. Fortunately, we also found that the use of copper alloys, instead of pure copper – along with small wire diameters – substantially mitigates the problem and provides reasonable strain operating margins in these high performance conductors. The data were used by Oxford Superconductor Technology to make immediate decisions regarding the conductor design for a new NMR system. • Copper Stabilizer Improves Coated Superconductors’ Strain Tolerance – High-temperature superconductor (HTS) wires are now being fabricated in kilometer lengths, providing the basis for a new generation of electric power devices, including high power-density motors and generators, transmission lines, and power conditioners. The development of HTS technology is expected to play a crucial role in maintaining the reliability of the power grid and upgrading power delivery to core urban areas. The most promising superconductor candidate for replacing ageing utility equipment is the highly textured Y-Ba-Cu-O (YBCO) compound deposited on buffered flexible metallic substrates. These “coated conductors” have a much higher current-carrying capacity compared to the Bi-Sr-Ca-Cu-O (BSCCO) tapes now commercially available. Whereas BSCCO tapes experience permanent damage when subjected to axial strains less than 0.2 percent, we demonstrated last year that the formation of cracks in the new YBCO system does not commence until subjected to strains higher than 0.38 percent, almost a two-fold increase in strain tolerance. This resilience of YBCO to strain is providing a strong motivation to produce commercial lengths of this “second generation” conductor, especially for the design of electric generators for which strain tolerance requirements have been raised to 0.4 percent. This year, we found that adding a Cu layer to the YBCO coated-conductor architecture extends the irreversible strain limit (εirr) of this composite even further, from 0.38 percent to more than 0.5 percent. This markedly widens the strain window for coated-conductor applications and takes it beyond even the most demanding benchmark for large-scale superconducting generators. These measurements were undertaken in close collaboration with conductor manufacturers American Superconductor (Westborough, MA) and SuperPower (Schenectady, NY), who are incorporating the stabilizer layers either by Cu-lamination or Cu-plating. The original motivation for adding the Cu layers was to improve the electric and thermal stability of the conductor; the strain-tolerance dividend was unexpected. We can relate this remarkable result to the mismatch of thermal contraction between Cu and the other components of the composite. During sample cooling from processing temperatures to the cryogenic operating temperatures, the Cu layer exerts an additional pre-compressive strain on the YBCO film, and hence extends the irreversible

ACCOMPLISHMENTS

Pre-compressive strain εmax versus Nb fraction for several niobium-tin wires with high niobium density. Data were obtained using a new measurement method developed by EEEL researchers for marginally stable superconductor wires.

• New Measurement Method for Marginally Stable Superconductor Wires – The next generation of particle accelerators for high-energy physics, and magnet systems for nuclear magnetic resonance (NMR) spectroscopy, will require the development of a new type of superconducting niobium-tin wire able to carry extremely high currents at high magnetic fields. One way to achieve high currents is to push the density of superconductor filaments in composite wires to new limits. Oxford Superconductor Technology (Carteret, NJ) has successfully demonstrated the feasibility of this concept. However, this could significantly reduce the beneficial “pre-compressive strain” in these conductors upon cooling, an important parameter for magnet design. Our superconductor electromechanical testing system is the only one in the U.S. that utilizes stress-free cooling, which is essential for a direct measurement of pre-compressive strain. Unfortunately, the new niobium-tin wires, owing to their relatively small amount of copper stabilizer, are only marginally stable, which makes electrical characterization extremely challenging. Hence, a new measurement technique was required that did not compromise the stress-free cooling advantage.


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strain εirr where permanent damage occurs. The Cu may also be acting as a crack arrester, which further improves the strain tolerance.

• Textbook on Cryogenic Measurement Apparatus and Methods – A new textbook has been written on experimental techniques for cryogenic measurements to be published by Oxford University Press. It covers the design of cryogenic measurement probes and provides cryogenic materials data for their construction. Topics include thermal techniques for designing a cryogenic apparatus, selecting materials appropriate for such apparatus, how to make high-quality electrical contacts to a superconductor, and how to make reliable criticalcurrent measurements. The textbook is written for beginning graduate students, industry measurement engineers, and materials scientists interested in learning how to design successful low-temperature measurement systems. The appendices are written for experts in the field of cryogenic measurements and include electrical, thermal, magnetic, and mechanical properties of technical materials for cryostat construction; properties of cryogenic liquids; and temperature measurement tables and thermometer properties. These appendices aim to collect in one place many of the data essential for designing new cryogenic measurement apparatus.

Normalized critical current density as a function of mechanical tensile strain for unlaminated and Culaminated YBCO coated conductor. The Cu stabilization layer extends the irreversible strain limit εirr of the composite from 0.38 percent to more than 0.5 percent.

Photograph of a new magnetoresistance probe designed to investigate carbon nanostructures. At the right end of the probe, the photo shows the steppermotor-controlled worm-gear system and sample stage, which allow precise angledependent, high-field measurements.

• New Magnetoresistance Apparatus to Probe Carbon Nanostructures – Electronic properties of materials change markedly as their dimensions approach those of a few atomic layers. Carbon nanostructures (including graphite sheets, singlewalled carbon nanotubes, and multi-walled carbon nanotubes) are prime examples of such potentially useful materials, although some of their very fundamental properties remain controversial. Characterization of these structures at high magnetic fields is one of the principal methods for determining the existence of ballistic conduction, for example, which could be the foundation for a new generation of nanoelectronic devices. We have designed and recently commissioned an apparatus to measure magnetoresistance of these highly directional structures in fields up to 18.5 teslas. (For comparison, the Earth’s magnetic field is only about 0.05 millitesla.) The apparatus automatically acquires data as a function of magneticfield magnitude, angle, and temperature. It was designed to also be compatible with the very-highfield magnet facilities at the National High Magnetic Field Laboratory at Florida State University, permitting the extension of EEEL’s measurements to fields up to 30 teslas. Magnetic field mapping has commenced for nanotubes fabricated at NIST and Rice University as well as for graphitic sheet structures manufactured by a nanotechnology research team at Georgia Institute of Technology. Magnetic-field angle can be varied with a resolution of better than 0.1 degree over a range of 130 degrees, and sample temperature can be varied over an extended range of 4.1 to 120 kelvins, with a stability of better than 3 millikelvins at 4.2 kelvins.

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105

A BASIC GUIDE TO THERMOGRAPHY Land Instruments International Infrared Temperature Measurement

THERMOGRAPHY

ELECTROMAGNETIC SPECTRUM

Thermography is a method of inspecting electrical and mechanical equipment by obtaining heat distribution pictures. This inspection method is based on the fact that most components in a system show an increase in temperature when malfunctioning. The increase in temperature in an electrical circuit could be due to loose connections or a worn bearing in the case of mechanical equipment. By observing the heat patterns in operational system components, faults can be located and their seriousness evaluated. The inspection tool Figure 1. The Thermal Image of electrical used by Thermographers is connector the Thermal Imager. These are sophisticated devices which measure the natural emissions of infrared radiation from a heated object and produce a thermal picture. Modern Thermal Imagers are portable with easily operated controls. As physical contact with the system is not required, inspections can be made under full operational conditions resulting in no loss of production or downtime. The Land Cyclops Thermal Imager is a device designed for plant condition Figure 2. The inspection of electrical equipment monitoring, preventative using a Thermal Imager maintenance and process monitoring applications. Potential applications include: • Inspection of electrical equipment • Inspection of mechanical equipment • Inspection of refractory lined structures

The energy from a heated object is radiated at different levels across the electromagnetic spectrum. In most industrial applications, it is the energy radiated at infrared wavelengths which is used to determine the object’s temperature. Figure 3 shows various forms of radiated energy in the electromagnetic spectrum including X-rays, Ultra Violet, Infrared and Radio. They are all emitted in the form of a wave and travel at the speed of light. The only difference between them is their wavelength which is related to frequency.

MEASUREMENT OF TEMPERATURE USING INFRARED METHODS When using a Thermal Imager it is helpful to have a basic knowledge of infrared theory.

BASICS PHYSICS An object when heated radiates electromagnetic energy. The amount of energy is related to the object’s temperature. The Thermal Imager can determine the temperature of the object without physical contact by measuring the emitted energy.

The human eye responds to visible light in the range 0.4 to 0.75 microns. The vast majority of infrared temperature measurement is made in the range 0.2 to 20 microns. Although emissions are mostly unable to be detected by a standard camera the Thermal Imager can focus this energy via an optical system on to a detector in a similar way to visible light. The detector converts infrared energy into an electrical voltage which after amplification and complex signal processing is used to build the thermal picture in the operator’s viewfinder on board the Thermal Imager.

ENERGY DISTRIBUTION Figure 4 shows the energy emitted by a target at different temperatures. As can be seen, the higher the target temperature the higher the peak energy level. The wavelength at which peak energy occurs becomes progressively shorter as temperature increases. At low temperatures the bulk of the energy is at long wavelengths. Figure 4. Infrared energy and distribution across the Electromagnetic spectrum

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EMISSIVITY

It can be shown that there is a relationship between emissivity and reflectivity.

The amount of energy radiated from an object is dependant on its temperature and its emissivity. An object which has the ability to radiate the maximum possible energy for its temperature is known as a Black Body. In practice, there are no perfect emitters and surfaces tend to radiate somewhat less energy than a Black Body. Figure 5 shows why objects are not perfect emitters of infrared energy. As energy moves towards the surface a certain amount is reflected back inside and never escapes by radiative means. From this example, it can be seen that only 60% of the available energy is actually emitted. The emissivity of an object is the ratio of the energy radiated to that which the object would emit if it were a Black Body.

Figure 5. The Infrared energy reflected at a body surface

Hence emissivity is expressed as

Emissivity is therefore an expression of an object’s ability to radiate Infrared energy.

EMISSIVITY VALUES The value of emissivity tends to vary from one material to another. With metals, a rough or oxidised surface usually has a higher emissivity than a polished surface. Here are some examples:

For an opaque object this is Emissivity + Reflectivity = 1.0

Hence, a highly reflective material is a poor emitter of infrared energy and will therefore have a low emissivity value.

EFFECTS OF EMISSIVITY If a material of high emissivity and one of low emissivity were placed side by side inside a furnace and heated to exactly the same temperature, the material with low emissivity would appear to the eye much duller. This is due to the different emissivities of the materials causing them to radiate at different levels, making the low emissivity material appear cooler than the high emissivity material, even though they are at exactly the same temperature. The Thermal Imager would see this in the same way as the eye and produce an error in making the temperature measurement. The temperature of an object cannot be determined by simply measuring its emitted infrared energy, a knowledge of the object’s emissivity must also be known. The emissivity of an object can be determined as follows: 1) Consult manufacturers literature (always ensure these have been evaluated at the operating wavelength of your Thermal Imager as emissivity can vary with wavelength). 2) Have the object’s emissivity evaluated by a laboratory method. There are two main ways to overcome the problem of emissivity. a) Mathematically correct the temperature measurement value. This is usually carried out within the signal processor of the Thermal Imager. Most modern Thermal Imagers have a compensation setting which can quickly and easily be set by the operator. b) It may be possible to paint the surface of a low emissivity target with a high and constant emissivity coating. This tends to elevate the target to a much higher emissivity level, but this may not be possible on all process plants. When carrying out Thermographic inspections, faults are often identified by comparing heat patterns in similar components operating under similar loads. This is an alternative to very precisely predicting the emissivity of each individual component and obtaining absolute temperature values.

Thermal Imager being used to inspect electrical equipment. With equal load and emissivities the temperature of the three measurement points should be the same.


THERMAL IMAGERS Thermal Imagers are sophisticated devices which measure the natural emissions of infrared radiation from a heated object and produce a thermal picture. Modern Thermal imagers such as the Land TI814 are usually very flexible containing many standard and optional features. Here are some of those of the TI814.

107 d) Parameter changes: parameters saved with the stored image may be changed within the software. These include emissivity, and background temperature. e) Image enhancements: filtering, and zoom facility. Figures 7 to 12 show some of the available temperature measurement modes.

OPTICAL: A motorised focus is used to obtain a clear image at different distances from the thermal imager. The focus distance is from 380mm/15 inches to infinity. An electronic zoom function enables 2X and 4X magnification of the image.

IMAGE DISPLAY: The real time thermal image is displayed in colour on a 102mm / 4 inch LCD screen. The image may be colourised by any one of the eight different palettes available. The real time thermal image is also displayed on the builtin high resolution colour viewfinder.

Figure 7. Measuring the temperature at several points in the scene

DIGITAL MEMORY: A built in non volatile memory system enables the simple capture of a large number of thermal images. Thermal images are stored on a removable compact flash memory card. This on board facility enables stored image recall to the viewfinder and selective image deletion. Several seconds of digital voice clip may be stored with each image and replayed or re-recorded on board the imager. The sound file can be replayed in by the imager or with image processing software. A 256MB card is capable of storing up to 1000 thermal images and up to an eight second digital voice clip with each image. Image file size including voice annotation is 256 KB. Transfer to image processing software for further image processing and report generation is via a USB Compact Flash memory card reader.

Figure 8. Measuring the average temperature within several rectangles in the scene

TEMPERATURE MEASUREMENT: Temperature measurement at single point in the scene is possible.

POST PROCESSING: This facility enables the generation of further temperature analysis in the imager viewfinder on stored images. A single movable point enables spot measurement at any point in the scene and a movable cursor generates a temperature profile trace.

Figure 9. Measuring the average temperature within several polygons in the scene

IMAGE PROCESSING SOFTWARE Frames of interest may be stored as an image file for record purposes, or be subjected to a range of processing functions as follows: a) File handling: save, delete and directory facility b) Image colouring: the image may be colourised using any one of five colour palettes. c) Temperature measurement: a variety of different modes are available to enable temperature measurement at any point in the scene, calculation of maximum, minimum or mean from within any defined area in the scene, profiles, histograms, and isotherms.

Figure 10. Measuring the temperature along several profiles in the scene

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THERMAL IMAGERS IN PREDICTIVE MAINTENANCE APPLICATIONS

Figure 11. Measuring the temperature distribution within a defined area in the scene

In today’s industrial plants it is essential that unplanned breakdowns and the resultant costly loss of production is kept to an absolute minimum. Predictive maintenance schemes have been introduced to identify potential problems and reduce downtime. Thermography in maintenance applications is based on the fact that most components show an increase in temperature when malfunctioning and faults steadily get worse before failure. Routine inspection programmes using Thermal Imagers can often offer the following benefits: Inspections can be made under full operational conditions and hence there is no loss of production. • Equipment life can be extended • Plant downtime may be reduced • Plant reliability may be increased • Plant repairs scheduled for the most convenient time • Quality of repair work may be inspected • Thermal Imagers are mainly used for industrial predictive maintenance in the following areas: • Electrical Installations • Mechanical Equipment • Refractory lined Structures

INSPECTING ELECTRICAL INSTALLATIONS Figure 12. Using Isotherm to highlight areas of the scene within a selected temperature band

The software system is menu driven, making it extremely easy to use. Report Writer: The image processing system provides a report writing facility. This may be used to provide a hard copy record of the thermal image accompanied by an imported photograph and any other information for reference purposes.

Faults in an electrical installation often appear as hotspots which can be detected by the Thermal Imager. Hot spots are often the result of increased resistance in a circuit, overloading, or insulation failure. Figure 14 shows a hot-spot created by a bad connection in a power distribution system.

Figure 14. Inspection of a power system

Some of the components commonly inspected are as follows: Connectors: When looking at similar current carrying connectors, a poor connection shows a higher temperature due to its increased resistance. Hot-spots can be generated as a result of loose, oxidised, or corroded connectors.

Figure 13. Typical items page in a report generated by the report writer facility

Figure 15. Inspection of connectors

Electricity Testing and Measurement Handbook – Vol. 7 Figure 15. Shows the fuses in the control panel of a machine. A faulty connection on the top of a fuse has created the hot-spot which can easily be seen by the imager. Three phase motors: Require balanced phases and correct operating temperatures. It has been shown that if correct operating temperatures are exceeded, the insulation life can be considerably shortened. Other commonly inspected components are: • Relays • Insulators • Capacitors • Switches

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INSPECTION OF REFRACTORY LINED STRUCTURES The refractory structures of process plants can often have an increased lifetime if the degree of wear and erosion can be assessed. Thermal patterns produced by viewing the outer walls of a structure can indicate hot-spots caused by worn refractories which may be corrected by appropriate maintenance.

INSPECTION OF MECHANICAL EQUIPMENT The type of mechanical equipment inspected is often rotating machinery. Increased surface temperatures can be the result of internal faults. Excessive heat can be generated by friction in faulty bearings due to wear, misalignment or inadequate lubrication. As with electrical installations, it is desirable to perform the inspection with the system in operation wherever practically possible. Interpretation of results should be based on comparison between components operating in similar conditions under similar loads or by trend analysis. Equipment commonly inspected using thermal Imagers is as follows: • Bearings • Gears • Drive Belts • Couplings • Shafts and Pumps.

Figure 16. Inspection of bearing housing

Figure 17. Inspection of a Kiln shell

Figure 17 shows an abnormal heat pattern on the wall of a cement kiln, which has been caused by erosion of the refractory brick liner. Equipment commonly inspected using Thermal Imagers is as follows: • Electric Arc Furnaces • Ladles, Heat Treatment Furnaces • Glass Furnaces • Rotary Kilns and Dryers

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BUYER’S GUIDE struction and engineering, street and parking lot lighting installation and maintenance. Now available - Power Quality field survey, monitoring and solutionsn to power quality problems. 3M Canada PO Box 5757 London, Ontario N6A 4T1 Tel: (800) 3M Helps Fax: (519) 452-6286 E-mail: [email protected] Web: www.mmm.com Description of products/services: • Terminations and splices, using Cold Shrink® Technology, moulded rubber, resin and heat shrink • Motor lead connection systems • Scotch® vinyl insulation tapes, splicing and terminating tapes, corrosion protection sealing and general use tapes • Scotchloc® terminal, wire connectors and insulation displacement connectors, lugs, copper and aluminum connectors • Scotchtrak® infrared heat tracers and circuit tracers • Fastening products, coatings and lubricants • Duct- , packaging, filament-, and masking tapes • Abrasive products • Personal safety products, sorbents.

B.G. High Voltage Systems Ltd. 1 Select Avenue, Units 15 & 16 Scarborough, ON M1V 5J3 Tel: (416) 754-2666 ext. 202 Fax: (416) 754-4607 E-mail: [email protected] www.bg-high-voltage.ca Contact: B. J. (Bert) Berneche, C.E.T., President Description of products/services: B.G. High Voltage Systems offers a comprehensive approach to electrical project management, providing design, construction and engineering services to meet all your requirements. We team up with our clients to ensure that all their needs are defined and met at each stage of the project. Our experts will coordinate with your engineering personnel to ensure minimal disruption to facility operations. As well as complete electrical project management we offer: material procurement, maintenance and training services, emergency repair, overhead and underground distribution con-

CD Nova Ltd. 5330 Imperial St. Burnaby, BC V5J 1E6 Tel: (604) 430-5612 Fax: (604) 437-1036 Contat: Don Bealle E-mail: [email protected] Web: www.cdnova.com CD NOVA companies distribute and service, in Canada, Energy and power Systems and devices, Transducer, Test and Measurement Instruments, Batteries, Chargers, UPS, Wireline and Wireless Comm. systems, SCADA systems, Power Quality Analysers and systems. Teleprotection, Transformers, Breakers Protective Relays, Gas and chemical Analysers, Stack sampline systems.

Duncan Instruments Canada Ltd. 121 Milvan Drive Toronto, Ontario M9L 1Z8 Tel: 416 742-4448 Fax:416 749-5053 Email: [email protected] www.duncaninstr.com Description of products/services: Duncan Instruments Canada is a leading manufacturers’ representative and master distributor for a wide range of utility and electrical instrumentation. We can offer you data loggers, power line analyzers power/energy/harmonics analyzers, power disturbance monitors and fused test leads/accessories. In addition to sales, Duncan Instruments Canada can also provide: calibration – traceable to NRC, technical product support and application training, instrument repair/modifications, and rental of selected electrical instruments. Registered to ISO 9001:2000

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Flir Systems 5230 South Service Road #125 Burlington, ON Tel: (905) 637-5696 Fax: (905) 639-5488 Web: www.flirthermography.com FLIR Systems Ltd. (Agema Inframetrics) designs, manufactures, calibrates, services, rents and sells many models of infrared imaging cameras and accessories. Complete predictive maintenance solutions include the ThermaCam PM 695 radiometric camera with thermaland visual images, autofocus, voice and text messaging and of course Reporter analysis software with "drag-n-drop" image transfer software. Level's 1, 2 and 3 Thermography training conducted on site or at ITC facility. Camera accessories, such as close-up and telescopic optics, batteries, etc. can be sourced directly from Canadian service/sales depot in Burlington, ON. Ask about trade in allowances.

FLUKE ELECTRONICS Canada LP 400 Britannia Rd. East Unit 1 Mississauga, ON, Canada

Electricity Testing and Measurement Handbook – Vol. 7 L4Z 1X9 Toll Free : 1-800-36-FLUKE Tel : (905) 890-7600 Fax : (905) 890-6866 Contact : Robin Bricker E-Mail : [email protected] www.flukecanada.ca Fluke Electronics Canada (www.flukecanada.ca) offers complete families of professional test tools, including power quality, thermography, digital multimeters, clamp meters, insulation resistance testers, portable oscilloscopes, thermometers, process testing equipment and accessories, as well as educational and training resources. A subsidiary of Fluke Corporation, Everett, Washington, Fluke Electronics Canada is headquartered in Ontario with offices across Canada. The Fluke brand has a reputation for quality, portability, ruggedness, safety and ease of use and Fluke test tools are used by technical professionals in a variety of industries throughout the world.

G.T. WOOD CO. LTD. 3354 Mavis Road Mississauga, ON L5C 1T8 Tel: (905) 272-1696

Fax: (905) 272-1425 E-Mail: [email protected] Website: www.gtwood.com/flash/splash.html Specializing in High-Voltage Electrical Testing, inspections, maintenance and repairs. Refurbishing and repair of New and Reconditioned Transformers, Structures, Switchgear and Associated Equipment. Infrared Thermography, Engineering Studies and PCB Management.

High Voltage, Inc. 31 Rt. 7A, P.O. Box 408 Copake, NY 12516 USA Tel : (518) 329-3275 Fax : (518) 329-3271 Contact : Bob Tighe, E-Mail : [email protected] Manufacturers of High Voltage Test Equipment. Products include portable AC-VLF, .1Hz, .05 and 0.2Hz Very Low Frequency hipots with sine wave output, switchgear and bottle testers up to 100 kVac. Portable DC hipots up to 300 kV DC. Aerial lift and bucket truck AC test sets up to 300 kVac according to ANSI standards. Controlled energy cable fault locators, oil test sets and burners also offered.

LIZCO SALES R.R. #3 Tillsonburg, ON N4G 4G8 Toll Free: 1-877-842-9021 Fax: (519) 842-3775 Contact: Robin Carroll Website: www.lizcosales.com We have the energy with Canada’s largest on-site directory: • New and Rebuilt Power/Padmount/Dry Transformers • New Oil-Filled “TLO” Unit Substation Transformers • New HV S&C fuses/loadbreaks/towers • High and low voltage: - Air Circuit Breakers – Molded Case Breakers - QMQB/fusible switches – Combination Starters • Emergency Service and Replacement Systems • Design/Build custom Application Systems

Megger 4271 Bronze Way Dallas, TX 75237-1088 USA Tel: 1-800-723-2861 Ext. 7360 (Toll Free)

Tel: 214-331-7360 (Direct) Fax: 214-331-7379 Email: [email protected] www.megger.com Megger is a leading provider of electrical test and measuring equipment for power, industrial, building wiring and communication applications. Its wide range of products extends from equipment to test protective relays and other substation electrical apparatus, to insulation resistance and ground testers. With three manufacturing facilities and sales offices located around the world, Megger is strategically positioned to provide customers with innovative products, hands-on technical assistance and superior service. For additional information, visit our web site www.megger.com.

OPTIMUM ENERGY PRODUCTS LTD. #333, 11979 - 40 St SE Calgary, AB T2Z 4M3 Toll Free (877) 766-5412 Main (403) 256-3636 Fax (403) 256-3431 E-mail: [email protected] Optimum Energy Products Ltd are specialists in Power Quality and Power Metering products. We represent Fluke, AEMC Instruments, Electro Industries, and many other manufacturers. We sell portable PQ instruments for engineers and troubleshooters in many industries. From Plug based voltage disturbance meters to three phase Class A Power Quality instruments. We also supply permanent power and power quality meters for use in residential, commercial and industrial applications. For complete product range and information, please visit our specialty websites: www.PQMeterStore.com www.PowerMeterStore.com www.ElectricityMetering.com www.MyMeterStore.com

Raytech USA 90 C Randall Avenue Woodlyn, PA 19094 Tel: 610-833-3017 Fax: 610-833-3018 email: [email protected] Web: www.raytechusa.com RAYTECH is an employee owned company that specializes in the design and manufacture of precision test equipment for the Electrical Industry. With extensive experience in the design and application of test equipment, RAYTECH offers products that truly meet the needs of the testing industry. Our durable products are used by Manufacturers, Rebuild Shops,

Field Test Crews, Utilities, Rural Electrical CO-OP's, Universities and Research Engineers.

RHCtest.com 610 Ford Drive Suite 248 Oakville Ontario L6J 7W4 Canada Tel : (905) 828-6221 Fax : (905) 828 -6408 Contact : John Riddell E-Mail : [email protected] RHCtest.com Inc. is a Canadian owned and operated Distributor of Electrical Test and Measurement Equipment. We carry various products lines such as Kyoritsu, Thurlby Thandar, Dataq Instruments, Topward Instruments, Nidec Shimpo, High Voltage and Midtronics. We distribute products such as; Multimeters, Voltage Testers, Clamp Meters, Clamp Adapters, Voltage and Current Loggers, Power Loggers, Power Analyzers, Insulation Testers, Earth Resistance Testers, Test leads, DC/AC Hipots, VLF Hipots, TAN Delta Cable Diagnostics, Thumpers, Cable and Fault locating products, Power Supplies, Spectrum Analyzers, RF Generators, DDS Generators, Arbitrary Waveform Generators, Function Generators, LCR Meters, Micro Ohm Meters, Frequency Counters, DMM’s DC Loads, Strobescopes, Hand Held Tachometers, Panel Mount Tachometers, Data Acquisition Starter Kits, Stand Alone Data Loggers, Thermocouple Data Acquisition Systems, DC Connected Data Acquisition Systems and Battery Testers.

SKM Systems Analysis Inc. 1040 Manhattan Beach Blvd. Manhattan Beach, CA 90266 USA Toll Free : 1-800-232-6789 Fax : 1-310-698-4708 E-Mail : [email protected] SKM Power*Tools software helps you design and analyze electrical power systems. Interactive graphics, rigorous calculations and a powerful database efficiently organize, process and display information. Associate projects with multiple one-line diagrams and TCC drawings with customized data fields. Generate better design with 'what if' scenarios by comparing study results in a single table. Also includes thousands of validated equipment libraries and the ability to export project data into AutoCAD DXF and XREF format. Multiple one-line diagrams can be associated with each project for better systems organization and presentation. Powerful drawing tools quickly create a structured, interactive one- line diagram system model.

SKM Systems Analysis, Inc. is a California-based corporation founded in 1972 with a desire to automate electrical design calculations. SKM has been a leader in the electrical engineering software industry for more than 30 years, providing quality software, training and support to thousands of satisfied customers throughout the world. SKM Systems Analysis, Inc. is also chosen by 39 of the top 40 Electrical Engineering firms in the world.

techniCAL Systems 2002 Inc. 436 Jacqueline Blvd. Hamilton, Ontario L9B 2R3 Canada: 1-86-MEASURE-1 (1-866-327-8731) Tel: 905-575-1941 Fax: 905-575-0386 E-mail: [email protected] Web-site: www.technical-sys.com techniCAL provides electrical contractors and utilities with Test, Measurement, Calibration, Control & Recording Instrumentation. Representing Best-of-Breed Manufacturers; techniCAL provides such products as; Power Quality Analyzers, Micro-Ohmmeters, Megohmmeters, Insulation Testers, Leakage Current Meters, Ground Resistance Testers, Data Loggers, High Voltage Ammeters, Power Transducers, Panel Meters, CT’s, PT’s, Shunts, etc…

Electrical Testing Measurement Handbook Vol 7

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