P ET ET RO RO N A S
GA S
TRAINING MODULE
ELECTRICAL
TITLE : MODULE NO :
LOW VOLTAGE SWITCHGEAR/ MCC E04
Capability & Improvement Dept.2004
DUTY NO 07: LOW VOLTGAE SWITCHGEAR & MCC OBJECTIVES
Upon completion of this module, the technician would be able to demonstrate knowledge and understanding on the following: 1. 2. 3. 4. 5. 6. 7.
low voltage electrical distribution system Construction of LV switchgear Functions of components LV switchgear Construction & operation of ACB. Functions of components of ACB Construction & operation of MCC Functions of components of MCC
8. 9.
Assembly drawings, schematic drawings. Preventive maintenance maintenance and testing of LV switchgear
10. 10. 11. 11.
Preventive maintenance maintenance and and testing of MCC. Automatic transfer scheme
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TABLE OF CONTENT Description
Page
1.0.0
Introduction
5
2.0.0
Low voltage power distribution. distribution.
6
2.1.0
The main components
2.2.0
LV switchgear and MCC
3.0.0
Metal enclosed low voltage switchgear
3.1.0
Construction
7 8 8
3.1.1. Structure
8
3.1.2. Bus bars
10
3.1.3. Circuit breakers
11
3.1.4. Protective devices
12
3.1.5. Metering
12
3.1.6. Wiring
13
3.2.0 Low voltage Metal enclosed Switchgear Specifications Switchgear Specifications 4.0.0
7
Air Circuit Breaker (ACB)
14 15
4.1.0
Theory of arc extinction
15
4.2.0
Construction
16
4.2.1. Main circuit and quenching mechanism
17
4.2.2. Operating mechanism
17
4.2.3. Secondary circuit
17
4.2.4. Interlocks and indications
18
4.3.0
5.0.0
Air Circuit breaker ratings
19
4.3.1. Rated operational voltage
19
4.3.2. Rated insulation voltage
19
4.3.3. Rated thermal current
19
4.3.4. Rated uninterrupted current
20
4.3.5. Rated short circuit making capacity
20
4.3.6. Rated short circuit breaking capacity
20
4.3.7. Rated short time withstand capacity
20
4.3.8. Short circuit performance categories
20
Motor starter/ controller
21
5.1.0
Construction
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21
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Description 6.0.0
Molded Case Circuit Breaker (MCCB)
6.1.0
23
6.1.1. Molded case
23
6.1.2. Operating mechanism
23
6.1.3. Trip element
24
6.1.4. Line and load terminals
25
6.1.5. Accessories
25
Ratings
25
6.2.1. Voltage rating
25
6.2.2. Continuous current rating
25
6.2.3. Interrupting capacity rating
25
6.2.4. Ambient temperature rating
25
6.2.5. Frequency rating
26
Isolator and HRC fuses
27
7.1.0
8.0.0
22
Construction
6.2.0
7.0.0
Page
Fuse rating
28
7.1.1. Voltage rating
28
7.1.2. Ampere rating
28
7.1.3. Interrupt rating
28
7.1.4. Current limiting rating
28
Contactors
29
8.1.0
Construction
29
8.2.0
Arc suppression
30
8.3.0
Contactor rating
30
9.0.0
8.3.1. Rated voltage
30
8.3.2. Rated current
30
8.3.3. Rated duty
31
8.3.4. Making capacity
31
8.3.5. Breaking capacity
31
8.3.6. Utilisation category
32
Thermal overload relay
33
10.0.0 Starter circuits and schematic drawing
34
10.1.0 Direct On Line (DOL) starter
34
10.2.0 Star Delta starter
34
10.3.0 Circuit description (Schematic diagram)
35
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Description 11.0.0 Testing and maintenance of Air Circuit Breakers
Page 37
11.1.0 Frequency of maintenance
37
11.2.0 Safety precautions
37
11.3.0 Maintenance procedure
38
11.4.0 Lubrication of circuit breakers
39
11.5.0 General guidelines for lubrication
39
11.6.0 Testing of Air circuit breaker
40
12.0.0 Testing and maintenance maintenance of Motor Control Control Centre C entre (MCC) (M CC)
41
12.1.0 Safety precautions
41
12.2.0 General guidelines
42
12.3.0 Visual inspection of MCC
42
12.3.1. Frame/ enclosure
42
12.3.2. Phase/ Neutral/ Ground bus
42
12.3.3. Bus supports
42
12.3.4. Panel indicators/ instrumentation/ control wiring
43
12.3.5. Cabinets/ cubicles/ interlocks
43
12.4.0 Cleaning of MCC/ starter unit
43
12.5.0 Mechanical checks
43
13.0.0 Testing and maintenance of motor starter/ controller
45
13.1.0 Preventive maintenance on MCCB
45
13.2.0 Routine maintenance tests on MCCB
45
13.2.1. Insulation resistance test
45
13.2.2. Milivolt drop test
45
13.2.3. Connections test
45
13.2.4. Overload tripping test
45
13.2.5. Instantaneous tripping test
46
13.2.6. Mechanical operation
46
13.3.0 Isolator (Disconnect switch) 13.3.1. Fuse holders
46 46
13.4.0 Contactor
46
13.5.0 Protective relays
46
14.0.0 Automatic transfer scheme
48
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1.0.0 Introduction Low voltage switchgear is designed to provide superior electrical distribution, control and protection, for the entire plant. The prime component of the switchgear is the Air circuit breaker. Switchgear is generally designed to maximize the functionality of the circuit breaker/MCC, which, in turn, delivers maximum uptime, system selectivity, and ease of maintenance and circuit protection. Low voltage switchgear is designed to operate within its rated voltage, current, and shortcircuit interrupting capacity. The successful operation of the equipment depends on proper handling, and maintenance. Neglecting the operational and maintenance requirements may lead to personal injury, as well as damage to electrical equipment or other property. Work on the switchgear requires training, experience and an understanding of the hazards involved. This module covers the basic underpinning knowledge required to perform the duties and task prescribed in POSS on LV switchgear/MCC for the electrical technicians. For gaining the expertise in the activities further study and hands on experience is necessary.
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2.0.0 Low voltage power distribution: Power requirement of the industry such as petrochemicals or fertilizers is very large. Therefore the power intake form the supply company/ utility is at high voltage (11 kV or above), but the majority of the load requires low voltage power supply. Hence every petrochemical or fertilizer industry has the typical low voltage power distribution system as shown in fig.1. Utmost importance is given to the reliability aspect of power supply while designing the power supply distribution system. Keeping in mind other qualities of good distribution system such as flexibility in operation and ease of maintenance a duplicate supply distribution system is adopted. Thus there are two feeders, two transformers instead of one for incoming supply. Each feeder and transformer has sufficient capacity to take entire load independently. BUS TIE
HV BUS
VCB VCB
VCB
TRANSFORMER
ACB
ACB BUS TIE
LV BUS
ACB MCCB CONTACTOR O/L RELAY
M
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To LDB
M
M
To LDB
M
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2.1.0 The main components a) HV feeders : These feeders fed the power to the transformer at high voltage (11 kV , 6.6 kV or 3.3kV) b) Transformers : It transform the power to low voltage (430/ 415V) and fed the load through LV switchgear and cables c) Switchgear/ MCC : It performs the basic function of switching and protection of the system d) LV cables : These cables fed the power to the equipment located inside the plant e) Electrical equipment (motors, lighting, power sockets, heaters, UPS, Battery chargers etc.) : These equipment convert the electrical energy to the required form of energy (Mechanical, heat, light etc.)
2.2.0 LV switchgear and MCC: Switchgear is the general term covering wide range of equipment concerned with switching and protection. Switchboard is an assembly consists of circuit breakers, switches, protective relays, instrumentation, and control. Switchgear equipment comes in various forms and rating depending on particular functions it is to perform. Metal enclosed switchgear is most commonly used for low voltage power distribution. MCC consists of motor starters backed up by HRC fuses or Molded Case Circuit Breaker (MCCB). As the low voltage motor starter circuits need small clearance, the several circuits can be conveniently arranged in the individual switchboard compartments. MCC essentially consists of contactor, HRC fuses or MCCB, protective relays, instrumentation, and control.
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3.0.0 Metal-Enclosed Low Voltage Switchgear It is completely enclosed on all sides and top with sheet metal. The assembly contains low voltage circuit breakers, switching or interrupting devices, with buses and connections. It may contain control, measuring, or protective devices. Metal-enclosed low-voltage switchgear includes the following equipment and features: 1) Low-voltage power circuit breakers that are mounted stationary or removable and contained in individual grounded metal compartments. 2) When the circuit breaker is removed, automatic shutters close off and prevent exposure of the primary conductors. 3) Bare bus and connections. 4) Instrument transformers 5) Instruments, meters and protective relays 6) Control wiring and accessory devices. 7) Circuit breakers may be controlled at the switchgear or from a remote point. 8) When the circuit breakers are removable, mechanical interlocks are provided for proper operating sequence Some typical arrangement of metal enclosed low voltage switchgear is shown in fig.2.
3.1.0 Construction There are two basic types of low voltage switchgears, those for use outdoors and those are use indoors. Indoor switchgear is, of course, considerably less expensive. Indoor types of switchgears are most commonly used for Oil & Gas or Petrochemical and Fertilizer industry. The transformers are located outside the switchgear building and connected to the switchgear with bus duct or cables. The following discussion is limited to indoor type of switchgear only.
3.1.1 Structures: The indoor switchgear will consist of a front section, which will contain the circuit breakers, meters, relays and controls; a bus section; and a cable entrance section. Each circuit breaker is isolated from all other equipment. Vents are provided in the circuit breaker compartments, for cooling and to allow escape of gases, which are formed when the circuit breaker opens to interrupt fault currents. Other sections of the switchgear are also ventilated to allow circulation of air for cooling. Barriers between the bus compartment and the cable compartment are provided for safety reasons, to permit connecting or disconnecting the cables without danger of contacting the live bus.
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Rear cable Compartment Bus Compartment
Base channel
Switch ear side view
Auxiliar instrument compartment
Through door circuit breaker
Circuit breaker compartment
Door latch Hinged door
Switchgear front view
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Switchgear rear view
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All cable terminations are insulated after installation, so that there will be no danger of contacting these live terminals when making changes in the cable connection. In double-ended substations the barrier between the two sections of the switchgear is provided. A horizontal barrier is placed between the upper and lower terminals of the tie breaker. A vertical barrier is provided to isolate the two buses. Vertical barrier is so as to prevent an arcing fault on one end of the substations from travelling to the other end and taking out both power sources. When drawout breakers are used, a hoist for removing the breaker is desirable. This may not be required if the overhead crane or other lifting facilities are available in the building.
3.1.2 Bus Bars: Copper is the better material for the bus bars than aluminum, except in few cases where corrosive atmospheres may have an advert effect of copper. Copper has a higher conductivity than
eutral
bus
Horizontal Cross bus
Ground
bus Bus Compartment (Rear view)
aluminum, it is more easily plated and bolted joints can be made more easily. Also the melting point of aluminum is lower than that of copper, so that more damage will be done to aluminum buses in case of arcing fault. However, copper is more expensive than aluminum and most switchgear LV Switch ear & MCC
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manufacturers now use aluminum for bus bars unless copper is specified, in which case they may charge a premium price. Copper joints are normally silver plated. Aluminum bolted joints are normally tin plated. Each vertical section of low voltage switchgear will contain from one to four circuit breakers. This requires branches and tap off the main bus supported on insulators. The insulating materials used should be flame retardant, track resistant and nonhygroscopic. It also must have high impact strength to be able to withstand stresses caused by the magnetic forces when fault occurs. Glass polyester is the best material available for this purpose. The insulators must be assembled to the bus in such a way that there are not continuous horizontal surfaces between bus bars. Such surface may collect dust that may form a high resistance path between the bars, which could develop into a fault. Supports for the bus bars should be placed at frequent intervals so that the bars will not be deformed when a fault occurs.
3.1.3 Circuit Breakers : Breaker with required frame size and the desired trip rating be installed. The interrupting rating is standardized with the frame size of breakers but some make breakers in other than standard frame sizes. For information on the Horizonta
Vertical
interrupting rating of these breakers the concern manufacturer may be consulted. Low Voltage Circuit Breakers may
Secondar y
be obtained with either fixed or drawout type. Switchgear with fixed breakers is less expensive. However, removal of a circuit breaker of the fixed type should not be attempted unless the main bus is deenergised. Therefore, if fixed type is used Circuit breaker compartment
a shut down may be necessary in case of trouble with any one of the circuit breakers,
and it will definitely be required for periodically for routine maintenance. For this reason the extra cost of the drawout type circuit breaker is usually justified. When drawout type circuit breakers are used a method is provided for moving the breaker from the fully withdrawn position to a test position, and to fully connected position. For safety of the operator, it should be possible to move the breaker from one position to another with the breaker compartment door closed and only when the breaker is tripped. Also it should not be possible to close the breaker when it is intermediate position between the test and the connected position. LV Switch ear & MCC
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When the breaker is in fully connected position, it is connected to the source and to the load side terminals, and the frame is connected to the ground bus. This is the position for normal operation. When in the test position, the breaker is disconnected from the source and from the load, but secondary circuits for control of electrically operated breakers, and for monitoring, are connected. In this position all secondary circuits may be tested and the breaker may be closed and tripped, either manually or electrically, without affecting the primary circuits. In the withdrawn position, all circuits to the breakers are disconnected. The frame of the breaker should be connected to ground in both the test and fully connected position. It may or may not be connected in the withdrawn position. Circuit breakers may be obtained either electrically or manually operated. Breakers must be electrically operated if they are to be operated from a remote location or if they are to be used in any automatic transfer scheme. Circuit breaker may be equipped with various auxiliary devices. All electrically operated breakers are equipped with auxiliary switches, some of which are used in the breaker control circuits. Undervoltage devices may be supplied on all the breakers. These devices will trip the breaker when the voltage on the source to which they are connected falls below a certain value. They are usually self- resetting so that the breaker may be reclosed as soon as voltage is restored, and may be obtained to trip the breaker instantly on loss of voltage, or after a time delay.
3.1.4 Protective Devices: Low-voltage circuit breakers may be obtained without overcurrent trip devices (nonautomatic), with magnetic trip devices, or with static trip devices. If supplied without overcurrent trip devices they may be used as a switch, or they may be supplied with a shunt trip device, which is operated through a relay to give desired protection. With this arrangement a source of power must be available for the shunt trip device. Batteries are the most reliable source of power, but control power may be taken from the source, which is feeding the switchgear, usually through a control power transformer. However, it is necessary to make sure that such power is available when needed. The magnetic-type overcurrent device was the standard type used by all manufacturers for many years, and it may still be available from some manufacturers. The device utilizes magnetic forces created by the current through the breaker to trigger the release mechanism, which allows the breaker trip. Various mechanical, hydraulic or pneumatic devices are used to delay the tripping, when desired. All manufacturers today offer static trip devices on their low-voltage large air circuit breakers. These devices are more reliable, and they have characteristic curves with narrow bands and LV Switch ear & MCC
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with shapes designed to match well with curves of fuses, relays, and molded case breakers. These static trip devices are fed by sensors, which are special current transformers, which monitor the current in each phase. The devices receives power from these sensors to trip the breaker when the current in the sensor is greater than the pickup current for which the device has been set. Timing in accordance with the characteristic curves is also accomplished by the electronic circuitry. Low-voltage distribution systems may be solidly grounded, grounded through a resister. The solidly grounded system is used most frequently. When it is used, some form of ground-fault protection should be provided.
3.1.5 Metering: The metering to be specified with low-voltage switchgear depends entirely on the needs for the information which metering can supply. Usually the minimum requirement is for voltmeter on the incoming power source and an ammeter to indicate total current. These instruments are of the single-phase type. They are usually supplied with switches to permit reading current and voltage of all three phases. If the voltage is greater than 240V, potential transformers are required, since standards do not permit voltages on the panels which exceed 250 V to ground. Current transformers are required for all ammeters. Ammeters and voltmeters may be supplied with either 1 or 2% accuracy. For most purposes the 2% instrument is satisfactory. The 1% instrument may be obtained at a moderate increase in price. Test switches or test blocks may be specified to permit plugging in portable instruments. This may be desirable to check the accuracy of the panel instruments or to obtain more accurate readings. Usually meters required for the incoming power will be mounted on a panel above the main breaker. Potential transformers and control power transformers will be mounted inside this compartment. Current transformers may be mounted in the rear, or if a main breaker is provided, they may be in the breaker compartment. Ammeters, and ammeter switches for the feeders can often be mounted on the breaker panel, if the meters are the panel type having 2% accuracy.
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3.1.6 Wiring: Wire used for metering and control circuits should not be smaller than No. 14 AWG. It should be insulated with a material rated not less than 90oC. Crimp-type terminals should be used, and
Vertica
Horizont Control Secondar disconnectin Shutter
Secondar terminal Secondar disconnectin
Secondary wiring system
terminals should be insulated. A detailed wiring diagram is required, showing the related location of terminals on various devices and on terminal blocks. When troubleshooting or making changes, wires can be identified by referring to this wiring diagram. The use of wire markers to identify each wire can be specified.
3.2.0
Low voltage Metal enclosed Switchgear Specifications The standards institutions publish the standard specifications on LV switchgear to cover
wider applications. These standards provide the guideline to the manufacturer for design and construction, whereas it is useful to the user for selection, erection and maintenance. The followings are general specifications prescribed for the LV metal enclosed switchgear. 1) Service : The switchgear suitable for service conditions such as indoor/ outdoor/ temperature 2) Power system : The type of the system, voltage and frequency 3) Metal enclosed assembly : The description of the features required. Following is the example: The assembly consists of breaker compartments with hinged front doors and sheet steel
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enclosure. The enclosure and welded components chemically treated and painted with light gray paint. Provisions for racking the breaker to “connected”, “test” and “disconnected” position with door closed. Mechanical interlocks to prevent the racking of breaker when it is closed. Arrangement of space heaters. Provision for the future expansion. 4) Circuit breakers : Types and specifications of the circuit breakers for the followings: 1. Main (incoming feeder) 2. Bus tie 3. Load (outgoing feeder) It includes the voltage, current ratings, interrupting capacity, type of the mechanism, arc quenching medium, different indicators, auxiliary switch, interlocks, and built in protective devices. 5) Bus : Material, current rating and short time withstand current rating of the main bus and neutral bus. Length of the ground bus. 6) Control power transformer : Type, capacity, voltage ratio, protection 7) Instrument transformer (CT/VT) : Type, voltage/ current ratio, burden 8) Indicating/ recording meters : Type, range, scale reading 9) Wiring : Installation method (metal channel or conduit), material, size and markers on the secondary wiring. 10) Accessories : Crank for manual operation of the breaker drawout mechanism, lifting yoke for each type of breaker element, test plugs/ blocks, test cables.
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4.0.0 Air Circuit Breaker (ACB) Air circuit breakers are most commonly used for low voltage industrial applications. The air at atmospheric pressure is used as an arc extinguishing medium. These breakers are generally indoor type and installed on vertical panels or draw out type switchgear. Air circuit breakers have several advantages:Ø
Less contact erosion from short circuit and load current operation, resulting in greater time between maintenance.
Ø Ø
No insulation handling Fire-risk is minimum
The main disadvantages are:Ø
The need to install them indoors
Ø
High initial cost
Ø
An increase insulation hazard due to free movement of insulation contamination
Ø
Overall dimensions are larger owing to air insulation
Ø
Limited fault clearing ability.
4.1.0 Theory of arc extinction : The resistance of the current path is increased gradually resulting in the increased voltage drop. The arc extinguished when the system voltage can no longer maintain the arc , due to high
Splitter
Arc runners
lates Arc chute
Arc Arcing contacts
Main contacts
Arc extinctio
value of the voltage drop. This principle is used in air break type a.c. circuit breakers. The air at atmospheric pressure is used as an arc extinguishing medium in Air Circuit Breakers. These circuit breaker employ the principle of high resistance interruption principle. In the air circuit breaker the contact separation and arc extinction takes place in air at atmospheric pressure. As the contacts of
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breaker are opened arc is drawn between them. The arc is basically consists of a plasma surrounded by the ionized particles of air. The arc is rapidly lengthened by means of arc runners and arc chutes. The resistance of arc is increased by cooling, lengthening and splitting the arc. The arc resistance is increased to such an extent the system voltage can not maintain the arc and the arc gets extinguished.
4.2.0 Construction : The circuit breaker main components are as follows:
Main circuit (poles) and insulators Arc quenching mechanism Operating mechanism. Secondary circuit (closing, tripping and control) LV Switch ear & MCC
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Interlocks and indication The frame or structure
4.2.1 Main circuit and arc quenching mechanism: There are two sets of contacts: Main contacts and Arcing contacts. Main contact conduct the current in closed position of the breaker. They have lower contact resistance and are usually silver plated. The arcing contact are hard, heat resistant and are usually of copper alloy. While opening the contact the main contact dislodge first. Then the current is shifted to the arcing contacts. The arcing contacts dislodge later and the arc is drawn between them. The arc is forced upwards by the electromagnetic action. The arc moves towards the arc chute where it is extinguished by splitting, lengthening and cooling.
4.2.2 Operating mechanism : The operating mechanism for air-break circuit breakers are generally with operating spring. The closing force is obtained from one of the following means : Solenoid : The solenoid mechanism drive power from battery supply or rectifiers. The solenoid energised by the direct current gives necessary force for closing the circuit breaker. Spring charged manually or by motor : The springs used for closing operation can be charged either by manually or by motor driven gears. At the time of closing the operation the energy stored in the spring is released by unlatching of the spring and is utilised in closing of the breaker. During the opening operation, the operating signal is given to trip coil. The movable system is unlatched and the energy of the opening spring is released to obtain the opening. The closing spring is automatically charged after each closing operation. Hence energy is always available for reclosing of breaker. Both opening and closing operations are initiated by high speed, electromagnetically operated latches.
4.2.3 Secondary circuit : This consists of the followings: Ø
Electrical operating mechanism: This includes the gear motor, a closing release, undervoltage release and spring charged limit switch. Some of the breakers are also equipped with the operation counter. This facilitate to read and record the total no. of breaker operating cycles. The manual mechanism is always available for emergency use. Generally the breaker is fitted with the time delayed undervoltage release. This will not allowed the breaker to close if the voltage is below (typically say, 85% of ) rated voltage. The closed breaker will open if the supply voltage drops below a value typically 35% and 70% of its rated
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voltage. To prevent the breaker tripping in the event of the voltage dips, the release provide the time delay. A closing release or relay is the device which releases the breaker closing mechanism when the spring is charged, Energisation can be maintained, as the closing release provides an antipumping function. Pumping of circuit breaker is alternate tripping and closing if the closing signal is available during a trip operation. After the breaker has been opened, either by fault trip or by manual or electrical operation, anti-pumping is provided by requiring cancellation of the initial closing command before reclosure of the circuit breaker is possible Ø
Auxiliary switches: This provides the additional contacts (NO & NC) to be used in the control/ protection or indication circuits. As a standard two normally open (NO) and two normally closed (NC) are provided. More contacts can be provided on request to the manufacturer.
4.2.4 Interlocks and indications : Various mechanical and electrical interlocks are provided on the breaker for safety. Some of the mechanical interlocks are listed below:
Padlocks for push buttons: This padlock device prevent the direct operation of the circuit breaker by covering the “on” and “off” push buttons. Position padlocking : The circuit breaker is usually provided with the mechanical interlock for “isolate” (disconnect) position. If requested, the breaker can also be provided with the interlocks for all the three position namely, isolate (disconnect), test and service (connect). LV Switch ear & MCC
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Door interlock: This interlock prevents the cubicle door from being opened when the breaker is in the service (connect) position. Racking interlock : This interlock prevents the insertion of the breaker when the breaker cubicle door is open. Withdrawal/ spring charged interlock : This interlock prevents the removal of the breaker when the breaker is closed or closing spring is charged. Shutter lock: The shutter automatically block the access to the disconnecting contacts when the breaker is isolated (disconnected) or in test position. The provision could be made to padlock the shutters when breaker is withdrawn from its cubicle. Electrical interlocks: The contacts of auxiliary switch or releases or protection relays can be used in the control circuit to provide the required electrical interlocks. The breaker are provided with the mechanical as well as electrical indicators. Mechanical interlocks could be to indicate: Ø
The breaker position indicators ♦
Service (connected)
♦
Test
♦
Isolate (disconnected)
Ø
The stored energy mechanism is charged/ discharged
Ø
The breaker is opened
Ø
The breaker is closed
Ø
Fault trip indication
Ø
Electrical indications are provided by mounting the indication lamps on the front door of the panel. The standard indications are as follows: Colour of the lamp Red Green Amber White
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Indication Breaker is closed Breaker is opened Breaker is tripped Breaker trip circuit is healthy
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4.3.0 Air Circuit breaker ratings: 4.3.1 Rated operational voltage: It is the value of the voltage to which the making and breaking capacities and short circuit performance categories refer.
4.3.2 Rated insulation voltage: It refers to the voltage to which the test voltages , clearances and creepage distances refer. It is generally the maximum operational voltage. (for three phase circuits, the rated voltage is the line voltage)
4.3.3 Rated thermal current: It is the maximum value of the a.c.(rms)current or steady value of d.c. current which breaker can carry in eight hour duty without abnormal temperature rise.
4.3.4 Rated uninterrupted current: : It is the maximum value of the a.c.(rms)current or steady value of d.c. current which breaker can carry in an uninterrupted duty without abnormal temperature rise.
4.3.5 Rated short circuit making capacity(*): It is the value of prospective peak current that the circuit breaker is capable of making. 4.3.6 Rated short circuit breaking capacity(*): It is the maximum value of a.c (rms) current that the circuit breaker is capable of breaking. 4.3.7 Rated short time withstand current (*): It is the maximum short-circuit current (rms) that the circuit-breaker can withstand for a short period of time (0.05 to 1 s) without its properties being affected. [ ( *) These currents are defined for a specific operational voltage rating]
4.3.8 Short circuit performance categories: Category P-1 P-2
Operating sequence for short circuit tests O - t - CO O - t - CO - t - CO
O - Breaking operation t - Specific time interval CO - Making operation followed by breaking
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5.0.0 Motor Starter/ Controller The motor starters/ controllers range from a simple toggle switch to a complex system using contactor, relays, and timers. The basic functions of a starter is to control and protect the operation of a motor. This includes starting and stopping the motor, and protecting the motor from overcurrent, undervoltage, and overheating conditions that would cause damage to the motor. There are two basic categories of motor starters: the manual and the magnetic. As the manual starter is seldom used in the plants, is not discussed here. A large percentage of control applications require that the motor starter to be operated from a remote location or operate automatically in response to control signals.
5.1.0 Construction: The basic parts of the motor starter are: Molded case circuit breaker (MCCB) or isolator with HRC fuses Magnetic Contactor Protection relay (Thermal Overload relay) Indicating instruments/ lamps All these components are connected electrically to each other to perform the basic function of the starter. A simple Direct On Line (DOL)starter circuit is shown below: This circuit can be divided functionally in two parts. 1) Power circuit and 2) Control circuit. The Power circuit contains all of the components that carry the full voltage and current to operate the motor. Besides the magnetic contactor, these commonly includes isolator-HRC fuses or MCCB and heater elements of the thermal overload relay. The control circuit is usually operated at lower voltage and contains all the components necessary to switch and monitor power to run the motor or to stop the motor under the predetermined condition and time. These commonly includes the devices like push buttons, protection relays, indicating devices, contacts of process instrument relay.
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6.0.0 Molded Case Circuit Breaker Molded case circuit breaker is used for overcurrent protection at the entrance of the starter circuit. The MCCB in the motor circuit provides the protection to that branch of the distribution system only. Like any other breaker its function is to make, break and carry the current in normal and abnormal or fault condition without any damage.
Line terminal
Arc chute Operating Knob
Moving Contact Quick make Quick break mechanis
Molded housing
Common tri bar
Magnetic trip ad ustment
Thermal trip ad ustment
Load terminal
The main difference in operation of ACB and MCCB is that MCCB can be closed manually and not automatically. But it can open automatically at a predetermined overcurrent. MCCB are available with thermal-magnetic protection or magnetic protection only. As most of the motor starter are equipped with thermal overload protection, for motor circuit MCCB with magnetic (overcurrent) protection only is preferred.
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6.1.0 Construction: Most common components of any MCCB are: the molded case, an operating mechanism, trip element, line/ load terminals and optional accessories such as shunt trip, undervoltage trip, auxiliary switch etc.
6.1.1 Molded case: The function of the molded case is to provide an insulated housing to mount all of the circuit
breaker components. The case is molded from a phenolic material which combines high dielectric strength with ruggedness. Following factors determines the strength of the casing: Maximum current, voltage and interruption capacity. Higher the rating, stronger the case.
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6.1.2 Operating mechanism: Most of the multi-pole breakers have a single operating handle. A circuit can be connected or disconnected using a circuit breaker by manually moving the operating handle to the ON or OFF position. All breakers, with the exception of very small ones, have a linkage between the operating handle and contacts that allows a quick make (quick break contact action) regardless of how fast the operating handle is moved. The handle is also designed so that it cannot be held shut on a short circuit or overload condition. If the circuit breaker opens under one of these conditions, the handle will go to the trip-free position. The trip-free position is midway between the ON and OFF positions and cannot be re-shut until the handle is pushed to the OFF position and reset. (Trip-free feature: If the breaker closing signal is present, the operating mechanism will start closing the breaker. Meanwhile if a protection relay closes the trip circuit and energises the trip coil. The trip-free mechanism will permit the tripping (opening) of the breaker, even if it is under the process of closing.) When the separable contacts of an air circuit breaker are opened, an arc develops between the two contacts. Different manufacturers use many designs and arrangements of contacts and their surrounding chambers. The most common design places the moving contacts inside of an arc chute. The construction of this arc chute allows the arc formed as the contacts open to draw out into the arc chute. When the arc is drawn into the arc chute, it is divided into small segments and quenched. This action extinguishes the arc rapidly, which minimizes the chance of a fire and also minimizes damage to the breaker contacts. Push-To-Trip is a another feature available for the MCCB. This permits the operator to manually trip the circuit without exposing the operator to live parts.
6.1.3 Trip element: Molded case breakers may have a thermal and magnetic trip units. MCCB used in motor starter circuits have a magnetic trip element only as the thermal protection is provided by a separate relay installed in starter circuit. Magnetic trip is an instantaneous trip. It is the part of a trip unit which contains an electromagnet assembly to trip the circuit breaker instantaneously at or above a predetermined value of the current. Usually each pole of the breaker has the magnetic trip element. This element responds to a given value of overcurrent. The magnetic unit utilizes the magnetic force that surrounds the conductor to operate the circuit breaker tripping linkage as LV Switch ear & MCC
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shown in fig. MCCB with fixed and adjustable type magnetic trip elements are available. The adjustment will set all the poles simultaneously for same value of tripping current. The adjustment is from approximately 5-10 times the breaker continuous current rating.
6.1.4 Line and Load terminals : All the circuit breakers have provisions for making line and load connections in the external electrical circuit. For industrial application, compression type lugs or mechanical type lugs are provided to facilitate these connections.
6.1.5 Accessories: A wide range of accessories are available for MCCB to suit it for particular application. Some of them are discussed below. Shunt trip: This is the mechanism operated by a solenoid when energised from separate source. It, in turn, will trip breaker. This solenoid circuit can be closed by an external relay or other means. A continuous flow of current through shunt trip coil may damage it because it is not rates for that continuous current. Therefore the coil clearing switch is included to break the solenoid circuit when the circuit breaker opens. Undervoltage trip: It is a device which trips the circuit breaker automatically when the main circuit voltage falls below 35-70% of its specified value. The breaker could be closed only if the 85% of rated voltage is available. An undervoltage trip with adjustable time delay unit is available to avoid nuisance tripping of breaker in the event of voltage dip or momentary fluctuations. Auxiliary switch: It is the mechanically operated switch by the breaker itself. They have normally open (NO) or normally closed (NC) contacts. It is used for signaling, interlocking and indicating contact position. Padlocking attachments: These attachment for the padlocking of breaker in OFF an/or ON position for safety reason. They do not interfere with the tripping of the breaker.
6.2.0 Ratings: 6.2.1 Voltage rating: A circuit breaker can be rated for either alternating current (AC) or direct current (DC) system application or both. It is the maximum system voltage on which they can be applied. While selecting the MCCB voltage one must consider the maximum voltage that could exist on the system. Some of the typical ratings are : AC- 120, 240, 480 and 600 Volts and DC- 125, 250 and 600 Volts
6.2.2 Continuous current rating: The maximum direct current or rms current in amperes at rated frequency which breaker should carry continuously without exceeding the specified temperature rise limit of any of its parts. LV Switch ear & MCC
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Breaker must carry this current indefinitely at rated ambient temperature in free air. The typical current ratings are: 100, 225, 400, 600, 800, and 2,000 amps
6.2.3 Interrupting capacity rating: The ampere interrupting capacity of a MCCB is the highest current at rated voltage that breaker is intended to interrupt under standard conditions. When the MCCB is rated for more than one voltage, the interrupting rating is specified for each voltage level.
6.2.4 Ambient temperature rating: The ambient temperature rating of a breaker is the temperature at which its continuous current rating is based. The typical reference ambient temperature for most of the Asian countries is 400C. The manufacturer’s application guide must be referred if an ambient temperature higher or lower than the rating.
6.2.5 Frequency rating: The standard rated frequency in Malaysia is 50Hz. If a MCCB rated 60Hz is applied it may affect the short circuit characteristic of the breaker. In that case the manufacturer’s application guide must be referred.
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7.0.0 Isolator and HRC fuses Most of the low voltage industrial motor control systems use either isolator-fuse (switch-fuse unit) or molded case circuit breakers to rapidly cut power to the system when overcurrent (short circuit fault) occur. An isolator (disconnect) is normally the first device in a motor branch circuit. A single operating handle mounted on the front of the enclosure makes and breaks power to all lines supplying power to the motor controller when repair or maintenance must be performed. Isolators have interlocks which prevent the enclosure door from being opened unless the isolator is in the off position. Once opened, power cannot be turned on without defeating the interlock. DO NOT DEFEAT THE INTERLOCK to re-energize the starter unless you are a qualified and authorised
person capable of working safely around energized circuits. Fuses are especially effective at interrupting overcurrent faults, even up to 200,000 amps. These faults must be interrupted within a small fraction of a second to prevent damage to the motor control system. Isolators are installed upstream of fuses so the fuses can be replaced safely. Fuses contain thin strips of metal which conduct the same current that runs the motor. These fusible elements melt when current exceeds acceptable levels. The size, design and composition of the fusible elements determine the current level at which they melt. The filler of the fuse, often quartz sand, helps to suppress the arc, dissipate the heat and speed up the process of interrupting the circuit. While many fuses respond almost instantly, most fuses used in motor control are dualelement time-delay fuses. They are designed to pass the high inrush currents drawn while a motor is starting. Time-delay fuses prevent nuisance fuse-blowing while still maintaining close protection for the system after the motor is running. They are often sized 15 percent above the full-load amperage of the motor. Single-element or one-time fuses are also common in motor control, but they must usually be sized at 300 percent of the FLA of the motor in order to pass inrush current. Protection for the system is substantially reduced. The fuse possess inverse time-current characteristics. It means that fuses work faster for high-level faults and more slowly for low-level faults. For example, the 100-amp fuse described by this curve will blow in five minutes at 160 amps, one second at about 1100 amps but well under onehundredth of a second at 5,000 amps. Since it is current-limiting, this fuse will blow much faster under very high faults, well under four thousandths (.004) of a second when subjected to a 200,000amp fault. These characteristics make fuses ideal for interrupting high-level faults, so their use is very common in circuits that have high available fault currents. However, their slower response at lower LV Switch ear & MCC
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fault currents means that the current that gets past the fuse before it blows - the let through current may be too great in some applications. That's one of the reasons MCCBs are often used in motor control systems.
7.1.0 Fuse Ratings There are four important ratings to consider when choosing replacement fuses: voltage, amperage, interrupt capacity and current-limiting ability.
7.1.1 Voltage ratings Voltage ratings of motor control fuses are usually 250 or 600 volts. The rating must match or exceed the voltage of the circuit where the fuse is used. Most 250-volt fuses are smaller than most 600-volt fuses, so fuses with different voltage ratings are usually not interchangeable. Some of them are, however -especially control fuses - so you should never assume the voltage is correct just because the fuse fits in the fuse clip.
7.1.2 Amperage rating Current-carrying capacity of fuses varies from an eighth of an amp to 600 amps. Fuses of the different amperage ratings can be installed in the same fuse base again, don't assume the amperage is correct just because it fits in the fuse base. For example, one manufacturer's 600-volt fuses can all be swapped with each other in the 35-amp to 60-amp range. While physical size does not prevent the mis-application of fuses, it does make gross mis-application less likely. This is important because undersized fuses will blow too easily while oversized fuses may not provide sufficient protection.
7.1.3 Interrupt capacity This is the total current which the fuse can interrupt without being damaged. Many fuses today have interrupt capacities as high as 200,000 amps because many industrial facilities have available fault currents that high. The typical common interrupt ratings are 50,000 and 100,000 amps.
7.1.4 Current limiting ability It is a measure of how much current is "let through" the system. Even a single cycle of a 200,000-amp current can severely damage motor control equipment. Current-limiting fuses must act much faster when currents are so high. The current-limiting ability of the fuse is expressed as a number, "K1" or "K5." K1 is more current limiting than K5.
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8.0.0 CONTACTORS The contactor is the central component of motor starter. During its useable life, it may switch power to the motor on and off thousands of times, or even millions of times.
A contactor, by definition, is a device capable of making, carrying and breaking currents under normal circuit conditions including operating overload conditions; the speed of make and break being independent of the operator. In simple language a contactor can be described as a special switch suitable for frequent remote operations. Contactors when used in motor circuit are usually backed up by MCCB or HRC fuses for overcurrent protection. The contactors can be classified into different categories according to its operating mechanism such as electro-magnetic contactor, pneumatic contactor, electro-pneumatic contactor and latched contactor and according to arc quenching medium, such as airbreak contactor and oilimmersed contactor. In line with the present day tendency to eliminate the use of oil., modern designs of contactors are of airbreak type. Most commonly used contactors are electromagnetic type. Hence all further discussion refers the electromagnetic contactor only.
8.1.0 Construction: Contactors have three basic parts: a set of stationary contacts, a set of movable contacts and an electromagnet. One side of the set of stationary contacts is wired directly to the power source while the other side is wired directly to the motor. A gap separates the two sides, keeping power from reaching the motor. The motor will not operate until the gap between the stationary contacts is bridged by the movable contacts.
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The movable contacts are mounted on a spring-loaded armature assembly. When the electromagnet is energized, the armature shifts the movable contacts into position across the gap between the stationary contacts, and power flows to the motor. When the electromagnet is deenergised, the armature assembly is released, the movable contacts return to their normal position, and the motor stops. The electromagnet is energized by a coil, powered through a separate circuit, the control circuit. When power flows through the control circuit, the coil is energized, the magnet attracts the armature, the movable contacts bridge the gap between the stationary contacts, and the motor is energized. In addition to the main power contacts, most contactors have sets of auxiliary contacts that are actuated as the armature closes. These contact sets are often used as seal-in contacts or as contacts for powering indicating lights or other parts of the circuit. The contacts are usually easily removed, so they can be changed from normally-open to norm ally-closed. Many contactors will also accept contact blocks with multiple auxiliary contact sets that may be configured in various combinations, such as normally-open, normally closed, on-delay and off-delay.
8.2.0 Arc Suppression Damage to contacts is most often caused by electrical arcs. They can develop whenever the circuit is made or broken. Arcs can develop at any voltage, usually when current is one ampere or more. Each arc vaporizes a little of the metal surface of the contact and heats up the contact pad, the mounting base, and the contactor. Suppressing arcs is an important part of making them work better. Arcs are usually suppressed with conductive arc chambers which cool the arc environment; arc chutes which divide the arc into smaller segments; arc horns which stretch the arc out In AC contactors, the arc will be automatically extinguished as current drops through the zero point if the arc environment has cooled enough to prevent re-ignition as the current rises again. Arc chambers and chutes are often sufficient arc suppression mechanisms for AC contactors.
8.3.0 Contactor Ratings 8.3.1 Rated Voltages -Rated operational voltage for three phase contactors. It is the rated voltage between phase. -Rated insulation voltage It is the voltage to which the dielectric rests, creepage distance are referred.
8.3.2 Rated Current -Rated thermal current: It is the maximum current the contactor can carry on eight-hour duty without the temperature rise exceeding the permissible limits. LV Switch ear & MCC
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-Rated operational current of a contactor is stated by the manufacture by taking into account the rated frequency, operational voltage, rated duty and utilization category.
8.3.3 Rated Duty (a) Eight hour duty. Contactors carry steady normal current for more than eight hours until the maximum temperature rise is attained. The rated thermal current of the contactor is determined on the basis of Eight hour duty. (b) Uninterrupted duty. Contactor remains closed without interruption for a prolonged time
(more than eight hours, weeks, months, and years). The dust, dirt, oxide coatings on contacts lead to progressive heating.
(c) Intermittent duty. Duty in which the contactors remains closed for periods having a
definite relation with no load period, both no load and load periods are too short to allow thermal equilibrium. In intermittent duty, the contactor is made on and off in such duration that the thermal equilibrium is not reached. For example In an intermittent duty, current of 200 A flows for four minutes in every fifteen -minutes. This intermittent duty may be stated as : 'Intermittent Duty : 200 A, 4 min 1/15 min. or as 'Intermittent Duty : 200 A, 4 operating cycles per hour,
8.3.4 Making Capacity. Rated making capacity of a contactor is the value of the current under steady condition which the contactor can make without welding or excessive erosion of contacts or display of flame. The rated making capacity of an a.c. contactor is expressed in terms of r.m.s. value of symmetrical component of current.
8.3.5 Breaking Capacity. The rated breaking capacity of a contactor is the value of current which the contactor can break without excessive erosion of contacts or display of flame. For a.c. contactors, the rated breaking capacity is expressed by r.m.s. value of symmetrical component of current.
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8.3.6 Utilization Categories of Contactors Category
Applications
AC-1
Non-inductive or slightly inductive loads, resistance furnaces
AC-2
Slip ring induction motors : Starting, plugging*
AC-3
Squirrel-cage induction motor : Starting, switching off
AC-4
Squirrel-cage motor : Starting, plugging, inching**
DC-1
Non-inductive and slightly inductive loads.
DC-2
Shunt-motors Starting, switching off
DC-3
Shunt motors Starting, plugging, inching
DC-4
Series motors Starting, switching off
DC-5
Series motors Starting, plugging, inching.
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9.0.0 Thermal overload relay This relay protects the motor from excessive current by monitoring current going into the motor. When current is too high for too long, current to the coil is interrupted and the motor shuts down. Most common type of thermal overload relay is “Bimetallic”
Trip Bar Circuit closed
Contact closed
Bimetal element
Circuit tripped
Contact Opened
In this type of relay a bimetal strip is employed for each pole. It comprises two metals, having different thermal co-efficient of expansion, firmly joined together. One end of strip is firmly held, whereas other end is free. When heated, the strip bends on account of different expansion of bimetal. The distance moved by the free end is proportional to the heat. The free end bends far enough to actuate a tripping mechanism which opens a set of the contacts in the control circuit. That interrupts the power to the coil of the contactor which opens the power contacts and stop the motor. In case of larger motor, the relay are connected in the secondary circuit of CT. Bimetal relay can usually be set in certain range. Most of them are provided with additional bimetal strip to enable ambient temperature compensation. The relay can be self resetting type or hand resetting type. In the latter, the trip mechanism locks itself in operated condition until reset mechanically.
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10.0.0 Starter circuits and schematics 10.1.0 Direct-on-line (DOL) starters With this type of starter, the stator windings of the motor are connected directly to the three phase mains supply. The motor starts and accelerates in a way determined by its own characteristics. Typically, the peak starting current is between 5 and 8times normal fullload current, and the peak starting torque is between0.5 and 1.5 times the motor’s nominal operating torque.
10.2.0 Star-delta starters This type of starter may only be used where access is possible to both ends of all three stator windings. In addition, the windings must be rated to withstand the full supply voltage when delta-connected. With star-delta starting, the peak starting current is typically between 1.5 and 2.6 times the normal full-load current, and the peak starting torque is between 0.2 and 0.5 times the motor's nominal operating torque. 9 On starting, the supply is first applied to the motor
with its stator windings star-connected. As the motor accelerates, its speed stabilises when its developed torque become equal to its load torque. This usually happens at about 75% - 80% of nominal speed. The star contactor is then de-energised, and the delta contactor energised to delta connect the stator windings. Each winding is now fed with the full supply voltage, and the motor adopts its normal operating characteristics. The run-up time with the windings starconnected is controlled by a timer which, typically, can be adjusted from 0 to 30 seconds. This timer is adjusted during commissioning to ensure that the star-delta changeover occurs, as closely as possible, at the point of torque equilibrium. LV Switch ear & MCC
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The typical schematic diagram of a DOL starter is shown in fig.
SCHEMATIC DIAGRAM FOR 415V MOTOR 415V 3PH. 50HZ REMOTE LINK FOR R
Y
B
N
A
CONNECTION TO
MCP
REMOTE AMMETER
Z CT T1
M
T2 T3 C
O/L
ST
CF
NL ELR MC P
ELR
ST RS O/L
STOP P/B
STOP
START
AUTO STOP
HR C
IR P
A
R C-1
LCS
C- 2
RS-1
G RS-2
A
10.3.0 Circuit description: The dark lines drawn indicate power circuit. Power circuit has MCCB, Electromagnetic contactor and heater elements of bimetallic thermal overload relay. Power is drawn from the LV bus and flows to the motor through MCCB and closed contacts of contactor, when the power circuit is energised and closing signal to the contactor is given. The control circuit has fuse, earth leakage relay, coil of contactor, auxiliary relay, hour-run meter, indicating lamps, start-stop push buttons and shunt trip solenoid of MCCB. After the energisation of power circuit, if the start pushbutton is momentarily pressed then the contactor coil is energised through series of contacts. The contact C1 of the contactor seal-in the start push button and contactor coil ( C ) remain energised even if start push button is released. The hour-run meter and red indicating lamp will get power supply as they are connected in parallel with the coil. The motor can be stopped by pressing the stop or emergency stop push button, thus breaking the circuit of coil ‘C’ at the same time green lamp will illuminate indicating motor has stopped. If the overload relay senses the excess current in the main (power) circuit, it will open its contact ‘O/L’ after a prescribed time delay and the coil ‘C’ will deenergise resulting opening of the LV Switch ear & MCC
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contactor. Therefore the motor will be stopped automatically. The operation of the overload relay will also energise the auxiliary relay ‘RS’ and its contact RS-2 will close. This will illuminate the amber lamp on the MCC panel. Thus indicating motor has tripped on overload. Earth leakage relay (ELR) will sense the unbalance in current due to earth fault through zero current transformer (ZCT) and will operate to close the contact in the branch where shunt trip solenoid is connected. Shunt trip will energise and open the MCCB to isolate the motor. Opening of MCCB will result in opening of its contact in series with shunt trip solenoid and thus the solenoid will denergised and will not be damaged.
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11.0.0 Testing and Maintenance of Air Circuit Breaker The maintenance of circuit breakers deserves special consideration because of their importance for routine switching and for protection of other equipment. Electric transmission system breakups and equipment destruction can occur if a circuit breaker fails to operate because of a lack of preventive maintenance. The need for maintenance of circuit breakers is often not obvious as circuit breakers may remain idle, either open or closed, for long periods of time. Breakers that remain idle for 6 months or more should be made to open and close several times in succession to verify proper operation and remove any accumulation of dust or foreign material on moving parts and contacts.
11.1.0 Frequency of maintenance Low-voltage circuit breakers operating at 600 volts alternating current and below should be inspected and maintained very 1 to 3 years, depending on their service and operating conditions. Conditions that make frequency maintenance and inspection necessary are: a. High humidity and high ambient temperature. b. Dusty or dirty atmosphere. c. Corrosive atmosphere. d. Frequent switching operations. e. Frequent fault operations. f. Older equipment. A breaker should be inspected and maintained if necessary whenever it has interrupted current at or near its rated capacity.
11.2.0 Safety precautions Following basic safety precautions should be taken be fore conducting any work on the circuit breaker Ø
NEVER work alone.
Ø
Obtain all necessary permits/ certificates required to perform the task/s.
Ø
Switch off all power supplying circuit breaker before working on it or inside its cubicle.
Ø
Always practice lock-out tag-out procedures.
Ø
Always use a properly rated voltage sensing device to confirm that all power is off.
Ø
Beware of potential hazards, wear personal protective equipment and take adequate safety precautions.
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Ø
Open circuit breakers and discharge all springs before performing maintenance work, disconnecting, or removing.
Ø
If the circuit breaker is to be left with it is in withdrawn position, both the bus bar and the circuit shutters must be padlocked, if they are not confirm dead.
Ø
If the circuit breaker is to be left in any position other than in service position, a warning notice must be displayed prominently.
Ø
When switchgear is isolated make sure it is earthed accordingly via integral earthing mechanism or earthing truck or by earthing/discharge stick with warning notice at both ends.
Ø
Verify that no tools or installation equipment are left inside the switchgear before turning on power to this equipment. Conduct electrical testing to verify that no short circuits were created during maintenance, or inspection.
Ø
Never insert a circuit breaker into a cubicle that is not complete and functional.
Ø
Replace all devices, doors, and covers before turning on power to the breaker.
Ø
The purpose of the isolation/de-isolation must be recorded in the substation daily logbook.
. ( Do n ot attempt to defeat any in terl ock and r efer to I nstru ction M anual wh en i n doubt)
11.3.0 Maintenance procedure Manufacturer's instructions for each circuit breaker should be carefully read and followed. The following are general procedures that should be followed in the maintenance of low-voltage air circuit breakers: a) An initial check of the breaker should be made in the TEST position prior to withdrawing it from enclosure. b) Insulating parts, including bushings, should be wiped clean of dust and smoke. c) The alignment and condition of the movable and stationary contacts should be checked and adjusted according to the manufacturer's instruction. d) Check arc chutes and replaces any damaged parts. e) Inspect breaker operating mechanism for loose hardware and missing or broken cotter pins, etc. Examine cam, latch, and roller surfaces for damage or wear. f)
Clean and relubricate operating mechanism parts such as pins, bearings, and the wearing surfaces of cams and rollers, etc.; with a recommended lubricant or equivalent.
g) Set breaker operating mechanism adjustments as described in the manufacturer's instruction book. If these adjustments cannot be made within the specified tolerances, it may indicate excessive wear and the need for a complete overhaul. h) Replace contacts if badly worn or burned i)
Inspect wiring connections for tightness.
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j) Check after servicing circuit breaker to verify the contacts move to the fully opened and fully closed positions, that there is an absence of friction or binding, and that electrical operation is functional.
11.4.0 Lubrication of circuit breakers A lack of lubrication continues to be a major factor in circuit breaker failures. Choosing and applying the correct lubricant to circuit breakers, while often overlooked, plays an essential role in the proper maintenance of electrical distribution equipment. Circuit breakers are sophisticated electromechanical devices. They are comprised of a complex combination of sliding and rotating parts, with various loads, mating surfaces and conductive and isolating materials. Circuit breakers are often required to operate in difficult environments, and are subject to a wide range of temperatures, moisture, dust, particulate, abrasive materials and corrosive atmospheric components. Ironically, a breaker that is in service is in a static condition. Only when an overcurrent situation arises are a circuit breaker’s moving parts required to move. The function of a lubricant is simple - reduce friction between moving metal surfaces. A lubricant coats surfaces and resists being displaced by pressure, keeping the metal parts separated. Lubricants also prevent corrosion, block contaminants and can serve as a coolant. A good lubricant flows easily under pressure and remains in contact with moving surfaces. It does not leak out from gravitational or centrifugal forces, nor does it stiffen in cold temperatures. There are several types of lubricants: Oils (Mineral and synthetic) Greases (Mineral and synthetic) Solid lubricants ( usually fine powders, such as Molybdenum Disulfide, and Teflon)
11.5.0 General guidelines for lubrication of ACB Circuit breaker lubrication recommendations should include three specifications: 1) the location of lubrication points, 2) the products to use for specific locations, and 3) terms of maintenance and overhaul. An equipment manufacturer's lubrication specifications, usually found in the maintenance section of the equipment operator's manual, should be followed without exception. However, you may find that the recommended lubricant is no longer available. In this case, the specific application should guide the selection of a substitute lubricant. Working temperature range is another important consideration. Because petroleum oils cannot handle temperature extremes, synthetic lubricants must be given preference where extreme LV Switch ear & MCC
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temperature could be a factor of operation. Synthetic lubricants last longer, which is an advantage over petroleum lubricants, particularly with today's requirements for extended service without maintenance. Be sure to remove all traces of the old lubricant before you apply a new lubricant product for the first time. Use a commercial cleaner - kerosene, mineral spirits, etc. If possible, soak the part in the solvent and use a soft-bristled brush to loosen the old lubricant. After cleaning, parts should be dried carefully and re-lubricated as soon as possible. While penetrating oils are useful for loosening stuck parts, they should not be used as a lubricant in electrical equipment because they attack, dissolve and wash out factory-installed lubricants, hastening equipment failure. Also, penetrating oils are flammable and should not be applied in areas where sparks or arcing may occur. Electrical contacts should not be lubricated with metal filled lubricants unless tested and proved to be effective long term. Many metal-filled lubricants can accelerate corrosion, create conductive paths and eventually cause failure. It is better to avoid graphite and Molybdenum Disulfide containing lubricants for electrical contacts, because it could cause a resistance rise after multiple operations. For switches that operate infrequently, keeping the contact just clean and dry with no lubricant might be a viable option.
11.6.0 Testing of ACB 11.6.1 Insulation resistance test – A megohmmeter may be used to make tests between phases of opposite polarity and from current-carrying parts of the circuit breaker to ground. A test should also be made between the line and load terminals with the breaker in the open position. Insulation resistance of control circuit, trip circuit and protection circuit should also be measured. Resistance values below 1 megohm are considered unsafe and the breaker should be inspected for possible contamination on its surfaces.
11.6.2 Contact resistance test – The d.c. resistance of each pole of the circuit breaker is tested with the breaker closed. The resistance across terminals of each pole may be measured by means of micro-ohm-meter. The contact resistance is of the order of a few tens of micro-ohms. Manufacturer’s manual may be referred for the recommended value and adjustments, if required.
11.6.3 Function test: The control and protection circuit of the breaker must be read and understood before carrying out the test. The circuit breaker is racked to the “Test” position. Control and protection circuit fuses are reinstated. Closing and tripping tests are carried out by giving appropriate signals. Various
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faults/abnormal conditions are simulated by shorting the relevant protection relay contact and tripping of circuit breaker is observed
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12.0.0 Testing and Maintenance of Motor Control Centre (MCC) Preventive maintenance is a scheduled periodic action that begins with the installation of the equipment. At that time, specific manufacturer’s instruction literature should be consulted, then stored for future reference. Follow-up maintenance should be at regular intervals, as frequently as the severity of duty justifies. Time intervals of one week, or one month, or one year may be appropriate, depending on the duty. It is also desirable to establish specific checklists for PM on MCC/Starter unit, as well as a logbook to record the history of incidents. Care must be exercised to comply with local, state, and national regulations, as well as safety practices of the operating unit. Authorized personnel may open a unit door of a motor control center (MCC) while the starter unit is energized. This is accomplished by defeating the mechanical interlock between the operating mechanism and the unit door. When servicing and adjusting the electrical equipment, refer to the applicable drawings covering the specific motor control center (MCC) and any other related interconnection drawings. Follow any instructions, which may be given for each device. Followings are the general guideline for the preventive maintenance. These instructions may not cover all details, variations, or combinations of the equipment, its installation, safe operation, or maintenance.
12.1.0 Safety precautions: Maintenance of control components requires that all power to these components be turned OFF by opening the branch circuit isolating means and withdrawing the unit to the partial
isolation position or removing the unit entirely from the MCC. When units are fully inserted into the MCC, the line side of each isolator is energized. Do not work on fixed units unless the main isolator for the MCC is OFF. When working on portions of a branch circuit remote from the MCC,
lock the disconnect means for that circuit in the OFF position to positively lock the operating mechanism in the OFF position. Separate control sources of power must also be disconnected. If control power is used during maintenance, take steps to prevent feedback of a hazardous voltage through a control transformer. Be alert to power factor correction capacitors that may be charged. Discharge them before working on any part of the associated power circuit.
Current transformer primaries must not be energized when secondary is open circuited. Short all CT secondaries Soot or stained areas (other than inside arc chutes), or other unusual deposits, should be investigated and the source determined before cleaning is undertaken. LV Switch ear & MCC
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12.2.0 General guidelines: The whole purpose of maintaining electrical equipment can be summarized in two rules: a. Keep those portions conducting that are intended to be conducting. b. Keep those portions insulated that are intended to be insulated. Good conduction requires clean, tight joints, free of contaminants such as dirt and oxides. Good insulation requires the absence of carbon tracking and the absence of contaminants such as salt and dust, which may absorb the moisture and provide an unintended circuit between points of opposite polarity.
12.3.0 Visual inspection of MCC 12.3.1 Frame/Enclosure Ensure that the nameplate/label data is legible. Inspect the overall exterior for missing screws, bolts, nuts, fasteners, retainers and keepers. Inspect for unused openings. Inspect for improper covers. Inspect for rust and corrosion. Inspect main lugs for signs of overheating and missing and defective parts. Inspect insulation structure for signs of overheating and deterioration. Inspect for proper alignment of each section. Check that cabinets are plumb and square. Record results on appropriate Inspection and Test Form.
12.3.2 Phase/ Neutral/ Ground Bus Inspect for signs of overheating. Inspect for rust and corrosion. Inspect for missing and defective parts. Inspect all connection points. Inspect insulation structure for signs of overheating and deterioration. Inspect for loose connections. Record results on appropriate Inspection and Test Form.
12.3.3 Bus Support Inspect for signs of overheating. Inspect for signs of deterioration. Inspect for chips, cracks and broken insulators. Record results on appropriate Inspection and Test Form.
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12.3.4 Panel Indicators/ Instrumentation/ Control Wiring Check all lens covers. Check all light bulbs for operation. Check all function switches. Inspect all meters. Inspect all control wiring. Inspect for signs of deterioration. Inspect for signs of overheating. Inspect for loose connections. Check all interconnecting wiring terminal blocks. Verify accuracy and legibility of all applicable wiring schematics and drawings. Record results on appropriate Inspection and Test Form.
12.3.5 Cabinets/Cubicles/ Interlocks Check all cabinets for interlock function. Check all pad lock systems. Check operation of all racking mechanisms. Record results on appropriate Inspection and Test Form.
12.4.0 Cleaning of MCC/ Starter units Vacuum or wipe clean all exposed surfaces of the control component and the inside of its enclosure. Starter unit may be blown clean with compressed air that is dry and free from oil. If air blowing techniques are used, remove arc covers from contactors and seal openings to control circuit contacts that are present. It is essential that the foreign debris be removed from the control center, not merely rearranged. Control equipment should be clean and dry. Remove dust and dirt inside and outside the cabinet without using liquid cleaner. Remove foreign material from the outside top and inside bottom of the enclosure, including hardware and debris, so that future examination will reveal any parts that have fallen off or dropped onto the equipment. If there are liquids spread inside, determine the source and correct by sealing conduit, adding space heaters, or other action as applicable.
12.5.0 Mechanical checks of MCC/Starter units Tighten all electrical connections. Look for signs of overheated joints, charred insulation, discolored terminals, etc. LV Switch ear & MCC
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Mechanically clean to a bright finish (don’t use emery paper) or replace those terminations that have become discolored. Determine the cause of the loose joint and correct. Be particularly careful with aluminum wire connections. Wires and cables should be examined to eliminate any chafing against metal edges caused by vibration that could progress to an insulation failure. Any temporary wiring should be removed, or permanently secured and diagrams marked accordingly. The intended movement of mechanical parts, such as the armature and contacts of electromechanical contactors, and mechanical interlocks should be checked for freedom of motion and functional operation.
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13.0.0 Testing and Maintenance of Motor starter (controller) 13.1.0 Preventive maintenance on MCCB Molded case circuit breakers are designed to require little or no routine maintenance throughout their normal lifetime. Therefore, the need for preventive maintenance will vary depending on operating conditions. As an accumulation of dust on the latch surfaces may affect the operation of the breaker, molded case circuit breakers should be exercised at least once per year. Routine trip testing should be performed every 3 to 5 years.
13.2.0 Routine maintenance tests on MCCB: Routine maintenance tests enable personnel to determine if breakers are able to perform their basic circuit protective functions. The following tests may be performed during routine maintenance and are aimed at assuring that the breakers are functionally operable. The following tests are to be made only on breakers and equipment that are deenergized.
13.2.1 Insulation resistance test – A megohmmeter may be used to make tests between phases of opposite polarity and from current-carrying parts of the circuit breaker to ground. A test should also be made between the line and load terminals with the breaker in the open position. Load and line conductors should be disconnected from the breaker under insulation resistance tests to prevent test measurements from also showing resistance of the attached circuit. Resistance values below 1 megohm are considered unsafe and the breaker should be inspected for possible contamination on its surfaces.
13.2.2. Millivolt drop test – A millivolt drop test can disclose several abnormal conditions inside a breaker such as eroded contacts, contaminated contacts, or loose internal connections. The millivolt drop test should be made at a nominal direct-current voltage at 50 amperes or 100 amperes for large breakers, and at or below rating for smaller breakers. The millivolt drop is compared against manufacturer's data for the breaker being tested.
13.2.3 Connections test – The connections to the circuit breaker should be inspected to determine that a good joint is present and that overheating is not occurring. If overheating is indicated by discoloration or signs of arcing, the connections should be removed and the connecting surfaces cleaned.
13.2.4 Overload tripping test – The proper action of the overload tripping components of the circuit breaker can be verified by applying 300 percent of the breaker rated continuous current to each pole. The significant part of LV Switch ear & MCC
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this test is the automatic opening of the circuit breaker and not tripping times as these can be greatly affected by ambient conditions and test conditions.
13.2.5 Instantaneous tripping test – Refer to manufacturer’s instructions for instantaneous trip time. The automatic opening of the circuit breaker is verified at the set value as per the current rating of the circuit breaker. It is also important to verify that the circuit breaker will not automatically open for the current below the set value.
13.2.6 Mechanical operation – The mechanical operation of the breaker should be checked by turning the breaker on and off several times.
13.3.0 Isolator (Disconnect switch): The external operating handle of the disconnect switch must be capable of opening the switch. If visual inspection after opening indicates deterioration beyond normal wear and tear, such as overheating, contact blade or jaw pitting, insulation breakage or charring, the switch must be replaced.
13.3.1 Fuse holders. Deterioration of fuse holders or their insulating mounts requires their replacement
13.4.0 Contactor Contacts showing heat damage, displacement of metal, or loss of adequate wear allowance require replacement of the contacts and the contact springs. If deterioration extends beyond the contacts, such as binding in the guides or evidence of insulation damage, the entire contactor may be replaced.
Irregular surface Replacement not
Worn out contact Replace
Curling
Repla
Conditions of the contacts
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13.5.0 Protective relays If burnout of the current element of an overload relay has occurred, the complete overload relay must be replaced. Any indication that an arc has struck and/or any indication of burning of the insulation of any of the protection relay also requires replacement of that protection relay. If there is no visual indication of damage that would require replacement of the relay, the relay must be tested using proper test set to verify the proper functioning of the relay contact(s).
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14.0.0 Automatic transfer scheme The power interruptions can result in the unplanned plant shutdowns which causes loss of production measured in thousands of dollars and may also cause severe damage to some of the critical plant equipment, where continuous process industry (Oil & Gas, Petrochemical, Fertilizers etc.) are involved. While power interruptions can not be totally eliminated by transfer scheme there duration and effect can be minimized by providing an alternate power source and some method of load transfer to the alternate source such as emergency generator. Electric supply companies are very conscious of the need of the service continuity to their customers and put their best effort to eliminate the factors contributing to the outage. Despite efforts made by the supply companies, outages do occur. Industrial power user must therefore do all that is possible to eliminate effects from outage. There are many ways to design auto transfer schemes; some are simple, and some are quite complicated and expensive. The main purpose of auto transfer scheme is to keep entire bus energized. This is accomplished by switching on/off the proper breakers and load shedding/reconnection at the proper time. The automatic transfer scheme becomes more complicated when motor loads are involved. The logic of such two schemes is shown in the diagrams on the next page.
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. Standby Genset OFF
ORMA
STANDBY
A: “normal” breaker B: “standby” breaker
AUTO A: closed B: open “normal” operation
Switch ositio
TRANSFER TO STANDBY SOURCE
RETURN TO NORMAL SOURCE
volta e restored
undervolta e Duration < T1
A: open B: closed “standby” operation
Duration > T1
Duration < T2
Duration > T2
Gen set startup request startup Gen set startup request
Genset voltage and frequency OK
startup
Transfer command
Load shedding command
A, B : open
Genset voltage and frequency OK
Load shedding command
After time delay T3
A: open B: open
A: closed B: open “normal” operation
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A: open B: closed “standby” operation
A: closed B: open “normal” operation
OPENING OF A
Load reconnection
After time delay T3 =0.25 s -30 s
After time delay T4 =0.25 s
CLOSING OF B
CLOSING OF A
A: open B: closed “standby” operation
A: open B: closed “standby” operation
A: closed B: open “normal” operation
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