THE INSTITUTION OF ENGINEERS SRI LANKA
DOMESTIC WIRING
MAY 2009
Technical Information Series Publication No: TIS001, Edition 1
THE INSTITUTION OF ENGINEERS SRI LANKA Technical Information Series Publication No: TIS001, Edition 1
DOMESTIC WIRING MAY 2009
Sponsored by:
LANKA ELECTRICITY COMPANY (PRIVATE) LIMITED 411,Galle Road Colombo 3.
Foreword
T
his publication on ‘Domestic Wiring’ is the first in a Technical Information Series that the Institution of Engineers, Sri Lanka (IESL) has decided to publish. It was during the Session 2003/ 2004 that the then Electrical and Electronic Engineering Sectional Committee of the IESL first decided on the need for a publication on this subject. This was considering the fact that many who presently undertake residential wiring and electrical installation work often overlook certain important factors that mostly affect the safety of the public. At present, electrical household wiring is designed and installed generally by unqualified “Electricians” whose knowledge and competency on the subject is questionable. In the bygone days, only the electricians who registered themselves with the Local Authorities were authorized to undertake household wiring. However, this rule is not being implemented any more. With the wide scale electrification of the country, a large number of unskilled workers have taken up work as electricians. The sub standard designs and installations done by these workers pose a grave risk not only to the occupants of the houses concerned but also to those living in the neighboured of those houses. To make matters worse, there is very little reading material available to educate and guide those who are interested to learn and update their knowledge so that they carry out their work according to laid out standards. Eng. J Mallikarachchi, who was a member of the Electrical and Electronic Engineering Sectional Committee during the Session 2003/2004 and who was well conversant with the subject is the author of this Monograph. He in compiling this publication has combined his own practical experience together with the theoretical knowledge acquired by reading various other publications available on the subject. The main focus of the publication has been on educating the reader on the IEE wiring regulations explained in simple terms while retaining their meanings. This publication on ‘Domestic Wiring’ will certainly fill this void. While the contents are simple and easy to understand
with no complicated mathematical calculations or drawings, it is comprehensive enough to cover all fundamental aspects that one needs to know on the subject. The Monograph is based on the 16th Edition of the IEE Wiring Regulations which was the standard that was being followed at the time of writing it although it is the 17th Edition that is now in practice. This however does not make the contents outdated in any way. We will also be shortly arranging for the translation of the book in to Sinhala so as to benefit a larger cross section of readers. Finally we take this opportunity to thank Eng. Ranjith Gunawardana, the Immediate Past President, who as the then Chairman of the Electrical and Electronic Sectional Committee during the Session 2003/2004 realised the need for this publication, the Author Eng. J Mallikarachchi, the Past Presidents, Eng. D. G. Senadhipathi and Eng. B. R. O. Fernando who assisted the Author in editing the publication, Eng. (Prof) J R Lucas, Eng. M Zubair and Eng. Ranil Senaratne for their valuable contributions in making the document more comprehensive. Finally we thank the Members of the Library, Publications and Publicity Committee (LPP&C) of the current session for their contributions towards making this monograph a reality. Eng. D P Mallawaratchie President of IESL – Session 2008/2009 Eng (Prof) AKW Jayawardane Vice President and Chairman LPP&C 2008/2009
CONTENTS Scope Introduction
11 12
Basics of electrical wiring Control of supply Protection Conductor Insulator Cables Additional protection for cables Stranding of conductors Flexible cords Electric circuits Relationship between voltage and current The basic circuit Waveform Types of available a. c. supplies Current carrying capacity Overcurrent Faults – short circuits Earth faults Direct and indirect contacts Prevention of fires and accidents Switching Switches Isolator Fuse Circuit breaker Emergency disconnection Voltage and current rating of equipment Earth fault protection Resistance area of earth electrode Earth loop impedance Earth leakage circuit breakers Voltage operated earth leakage circuit breaker Residual current circuit breaker Residual current device Distribution inside the house Distribution board Final circuit Fixed wiring Socket outlet Electrical point
13 13 13 13 13 13 13 14 14 14 15 15 16 16 16 17 17 17 18 18 19 19 19 19 19 20 20 20 21 22 22 22 23 24 24 24 24 24 25 25
Different types of final circuits Sequence of supply controls Declared or nominal voltage Accessory Diversity The consumer unit Safety precautions Types of switches Wiring systems – additional protection for cables Looping in system Identification (Colour code) Single line diagram IEE Regulations
25 26 27 27 27 27 27 29 30 30 31 31 33
Summary of Regulations relevant to domestic wiring
35
Fundamental Principles Protection for safety Protection against electric shock Protection against thermal effects
35 35 35 35
Requirements for safety Conductors and equipment Fuses and circuit breakers Connection of switches Emergency disconnection General provision for earthing Installation of equipment Special conditions Addition to an installation Testing and inspection Control distribution and over-current protection Protective equipment Control of supply Isolation Overcurrent protection Protection against electric shock Protection against direct contact Protection against indirect contact Earthed equipotential bonding Circuit protective conductors Connection with earth Labelling Maximum demand and diversity Nature of demand Final circuits Conductors and cables: choice and construction
35 35 36 36 36 36 36 36 36 36
37 37 37 37 38 39 39 39 39 40 41 41 41 41 42 42
Voltage rating Current rating Voltage drop Cross-sectional area of conductors Protection against mechanical damage Identification of conductors Terminations Installation of equipment Lamp holders Lighting points Ceiling roses Lighting fittings Plugs and socket-outlets Switching for safety
42 42 42 42 42 43 43 43 43 44 44 44 44 46
EXCERPTS FROM PREVIOUS EDITIONS Earth fault currents Detached buildings Diversity Overcurrent protection Provision of protective devices Final circuits Final circuits of rating exceeding 15A Domestic ring and radial circuits Bending radius of cables Terminations Conduit systems Metal conduit systems Flexible conduit systems Suspension from non-metallic boxes Equipment liable to cause overheating Ceiling roses Isolated metal Electrical discharge circuits Earth fault protection Room containing a bath From the “ON-SITE GUIDE” Maximum demand and diversity Conventional circuit arrangements Final circuits using 13A socket-outlets Circuit protection Conductor size
47
47 47 47 47 47 48 48 48 49 49 49 50 50 51 51 51 51 51 51 52 53 53 55 55 55 56
Cooker circuits in household premises Correction factors for current carrying capacity
56 56
Selection of a System Conductors Insulation Wiring systems Overcurrent Protection Means of isolation Earth fault protection Distribution boards Layout Type of final circuits Selection of cables Voltage drop calculation Lighting circuits Socket outlet circuits Conduit capacities
59 59 59 60 60 61 62 62 62 65 65 67 68 68 69
Planning the installation Example Installation Conduits on walls and ceilings Conduits for switches Conduits for socket outlets Conduits for distribution boards Wiring Example Single line diagram Conduit layout Wiring procedure
71 75
Testing Tests to be carried out Continuity of earth conductors Continuity of ring final circuits Insulation resistance Earth electrode resistance Operation of residual current devices
95 95 97 98 100 102 103
83 84 84 85 86 86 88 89 89 90
New colour code for cable cores 105 Regulation on “Warning notice – Non standard colours” 106
FROM THE APPENDIX: “HARMONIZED CABLE CORE COLOURS” Introduction Alteration or addition to an existing installation Single phase Two- or three-phase installation Switch wires Intermediate and two-way switches Phase conductors Changes References
107 107 107 107 107 108 108 108 108 112
List of Diagrams Fig.1 Fig.2 Fig.3 Fig.4 Fig.5 Fig.6 Fig.7 Fig.8 Fig.9 Fig.10 Fig.11 Fig.12 Fig.13 Fig.14 Fig.15 Fig.16 Fig.17 Fig.18 Fig.19 Fig.20 Fig.21 Fig.22 Fig.23 Fig.24 Fig.25 Fig.26 Fig.27 Fig.28 Fig.29 Fig.30 Fig.31 Fig.32
Components of a cable Stranding of conductors Series and parallel circuits Basic domestic circuit A.C. Voltage Wave Electrical faults Direct and indirect contacts Multi pole switches Earth conductor Resistance area Voltage operated earth leakage circuit breaker Residual current circuit breaker Ring and radial circuits Sequence of supply controls Polarity: position of fuses (mcbs) and switches Polarity: connections to screw type lamp holders Polarity: connections to socket outlets Circuits with two-way and intermediate switches Looping in wiring system Single line diagram House – Electrical points Upper floor distribution board Ground floor distribution board House – Electrical layout Conduit junction boxes Sunk box Conduit connections to lamps and a switch Termination for a switch on wall Conduit connection to socket outlets at skirting level Conduit entries to surface mounted distribution boards Method of connecting cable to draw wire Cables entangling in a junction box
13 14 15 15 16 17 18 19 21 22 23 23 25 26 28 28 29 29 30 31 73, 74 79 80 81, 82 83 83 84 84 85 86 87 87
Fig.33 Fig.34 Fig.35 Fig.36 Fig.37 Fig.38 Fig.39 Fig.40 Fig.41 Fig.42 Fig.43 Fig.44 Fig.45 Fig.46 Fig.47 Fig.48
Wiring points in a house Single line diagram Wiring layout Conduit layout Example wiring using looping in method Connections to switches and ceiling roses Typical layout for 13A socket outlets Continuity test for earth conductor (Method 1) Continuity test for earth conductor (Method 2) Continuity test for ring final circuits (Method 1) Continuity test for ring final circuits (Method 2) Insulation resistance between phase and neutral conductors Measurement of earth electrode resistance Testing RCCB with a lamp Addition and alteration to an existing installation Addition of a socket-outlet using new cables
88 89 89 90 91 93 94 97 97 98 99 101 102 104 109 110
List of Tables Table 1 Table 2 Table 3 Table 4 Table 5 Table 6 Table 7 Table 8 Table 9 Table 10 Table 11 Table 12 Table 13 Table 14 Table 15 Table 16
Identification of conductors 43 Overcurrent protection of lampholders 44 Plugs and socket-outlets 45 Minimum internal radii of bends in fixed wiring 49 Current demand to be assumed for points and equipment 53 Allowances for diversity 54 Final circuits using 13A socket-outlets 55 Correction factors for groups of more than one circuit 57 Current ratings applicable when protected by circuit breakers 66 Current ratings applicable when protected by rewirable fuses 67 Data on Flexible Cords 67 Conventional final circuits 69 Conduit capacities for PVC / PVC cables 70 Example of conductor marking at the interface 107 Changes to cable core colour identification - rigid cables 108 Changes to cable core colour identification - flexible cables 108
10
Scope: This Guide is meant for electricians and covers only domestic installations not exceeding 100 A per phase This Guide is restricted to installations:
(a) At a supply frequency of 50 Hz.
(b) At a nominal voltage of 230 V a. c. single phase and 400 V a. c. three phase (c) Fed through a Supply Authority’s cut-out having a fuse or m. c. b. of 100A or less
This Guide also contains information, which may be required in general installation work, such as conduit capacities, bending radii of cables etc. in order to eliminate the need for detailed calculations.
11
Introduction The electricity supply authorities are responsible for providing a supply of electricity to suitable terminals on a consumer’s premises; the electrical installation in the premises provides the means of conveying the electricity to the equipment where it is to be used. The equipment installed has to be connected by means of cables and controlled by suitable switchgear. No equipment can be used safely unless the installation work has been carried out correctly. Like fire, electricity is a good servant but a bad master and so before anyone can install a safe and efficient electrical system, it is essential for him to be familiar with the nature of electricity and the dangers inherent in its use. The two main hazards involved with electricity are electric shock and fire. Both of these could be reduced to negligible proportions by using suitable materials and correct methods of installation. Because of the vital need to maintain high standards in carrying out installation work various lists of regulations, requirements and codes of practices are published, some are enforceable by law while others are mere recommendations but accepted as standards to which every installation should be constructed. The most important set of regulations concerning electrical installation is known as I.E.E. Regulations and no electrician can claim to be fully competent if he is not familiar with its contents. The I. E. E. Regulations are enforceable by law in Sri Lanka under the Electricity Act.
12
Basics of Electrical Wiring Control of supply In electrical engineering, control generally means the ability to isolate, connect, disconnect, direct or restrain the flow of electricity. Protection Protection in electrical engineering means the protection of cables and equipment, usually against damage due to overcurrent or earth faults. Conductor Any material which allows free passage of electric current is known as a conductor. Conducting materials vary in the degree to which they can conduct electricity. Good conductors are required for electric circuits so that they convey electricity with a minimum loss of voltage. Insulator Any material which does not allow free flow of electric current is known as an insulator. Insulators are used to confine electric current to the conductors in which they are intended to flow and to prevent leakage of electricity to adjacent conducting materials which are not intended to become “live”. Insulators are also needed to prevent unintended interconnections between various parts of an installation. Cables: Major part of the domestic wiring installation involves cables. Cables consist essentially of conductors to carry electric current and insulation to prevent leakage of current from conductors. They are usually provided with some form of protection against mechanical damage.
Sheath for mechanical protection
Insulation
Conductor (Solid or stranded)
Fig. 1. Components of a Cable Additional protection for cables: Cables may of course be enclosed in metal or plastic conduits. A conduit may be defined as a tube or channel. In electrical installation work conduits refer to metal tubing of comparatively light gauge or to non-metallic tubing. Although not considered as a part of the cables, these are a method of cable protection. The same applies to trunking and ducting. 13
Stranding of conductors: To give flexibility conductors of cables are stranded. The number of strands in a cable is chosen to have a near circular shape when all strands are combined. This is done by having layers of 1, 6, 12, 18, … resulting 1, 1+6, 1+6+12, 1+6+12+18, … stranded conductors. This makes number of strands making up a cable 1, 7, 19, 37, and so on. Conductors with 3 strands were included in imperial standards but not in metric standards.
1
3
1+6
1+6+12
1+6+12+18
Fig.2 : Stranding of Conductors
Single solid wire may be used in smaller metric cables of sizes below 4 mm2 Conductor sizes are expressed either by the number of strands and diameter of the strand or by the cross sectional area. For example: 1/044 (Imperial), 1/1.13 or 1 mm2 (Metric) cable has one strand of 0.044 inches or 1.13 mm diameter and a cross sectional area of 1 square millimetre. Flexible cords Conductors of flexible cables consist of large number of fine wires. Smaller flexible cables in which the cross sectional area does not exceed 4 mm2 are known as flexible cords. Flexible cords used normally consist from 16 to 56 strands each from 0.2 to 0.3 mm diameter. Electric circuits An electric circuit is an arrangement of conductors for the purpose of flow of electricity. Before an electric current can flow in a circuit two conditions must be fulfilled. Everybody is aware that a height difference is required for water to flow from one point to another. Similarly for electricity to flow from one point to another an electrical potential difference is required between the two points. The other condition is that there must be a complete path of conducting materials through which the current can flow. This potential difference is known as the voltage and the rate at which the electricity flows is known as the current. There are two basic types of electric circuits. Arrangement in which circuit components are connected in series so that the same current flows through every one of the components is known as a series circuit. Arrangement in which circuit components are connected in parallel so that same voltage is applied to each component is known as a parallel circuit. In a parallel circuit current supplied is shared by each component. In domestic wiring parallel circuits are used. 14
Current 1 Voltage 1
Voltage 2
Lamp 1
Lamp 2
Voltage 3
Total Current
Current 2
Lamp 3
Total Voltage Current
Current 3
Lamp 1 Lamp 2
Lamp 3
Total Voltage
Voltage source
Voltage source
Fig.3 Series and Parallel Circuits
Relationship between voltage and current: Any conductor in which a current flows offers some resistance to the flow of current. Consequently there is a drop in voltage between the two ends of the conductor. This voltage drop is said to be proportional both to the current as well as to the resistance. In d. c. circuits this relationship is given by Ohm’s law as V=IR where V is the voltage between the two ends, I is the current and R is the resistance of the conductor. In a .c. circuits there are two more phenomena known as inductance and capacitance which oppose rapid changes in current and voltage respectively. The combined effect of resistance, inductance and capacitance is known as impedance and is denoted by Z. The relationship of voltage current and impedance of a. c. circuits is given by V=IZ. I = Current
Line
Phase (Line) Conductor Supply Transformer
Z = Impedance V = Voltage
Neutral Conductor Transformer neutral earth point
Neutral
Flg.4: Basic Domestic Circuit
The basic circuit: The basic a. c. circuit used in domestic wiring has two conductors and the current consuming equipment. The equipment is generally known as the load. The conductors are connected to the transformer at the supply end. One of the conductors is connected to the earth at the transformer for safety reasons. It is known as the neutral point as its voltage with respect to earth is zero. The conductor between the neutral point and load is called the Neutral Conductor while the other is called the Phase Conductor. (Earlier the phase conductor was also known as the Live Conductor, but the current IEE Regulations considers both Phase and Neutral conductors as Live Conductors). 15
Waveform: The voltage between the phase conductor and the neutral rapidly alternates between two values Vm and -Vm. (Vm is known as the Peak Voltage). Time taken to travel from a particular value to the two extremities and returning to the original value is known as a cycle. Number of such cycles per second is called the frequency of the supply. Voltage, current and resistance are measured in volt, ampere and ohm respectively and frequency is measured in hertz or cycles per second.
VOLTAGE
1 Cycle
TIME
Fig. 5: A.C. Voltage Wave
Types of available a. c. supplies: There are two types of a. c. supplies, available in Sri Lanka, which are known as single phase 2 wire and three phase 4 wire systems. Single phase supply has one phase (or line) conductor and a neutral conductor while three phase supply has three phase conductors and a neutral conductor. The three phases are designated as Red (R), Yellow (Y) and Blue (B) by convention. (This would be changed to Phase 1, Phase 2 and Phase 3 with the change of cable core colour code explained later). Current carrying capacity: Like any conductor a cable offers some resistance to flow of electric current. This produces heat. The amount of heat produced depends on the value of the current and the duration it flows. The insulation of the cable traps this heat, which causes the temperature of the insulation to rise. Temperature of the cable insulation cannot be allowed to rise to very high values since it causes deterioration of insulation properties. When the current flow is very high, overheating and possible destruction of the insulation could result. The maximum current that can be taken through a cable without damaging it is known as the current carrying capacity of the cable. The current carrying capacity of a cable is determined by the cross section area of its conductor, by the type of insulation and by the condition of the installation. 16
Overcurrent: If the insulation of a cable is damaged conducting parts could make unwanted contacts. Such contacts sometimes draw very high currents causing further overheating and deterioration. These could heat up the surrounding materials and end up in fires. When the current drawn in a circuit is more than the current carrying capacity of the cable, it is known as an overcurrent. Faults - short circuits: Contacts made between conductors bypassing the load are known as faults. If the contact is made between phase conductors (In 3 phase circuits) or between phase and neutral conductors bypassing the load extremely high currents can flow. These are called short circuits. Over currents caused by short circuits are called short circuit currents.
fault
Earthed metal parts fault
(b) Earth fault
(a) Short circuit
Fig. 6: Electrical Faults
Earth faults: Earth is a very good conductor offering practically no resistance to flow of current. Since the neutral point is already earthed, if a phase conductor accidentally makes contact with earth, the current will flow through the earth to the neutral point bypassing the load. Metal parts in contact with earth will also help to carry these currents. The value of current varies depending on the resistances of the contacts. Therefore if a current carrying conductor makes contact with such metals fault currents could result. Such fault currents are called earth fault currents. When contact between the metal part and earth is not perfect (where the resistance between the two is more than zero) metal part could gain a voltage due to the earth fault current. If a person touches such a metal part he could get an electric shock. 17
Phase
conductor
To lamp
Direct and indirect contacts: Electric shock is caused by touch. It can be divided into two main groups. If a person touches a current carrying component direct, it is called direct contact. An example would be a person removing a switch plate and accidentally touching the phase conductor. If a person touches an exposed metal which is not normally live, but has become so under fault conditions, it is called indirect contact. An example would be contact with the metal case of an electric iron having both a phase to earth fault and a significant resistance to earth.
Phase Neutral Earth (High Impedance)
Switch
Earth Fault
(a) Direct Contact
(b) Indirect Contact
Fig. 7: Direct and Indirect Contacts
Prevention of fires and accidents: In electrical installations fires could be started due to three reasons. They are overloads, short circuits and earth faults. Overloading is caused by drawing electricity through a circuit more than it is designed to carry, while, short circuits or earth leakages are caused by insulation failures. If electricity is allowed to flow through a fault for a considerable time, it could result in a disaster. So, one of the prime requirements of an electrical installation is to have a device in a readily accessible position to disconnect the supply in case of an emergency. On the other hand an overload or an earth fault may not be readily noticeable but still could lead to a disaster. Therefore it is necessary to provide devices to automatically disconnect the supply in such cases. Switching: It must be evident now that there should be a complete path of conducting materials from the phase point to the neutral point of the transformer for a current to flow. Any discontinuity of the arrangement of conductors would stop the flow of current. This fact is utilised to control electric circuits. The simplest device used to achieve this function is called a switch while switchgear is a broad term applied to equipment used for controlling the flow of electricity. 18
Switches The switch is used to manually open or close a circuit carrying a normal current. It is capable of closing and opening a circuit under specified overload conditions also. There are two types of switches employed in electric wiring. One type opens only one conductor (known as single pole switches) and the other opens all the conductors (2 pole, 3 pole and 4 pole switches). A switch in which it is arranged to open all the conductors together is known as a linked switch. Phase 1
Phase 2
Phase Single Pole Switch
Phase
Phase 3
Neutral
Neutral Double Pole Switch
Four Pole Switch
Fig. 8: Multi-Pole Switches
Isolator: This is a mechanical switching device capable of opening or closing a circuit under condition of no load or negligible current. It is used to provide the function of isolation. Isolation is to cut off the supply from all or a section of the installation by separating the installation or the section from every source of electrical energy. An isolator defers from a switch in that it is to be opened only when the circuit concerned is not taking current. Its purpose is to ensure that the supply cannot be restored by switching the circuit on from somewhere else. It implies that some other device needs to be incorporated in the circuit along with the isolator to control the circuit in normal use. Fuse: This is a device inserted in a circuit to interrupt an overcurrent flowing in it. It opens the circuit by melting the fuse-element by such overcurrent. A fuse element is a conductor designed to melt when an overcurrent flows. Thus a fuse can be used to automatically open the circuit when an overcurrent flows, but it has to be replaced (after eliminating the fault) to restore the supply. If it is necessary to open and close the circuit a switch is also required. Thus a commonly used device to control circuits is a switchfuse which incorporates a switch and a fuse (or fuses) together. Circuit breaker: This is a mechanical device for making and breaking a circuit, both under normal conditions and abnormal conditions, such as those of a short circuit. It can be used to open and close a circuit in normal use as well as 19
to automatically open a circuit in case of faults. There are many types of circuit breakers such as oil circuit breakers (OCB), air circuit breakers (ACB), moulded case circuit breakers (MCCB), miniature circuit breakers (MCB) etc. in use. Emergency disconnection: The device to cut off the supply in case of an emergency could be a switchfuse or a circuit breaker. Fuses or circuit breakers could be used to automatically disconnect the supply in cases of overloads and short circuits. The same devices could be used for earth faults as well when the earth fault current is high, but some other device need to be employed to automatically disconnect earth faults when the earth resistance is high. Voltage and current rating of equipment: One of the precautions that can be taken to prevent development of faults during normal use is to use proper materials. There are standards set up for selection of proper materials. Voltage rating and current rating are two of such standards of importance in electrical wiring. The voltage rating of equipment or cables indicates their safe working voltage. In case of cables it is determined by the type and thickness of the insulation. Standard voltage rating of most wiring cables is 600/1000V. 600V is the safe working voltage to earth and 1000V is that between conductors. (It must be mentioned here that safe working voltage does not mean that one can safely touch a conductor energized to that voltage, but one can safely touch a cable insulated to 600/1000V whilst its internal conductors are energized to 600V to earth). The maximum current that can be permitted to flow in equipment without overheating is known as the current rating. The current rating of a cable is determined not only by its physical properties but also the conditions of installation. Whether it is installed in direct contact with air, touching a surface, in a duct or a conduit, embedded in wall or buried underground, temperature of the surroundings, thermal insulation properties of surrounding materials and whether there are other cables around it, are some of those properties which affects the current rating. Current rating of a fuse is the maximum current, which can continuously flow in the circuit, and not the current at which the fuse will blow off. A fuse normally blows off almost immediately at around 3 times the rated current. This value is known as the fusing current. Earth fault protection: A shock risk arises whenever accidental contact is made between a live conductor and exposed metal work. At the same time earth fault currents can give rise to fire risks. Therefore reliable protection is required against possibilities of electric shock and fire risks from earth faults. These risks can be guarded against by efficient earthing. 20
Method adopted in domestic wiring to minimise shock risk is to earth all accessible metal parts and arrange to cut off the supply when the metalwork is liable to be dangerously live due to a fault. For this purpose a separate conductor is run throughout the installation along with the phase and the neutral conductor. One end of this is earthed and it is known as the earthing conductor. L
Phase (Line) Conductor
Phase R Supply Transformer Phase B
Phase Y Transformer neutral earth point
L O A D
Earthing Conductor
Main Earthing Terminal
Protected Metal Work
Neutral Conductor Circuit Protective Conductor (cpc)
Earth return path SUPPLY AUTHORITY
CONSUMER
Fig. 9: Earth Conductor
Resistance area of earth electrode: The term “earthed” implies effective connection to the general mass of the earth with adequate area of contact. This contact is usually achieved by means of an earth electrode which could be a metal pipe, metal rod or a metal plate or some other conducting object. Every earth electrode has a definite electrical resistance to “true” earth. The current flow to the general mass of the earth has to overcome not only the resistance of the conductor forming the earth electrodes at the consumer’s end as well as the transformer end, but also the resistance of the surrounding soil. Soil is a relatively poor electrical conductor. As the current leaves the electrode it fans out and as the current travels farther from the electrode, the effective cross sectional area of the soil through which it flows is increased. This causes the resistance to decrease. The overall effect is to produce a graded resistance concentrated mainly in the soil surrounding the electrode and diminishing to near zero around 1.5m away. The area containing virtually all the resistance is known as the resistance area of the earth electrode. As in the case of any conductor, when a fault current flows through the earth electrode to the general mass of the earth, a voltage gradient is produced in this resistance area. When the resistance and also the fault current are high, the associated voltage difference may be large enough to give a lethal shock through the two feet of a person standing on this area. For this reason it is advisable to bury the electrode (about 150mm) beneath the surface of the ground and the earthing lead connected to the electrode needs to be insulated. Vertical electrodes to be most effective should cover this total resistance area, which means they should be buried up to a depth of about 1.5m. 21
Ground
Fault Current
Soil Layers
Earth Electrode
Soil Layers
Paths of Fault Current (a) Sectional Elevation Fig. 10: Resistance Area
(b) Plan View
Earth loop impedance: The earth electrode is only one part of the earth fault current loop which determines the value of the current drawn. The fault current flows from the transformer through the phase conductor to the fault and back to the transformer neutral point through the earth conductor, earthing terminal, earthing lead, earth electrode and soil path to neutral earth. The overall resistance of this path will determine the magnitude of the fault current. Since the voltage applied is alternating, it will depend not only on the resistance, but also on the overall inductance and capacitance as well. The overall impedance of the path that determines the magnitude of the earth fault current is called the earth loop impedance. Earth leakage circuit breakers; A fuse or a circuit breaker can be used to disconnect the supply, only if, a sufficiently high current flows long enough to operate them. When the earth loop impedance is very high a fuse or a circuit breaker may not provide the necessary earth leakage protection. In such an instance, a device known as an earth leakage circuit breaker (e. l. c. b.) (Popularly known as a trip switch) is used. The reason for e. l. c. b. s to be more effective when connected to circuits with relatively poor earths is that they operate on very low leakage currents. There are two types of e. l. c. b. s: voltage operated and current operated (or current balance). Voltage operated earth leakage circuit breaker In the voltage operated e. l. c. b. the trip coil is connected between the metalwork and the earth electrode. If a fault occurs from a current carrying part to the metal frame, the fault current would flow through the earth conductor, trip coil, insulated earth lead, consumer’s earth electrode to earth. When the fault current is large enough the trip coil would operate and disconnect the supply from the load. 22
Supply Test Resistance
Protected Metal work
L O A D
Trip Coil
Insulated Earth Lead
Auxiliary Earth Electrode
Consumer’s Earth Electrode
Fig. 11: Voltage Operated Earth Leakage Circuit Breaker
(Note: It should be noted that the current IEE Regulations do not permit the use of voltage operated e.l.c.b. s and therefore they shall not be incorporated to new installations.) Supply
Multiplier Search Coil
Trip Coil
Test Button Test Resistance
L O A D
Protected Metal Work
Fig. 12: Residual Current Circuit Breaker Earth Electrode
Residual current circuit breaker The current operated type e. l. c. b. operates by detecting the out of balance current. The load is fed through two equal coils wound on a common core on which a third coil is also wound. The third coil which is known as search coil is connected to the trip coil of the e. l. c. b. through a voltage amplifier. Under normal conditions the phase and the neutral current are equal and there is no current flowing in the trip coil. When there is an earth fault all the current flown through the phase conductor will not flow through the neutral conductor as a portion will leak to the earth through the fault. Hence there will be an imbalance and a current will flow in the search coil and trip coil. This is known as the residual current. When the residual current is large enough the trip coil will operate the e. l. c. b. Though this device was initially called current operated e. l. c. b. the earth conductor is not going through it and the term is somewhat erroneous. It is now known as a Residual Current Circuit Breaker (r. c. c. b.). 23
Residual current device: This is a mechanical device or a combination of devices which opens the contacts when the residual current attains a given value under specified conditions. The r. c. c. b. is a common particular implementation of the residual current device. Distribution inside the house: Theoretically the whole installation in a house starting from the supply authority’s fuse and ending up at its neutral terminal could be a single circuit. Such a circuit could be very complex offering many parallel paths and difficult to understand. Therefore it is usually divided into several sub circuits. This is done by introducing one or more distribution boards with a number of fuses on the live side and equal number of terminals on the neutral side. Each sub circuit starts from a fuse and ends up at the corresponding neutral terminal. (IEE Regulations define each of these sub circuits as a final circuit.) Distribution board: This is an assembly of parts including one or more fuses (or other protective devices) arranged for the distribution of electrical energy to final circuits or other distribution boards. Every distribution board must be connected to and controlled by the main switchgear controlling the supply or a separate way on a larger distribution board. Final circuit: The portion of the circuit between the distribution board fuse and the neutral terminal is known as the final circuit. Every final circuit is connected either to one way of a distribution board or to a switchfuse. The sizes of the circuit fuses in a distribution board depend upon the ratings of the final circuits. Fixed wiring Electricity consuming equipment in a household can be broadly divided into two categories. Items like lights, ceiling fans etc. which generally need not be moved about are known as fixed equipment. Other items like table lamps, table fans, immersion heaters, electric irons, refrigerators etc. which are moved around are called movable equipment. (Out of these items which are not heavy and frequently moved items are known as portable equipment). Usually fixed equipment are directly connected to the wiring system while movable equipment are wired up to a certain point and are provided a means to connect and disconnect the movable portion at will. 24
Radial Circuit
Ring Circuit Distribution Board Spur Fig. 13: Ring and Radial Circuits
Socket-outlet The part of the facility to connect and disconnect the movable equipment connected to the fixed wiring portion is known as a socketoutlet. A socket-outlet is usually provided with protected contacts to make connections for line, neutral and earth conductors. The part of the facility connected to the movable equipment is known as a plug. It has protruded pins to make connections with the line, neutral and earth contacts of the socket-outlet. The plug is usually connected to the movable equipment by a flexible cable or flexible cord. The movable equipment is usually required to be used at different locations at different times. Therefore socket-outlets need to be provided at all those different locations. As there is a wide range of equipment drawing varying currents there are socket-outlets of different capacities to choose from. The most common socket-outlets found in domestic wiring in Sri Lanka are either 5A or 15A. Also there are 13 A socket-outlets and fused plugs which can be used in place of both 5A and 15A socket-outlets with suitable selection of the fuse in the plug. In a ring circuit components of the circuit are arranged in a ring and both ends of the ring are connected to a single point of supply. If the socketoutlets are arranged in a chain with only one point connected to the supply point, it is known as a radial circuit. Electrical point: Termination of the fixed wiring intended for the attachment of a lighting fitting or of a device for connecting to the supply a current-usingappliance is known as a point. Different types of final circuits: In configuring final circuits it is usual to group similar type of equipment separately (such as lamps, 5A socket-outlets, 15A socket-outlets, 13 A socket-outlets etc.) to final circuits so that fuses and cables could be chosen appropriately. 25
Final Circuits
Sub Distribution Board Sub-main Cables to another Sub-distribution board Final Circuits Service Main
Main Distribution Board
Service Fuse Residual Current Device Main Switchfuse or DP MCB
Meter Consumer’s Terminals
Supply Authority’s Terminals
ORIGIN OF THE INSTALLATION
Load Cables
Supply Authority’s Equipment
Consumer’s Earth Electrode
Consumer’s Terminals
Fig. 14: Sequence of Supply Controls
Sequence of supply controls: The supply authority sells the electricity to the consumers. Therefore a prime requirement for them is to provide an energy meter to measure the amount of electricity consumed. They also provide a fuse cut out or a circuit breaker to protect the meter. These items are sealed to prevent tampering with the meter. Though mounted inside the consumer’s premises, equipment up to and including the meter belongs to the supply authority. The cable connecting the meter to the main switch is known as the meter tails or load cables. Terminals of the meter connected to the load cable are known as supply authority’s terminals, while terminals of the main switch connected to the load cable are known as consumer’s terminals. This is the position at which electrical energy is delivered to the consumer and it is known as the origin of the installation. 26
Declared or nominal voltage: As in the case of any conductor there will be a drop of voltage along the service line from the transformer to the supply authority’s terminals. As the distance from the transformer varies from house to house, the voltage available at the consumer’s terminal varies due to this voltage drop. In order to overcome this problem service lines are designed to maintain the voltage within a reasonable range rather than at a constant value. The voltage expected at the origin of the installation is known as the Declared Voltage (or Nominal Voltage). The declared voltage in Sri Lanka is 230 V Single phase and 400 V three phase. The voltage between any two phases is 400 V and between any of the phases and the neutral is 230 V. Accessory: Any device other than a lighting fitting, associated with the wiring of the installation, e.g. a switch, a fuse, a plug, a socket-outlet, a lamp holder or a ceiling rose is known as an accessory. Diversity: In normal use it is very unlikely to switch on all the lamps and other current consuming appliances (such as electric oven, hot plate, kettle, water heater, iron, toaster, water pump, etc.) at the same time. Therefore it is not necessary to provide cables and switchgear large enough to supply the maximum possible load. Thus, it is possible to reduce the size of cables and switchgear to cater to the maximum likely load. This is known as allowing for diversity. The consumer unit: In the past individual items with rewireable fuses were used as main switches and distribution boards, while voltage operated earth leakage circuit breakers were used for earth leakage protection. The present practice is to use miniature circuit breakers (MCB) instead of fuses and residual current circuit breakers (RCCB) for earth fault protection. Another popular item in use is the Consumer Unit with main switch, RCCBs and MCBs in one enclosure instead of three separate units. Thus a distribution system in a house would consist of A Main Switch, a Residual Current Device, One or more Distribution Boards, (Or a consumer unit), Sub Main Cables and final circuits. Safety precautions If a fuse or a circuit breaker is fitted on the neutral side of a distribution board it will interrupt the supply in case of an overload, but it will not be effective for other faults. Even when the circuit is disconnected at the neutral fuse, the wiring and fittings will be live and there is a danger of receiving a 27
shock by anyone attempting to repair an electrical item. Therefore fuses or single pole circuit breakers are never connected on the neutral side. However there was a practice long ago to have fuses on the neutral side for additional safety. But it has been discontinued now because sometimes the neutral fuse blows without affecting the fuse on the live side creating the dangerous condition described above. A switch connected in any position in a lamp circuit can be used to switch on and off the lamp. But if the switch is on the neutral side from the lamp, the wiring and the fitting will remain “live” even when the switch is off although the lamp will also be off. Therefore there would be a considerable risk of electric shock if any one attempts to replace the lamp with only the switch off. On the other hand, if the switch is on the live side of the lamp the circuit will be dead when the switch is in off position and it will be possible to replace the lamp or do other work on the circuit without electric shock. Therefore switches are always connected on the live side (phase conductor) in the interest of safety.
Correct
Switch on Neutral side Incorrect
MCB on Neutral side Incorrect
Fig. 15: Polarity: Position of Fuses (mcbs) and Switches
For similar reasons, when connecting socket-outlets and plug tops, live conductor is always connected to the terminal marked “L”, Neutral conductor to the terminal marked “N” and earth conductor to the terminal marked “E”. Similarly live conductor on the screw lamp holders must be connected to the centre contact in order to reduce danger of shock should the fitting touched while the lamps are on.
Phase to centre contact Neutral to Screw
Fig. 16: Polarity: Connections to screw type lamp holders
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Fig. 17: Polarity: Connections to socket outlets
Type of switches Lighting circuits are generally controlled by ( i ) One way switches, ( ii ) Two way switches and ( iii) Intermediate switches. One way switch is the most commonly used item. This has a single contact point which makes (on) and breaks (off) depending on the position of the toggle. Two way switches have three contact points with one point making or braking contact with other points depending on the position of the toggle. These are used to control lamps from two different positions such as from either floor of a two-storied building (Usually located at the entrance to the staircase). Intermediate switches have four contact points which changes contact position from two parallel lines to a cross (see figure) as the toggle is operated. They are used to control lamps from three or more points in association with two, two way switches.
Two way switch
Two way switch
Lamp
Intermediate switch
SWITCHING POSITIONS
Two way switch
Lamp
Lamp
Lamp
(b) Two way and Intermediate swiches (a) Two way swiches Fig. 18: Circuits with two way and intermediate switches
29
Wiring systems – additional protection for cables There are many types of wiring systems that can be used to provide safe, efficient and economical installations. The principal types in use are: Screwed metal (Steel, Copper or Aluminium) conduit, Plain unscrewed metal conduit, Non-metallic (PVC,PE) conduit, Armoured cable, Mineral insulated copper sheathed cable, Cable duct, Bus-bar and Wood casing systems. Looping in system An opening in cable insulation always makes a weak point and reduces the insulation resistance. It is also necessary to minimise the number of joints in an electrical installation since in addition to the aforementioned, the effective current carrying capacity of the cable reduces at the joint. In practise this is achieved by avoiding midway joints and making them only at switches, socket-outlets, ceiling roses, lamp holders and similar accessories. This will require more length of cables than if midway joints are allowed, but extra cost is justifiable in the long run. This is called the looping in system. The term, looping in, gives the erroneous impression that one length of cable is bared at intervals and looped in at switch and lamp terminals. It is not practicable. In practice, when wiring in conduit, necessary numbers of cables are drawn through the conduits and the joints are made at the switches, lamps or other terminals. (Note: In ring circuits it is necessary to keep the loops avoiding joints as much as possible, in order not to reduce the current carrying capacity of the cables.)
To Upper Floor Circuit
Distribution Board
Fig. 19: Looping in Wiring System
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Identification (Colour code). There are three kinds of conductors involved in electrical installations described earlier. They are the phase (line), the neutral and the earth conductors. When there are a large number of cables involved, it is useful, if these can be readily identified. This is achieved in practice by having a different colour for each type of conductor. The colours used at present are Red (Phase or Line), Black (Neutral) and Green or Green-and-Yellow (Earth). In the case of flexible cables Brown (Phase), Blue (Neutral) and Green-andYellow (Earth) is adopted. (The latest amendments to IEE Regulations have changed the colour coding as Brown (phase), Blue (neutral) and Green-andYellow (Earth) for rigid cables as well, effective from 1st April 2006.) Single line diagram The readers may have already noted that it is easier to present the items in an electrical installation in a diagram rather than a verbal description. Further such information can be presented more clearly in symbols rather than drawing the actual items. The connection details are usually shown in what is called a “Single Line Diagram”. Here the circuits are shown in single line form irrespective of the number of conductors involved. The actual number is usually indicated by a set of short cross lines. Layout drawing has been already shown in Fig. 19 and a typical single diagram is shown in Fig 20 and a set of symbols used is shown overleaf.
2x6
30 A DP MCB
RCCB
40 A DP 30 mA
2x1
Lamps
2 x 2.5 Ring Socket Outlets
Fig. 20: Single Line Diagram
31
STANDARD SYMBOLS Symbol
Description Switch (General) Triple Pole Switch Circuit Breaker Single Pole MCB Fuse Link ( Neutral) On Push Button Off Push Button Filament Lamp Distribution Board Residual Current Device Electricity Meter Pendent or Ceiling Lamp Wall Bracket Single Fluorescent Lamp Twin Fluorescent Lamp Ceiling Fan 13A Socket Outlet 5A Socket Outlet 15A Socket Outlet One way Switch Two way Switch Electric Bell Bell Push Earth Connection 32
I.E.E. Regulations
The I.E.E. Regulations (or more specifically, BS 7671: 2001 Requirements for Electrical Installations, will hereinafter be referred to as the Regulations) provides a comprehensive list of the requirements which experience has shown to be necessary for a safe and efficient electrical installation. They cover the whole field of installation work and if an installation complies with the regulations it can be certain that the installation will be acceptable to every one concerned. Since, the Regulations are recognised as a National Code in the United Kingdom it has been necessary to write them in precise form. Therefore the first time readers may find the formalised wording difficult to understand. Further, the Regulations themselves state that they are not intended to instruct untrained persons. An average electrician sometimes may have difficulty in knowing what a regulation requires as well as the reason why a particular requirement is necessary. Therefore, a summary of more important points of the Regulations are included in this chapter in order for an electrician to get a general idea of the requirements. However more enthusiastic readers are advised to refer to the Regulations to gain a more precise knowledge. First edition of the Regulations was issued more than 100 years ago in 1882 by the Institution of Electrical Engineers, London. Thereafter, several editions and amendments have been issued from time to time to revise and update to suit changes in technology and attitudes towards safety. Its 15th Edition issued in 1981 drastically changed the layout maintained up to the 14th Edition, but there was no appreciable change of the contents from the 14th Edition. This was done in order to fall in line with international bodies like International Electro-technical Commission (IEC) and European Committee for Electro-technical Standardisation (CENELEC) aiming at a common set of wiring regulations. Then another change took place in 1992 when the 16th Edition was published as a British Standard jointly by the British Standards Institution and I.E.E. There were several subsequent amendments and a document has been published incorporating amendments up to 2004. It is the latest edition at present and it is referred to as, “BS 7671: 2001 (2004), Requirements for Electrical Installations, formerly the I.E.E. Wiring Regulations”. 33
Apart from the small changes that took place to cater to the changes in technology and attitudes towards safety, the fundamentals of 14th and 16th Editions are the same. The layout used in the 14th Edition (Metric Supplement issued in 1970) is in a more logical sequence and easier to follow for an average electrician engaged in domestic wiring and it is used in presenting the summary of the Regulations in this Chapter. But changes have been made to incorporate the alterations and additions included in the 16th Edition. There are some clauses in the 14th Edition which cannot be correlated directly to any clause in the 16th Edition. But closer inspection of some clauses in the 16th Edition permits, application of those clauses in the 14th Edition for domestic wiring in Sri Lanka. These clauses have been included separately at the end of the Chapter. Note: The IEE Regulations have been revised and 17th Edition has been issued after this monograph was drafted but before it was printed. The current edition is referred to as BS7671 (2008), Requirements for Electrical Installations IEE wiring Regulations. 17th Edition.
34
Summary of Regulations Relevant to Domestic Wiring Fundamental Principles Protection for safety
These requirements are for providing safety to people and property when using electricity from: i. shock currents ii. high temperature iii. moving electrical equipment iv. explosion
Protection against electric shock
Persons shall be protected against dangers that may arise from contact with (a) live parts of the installation and (b) exposed metal parts during a fault. Protection can be achieved by: i. preventing a current from passing through the body ii. limiting the current which can pass through a body to a safe value iii. automatic disconnection of supply under fault conditions
Protection against thermal effects
The electrical installation shall be arranged to avoid causing fires due to high temperature or electric arc. Persons, equipment and material adjacent to electrical equipment shall be protected against, burns, fires, and harm due to heat emitted by the electrical equipment. Requirements for Safety
Conductors and equipment
All electrical conductors shall be of sufficient size and current rating for the purpose for which they are to be used. All equipment shall be suitable for the maximum power demanded by the current using equipment. All conductors shall either be insulated or so placed to prevent danger. Every electrical joint and connection shall be of proper construction as regards conductance, insulation, mechanical strength and protection. 35
Good workmanship and proper materials shall be used. All equipment shall be constructed, installed and protected to prevent danger. Fuses and circuit breakers
Every Electric circuit and sub-circuit shall be protected by a suitable fuse or a circuit-breaker. If an installation is protected by a single residual current device, this shall be placed at the origin of the installation. No fuse or single pole circuit breaker shall be inserted in the neutral conductor.
Connection of switches
All fuses and single pole control devices such as switches, circuit-breakers, thermostats etc. shall be connected in the phase conductor only.
Emergency disconnection
There should be a main switch or a circuit breaker at a readily accessible position to cut-off the supply in case of an emergency.
General provision for earthing
In every installation, a consumer’s earthing terminal shall be provided adjacent to the consumer’s terminal. An earth conductor (now known as circuit protective conductor) shall be provided through out the installation which shall be connected to the earthing terminal.
Installation of equipment
Every piece of equipment which requires operation or attention of a person shall be so installed that adequate and safe means of access and working space are available.
Special conditions
All equipment likely to be exposed to weather shall be so constructed and protected as may be necessary to prevent danger from such exposure.
Additions to an installation
Before adding any new load to an existing installation the current rating and condition of the cables and switchgear should be checked to make sure that they could carry the additional load. On completion of an installation or an extension or alteration of an installation appropriate tests and inspection shall be carried out.
Testing and Inspection
36
Control, Distribution and Overcurrent Protection
Protective equipment
Protective equipment shall be selected to provide: i. overcurrent (overload, short circuit) protection ii. earth fault protection iii. overvoltage protection iv. undervoltage and no-voltage protection. The protective devices shall operate at values of current, voltage and time which are suitably related to the circuits and danger.
Control of supply
Every consumer’s installation should have switchgear at a readily accessible position with means of isolation, overcurrent protection and protection against electric shock.
Isolation
A linked main switch or circuit-breaker shall be provided as near as practicable to the origin of every installation as a means of switching the supply (on and off) on load and as a means of isolation. Where an installation is supplied from two sources, a main switch shall be provided for each source of supply and a notice shall be fixed near these switches to indicate that both should be switched off to isolate the installation. Alternatively a suitable interlock system shall be provided. The device for isolation shall isolate all live supply conductors from the circuit concerned. A main switch intended for operation by unskilled persons e. g. household or similar installation, shall interrupt both live conductors of a single phase supply. Means of isolation shall preferably be provided by a multipole device which disconnects all poles of the relevant supply, but use of single pole devices situated adjacent to each other is not excluded. Provision shall be made for disconnecting the neutral conductor. Where there is a joint, it shall be such that it is in an accessible position, can only be disconnected by means of a tool, is mechanically strong and will reliably maintain electrical continuity 37
Where an installation is supplied from two sources and one of them requires independent earthing and it is necessary to ensure that only one means of earthing is applied at anytime, a switch may be inserted between the neutral point and the earthing, provided that the switch is a linked switch arranged to disconnect and connect the earthing conductor for the appropriate source at the same time as the live conductors. Overcurrent protection
A fuse or a circuit-breaker shall be provided to break any overload current flowing in the circuit conductors before such a current causes a damage. Every circuit shall be designed so that a small overload of long duration is unlikely to occur. The fuse or the circuit-breaker shall satisfy the following conditions: i. nominal current of the fuse or current setting of the circuit-breaker is not less than the design current of the circuit ii. nominal current of the fuse or current setting of the circuit-breaker does not exceed the current carrying capacity of the lowest-rated conductor of the circuit. iii.tripping current of the protective device does not exceed 1.45 times the current carrying capacity of the lowest-rated conductor of the circuit. For a circuir-breaker compliance with condition (ii) above also results in compliance with condition (iii). For a rewirable fuse to satisfy the condition (iii), its nominal current shall not exceed 0.725 times the current rating of the lowest-rated conductor in the circuit. The protective device shall be placed at the point where a reduction occurs in the value of current carrying capacity of the conductors of the installation due to a change in cross-sectional area, method of installation, type of cable or 38
conductor, or in environmental conditions. This requirement does not apply for a conductor of length not exceeding 3m, when the overload protective device is placed along the run of the conductor, provided there is no branch circuit or a current using outlet between the point where the current carrying capacity of the conductor is reduced and the position of the protective device. Protection against electric shock
Protection against electric shock shall be provided by appropriate measures providing protection against direct contact and indirect contact.
Protection against direct contact
For protection against direct contact one or more of the following measures shall be used: protection by i. insulation of live parts ii. barrier or enclosure iii. obstacles iv. placing out of reach
Supplementary protection can be provided by a residual current device but it shall not be used as the sole means of providing protection against direct contact. Such a residual current device shall have a maximum sensitivity of 30 mA.
Protection against indirect contact
For protection against indirect contact one of the following measures shall be used: protection by i. earthed equipotential bonding and automatic disconnection of supply ii. Class II equipment iii. Non-conducting location iv. earth-free local equipotential bonding v. electrical separation
Earthed equipotential bonding
This measure is generally applicable and is intended to prevent occurrence of a voltage between two simultaneously accessible metal parts that could give electric shock. All non-current carrying metal work shall be connected via main earthing terminal to a common earth electrode. All exposed metal parts of other services (water, gas etc.) in within reach should be connected to the main earthing terminal. 39
One or more of the following types of protective devices shall be used: i. residual current device ii. an overcurrent protective device (Residual current device is preferred) The following condition should be fulfilled for each circuit: RA Ia ≤ 50V Where RA is the sum of the resistances of earth electrode and the protective conductors connecting to the exposed metal part. Ia is the current causing the automatic operation of the protective device within 5 s. In case of a residual current device, Ia is the residual operating current Every socket-outlet circuit shall be protected by a residual current device. A circuit protective conductor shall be provided in every final circuit. Earthing terminal of every socket-outlet shall be connected to the circuit protective conductor. All lighting points shall be provided with an earthing terminal and connected to the circuit protective conductor. All switches shall be provided with an earthing terminal and connected to the circuit protective conductor. Circuit protective conductors
Cross sectional area of every circuit protective conductor not contained in a multi core cable shall have the cross section as follows:
40
Cross sectional area of phase conductor (S) mm2 S ≤ 16 16 < S < 35 S ≥ 35
Cross sectional area of circuit protective conductor (Sp) mm2 S 16 S/2
(A complicated formula is available in the Regulations to determine a more accurate smaller cross sectional area for earth conductor to be used by competent persons if desired.) Connection with earth
Main earthing terminal shall be connected via an earthing conductor to an earth electrode. Every protective conductor up to and including 6 mm2 other than copper strip shall be insulated. The connection of an earthing conductor to an earth electrode should be soundly made and be mechanically and electrically satisfactory. Every connection of an earthing conductor to an earth electrode shall be readily accessible.
Labelling
When the purpose of a switch or a circuit-breaker is not obvious, a label to indicate the equipment it controls shall be provided. An indication of the circuit protected and the appropriate current rating of the fuse or circuit-breaker shall be provided in the cover / case of every distribution board.
Maximum demand and diversity
The maximum demand of an installation shall be assessed. Diversity may be taken into account for the purpose.
Nature of demand
The number and type of circuits shall be determined from the knowledge of: i. location of points of power demand ii. loads to be expected on the various circuits. 41
Final circuits
Where an installation has more than one final circuit, each final circuit shall be connected to a separate way of the distribution board. The wiring of each final circuit shall be electrically separate from that of every other final circuit.
Conductors and Cables: Choice and Construction Voltage rating
The voltage rating of every cable shall be not less than the declared or nominal voltage of the circuit.
Current rating
The current rating of every cable shall be suitable for the design current and the current which will likely to flow through it in abnormal conditions, for such periods of time as are determined by the characteristics of the protective devices concerned.
Voltage drop
The size of every conductor shall be such that the drop in voltage from the consumer’s terminal to any point in the installation does not exceed 4 % of the declared or nominal voltage when the conductors are carrying the full load current.
Crosssectional area of conductors
The cross-sectional area of conductors shall be determined according to: i. the maximum temperature ii. the voltage drop iii. effects of short-circuit and earth fault currents iv. other mechanical stresses v. the maximum impedance for operation of short- circuit and earth fault protection
Protection against mechanical damage
A wiring system shall be selected and erected so as to minimize damage to sheath and insulation of cables and insulated conductors and their terminations, during installation, use and maintenance. The conduits for each circuit shall be completely erected before any cable is drawn in. The radius of every bend in a wiring system shall be such that conductors and cables shall not suffer damage. A flexible wiring system shall be installed so that extensive tensile and torsional stresses to the conductors and connections are avoided. 42
Identification of conductors
Every single core cable and every core of a twin or multi core cable shall be identifiable at its terminations and preferably throughout its length as prescribed in Table 1. TABLE 1. Identification of conductors
Function Colour Identification Protective (including earthing) conductor Green or Green and yellow Phase of a. c. single phase circuit Red (or yellow or blue*) Neutral of a. c. single phase or three Black phase circuit Phase R of a. c. three phase circuit Red Phase Y of a. c. three phase circuit Yellow Phase B of a. c. three phase circuit Blue * As alternative to the red in large installations, on the supply side of the final distribution board. (Information from TABLE 51A, BS 7671: 1992)
The latest amendments to the Regulations have changed the colour coding as indicated in Table 1A. It will be mandatory to follow this new colour code for installations commencing on site after 31st March 2006. TABLE 1A. Identification of conductors (Mandatory for new installations from 1st April 2006) Function Protective (including earthing) conductor Phase of a. c. single phase circuit Neutral of a. c. single phase or three phase circuit Phase 1 of a. c. three phase circuit Phase 2 of a. c. three phase circuit Phase 3 of a. c. three phase circuit
Colour Identification Green and yellow Brown Blue Brown Black Grey
(Information from TABLE 51, BS 7671:2001(2004)
Terminations
All terminations of cable conductors shall be mechanically and electrically sound. Installation of Equipment
Lamp holders
A lamp holder shall not be connected to any circuit where the rated current of the fuse or circuit-breaker exceeds value given in Table 2. 43
TABLE 2. Overcurrent protection of lampholders Type of Lampholder Maximum rating (amperes) of fuse or circuit-breaker Small Bayonet Cap B15 SBC 6 Bayonet Cap B22 BC 16 Small Edison Screw E14 SES 6 Edison Screw E27 ES 16 Goliath Edison Screw E40 GES 16 (Information from TABLE 55B, 16th Edition of IEE Regulations)
A lampholder for a filament circuit shall not be installed in a circuit operating at a voltage exceeding 250 V. The outer contact of every Edison screw or single centre bayonet cap type holder shall be connected to the neutral conductor. Lighting points
At each of the fixed lighting point one of the following shall be used: i. a ceiling rose ii. a luminaire supporting coupler iii. a batten lampholder iv. a luminaire designed to be connected directly to the circuit wiring A lighting accessory or luminaire shall be controlled by a switch or combination of switches.
Ceiling roses
Ceiling roses shall not be installed in circuits operating at a voltage exceeding 250 V. Not more than one flexible cord shall be connected to a ceiling rose.
Lighting fittings
Accessories used in pendent lights shall be suitable for the weight of the suspended light.
Plugs and socket-outlets
Every plug and socket-outlet shall confirm with the applicable British Standard listed in Table 3. 44
TABLE 3. Plugs and socket-outlets Type of plug and socket-outlet Rating (amperes) Fused plugs and shuttered socketoutlets, 2-pole and earth 13 Plugs, fused or non-fused, and socket-outlets, 2-pole and earth 2,5,15,30 Plugs, fused or non-fused, and socket-outlets protected type, 2-pole and earth 5,15,30 Plugs and socket-outlets (industrial type) 16,32,63,125
Applicable British Standard BS 1363 BS 546
BS 196 BS EN 60309-2
(Information from TABLE 55A, 16th Edition of IEE Regulations)
A plug and socket-outlet not complying with above may be used for: i. the connection of an electric clock, provided that it is designed for the purpose and the plug incorporates a fuse of rating not exceeding 3 amperes. ii. the connection of an electric shaver, provided that the socket-outlet is incorporated in a shaver supply unit. Every plug and socket-outlet shall comply with the following: It shall not be possible for any pin of a plug to make contact with any live contact of the socket-outlet while any other pin of the plug is exposed. It shall not be possible for any pin of a plug to make contact with any live contact of any socket-outlet within the same installation other than for which the plug is designed. Every plug and socket-outlet shall be of the non-reversible type, with provision for connection of a protective conductor. Every socket-outlet for household use shall be of the shuttered type. A socket-outlet on a wall or similar structure shall be mounted at a height above the floor or any working surface to minimize the mechanical damage to the socket-outlet or the plug. 45
Socket-outlets shall be provided at conveniently accessible places, where portable equipment is likely to be used. Switching for safety
A fireman’s switch shall be provided in the low voltage circuit supplying; i. exterior electrical installations ii. interior discharge lighting operating at a voltage exceeding 1000V. For an exterior installation, fireman’s switch shall be outside the building and adjacent to the equipment. For an interior installation, fireman’s switch shall be in the main entrance to the building
46
Excerpts from Previous Editions th th The Regulations listed below were in the 14 /15 Editions but cannot be th directly referred to a clause in the 16 Edition. But these can be considered still applicable to domestic wiring in Sri Lanka, when the Regulations in the 16th Edition are studied in detail. Earth fault currents
Every circuit shall be arranged so as to prevent dangerous earth fault currents. Where the earth resistance is high a residual current device or an equally effective device shall be provided.
Detached buildings
Where a consumer’s installation is in two or more detached
Diversity
When calculating the size of the conductors and switchgear
buildings, separate means of isolation shall be provided.
of final circuits, total current of the connected load shall be taken into account. Overcurrent protection
Every means of overcurrent protection shall be suitable for
Provision of protective devices
Every conductor in the installation shall be protected
the maximum short circuit current attainable.
against overcurrent by either a fuse inserted in every live conductor of the supply or a circuit-breaker having an overcurrent release fitted in each phase conductor of the supply. The devise shall be fitted at the origin of the circuit which the conductor forms part. The current rating of every fuse used for this purpose shall not exceed, that of the lowest rated conductor in the circuit protected. Every circuit-breaker used for this purpose shall operate when the circuit protected is subjected to a sustained overcurrent of 1.5 times the rating of the lowest rated conductor.
47
Exemptions: i. circuits in which omission of overcurrent protection is required to prevent damage: e.g. the shunt trip circuit of a circuit-breaker ii. auxiliary circuits of equipment contained entirely within the equipment iii. radial or ring circuits and spurs installed in accordance with the Regulations iv. 0.5 mm2 flexible cord protected by a 5A fuse or circuit-breaker in a distribution board v. 0.75 mm2 or 1.0 mm2 flexible cord protected by a 13A fuse in a 13A fused plug or a fused spur-box 2 vi. 1.0 mm flexible cord protected by a 20A circuitbreaker in a distribution board Final circuits
To facilitate disconnection of each final circuit for testing, the neutral conductors shall be connected at the same order as that in which the live conductors are connected to the fuses or circuit breakers.
Final circuits of rating exceeding 15A
The circuits shall not supply more than one point, with the following exceptions: Final circuits of ratings in the range of 15 to 30A may be used to supply a number of 13A socket-outlets with fused plugs
Domestic ring and radial circuits
Radial or ring circuits may be installed to serve 13A Socketoutlets. Each circuit conductor of a ring final circuit shall be run in a form of a ring, commencing from a way in the distribution board, looping into the terminals of the socket-outlets and joint boxes (if any) connected in the ring and returning to the same way of the distribution board. The circuit protective (earth) conductor shall also be run in the form of the ring having both ends connected to the earth terminal at the distribution board. Each ring final circuit conductor shall be looped into every 48
socket-outlet or joint box and shall remain unbroken throughout its length or, alternatively, if the conductor is cut its electrical continuity shall be ensured by appropriate joints complying with the regulations. Bending radius of cables
The internal radius of every bend in a cable shall not be less than the appropriate value stated in Table 4. TABLE 4. Minimum internal radii of bends in fixed wiring
Insulation Finish Rubber or PVC Non-armoured Armoured
Overall diameter Not exceeding 10 mm Between 10 and 25 mm Exceeding 25 mm Any
Minimum bending radius 3 times the overall dia. 4 times the overall dia. 6 times the overall dia. 6 times the overall dia.
(Information from TABLE B1.M, 14th Edition of IEE Regulations)
Terminations
Every connection at a cable termination shall be made by means of a terminal, soldering or compression type socket. The socket shall securely contain and anchor all the wires of the conductor and the cable shall not impose any appreciable mechanical strain on the terminal or the socket.
Conduit systems
Conduits shall be securely fixed and adequately protected where they are liable to mechanical damage. Inspection boxes, draw in boxes, etc. should be so situated that they remain accessible throughout the life of the installation. The number of cables run in a conduit shall be such as to permit easy drawing-in. The number and sizes of cables shall be such that a space factor of 40% is not exceeded. The inner radius of a conduit bend must not be less than 2 1/2 times the outside diameter of the conduit and also should not be less than 4 times the diameter of the largest cable installed. Solid elbows and tees may only be used at ends of conduits immediately behind accessories or lighting fittings. 49
All burrs shall be removed from the ends of lengths of conduits and outlets must be bushed to guard against the possibility of abrasion of the cables. Where joints in the cables are required substantial boxes should be used. At every termination the cables must be enclosed in an enclosure of incombustible material. Conduit system shall be self-ventilating and drainage outlets should be provided wherever condensed moisture might collect. Metal conduit systems shall be efficiently earthed. Metal conduit systems
In metal conduit systems all phase conductors, the neutral conductor and the circuit protective (earth) conductor of each circuit must be drawn into the same conduit. Where the conduits themselves are particularly liable to mechanical damage adequate extra protection against the damage should be provided. Where conduits pass through floors, walls, partitions or ceilings the holes should be made good with cement or similar material. Where conduit is installed in a damp situation or is exposed to the weather, heavy gauge conduit should be used and this should have a corrosion resistant finish.
Flexible conduit systems
Flexible metal conduit is not suitable for use as a circuit protective (earth) conductor. A separate circuit protective conductor is required. Flexible cable or cords shall be used for all connections to moving apparatus Flexible cable or flexible cords shall not be used as fixed wiring unless contained in earthed metal or other noncombustible and mechanically strong enclosure. This requirement does not apply to short lengths of flexible cables or cords used for final connections to fixed apparatus. Non-sheathed flexible cables or cords shall not be drawn into conduits or ducts. 50
Suspensions from nonmetallic boxes:
Where a non-metallic outlet box of thermo plastic material (e.g. PVC) is used for suspension of a lighting fitting, care shall be taken to ensure that the temperature of the box 0 does not exceed 60 C. The mass suspended from the box shall not exceed 3 kg.
Equipment liable to cause overheating
All filament lamps shall be placed or guarded against ignition of any flammable material.
Ceiling roses
No terminal shall remain live in a ceiling rose when the switch is off.
Isolated metal
Following metal components may be isolated without connecting to the circuit protective (earth) conductor: Short isolated lengths of metal used for protection of cables with non-metallic sheath Metal cable clips Metal lamp caps Screws or Nameplates Metal chains used to suspend lighting fittings Metal lighting fittings which cannot be touched
Electrical discharge circuits
Every switch used to control a discharge lighting circuit (including fluorescent lighting) shall have a current rating of not less than twice the total steady current which it is required to carry.
When a distribution board without a back or side is installed, it shall be fitted only to a non-combustible surface.
In a discharge lighting circuit the neutral conductor shall have a cross section area not less than that of the phase conductors. Earth fault protection
Earth fault protection may be given by fuses or circuit breakers if the leakage current is more than: (a) 3 times the current rating of any semi enclosed fuse (b) 1.5 times the tripping current of any circuit breaker 51
Room containing a bath
The latest amendments to the 16th Edition of the Regulations have changed the requirements apply to rooms containing baths, showers and cabinets containing shower and/or bath. The areas are divided into four zones basically containing the bath or shower basin, immediate vicinity and two outer areas, allowing some submersible, jet-proof and waterproof equipment and accessories to be installed in these areas. However, for domestic wiring utilising normal accessories the previous requirements stated below could be considered valid: No switches or socket-outlets should be installed in a bathroom. Switches operated by insulated cords and shaver outlets incorporating isolating transformers are permitted. Electric shavers with isolating transformers may be installed in a room containing a bath or a shower.
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From the “On-site Guide” When the Regulations became a British Standard some of the appendices published up to then with the Regulations were removed as they were not falling in line with the format of British Standard publications. IEE published a separate document called “On-site guide” to expand and clarify some aspects of the 16th Edition incorporating the material not included in the Edition, but which was included in earlier issues. The following are some excerpts. Maximum Demand and Diversity The current demand of a final circuit is determined by adding up the demands of all points and equipment in the circuit and where appropriate making an allowance for diversity. Typical current demands to be used in this summation are given in Table 5. TABLE 5. Current demand to be assumed for points and equipment Point or equipment Current demand to be assumed 15A Socket-outlet 15 ampere 13A Socket-outlet 13 ampere* 5A Socket-outlet 5 ampere Lighting point** Current equivalent to the connected load with a minimum of 100 watt per lamp holder Household cooking appliance The first 10A of the rated current plus 30% of the remainder plus 5A if a socket-outlet is incorporated in the control unit Electric clock, or electric shaver supply unit, bell transformer May be neglected All other fixed or free standing appliances British Standard rated current Note:
* For ring and radial final circuits with many 13A socket-outlets current demand to be assumed is the rating of the fuse or m. c. b. of that particular circuit. ** Final circuits of discharge lights are arranged so as to be able to carrying the total steady current (current drawn by the lamp, controlgear and harmonic currents). Where more exact information is not available, the demand in volt-amperes is taken as the rated lamp watts multiplied not less than 1.8. (Source: Table 1A, Appendix 1, “On-Site Guide”)
The current demand of a circuit supplying a number of final circuits may be assessed by using the allowances for diversity given in Table 6. This should be applied to the total current demand of all the equipment supplied and not by adding up the current demand of individual final circuits. 53
TABLE 6. Allowances for diversity Types of premises
Purpose of final circuit
Small shops, stores, Small hotels, offices boarding houses, guest houses
Houses
1. Lighting
66% of total current demand
90% of total current demand
75% of total current demand
2. Cooking appliances
10 amperes + 30% of full load of cooking appliances in excess of 10 amperes + 5A if socket-outlet incorporated in unit
100% full load of largest appliance + 80% f. l. of 2nd largest appliance + 60% f. l. of remaining appliances
100% full load of largest appliance + 80% f. l. of 2nd largest appliance + 60% f. l. of remaining appliances
100% full load of largest motor + 80% f. l. of 2nd largest motor + 60% f. l. of remaining motors
100% full load of largest motor + 50% f. l. of remaining motors
100% full load of largest appliance + 100% f. l. of 2nd largest appliance + 25% f. l. of remaining appliances
100% full load of largest appliance + 100% f. l. of 2nd largest appliance + 25% f. l. of remaining appliances
3. Motors
4. Water Heaters (instantaneous type)
100% full load of largest appliance + 100% f. l. of 2nd largest appliance + 25% f. l. of remaining appliances
5. Water heaters (thermostatically controlled)
No diversity allowable
6. Conventional arrangement of final circuits as given below
100% current demand 100% current demand of largest circuit + of largest circuit + 40% 40% of the current demand of every other of the current demand circuit of every other circuit
7. Socket-outlets other 100% current demand than those included in of largest point + 40% of the current demand 6 above of every other point
(Source: Table 1B, Appendix 1, “On-Site Guide”)
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100% current demand of largest point + 75% of the current demand of every other point
100% current demand of largest point + 75% of the current demand of every point in main rooms (dining rooms etc.) + 40% of the current demand of every other point
Conventional circuit arrangements; Conventional circuits given below satisfy the Regulations for overload protection, isolation and switching, together with requirements as regards current-carrying capacities of conductors. Final circuits using 13A socket-outlets A radial or ring circuit with spurs if any permanently connected equipment and unlimited number of socket-outlets. The floor area served by the circuit is determined by the known or estimated load but does not exceed the value given in Table 7. For household installations a single 30A ring circuit may serve a floor area up to 100 mm2 but consideration should be given to the loading in kitchen which may require a separate circuit Each socket-outlet of a twin or multiple socket-outlet unit should be counted as one socket-outlet Diversity between socket-outlets and fixed equipment has already been taken into in Table. 7 and no further diversity should be applied. TABLE 7. Final circuits using 13A socket-outlets Overcurrent protective Type of Minmum Maximum floor device circuit conductor size* area served Rating Type A mm2 m2 A1 Ring 30 or 32 Any 2.5 100 Cartridge fuse or A2 Radial 30 or 32 4 50 circuit-breaker
A3 Radial
20
Any
2.5
20
* The tabulated values of conductor size may be reduced for fused spurs. (Source: Table 9A, Appendix 9, “On-Site Guide”)
Where two or more ring final circuits are installed the socket-outlets are reasonably distributed among the circuits. Circuit protection The overcurrent protective device is of the type, and has the rating, given in Table 7 55
Conductor size The minimum size of the conductor in the circuits and non-fused spurs is given in Table 7. However, if the cables of more than two circuits are bunched together or ambient temperature exceeds 300C, the size of the conductor is increased and is determined by applying the appropriate correction factors from Table 8, so that the conductor size is then corresponds to a current-carrying capacity not less than: i. 20A for circuit A1 ii. 30A or 32A for circuit A2 iii. 20A for circuit A3 The conductor size of a fused spur is determined from the total current demand served by that spur, which is limited to a 13A. iv. when such a spur serves socket-outlets the minimum conductor size is 1.5 mm2 for pvc insulated cables with copper conductors Cooker circuits in household premises The circuit supplies a control switch or a cooker unit which incorporates a socket-outlet. The rating of the circuit is determined by the assessment of the current demand of the cooking appliance(s) and control unit socket-outlet if any, in accordance with Table 6. A circuit of rating exceeding 15A but not exceeding 50A may supply two or more cooking appliances where these are installed in one room. The control switch should be placed within two metres of the appliance. Where two stationary appliances are installed in one room, one switch may be used to control both appliances provided that neither appliance is more than two metres from the switch. Correction factors for current carrying capacity The current-carrying capacity of a cable for continuous service is affected by ambient temperature, by grouping and by enclosing in a thermal insulation material. The cables are rated for 30oC and domestic wiring in Sri Lanka is unlikely to exceed this temperature. Enclosing in a thermal insulation would also be a rare occurrence. The correction factors for grouping are given in Table 8.
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TABLE 8. Correction factors for groups of more than one circuit Correction factor
Method of Installation
Number of circuits
2
3
4
5
6
7
8
9
10 11 12
Single core cables in conduit or 0.8 0.7 0.65 0.60 0.57 0.54 0.52 0.50 0.48 0.45 0.43 trunking on a wall or ceiling
Single layer clipped to a non metallic surface (Touching)
0.85 0.79 0.75 0.73 0.72 0.71 0.70 -
Single layer clipped to a nonmetallic surface (Spaced)
0.94 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.90
(Source: Table 4B1, Appendix 4, 16th Edition of IEE Regulations)
57
-
-
-
58
Selection of a System In deciding on a wiring system factors such as type of load to be supplied, type of building in which the installation is to be done, cost, durability, appearance, any adverse conditions and expected life of the installation should be considered. To facilitate this task one should have some knowledge of materials and systems employed in this kind of work. Conductors Some of the materials used as conductors in electrical engineering are silver, cooper, aluminium, brass, nichrome, manganin, tungsten and carbon. Silver is the best known conductor of electricity, but it is too expensive for general use. The contacts of some switches are plated with silver to make better contact. Copper is the material widely used in electric wires and cables. It has a low resistance, not very expensive, can be easily formed into wires and can be soldered easily. Aluminium is also used in electric wires and cables, but it is not suitable for smaller sizes. Brass is often used for terminals and various electrical fittings and accessories. Brass is harder than copper, can be easily machined, readily cast and easily soldered. Nichrome and manganin are used in heaters and resisters. Tungsten is used in electric lamp filaments and carbon is used in brushes. Insulation Rubber, PVC, XLPE, ceramic, bakelite and Perspex are some of the insulating materials used in electrical engineering. Most widely used cable insulation for many years was rubber (Natural rubber vulcanized to give strength). Poly Vinyl Chloride (PVC), which has now practically replaced rubber, is a plastic compound impervious to moisture and resistant to most chemicals. It has the disadvantage that it softens at high temperatures and it becomes brittle at very low temperatures. Though it can be produced in a form which is suitable for higher temperatures, it cannot be used in very low temperatures. Cross Linked Polyethylene (XLPE) is also now used as cable insulation. Some of the materials used as cable insulation can be modified for use as protective sheathing. Toughened rubber was used as a sheath in rubber insulated cables and PVC modified to make it mechanically stronger is used for PVC cables. PVC has the advantages being resistant to oils and most chemicals and being self extinguishing. PVC sheathing is often of a softer composition than PVC insulation, which facilitates its stripping. 59
Wiring systems Bare conductor, cleated wiring, wood casing, armoured cable, mineral insulated copper sheathed cable (MICC), catenary supported wiring, metal conduit, insulated conduit, cable duct, bus-bar trunking and under floor concealed ducts are some of the wiring systems in use. The most popular wiring system for permanent wiring is the conduit system. The practice of threading the cables through the conduits and conduit fittings and then connecting elbows and tees to the conduit was customary in the early days of electrical wiring where the first cost was the only consideration. After the completion of the building it was quite impossible to withdraw or rewire any of the cables when faults developed, as they frequently did. This method of wiring is contrary to the Regulations which require that the conduit system to be complete, before the cables are drawn in. The screwed steel conduit system is popular for industrial applications. Screwed steel conduits are made to standard sizes with standard threads, standard conduit fittings, saddles and other accessories. Advantages of screwed steel conduit system are that it affords good mechanical protection, permits easy re-wiring when necessary, minimizes fire risks, provides efficient earth continuity and presents a pleasing appearance if properly installed. But in domestic wiring screwed steel conduits are hardly used nowadays. The advantages of PVC conduit system are that it is less costly, can be installed much more quickly than steel conduits, non corrosive, impervious to most chemicals, weather proof and will not support combustion. The disadvantages are that it is not suitable to be exposed to temperatures below o 15oC or above 65 C and will not withstand blows and strains as much as steel conduits. PVC conduits are available in sizes of 16mm(5/8”), 20mm(3/4”), 25mm(1”), and 32mm(1 1/4”) of nominal diameter. It is generally the insulated conduits used in domestic wiring these days. In this system circuits are wired with single core PVC insulted cables in PVC conduits for mechanical protection. The Regulations allow using PVC cables without a sheath in conduits. But this type of cables is not readily available in the market in this country and is not normally used in domestic wiring. What is generally used is PVC insulated PVC sheathed cables with copper conductor in PVC conduits. Overcurrent protection The ordinary rewirable fuse is still used in many switchgear. Its advantages are that it is cheap and costs practically nothing to replace but there is nothing very much more that can be said in favour of them. It is unreliable as it is subject to deterioration due to oxidation and scaling, and this may result in reducing its current carrying capacity and which in turn might cause overheating, and further deterioration and overheating 60
until serious damage is caused to the switchgear before the fusing element reaches sufficient temperature to melt and open the circuit. Some of the other disadvantages are that inability to clear heavy faults and the ease with which the fuse may be replaced by an unskilled person using one of incorrect size. Fuse elements which are made of the same material and having the same thickness and length do not always blow off at the same value of the current. One reason is that they can carry some overcurrent for some time. Therefore they blow off at different times depending on the value of the overcurrent. The other reason is ageing. The fuse element is heated up to some extent when carrying current and cooled again when the flow is stopped. This causes deterioration and ultimate blowing of the fusing element at a lesser current, sometimes even below the rated value. Modern circuit breakers of reputable makes are designed to handle safely heavy short-circuit currents within their rated breaking capacities. Circuit-breaker has several advantages over any type of fuse. The major two are: 1. In an event of overload all poles are simultaneously opened. 2. The circuit can be closed again quickly. A remarkable difference between a fuse and a circuit-breaker is the time that it takes to operate in an overload. The circuit-breakers can be designed to operate at a predetermined time for a given overcurrent. A fuse will take more than four hours to operate when an overcurrent of 1.5 times the rated current flows continuously. On the other hand a circuit-breaker would trip in less than four hours in a similar situation. The time taken would vary depending on the type of the circuit-breaker. This fact is taken into account in the Regulations and it requires the current carrying capacity of a cable protected by a semi-enclosed fuse to be reduced to a half of the capacity of the same cable protected by a circuit-breaker of the same rating. The miniature circuit breaker is more expensive in first cost but it has much to commend it especially as it can be made to incorporate an earth leakage trip. If they operate due to overload they can readily be reset. In domestic wiring the main switch used nowadays is generally a MCB. MCCB is also sometimes used in large houses. Means isolation The Regulations allow the consumer not to provide a main switch if the supply authority agrees for the consumer to use their equipment to protect the installation. But the consumer needs to provide a means to isolation at a readily accessible position. Therefore it is allowed to use an isolator which opens all the poles (i.e. double pole for single phase and four pole for three phase supplies) in place of the main switch. But it must be noted that most isolators may not effectively open the circuits in case of a heavy short circuit and it is not advisable to use an isolator for the purpose. 61
Earth fault protection In the early days, current operated e.l.c.b.s were much more expensive and less sensitive than voltage operated e.l.c.b.s and the latter were extensively used. With the development of electronic devices, sensitivity of the residual current detection devices was greatly improved and the cost has been appreciably reduced. Now there are devices combining both overcurrent and earth fault protection in the same unit. The device generally used for earth fault protection in domestic wiring nowadays is RCCB, but voltage operated earth leakage circuit breakers are still in use in most of the old installations. (Note: 15th edition of the Regulations refer to them as Fault Voltage Operated Circuit Breakers, but 16th Edition makes no reference at all implying that the use of voltage operated circuit breakers is now obsolete). Distribution boards There are many types of distribution boards available in the market and it is wise to select one which provides plenty of wiring space and is of sufficient size to accommodate the cables, which have to be connected to them. Two main categories are those that are surface mounted and those mounted flush with the surface. The selection in this regard lies basically with the likings of the owner of the dwelling. Distribution boards can be divided to three different categories when based on the protective device used for final circuits: Those fitted with (1) rewirable fuses, (2) HRC fuses and (3) miniature circuit-breakers (MCB). HRC fuses are not likely to be present in domestic installations in Sri Lanka. The trend now is to use distribution boards with MCBs in place of rewirable fuses. However fuses are still used in some installations. An interesting point about fusing and fuse protection is that circuit fuses protect the circuit cables being overloaded and should also prevent main fuses from operating in case of a local short circuit. There is however no guarantee that circuit fuses will protect any current consuming device from becoming overloaded, especially on circuits consisting of more than one point. Layout After the positions of the electrical points, switches, DB etc. are finalized it is possible to decide on the conduit layout which gives the shortest cable paths. When choosing the route to be followed by the wiring in a building the following factors must be taken into account as far as possible: i. Cables should not be located in positions where they are subject to the risk of mechanical damage or liable to deteriorate because of vibrations, moisture, heat, corrosive environment etc. ii. It is an advantage if the runs are easily accessible both for installation and maintenance, at the same time wiring should be as unobtrusive as possible. 62
iii. The routes chosen should run as directly as possible, so avoiding the use of an excessive quantity of materials and keeping voltage drops to a minimum. The relative importance of these factors varies so much from one installation to another and no hard and fast rule can be laid down. The Regulations state that every cable and flexible cord shall have a current rating either not less than that of the fuse element or not less than two-thirds of the operating current of an instantaneous circuit breaker. If a circuit designed to carry 15A wired with 2.5 mm2 cable it could be protected by a fuse rated to carry 15A (fusing current of a 15A fuse element would be around 29A). If a pendent consisting of a 0.5 mm2 flexible cord is connected to this circuit and a 15A heater is connected to the pendent by an adapter, the fuse would not operate, but the flexible cable would get dangerously hot, as it is rated to carry only 3A. It is a well known fact that this kind of heater should not be connected to a lamp holder, but it is very often done and circuits must be protected to ‘prevent danger’ from such misuse, by choosing a suitable fuse rating to protect the flexible cord. The Regulations divide the final circuits to two broad categories: those rated less than 15A and those rated equal or more than 15A. A circuit feeding more than 15A is not permitted to be connected to any other point. (Except in the case of 13A socket-outlets and cookers). Therefore a separate circuit is required for each point drawing more than 15A. If the lighting load in a house does not exceed 15A, all points may be wired on one circuit. Then the circuit need to be wired with 1.5 mm2 cable and protected by a 15A fuse, but it would also be necessary to use 1.5 mm2 flexible chord for all pendants. This rather extreme example shows that although the Regulations would permit such a circuit, it would prove very inconvenient to the user and there would be no saving in materials or labour. Such single circuits are generally discouraged in domestic installations. In actual practice domestic lighting circuits are now generally rated at 5A. This means that they are controlled by a 5 or 6A fuse or a MCB, wired with 1.0 mm2 cable and 0.75 mm2 flexible cords together with ordinary B.C. lamp holders. (For pendent lamps 0.5 mm2 flexible cords are allowed as an exception). If all lights in a house are wired on to a single circuit the whole premises will be plunged into total darkness when the final circuit fuse or MCB operates. Therefore it is always advisable to provide at least two lighting circuits, however small the premises is. If 5A socket-outlets are to be installed then only one may be installed to a 5A circuit, two to a 10A circuit and three to a 15A circuit. The usual practice is to provide two 5A socket-outlets protected by a 10A fuse or a MCB in a circuit wired with 1 mm2 cable. 63
In the case of final circuits with rated capacity exceeding 15A, no diversity factor can be applied as it serves only one point. But cable size may have to be increased to prevent excessive voltage drop. The cables must be rated to take the full current of the point or equipment which will be fed by the point, whichever is greater. There are socket-outlets and plugs of different ratings used for various current ratings of equipment in use. This had been a nuisance for the average householder in finding the correct type of accessory for each kind of equipment. An opportunity arose to get rid of this problem in the United Kingdom soon after the Second World War when most of the buildings had to be reconstructed. The 13A socket-outlet with a fused plug was introduced for the first time as a solution and to keep the cost of wiring as low as possible. Having long lengths of flexible cable leads from socket-outlets to portable equipment spreading all over the house is a nuisance and bound to create accidents. In order to avoid this, one would like to have as many socket-outlets as possible, close to positions where such equipment is in use. For example, in a kitchen or a pantry, every worktop should be provided with a number of outlets and in a living room it will be convenient and safe to have different outlets for the TV set, the booster, the radio, the audio equipment, the table lamp, the table fan and so on. These suggest a large number of closely spaced outlets. If they are to be wired back separately, the cost of cables would be prohibitive. Therefore many people are not willing to install the necessary number of outlets. Since these outlets, if provided, are closely spaced, most probably only a small portion of them would be in use at any given time. Thus, the principle of diversity could be applied to this situation to bring down the size of the feeding cable. Because the outlets are spread over a large area, it is possible to form them into a ring by bringing the both ends of the cables to the distribution board by introducing small lengths of cables. This makes each outlet to have two parallel paths of supply, which enables to reduce the size of the cable further. Thus a 30A fuse or a MCB could easily feed a circuit with twenty outlets. This makes it necessary to provide a fuse of appropriate rating at the outlet to protect the flexible lead to the portable equipment and therefore a fuse is inserted inside the plug top. By selection of the rating of this fuse suitably, both the flexible cable and the equipment could be protected. This could be used for any equipment drawing less than 13A by changing the fuse in the plug to an appropriate rating. The main advantages are that any appliance can be with a load not exceeding 3 kW could be connected with perfect safety to any socket and as 64
many as ten socket-outlets could be wired to a circuit. With this system cost of providing three 13A socket-outlets in a room would not exceed the cost of installing one 15A socket-outlet plus a three-way adapter. The Regulations regarding circuits containing 13A socket-outlets have been changed from 14th to 16th editions. Earlier there was a limit to the number of outlets connected to a particular circuit, now it has been amended to limit the area served instead of outlets connected. The Appendix relevant to this stipulation has now been shifted from the Regulations proper to the “On Site Guide” as per British Standards Institution requirements. (The relevant particulars were given earlier). Types of final circuits Final circuits found in domestic wiring can be divided into the following groups and each of which needs different treatment at the planning stage: Those feeding lighting points Those with 5A socket-outlets Those with one 15A socket-outlet Those with 13A socket-outlets Note: One 5A socket-outlet could be incorporated into a lighting circuit provided that the total demand does not exceed the rating of the circuit. Selection of cables The current ratings of cables depend on their physical properties as well as the installation conditions. In deciding the size of the cable for a particular purpose, it is necessary to consider the voltage drop in addition to the current rating. IEE Regulations include tables giving the values of voltage drop per ampere per metre. It is indicated that the values have to be modified to allow for ambient temperature, grouping of cables, installing methods and the Regulations give correction factors which have to be used to multiply the given value to arrive at the corrected value. It also requires applying different ratings on the type of protective devices. If MCBs are used to provide overcurrent protection, supplied tables can be applied direct and when semi-enclosed fuses are used the values have to be multiplied by a factor of 0.725. Table 9 gives the relevant particulars for most commonly used cables for domestic wiring.
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TABLE 9. Current ratings applicable when protected by circuit breakers Clipped direct to surface or cable tray Nominal Number Two Cables 3 or 4 Cables 3 or 4 Cables Two Cables and cross Three phase Three phase Single phase Single phase sectional diameter Volt area of wires Current Volt drop Current Volt drop Current Volt Current drop 2 mV per drop rating mV per mm No./mm rating rating mV per rating Amp per mV per A Amp per A Amp A A Metre Amp per Metre per Metre Metre 1 1/1.13 14 42 12 37 17 42 16 37 1.5 1/1.38 17 28 14 24 21 28 20 24 2.5 1/1.78, 24 17 21 15 30 17 26 15 7/0.67 4 7/0.85 32 11 29 9.2 40 11 36 9.2 6 7/1.04 41 7.1 37 6.2 50 7.1 45 6.2 10 7/1.35 55 4.2 51 3.7 68 4.2 61 3.7 16 7/1.70 74 2.7 66 2.3 90 2.7 81 2.3 25 7/2.14 97 1.7 87 1.5 118 1.7 106 1.5 35 19/1.53 119 1.3 106 1.1 145 1.3 130 1.1 Installed in conduits or trunking
(Source: IEE Regulations - 15th Edition)
(Note: Tables published in the 16th Editions have a slightly different format. Current ratings and volt drops are given in two separate sets of tables and for cables above 16 mm2 voltage drop has been divided into resistive and reactive components.) Table 10 which was extracted from the data published in the 14th edition of the Regulations can be applied direct when rewirable fuses are used for protection of final circuits.
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TABLE 10. Current Ratings applicable when protected by semi-enclosed (rewirable) fuses 3 or 4 cables in conduits or trunking Volt drop Current Rating mV per Amp A per Metre
3 or 4 cables clipped direct to a surface or cable tray Volt drop Current Rating mV per Amp A per Metre
Nominal cross section Area mm2
Number and diameter of wires No./mm
1 1.5 2.5 4
1/1.13 1/1.38 1/1.78, 7/0.67 7/0.85
9 11
35 23
12 15
35 23
16 22
14 8.8
20 27
14 8.8
6
7/1.04
28
5.9
34
5.9
10 16 25 35
7/1.35 7/1.70 7/2.14 19/1.53
39 53 71 88
3.9 2.2 1.4 1.0
46 61 80 98
3.5 2.2 1.4 1.0
(Source: IEE Regulations - 14th Edition)
TABLE 11 Data on Flexible Cords Number and diameter of wires No./mm 0.5 0.75 1.0 1.5 2.5 4.0
Current Rating Maximum weight supported by A twin flex cord Kg.
Nominal cross section Area mm2 16/0.20 24/0.20 32/0.20 30/0.25 50/0.25 56/0.30
3 6 10 15 20 25
2 3 5 5 5 5
Voltage drop calculation In calculating the voltage drop in the cables in the following example, it is assumed that the circuits are wired from the distribution board which is located at the origin of the installation. If there is a long length of sub-main cable from the origin of the installation, then the voltage drop along the submain cable also has to be calculated in a similar manner and added to the voltage drop of the circuit wiring. The total voltage drop from the origin to the end of the circuit should not exceed 4% of the nominal voltage to comply with the 16th Edition of the Regulations. For 230V supply this amounts to 9.2V. The voltage drop in a circuit is ascertained by first multiplying the length 67
of the run in the circuit in metres by the current in the circuit in amperes. This gives a product in Metre Amperes. When 9.2V is divided by this product, the maximum allowable voltage in volts per ampere per metre is found. As the values in the table are given in millivolt the result should be multiplied by 1000 to select the cable. The smallest cable having the voltage drop per ampere per metre value less than the figure arrived from the calculations should be selected. (The normal practice would be to tentatively select the cable on the basis of current carrying capacity and calculate the voltage drop at the rated current. If the voltage drop is excessive a larger cable is selected using the aforementioned calculation method.) Example: Find a suitable cable to carry a current of 20A in a circuit with a length of 25 metres connected to 230V supply. The metre-ampere figure will be 20*25 = 500. Therefore maximum allowable mV/A/m will be 9.2 * 1000 / 500 = 18.4. From the table for two wire single phase wiring in conduits the smallest cable satisfying the requirement will be 2.5 mm2 (1/1.78 or 7/0.67) having a voltage drop of 17 mV/A/m. The voltage drop when carrying rated current will be 17*20*25 = 8.5V. (Before using this value it has to be checked whether it can safely carry 20A under installation conditions. Example is to show voltage drop calculation only.) Lighting circuits The major component of domestic wiring involves lamps which are normally wired into 5A circuits using 1 mm2 cable. As per the Regulations, each lamp holder needs to be allowed 100 W. Therefore maximum number of lamp points that can be allowed to a 5A circuit will be 11 Nos. Allowing for 20% for future use, only 8 o 9 lamp points can be connected to a circuit. Voltage drop of a 1 mm2 cable is 37 mV per ampere per metre. Thus the maximum length that is permissible for a 5A circuit is about 50 metres. This is applicable only to final circuits from the main switchboard. For final circuits of sub distribution boards, voltage drop along the sub-man cable also need to be taken into account. Thus a lamp circuit in a house could consist of 8 lamp points and wired up to a maximum length of 50 metres measured along the cable path. Using this criterion the number of circuits to be wired and the position of the distribution board can be decided. A ceiling fan also consumes about 100 W and can be allocated to circuits on the same basis. Socket-outlet circuits If 13A socket-outlets are to be wired, a ring circuit using 2.5 mm2 cable can be provided for each 100 square metres of area. However, it would be better to provide the kitchen with a separate circuit. 68
5A socket-outlets can be wired with 1 mm2 cable protected by a 10A fuse/mcb with one or two outlets in the circuit. Three 5A socket-outlets can be wired with 1.5 mm2 cable protected by a 15A fuse/mcb. The maximum length of the circuit will depend on the number of outlets as the Regulations require us to assume a demand of 5A for each outlet. 15A socket-outlets can be wired using 1.5 mm2 cable up to 13 metres and using 2.5 mm2 cable up to 21 metres. Cable sizes need to be increased for longer lengths to compensate for the voltage drop. It is not intended to mix 5A and 15A socket-outlets with 13A socketoutlets in the same premises. Using air conditioners with 13A socket-outlets has been a problem, due to the fuse being blown sometimes due to the starting current. To overcome this problem a 20A DP switch can be incorporated to the ring circuit to feed air conditioners. Schedule of Final Circuits Details of the components related to final circuits can be summarized as given in Table 12. TABLE 12. Conventional final circuits Rating of the Fuse or M.C.B. Amperes 5 10 13 15 20 30 30
Minimum size of Cable
Minimum size of Flexible Cord
Square Millimetre
Square Millimetre
1.0 1.0 1.5 2.5 2.5 2.5 (ring) 4.0 (radial)
0.75* 1.0 1.5 1.5 2.5 - -
Suitable Socketoutlet Amperes 5 5 5 5/15 13/15 13 13
* It is permissible to use 0.5 sq. mm. flexible cord for a pendent which is connected to a circuit controlled by a 5A fuse / mcb as an exemption. Conduit Capacities The “On-Site Guide” include tables giving details to determine the number of each size of cable that can be drawn into various sizes of conduits. These tables have been prepared allowing 40% free space so that the cables can be drawn into the conduits easily. But these are not directly applicable since they are based on the dimensions of PVC insulated (only) cables, where as cables used in Sri Lanka are PVC insulated and sheathed cables. However these tables could be modified and used taking the overall diameter of the cables into consideration. 69
The Tables in the Regulations provide different values for different sizes of conduits and cables for different distances between the adjacent draw-in boxes. They are further divided based on the nature of the run, straight, with one ninety degree bend, with two ninety degree bends and so on. A table can be prepared for insulated and sheathed cables interpolating the values given for the insulated cables using the overall diameter as the base. Table 13 indicates the number of such cables of each cross section area that can be drawn into the indicated sizes of conduits for straight runs and incorporating three, ninety-degree bends between draw-in boxes where the draw-in boxes are not more than three meter apart. TABLE 13. Conduit capacities for PVC / PVC cables Cable size in Sq. mm. 1 1.5 2.5 4 6
Straight runs
16 5 4 2 1 1
20 8 7 4 3 2
With Three Bends Conduit diameter in mm
25 14 12 7 5 4
32 25 22 12 9 8
16 4 3 1 1 -
20 7 5 2 2 1
25 12 10 5 3 2
32 22 18 9 6 5
When light fittings are installed underneath PVC conduits, care should be taken to ensure that the heat from the lamp does not result in the o PVC being subject to a temperature exceeding 65 C. For surface installations it is recommended that the saddles should be fixed at intervals of 800 mm for up to 20 mm diameter and at 600 mm for larger sizes. Flexible PVC conduits are also available and they can be used where there are awkward bends and other locations where rigid PVC would be difficult to install.
70
Planning the Installation As the first step in starting wiring an installation one has to check from the supply authority whether the required supply is available and whether there are any temporary limitations on the load which can be connected. Then it is necessary to find out, whether the service cable will be taken underground or overhead, as this will influence the position of the meter and main switch. The meter and the main switch should be located so as to give easy access at all times and also to avoid positions which cause disfigurement. For example a cupboard under the stairs does not always provide easy access, whilst a meter and cut-outs mounted at the entrance look unsightly. In deciding the position of the meter, the direction from which the service lines of the supply authority are likely to enter the premises also must be considered. It is also necessary to remember that the supply authority needs the access at all times. Any apparatus used to control the supply in a house should be readily accessible to the occupant because rapid disconnection from the supply could be required in case of an emergency. The Regulations require the main switch to be at the origin of the installation. The main switch should be located inside the house as close as possible to the meter cut-out, so that the meter tails are kept as short as possible. But the location of the distribution board will depend upon the size of the installation and other factors. For small houses it is preferable for the distribution board to be near the main switch, but for larger houses it may be preferable for the distribution board to be located centrally to enable all final circuits to be kept as short as possible. This will be a matter of balancing the cost of sub-main cable length against the saving of additional lengths of final circuit wiring. Other factors which will help to decide the best position are the availability of a suitable wall or stanchion, the ease with which circuits can be run to the position chosen, accessibility for replacement of fuses, freedom from dampness and adverse conditions etc. It must be remembered that the regulations require that all equipment must be sufficient in size and power for the work they are called upon to do. This specially applies to main switchgear. The size and type of main 71
switchgear to be installed will depend on the anticipated load. To determine the size it is necessary to add up the total connected lighting, heating and cooker loads and then calculate the maximum current which will normally flow in the installation. This will depend upon how the premises will be used, the number of people who will use the premises, whether there are alternative or supplementary means of cooking and many other considerations which can be learnt by experience. To estimate the maximum load a schedule should be made of the lamps and other current using equipment which are likely to be installed. From this schedule it is possible to calculate the total demand using the value given in the Table 5. “Current demand to be assumed for points and equipment” which was given earlier. The diversity factor may be applied to obtain the possible maximum demand of the installation. The applicable diversity factors are given in Table 6. It must be remembered that the diversity factors should be applied to the totals of the whole installation and not to individual final circuits. Another 20% should be added to this to cater to any future additions. Capacity of the main switch can be decided from this maximum demand. For an average house 30A single phase supply would be sufficient. In case of large houses it may be necessary to bring in a 3 phase 4-wire supply and to balance the single phase loads. In the case of 3 phase supplies special precautions have to be taken to avoid shock risk at 400 V as most of the people would not expect this voltage in a house where only single phase appliances are used. The voltage between conductors connected to two different phases would be 400 V and therefore taking two different phases to nearby places should be avoided. When more than one phase has to be brought into a multi-gang box, the accessories and wiring connected to each phase should be separated by fixed barriers. When planning an installation, at least 20 per cent should be left as spare ways in every distribution board, and the metal clad type should be provided with plugged holes suitable for accommodating additional conduits for the future circuits.
Deciding on the position of lamps in a domestic dwelling will be done
by the occupant mainly on aesthetic considerations rather than illumination. Therefore the lamp points need to be decided accordingly. In deciding 72
the positions of socket-outlets, the object should be to provide outlets at convenient positions to feed all portable lamps, table fans, electric irons and other appliances that are likely to be used. If 5A and 15A socket-outlets are to be used it would be expensive to provide an outlet at each possible location, but if 13A ring circuits are to be used it would be economical. The size of each conduit run can be decided depending on the sizes and the number of cables to be drawn in each length. Once the correct sizes of the cables for the circuits are determined it will be necessary to decide the size of conduits to be used to accommodate the cables.
UPPER FLOOR PLAN Fig. 21a House - Electrical points
73
GROUND FLOOR PLAN Fig. 21b House - Electrical points
74
Example Let us now consider designing electrical installation for the two storey house shown in Fig. 21. This is a true plan prepared by an architect and the electrical points are designed according to his wishes and may be representing a house existing in Colombo. This house has eighty two lamps points, six ceiling fans, one electric bell, one cooker control unit, thirty six 13A socket-outlets, four 20A DP switches and four shaver outlets. The lamps numbered 1-6, 15, 43-52, 55, 56, 59, 71, 76 and the bell push are exposed to weather and they need to be weatherproof. Power supply would be brought along the road and the meter point has been chosen near a corner in the garage. The main switch could be located on the wall in one of the positions marked as A, B, C or D. When the lengths of final circuits are considered most economical position would be somewhere on the wall A as it is closer to the centre of the building. But it is very unlikely that the occupant would like to have the switchgear in his living room. If the client has no objection you could locate the main switch there. The next best position on economic consideration would be on the wall along point B, but this is on the landing and may not be convenient in accessing in case of an emergency as you need to climb several steps to reach it. Besides this is in the lobby for the first floor entrance and there is a possibility of the door being locked at times. Thus it could not be described as a readily accessible position as required by the Regulations. The next best is the location C. Presuming that the occupant has no objection in locating the switchgear in his dining room, this position is selected for our design. If the client objects you could choose the wall D. On the other hand, kitchen being a place with heavy consumers it could be the load centre and the most suitable place. Let us now estimate the maximum demand. Assuming that the house belongs to an affluent owner (results in a very high demand), the list of equipment likely to be used in this house could be as follows: 1. Incandescent lamps 73 Nos. 2. 1 x 20w fluorescent lamps 7 Nos. 3. 1 x 40w fluorescent lamps 2 Nos. 4. Ceiling fans 6 Nos. 5. Electric bell 1 No. 6. 4 burner cooker with oven 1 No. 7. Refrigerator 1 No. 8. Deep freezer 1 No. 9. Electric kettle 1 No. 10. Blender/Grinder 1 No. 11. Mixer 1 No. 12. Toaster 1 No. 13. Washing Machine 1 No. 14. Television 5 Sets 15. Audio 1 Setup 16. Electric Iron 1 No. 17. Air conditioner 4 Nos. 18. Computer 4 Nos. 19. Electric shaver 4 Nos. 75
The maximum demand occurs when the most of the current consuming equipment are in use at the same time. In Sri Lanka this normally occurs during the period 6.00 to 9.00 p.m. and equipment likely to used at that time need to be considered for computing maximum demand. Lighting Incandescent lamps 73 Nos. Regulation requires to assume a minimum of 100 W for each lamp holder (assuming 2 chandeliers have 3 holders each) Total load = 7700 W Fluorescent lamps 1 x 20 W 7 Nos. = 140 W x 1.8 (for control gear) 1 x 40 W 2 Nos. = 80 W x 1.8 Total lighting = 8096 W Allowance for diversity = 8096 x 66/100 W = 8096 x 66 / 100 x 230 A = 23.23 A Ceiling fans 6 Nos. Since air conditioners are available it is unlikely that ceiling fans in the bed rooms will be used. Therefore only two fans need to be considered. Maximum demand = 2 x 100 /230 A = 0.87 A) Electric bell may be neglected. 4 Burner cooker with oven Assume cooker has two 1500W and two 1000W burners and an oven with 1500W element and a socket-outlet at the control unit Total load = 6500 W = 6500/230 A = 28.26 A Allowance for diversity = 10 + (28.26-10) x 30/100 + 5A = 20.48 A Refrigerator (150 W) 1 No. Load = 150 / 230 = 0.65 A Deep freezer (300 W) Load
1 No. = 300 / 230
= 1.3 A
Electric kettle (1000 W) Load
1 No. = 1000 / 230
= 4.35 A
Blender/Grinder (100 W) 1 No. Load = 100 / 230 = 0.44 A Mixer (100 W ) 1 No. Toaster (400 W) 1 No. May be ignored because it is unlikely that these could be used with others at the same time 76
Washing Machine (1000 W) Load = 1000 / 230
1 No. = 4.35 A
Television (200 W) Load = 200 x 5 /230
5 Sets = 4.35 A
Audio (100 W) 1 Setup May be ignored because it is unlikely that audio will be used when 5 TV sets are in use. Electric Iron Load
= 1000 / 230
Air conditioner (1500 W) Load
4 Nos. = 1500 x 4 / 230 = 26 A
Computer (200 W) Load
= 200 / 230
Electric shaver May be neglected.
1 No. = 4.35 A
1 No. = 0.87 A
4 Nos.
Total demand = 23.23+0.87+20.48+0.65+1.3+4.35+0.44+4.35+4.35+ 4.35+26+0.87A = 91.34 A Another 20% need to be added to this total to allow for future extensions. (A water pump, hot water geezers, air conditioning living and dining rooms, flood lighting the garden, swimming pool or a water pond equipment etc. could be considered as future additions and also alterations and additions to the building also could take place.) Estimated maximum demand = 91.34 x 120/ 100 = 109. 61 A It is obvious that the supply authority would not allow such a big single phase supply and the house needs a three phase supply with capacity of 36.54 A per phase. The supply authorities usually provide either a 30A or a 60A three phase supply and a 60A three phase supply need to be obtained for this house. Therefore a 60A TP MCB shall be provided as the main switch. At this stage it is necessary to verify from the supply authority whether a 60A three phase supply could be obtained from the location and if it is not possible the client should be advised to curtail his demand to reduce to 30A three phase. Perhaps it may be necessary to delete some of the socket-outlets proposed to avoid simultaneous use of some equipment (e.g. in the kitchen). When the distribution is considered it can be seen that a fair amount of points are in the upper floor and a three phase supply can be taken there. The distribution board for the upper floor can be located in the lobby at 77
F since it is more or less at the centre. Repeating the calculation of the maximum demand for the upper floor would result a need of 12A three phase supply. However, there are 13A socket-outlets and size of the main switch need to be considered only after deciding the final circuit arrangement. It is necessary to divide the load among the phases as much as possible. When you examine the layout you would notice that the three bedrooms are similar and the equipment in each room could be assigned to final circuits connected to the three different phases. All the lamp points in a bed room could be allocated to a single circuit. The ceiling fan and the shaver-outlet could also be assigned to the same final circuit. The total demand of these circuits would be less than 6A. Thus they can be protected by 6A MCBs and wired with 1 mm2 cable. The floor area of each bedroom is more than 20 m2 but is less than 50 m2. Therefore the socket-outlets in each room could be assigned to a final circuit protected by a 30 or 32A MCB. It could be wired as a radial circuit using 4 mm2 cable or a ring circuit using 2.5 mm2 cable. The 20A DP switch provided for the air conditioner also can be connected to this circuit. As the socket-outlets are at least on three walls of the room wiring in a ring circuit with cables of lesser cross-section area would be more economical and it would permit easy addition of future extensions. Therefore they are assigned to a ring circuit protected with a 30A MCB and wired with 2.5 mm2. What remains is the lobby area and the lamps of the area could be served by a separate final circuit protected by a 6A MCB and wired with 1 mm2 cable. The two socket-outlets could be connected to the ring circuit of the adjacent room. Thus we would end up with seven final circuits divide as 3, 2, 2 to the different phases. The Regulations require to provide a further 20% for the future additions and this could be satisfied by adding two more final circuits making it possible to balance the number of circuits connected to each phase. The three ring circuits provided cover the entire upper floor and each circuit serves a floor area of less than 100 m2 and therefore there is no need to provide spare circuits to connect 13A socket-outlets. Therefore two 6A MCBs could be provided to protect the spare circuits. Since there are socket-outlet circuits needing protection with 30A breakers, the main switch also need to be a 30A TP MCB. To provide protection against earth fault currents a residual current device need to be provided. This could be a 4 pole RCD connected after the main switch. But as all the loads are single phase, it would be more prudent to provide three double pole RCDs on the three phases so that only one phase is affected in case a RCD trips due to an earth fault. An important thing to remember in this type of arrangement is to separate the three neutral conductors connected to each phase throughout the installation beyond the RCDs. Three separate neutral bars shall be provided at the distribution board for this purpose. The complete distribution board for the upper floor is shown in FIG. 22.
78
From DB - G 4x6 mm2 + 6 mm2E
DB-U 30A TP MCB
RCCB
40A DP 30mA
(6A) (30A) (6A)
RCCB
Lamps 57-65, 1CF, 1Sh S/O 23-28, 1 AC spare
40A DP 30mA
RCCB
40A DP 30mA
(6A) Lamps 66-73, 1CF, 1Sh
(6A)
(30A)
(30A)
(6A)
S/O 29-32, 1 AC Lamps 49-56
Lamps 74-82, 1CF, 1Sh S/O 33-36, 1 AC
(6A) Spare
Fig. 22 - Upper Floor Distribution Board
A sub-main cable need to be drawn from the main switch at the ground floor to the distribution board to supply electricty. The length would be around 15m and the maximum current would be about 15A (allowing for future expansions as well). This gives a product of 225 metre amperes. The voltage drop allowable is 4% of the nominal voltage from the main switch to the end of the circuit. So in this case let us assume half of that is allowed to drop along the sub-main cable leaving the other half for the final circuit. So the voltage drop allowable is (230 x 2/100 =) 4.6V. Thus the maximum allowable voltage drop would be 4.6/225 = 0.0204 V/A/m = 20.4 milli volts per ampere per metre. This would allow us to use a cable with a cross-section area of 2.5 mm2. But since the main switch of the distribution board is 30A the submain cable need to be capable of drawing that current. Therefore the cable cross-section need to be at least 6 mm2. (It is presumed that the sub-main cable is drawn in a separate conduit and no external influences requiring to apply derating factors are present.) Since the sub-main cable is of more than 3 metres in length another 30A TP MCB need to be provided at the origin of the installation (Distribution board at the ground floor). A separate earth cable of the same cross-sectional area (6mm2) of green-and-yellow colour also needs to be drawn along with these cables to connect the to earthing terminals of the distribution board Maximum demand for the ground floor distribution could also be similarly computed to be around 25A. There are 48 lamp points and these could be wired to six final circuits each having around eight lamps per circuit. It should be noted that the Lamps No. 9 and 10 are chandeliers and they would demand more current. Therefore the circuit containing them should have less number of points. (each holder need to be allowed 100W). This would allow them to be wired with 1 mm2 cable protected with 6A MCBs and easily be balanced between the phases. Another important point to be considered in dividing lighting points to final circuits is the location of their switches. 79
It is better if only one circuit is assigned to switches ganged at one place. (There is no Regulation to prevent such use except to say that a warning notice need to be provided if 400V could be present at adjacent switches.) Assignment final circuits can be done as follows: 1. Lamps 1-6, 11, 12 and 15 2. Lamps 7-10, 13 and 14 3. Lamps 16-23 and two ceiling fans in the living room 4. Lamps 28-33, ceilng fan and the shaver-outlet in the bed room 5. Lamps 34-42 6. Lamps 24-27, 43-48 7. The cooker control unit 8. Socket-outlets (in the Kitchen)16-18 9. Socket-outlets 1-8 10. Socket-outlets 9-15 and the AC in the bedroom Requirement for the additional circuits for future extensions could be fulfilled by adding another two circuits. The cooker control unit needs a 30A MCB for protection and should be wired with 6 mm2 cable. Socket-outlets in the kitchen can be wired as a radial circuit using 4 mm2 cable with 30A MCB protection and other two socket-outlet circuits could be wired as ring circuits using 2.5 mm2 cable with 30A MCB protection. Additional circuits could be protected with 6A MCBs allowing for lamp circuits, since the entire floor has been covered with13A circuits as in the first floor. When distributing these final circuits among the phases, it is better if the cooker control and the socket-outlets in the kitchen are assigned to the same phase to avoid presence of 400V. At the same time it is better if the lamps in the kitchen are assigned to a different phase so that even if the RCD connected to the kitchen appliances trip due to a fault there would be light in the kitchen. Thus the distribution could be as shown in Fig.23. To meter point 2
4x16 mm
DB-G
60A TP MCB
30A TP RCCB 40A DP 30mA RCCB 40A DP 30mA RCCB 40A DP 30mA MCB (6A) (6A) Lamps 34-42 (6A) Lamps 1-6, 11, 12,15 Lamps 28-33, 1CF, 1Sh To DB U (6A) Lamps 7-10,13,14
(6A) Lamps 24-27, 43-48, 1bell
(6A)
(30A)
(30A)
(30A) 1 Cooker
(6A)
S/O 1-8 Lamp 16-23, 2 CF
S/O 9-15, 1AC (6A) Spare
Fig. 23 - Ground Floor Distribution Board
80
Spare
(30A)
S/O 16-22
Since the main switch for the whole house is installed in this distribution board, there is no need to provide another device to isolate the ground floor installation. As explained in the first floor, earth fault protection is preferably provided to individual phases. Considering the diversity of individual phases a 40A DP RCD shall be provided for each phase. As socket-outlets are fed from all RCDs their sensitivity should be 30mA. The arrangements of these circuits are shown in the layouts in Fig. 24. The load cables from the meter point should be able to carry 60A and 16 mm2 cables should be used for the purpose. The earthing conductor should also be 16 mm2. The earth electrode could be located at E in the garden behind the kitchen.
F
UPPER FLOOR PLAN Fig. 24a House - Electrical Layout
81
GROUND FLOOR PLAN Fig. 24b House - Electrical Layout
82
Installation Conduits are mainly run concealed in walls and floors, above ceiling spaces and sometimes on the surface. PVC casing and capping is now available and they are used for surface applications. Surface wiring system is fairly straight forward and is not described any further. However a good deal of planning and skill is required to produce a first class installation with a neat appearance. PVC conduits can be easily formed into required shape by heating. However it is important to avoid forming wrinkles as it would make the drawing in of cables difficult. As the walls of the conduits are thin, usually sand is rammed in and both ends are plugged before heating to retain the circular shape. Area to be bent is uniformly heated up by carefully exposing to a mild heat source and formed the required shape and then cooled with a wet cloth or alike so that the shape is retained. However this is a tedious process and some skill is required to avoid deforming or burning the conduits.
TWO-WAY
THREE-WAY
FOUR-WAY
END
ANGLE
Fig. 25 - Conduit Junction Boxes
Now there are manufactured bends and junction boxes to suit the requirements. Junction boxes are available with 2, 3 or 4 holes so that conduits can be connected in the required direction. Boxes are also available to terminate conduits at the wiring point. Sunk boxes are now available with knock out on all side walls as well as on the back. These boxes are designed to accept standard switches and socket-outlets. Good quality sunk boxes have two screwed metal holes to accept standard screws of the switch or socket-outlet plate.
Flg. 26: Sunk Box
83
The conduits in the concealed wiring system is usually installed during the building construction in walls and floors in such a manner that the cables can be drawn in at any time after the completion of the building. In installation of conduits it is required to decide the most suitable runs for the conduits. The routes should be chosen so as to keep the conduits as straight as possible. However it is a good practice to keep the concealed conduits in walls only on horizontal and vertical direction in order to minimize subsequent damage. It must be remembered that the Regulations do not permit to draw cables through more than two right angle bends or their equivalent. Therefore additional junction boxes may be required at places other than where accessories or wiring points are present. Conduits on walls and ceilings When laying conduits on floors to serve the room above it is not practicable to leave junction boxes on the floor. Therefore the conduit system has to be arranged so that cables can be drawn through ceiling or walls.
Floor
Junction Box
Junction Box Wall
Sunk Box
Fig. 27 - Conduit connection to lamps and a switch
Conduits for switches
Sunk Box
CORRECT METHOD
Conduit set out to clear wall
Conduit Bend
INCORRECT METHODS Fig. 28 - Termination for a switch on a wall
84
Box buried too deep
The Regulations require that at the switch points the conduit is terminated a box or similar enclosure. When run buried, taking the conduit out to the surface through a bend or a set in the conduit is not appropriate. On the other hand the sunk box shall not be buried too deep leaving a gap between the box and the switch plate. When conduits are run to switches or other positions on a wall, they are usually run in chases cut into the wall. These chases, which will be filled after the conduits are laid, must be deep enough to allow at least 10 mm of plaster covering the conduit, to avoid plaster cracks appearing in a later stage. Sunk boxes fitted to these conduits should be placed allowing for the plaster cover. These will be applicable to conduits and sunk boxes on ceiling soffit as well. Conduits for socket-outlets Socket-outlets near the skirting level shall be preferably be fed from floor above rather than from floor below, in order to avoid traps where moisture could get collected. On reinforced concrete floors conduits have to be laid on shuttering and secured in position before the concrete is poured. Care must be taken while concrete is being poured, because, if not securely fixed, conduit may move out of position or lift and then, once the concrete is set, it will be too late to rectify matters. Whenever conduits are to be buried in cement, special care should be taken to see that the joints are tight. Otherwise liquid cement may enter the conduit and form a solid block inside. Junction Box
Junction Box Floor
Incorrect Method
Trap for Moisture Floor above
Looping Box
Looping Box
Box for S/O
Box for S/O
Correct Method Fig. 29 - Conduit connection to socket outlets at skirting level
85
Floor Below
Conduits for distribution boards When surface mounted distribution boards are used with a buried conduit system, the best method to take the cables from the conduits to the DB is to fit an adaptable box in the wall to take the conduits into it. The conduits can be taken out of the plaster and enter the DB direct with a set on the conduits but this method is not recommended. Alternatively adapter boxes can be mounted on top or bottom or both on the DB to take the cables. Adapter box can be mounted partially buried so that the conduit can enter it easily.
Flush mounted box Distribution Board
Best Method
Conduit set-out to enter DB
Partially sunk Adapter box
Distribution Board
Not Recommended
Distribution Board
Recommended
Fig. 30 - Conduit Entries to Surface Mounted Distribution Boards
Before drawing the cables, the installation of the conduits must be complete and also must be clean and dry. No attempt should be made to wire conduits which are buried in concrete until the building has been dried out and conduits swabbed out to remove any moisture and obstructions which may have entered the conduits. A draw wire with a swab at the end should be drawn through the conduit for this purpose. It is a good practice to keep the open ends of the conduits plugged as soon as they are laid before concreting to prevent any foreign matter entering them. Wiring Wiring is carried on each conduit run from draw-in box to draw-in box in sequence usually starting from a mid-point in the conduit system so as to reduce the length of cables which have to be drawn-in. A draw-wire has to be inserted into the conduit run to draw in the cables. It is a good practice to keep a draw-wire drawn inside the conduit whilst it being laid. However, o if the number of bends do not exceed the stipulated equivalent of two 90 bends and the length is not excessive, it is not difficult to insert the wire at a later stage. If it is difficult, two wires from either end of conduit run, with small hooks at the ends could be inserted. Once the two ends reach each other wires could be twisted so that the two hooks get entangled and one wire could be pulled out from one end until the draw-wire fed from the other end comes out. Obviously two persons are required to do this operation. 86
Once the draw-wire is inserted and the conduit is cleaned if necessary, the required number of cables should be tied to one end of the draw-wire. The ends of the cables to be tied must be bared for a distance of about 50 mm and threaded through a loop in the draw wire for this purpose. When drawing in a number of cables, they must be carefully fed in from the sending end whilst one pulls them at the receiving end. Draw
wire
Cables
Fig. 31 - Method of connecting cables to draw wire
Before drawing in the cables into the conduits they must be run off the reels. If the cables are allowed to spiral off the reels they would become twisted and kinked inside the conduits. The cables should be fed into the conduits in such a manner as to prevent any cables crossing and also to avoid them being pulled against the sides of the openings of the draw-in boxes. Always some slack has to be left at the draw-in boxes and it has to be ensured that the cables are fed to the conduit in such a way that no twisted cables will be left at the draw-in point. This is particularly applicable when the cables have to be pulled out from one conduit and fed to another connected to the same draw-in box.
Fig. 32 - Cables entangling in a junction box
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This operation needs care and synchronization between the persons who are feeding and pulling. Sometimes a third person may be required to be stationed mid-way to relay messages if the persons at both ends are not within earshot. If the cables are not drawn carefully in this manner, they are almost certain to become crossed and might result in the cables becoming jammed inside the conduit. Also the insulation of the cable is liable to get damaged in the process.
GROUND FLOOR Fig. 33 - Wiring points in a house
UPPER FLOOR
Example Let us now consider wiring of the two-storied house shown in Fig. 33 as an example. This house has 9 lighting points in the ground floor and 5 in the upper floor. The light in the staircase is to be controlled by two-way switches in both floors. The obvious choice of lighting circuits will be to have one for the ground floor nine lights and another for the five lights in the upper floor. There are four 13A socket-outlets in the kitchen and another sixteen in other areas. Thus the logical distribution of the socket-outlets will be to have one final circuit for the kitchen and to have another for other areas. Allowing another circuit for future expansions the distribution board can consist of five final circuits. Thus the final circuits of the house would be as follows: Lighting circuits with lamps No. 1 to No. 9 wired using 1 mm2 cable fed through a 5 or 6A MCB Lighting circuits with lamps No. 10 to No. 14 wired using 1 mm2 cable fed through a 5 or 6A MCB Four socket-outlets in the kitchen wired using 4 mm2 cable in a radial circuit fed through a 30 or 32A MCB Other socket-outlets wired using 2.5 mm2 cable in a ring circuit fed through a 30A MCB. (This would be possible only if the floor area served by the socket-outlets other than the kitchen is not more than 100 m2). This will usually be fed from a 30 or 32A DP MCB or an isolator and a 40A DP 30mA RCCB (because 30A RCCB is a non standard unit) using 88
6 mm2 cable. (Use of an isolator is not recommended. Actual required capacity of the main switch need to be determined by estimating the maximum demand considering the equipment to be used in the house.) Single line diagram The arrangement can be shown in a single line diagram as in Fig. 34. 2x6 30 A DP MCB 40 A DP 30 mA
RCCB
(6A) (6A)
2x1 2x1
(30A) 2 x 4 Radial
LIGHTS 1-9 LIGHTS 10-14 SOCKET OUTLETS IN KITCHEN
(30A) 2 x 2.5 Ring OTHER SOCKET OUTLETS (6A)
SPARE Fig. 34 - Single Line Diagram
Conduit layout A suitable method of wiring the lighting circuits and connected conduit layout is shown in Fig. 35 and 36. Numbers of cables between respective points are shown in by the number of lines in the Fig. 35 and the number inside the parenthesis in Fig. 36. A three dimensional view of the conduits for the lighting circuits are shown in Fig.37.
GROUND FLOOR
Fig. 35 - Wiring Layout
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UPPER FLOOR
GROUND FLOOR
UPPER FLOOR Fig. 36 - Conduit Layout
Wiring procedure The practical method of wiring a conduit installation such as this is briefly as follows: A start is usually made from the three way box nearest to the DB (D). There will be four red cables going from this box to the switch (C) and one black cable going to the DB. Therefore one more draw-in box (E) is required at the wall above the switch position to separate the two sets of cables. So the distance along the conduit path from (D) to (C) is measured and four red cables are reeled off equal to this length and some extra slack to allow for the connection at the switch and tying to the draw-wire. Then the distance along the conduit path (D-E-A) from (D) to the DB (A) is measured and a black cable is reeled off to the corresponding length with some slack. A draw-wire is inserted into the conduit length from three way box (D) to the draw-in box (E) on the wall if not done already. The ends of the five (4 red and 1 black) cables are then bared and tied to the draw-wire. Then the chosen lengths of cables are drawn into the conduit. The excess lengths are drawn out of the draw-in box (E) on the wall so that all the cables are drawn in up to the measured length ending at the three way box (D). Another red cable is required to connect the switch (C) to the DB. This length is measured and a red cable is cut to the length plus the required slack for connecting and tying. The draw-wire connection to previous bundle is untied and red and black cables are separated. Then the end of the red cable going to the DB is bared and tied to the draw-wire together with the end of the black cable. These two cables are then drawn into the conduit connecting to the DB. Then the other end of the red cable is bared and together with the ends of the other four red cables are tied to the draw-wire and pulled through the conduit going to the switch. Care should be taken to avoid kinks and entanglements forming inside the draw-in box in this operation. 90
Fig. 37 - Example wiring using looping in method
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Once the cables are drawn in to the conduit it will be difficult to identify the individual ends at a later stage. Therefore it is customary to make some marks at the ends of the cables before they are drawn into the conduits. This is normally done by cutting off a small portion of the sheath (usually in the shape of a diamond) exposing the coloured insulation. Identification is done by the number of such marks on each cable end. There will be five red cables drawn into the switch position (1, 2). One is coming from the DB and is already cut to the required length. Other four should be terminated at switch (3), lights (1), light (2) and switch (4, 5). The lengths are measured along the conduit paths and cut with the required slack. The black cable also goes to the light (1) and can be cut to length. Otherwise it may be extended up to lights (2) or (3) if desired allowing additional loop lengths to make connections at the lights. The chosen cable lengths should be cut and the cables to the next conduit run (between three-way boxes at (D) and (F)) can be drawn in now. There are three cables to be drawn into this conduit which are already cut to length. (If the black cable was terminated at light (1) two new black cables have to be drawn to connect neutral of light (1) to neutrals of light (3) and light (4). Once three cable ends up to (F) are drawn, wiring up to switch (3) and light (2) are done in a similar manner introducing a new red cable length from switch (3) to light (3) and a black cable length from light (2) and light (3). Now the two cables (The red cable between switches (1, 2) and (4, 5) going in the circuit run between lights (D) and (I) can be drawn in. This process should be repeated until the whole circuit is wired. There are five red cables drawn into the two ganged switch at (C). The cable coming from the DB and the one going to switch (3) shall be connected to one side of the switch (1). (One side of switch (2) also should be connected to the same side using a red jumper, if not already connected by the manufacturers.) Then the other two cables going to the two lights (1) and (2) shall be connected to the other side of the respective switches. At the position of the light (1) there will be three black cables (or one end and a loop) and a red cable. The black cables should be bared and connected to one of the ends of the ceiling rose and the red cable should be connected to the other which will form the lighting point. All other connections also need to be done in a similar manner when the cables are drawn into each position. Note: The Regulations require a circuit protective (earth) conductor to be drawn to connect all switch plates and lighting points up to the ceiling roses. But this is generally not practised in domestic wiring in Sri Lanka and therefore not shown in this illustration. 92
To SWITCH (3) at H
To LIGHT (2) at G
To LIGHT (1) at D
From MCB 1 of DB
To SWITCH (4) at L
Phase to LIGHT (1)
Red cable from SWITCH at C
Neutral to LIGHT (1)
Black cables from Terminal 1 of Neutral bar To LIGHT (3) at F To LIGHT (4) at I
(a) Connections of SWITCH (1,2)
(b) Connections at Ceiling Roses of LIGHT 1
Fig. 38 - Connections to Switches and Ceiling Roses
Although it is shown that three cables can be connected to one terminal of a switch or a ceiling rose in this illustration in order to arrive at the least cost installation, readers may find that in practice it is not very convenient to insert three 1 mm2 cable ends to one terminal of the most of the ceiling roses available in the market. Therefore it will be more prudent to plan the installation to avoid such connections. In this example it could be achieved by obtaining the connection to the switch (4, 5) at L from the switch (3) at H instead of the switch (1, 2) at C and to the light (4) at I from the light (2) at G instead of the light (1) at D. This will require to draw a red cable from H to L instead of C to L and a black cable from G to I instead of D to I. It will alter the number of cables in the conduit runs C-D-E (4), DF (5), FG (3) and FH (3). Some of the conduit connections to junction boxes in this illustration are drawn at some odd angles. If manufactured accessories are to be used this kind of connections may not be practicable. Therefore it may be necessary to form off-sets in the conduits to make them enter the junction boxes at right angles. However if the junction boxes are sufficiently far apart, connections could be made utilizing the flexibility of conduit to form an arc rather than making a permanent set in it. All the conduiting and wiring to socket-outlets are not shown but the procedure is similar. Cables to socket-outlets also can be drawn in the same conduits. However if 13A socket-outlets are involved it will be convenient to use separate conduits for them. Normally 2.5 mm2 twin flat cables together with green or green-and-yellow earth cable is used for wiring 13A socketoutlets in ring circuits. 93
In the case of 13A socket-outlets, most of them are installed at skirting level while in kitchen and pantries they are installed at the table top level. Therefore it will be more convenient to run the conduits along the walls at the level of the socket-outlets. However they need to be taken up at the door crossings. The conduiting can be as shown in Fig. 39. It should be noted that all socket-outlets except those in the kitchen are grouped into one circuit to reduce the cost. However, it would be more convenient to the user if the upper floor socket-outlets are wired in a separate circuit. Further it would be much better if the upper floor lighting and socketoutlets are wired from a separate distribution board installed in the upper floor itself on safety considerations. Increase in cost in this regard would be only marginal.
Fig. 39 - Typical Layout for 13A Socket Outlets
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Testing On completion of wiring it is necessary to inspect and test the installation thoroughly before it is permanently connected to the supply in order to be sure that it will function correctly and safely. In many cases compliance with the regulations can be determined only on results of tests. The regulations require a detailed inspection carried out before testing and must normally be done with the part of the installation under inspection disconnected from the supply. The Regulations indicate nearly twenty items to be inspected by the person (who could be a complete outsider) carrying out the inspection and testing. Most of them would be already covered by the person doing the installation work, and in case of domestic wiring inspection of the following would be sufficient on completion of the work. i connection of conductors ii identification of conductors iii connection of single pole devices for protection or switching on phase conductors only iv correct connection of socket-outlets and lamp holders The purpose of the inspection is to see that the installation conforms to the Regulations and is not visibly damaged or defective so as to impair safety. In some cases a test itself will raise the voltage of the earth conductor to a high value during testing. If the protective system is defective and supply is not disconnected, the test itself could cause danger. The test circuit of a voltage operated circuit breaker, which disconnects the earthing conductor from the earth electrode when the push button is pressed before applying the test voltage is a good example. It is important that the tests are carried out in the correct sequence. For example the continuity of the earth conductor and its effectiveness should be tested before carrying out the insulation resistance tests. An open circuit on the earth conductor together with a very low insulation resistance in a circuit could make the whole earth conductor system live at 500V (test voltage) during the insulation test of the faulty circuit. Another example is the testing of operation of residual current devices. This need to be carried out after the supply is connected to the system. There is obvious danger in providing a supply to an installation without checking the protective system, insulation resistance and polarity. Therefore testing has to be carried out in a certain order so that the items which need to be correctly functioning for other tests to be safe and satisfactory are tested first. Tests to be carried out: On Site Guide to the 16th Edition Wiring Regulations published by the Institution of Electrical Engineers, London indicates that the testing should be carried out in the following sequence 95
Before the supply is connected: i continuity of earth conductors ii continuity of ring final circuits iii polarity iv insulation resistance v earth electrode resistance With the supply connected: i re-check polarity before further testing ii operation of residual current devices The person carrying out the tests as well as the users must be quite clear as to how the installation is arranged to carry out its functions. For this purpose installation must be provided with: i Labels to indicate the purpose of the switchgear and control gear (This requirement can be met by durable paper labels) ii Correct identification of conductors, cable cores, and earth conductors iii Warning notices to indicate the presence of 400V where n o t expected (for example separately enclosed distribution boards connected to different phases but which can be simultaneously touched) iv Labelling on distribution boards, arranged so that the circuits protected may be quickly and easily identified. v Diagrams, charts or tables to show the arrangement of circuits, as well as the identification and location of fuses, circuit breakers, isolators and so on. (Please see Fig. 34 in the previous Chapter for the format of the diagram required.) Test procedure To test the continuity with supply disconnected a continuity tester or a lamp and a battery set or a bell and a battery set could be used.
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Continuity of earth conductors Method 1. 1. Temporarily connect the phase bus-bar to the earthing terminal at the Distribution Board 2. Check the continuity between phase and earth terminals at each outlet in Continuity Tester the circuit Temporary Connection
Fig. 40 - Continuity test for earth conductors: Method 1
Method 2. 1. connect two long leads to the continuity tester 2. connect one lead to the consumer’s main earthing terminal 3. check continuity of earth conductor at various points on the circuit such as socket-outlets, switches, luminaries etc. with the other testing lead. Continuity Tester
Consumer’s Main Earthing Terminal
Fig. 41 - Continuity test for earth conductors: Method 2
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Continuity of ring final circuits This test is required to see that phase, neutral and earth conductors of every ring final circuit are connected in complete rings. It should be noted that even if the ring is open at one point, all the socket-outlets will have supply, but trouble may develop later due to conductor overloading. For this test a low reading ohmmeter is required and the socket-outlets connected to the mid point of the circuit needs to be identified. Method 1 1. Connect the meter leads to each end of the earth conductors of the ring circuit to be tested at the distribution board and note down the reading (Say A). 2. join the two ends of the earth conductor at the DB together and connect to one lead of the meter. 3. connect the other lead to the mid point of the earth conductor of the ring and note down the reading (Say B). 4. connect the testing leads together and note down the reading (Say C). 5. check whether A/4 ≈ B-C. 6. the continuity of the phase and neutral conductors shall be checked in a similar manner. Continuity tester
Continuity tester
(b) Measurement between closed ends and mid-point
(a) Measurement between ends of earth electrodes Continuity tester
(C) Measurement between extended test leads Fig. 42 - Continuity test for Ring Final Circuits Method 1
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Method 2 1. Connect the meter leads to each end of the earth conductors of the ring circuit to be tested at the distribution board and note down the reading (Say A) 2. Connect the meter leads to each end of the phase conductors of the ring circuit to be tested at the distribution board and note down the reading (Say B) 3. Connect the meter leads to each end of the neutral conductors of the ring circuit to be tested at the distribution board note down the reading (Say C) 4. Temporarily connect phase and earth conductors together at the socketoutlet at mid point 5. Connect one of the meter leads to paralleled end of the phase conductors of the ring circuit at the distribution board. 6. Connect the other meter lead to paralleled end of the earth conductors of the ring circuit at the distribution board and note down the reading (Say D ) 7. D ≈ A/4+B/4 8. Remove the connection between phase and earth conductors and temporarily connect phase and neutral conductors together at the socket-outlet at mid point 9. Connect one of the meter leads to paralleled end of the phase conductors of the ring circuit at the distribution board. 10. Connect the other meter lead to paralleled end of the neutral conductors of the ring circuit at the distribution board and note down the reading (Say E ) 11. E ≈ B/2 ≈ C/2
Temporary Connection
Continuity tester
Fig. 43 - Continuity Test for Ring Circuits - Method 2 (Steps 8-10)
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Insulation resistance To comply with the Regulations the insulation resistance between the live conductors and also between the live conductors and earth shall be measured with an instrument giving a d.c. test voltage of 500 V. The insulation resistance shall be measured on each distribution board with all its final circuits connected but with current using equipment disconnected. The insulation resistance shall not be less than 0.5 megohms. Tests should be carried out using an Insulation Resistance Tester (Megger). Before testing it should be checked to see if any pilot or indicator lamps and capacitors are connected in the section to be tested. If any, they should be disconnected to avoid inaccurate results. It should also be checked to see if any voltage sensitive electronic devices such as residual current devices with electronic amplifiers, dimmer switches, touch switches, timers, power controllers, electronic starters for fluorescent lamps etc. are connected. If present they also are necessary to be disconnected so that they are not subjected to the test voltage. Test procedure 1. make sure ends of meter tails are separated and not touching anything 2. disconnect all pilot and indicating lamps 3. disconnect devices with electronic circuits 4. check whether all fuses are in place 5. close all MCBs and switches 6. remove all lamps and other current using equipment. (If it is not practicable to remove any lamp or disconnect any current-using equipment, the local switch controlling such lamp or equipment should be open.) 7. measure the resistances between phase, neutral and earth conductors at the distribution board. It could be carried out by measuring the insulation resistance of the following: Single Phase 1 between phase and neutral conductors 2 between phase and earth conductors 3 between neutral and earth conductors Three Phase 1 between phase 1 and phase 2 conductors 2 between phase 2 and phase 3 conductors 3 between phase 3 and phase 1 conductors 4 between phase 1 and neutral conductors 5 between phase 2 and neutral conductors 6 between phase 3 and neutral conductors 7 between phase 1 and earth conductors 8 between phase 2 and earth conductors 9 between phase 3 and earth conductors 10 between neutral and earth conductors Where it is more convenient, conductors may be joined together for this test. 100
Switches Closed
Lamps Removed
All fuses or mcb closed
Insulation Resistance Tester
RCCB
Plugs Removed
Closed or Bypassed and disconnected Main Switch Closed
Fig. 44 - Insulation Resistance test between phase and neutral conductors
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Earth electrode resistance The measurement is to be made using an Earth Electrode Resistance Tester. It should be noted that instructions on the use of these instruments vary from manufacturer to manufacturer and also on the model. The procedure described below is applicable to a popular instrument with three outgoing leads marked E (common), C (current) and P (voltage) terminals to connect the outgoing leads. Test Procedure 1. disconnect earthing conductor from the earth electrode 2. connect common lead of the test instrument to the earth electrode 3. plant one test probe about 30 metres away from the earth electrode 4. connect current lead of the test instrument to the test probe 5. plant the other test probe about 10 metres away from the earth electrode in line with the current probe and earth electrode as shown below. 6. connect voltage lead of the test instrument to this test probe 7. read the earth resistance value from the meter.
Earth Resistance Tester Common
Current Voltage
Voltage Probe
Ground
Earth Electrode
About 10m About 30m Fig. 45 - Measurement of Earth Electrode Resistance
102
Current Probe
Operation of residual current devices Operation of residual current devices can be checked with the supply connected by pressing the test button provided on the device. However this does not check the continuity of the earth conductor or the effectiveness of the earth electrode which are required for the device to provide the intended protection. The operation can be easily checked at socket-outlets by inserting a plug with a lamp connected between its phase and earth pins. If the sensitivity of the RCD is 30mA, the lamp used should be around 6.9 watts to get accurate results. Test procedure: 1. Obtain a lamp holder, a plug top matching to the socket-outlet to be tested, a low wattage lamp and two pieces of flexible cables 2. Connect two ends of the cables to the lamp holder 3. Connect one of the remaining ends to the earth pin of the plug top 4. Connect the other end to the live pin of the plug top 5. Insert the lamp in the holder 6. Insert the plug top into the socket-outlet to be tested 7. The RCCB would trip if the conductor connected to the earth pin is properly earthed and RCCB is in working order. Note: 1. In new installations electricity supply necessary to do this kind of testing may not be available. 2. In a complete test on a RCD it is required to ascertain the tripping time. There is an instrument called a RCD tester which could be used to do a complete test, but it is not described in here because it is unlikely that average reader would come across such a tester in normal practice.
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Short flexible cable leads
Plug top with cover removed
Socket outlet to be tested Fig. 46 - Testing RCCB with a lamp
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Lamp
New Colour Code for Cable Cores In the past the cable core colour codes used in the United Kingdom and other European countries were different. The United Kingdom changed its colour code for flexible cables as far back as 1969 to remove trade barriers in Europe for its domestic and similar equipment. The countries in the European Union decided to have one colour code throughout Europe. When the United Kingdom joined EU it had to follow suit. By 1996 all countries in Europe except the United Kingdom had the core colours of brown or black for phase conductors, blue for neutral conductor and green-and-yellow for earth conductor. Thus the United Kingdom had two conflicting situations with regard to cable core colours, one between its own rigid and flexible cables and the other between rigid cables of the United Kingdom and of the other European countries. The colour codes were as follows: BS Colour code for cores of flexible cables: Phase Brown Neutral Blue Earth Green-and-Yellow BS Colour code for cores rigid cables: Phase Red, Yellow or Blue (or 3 Reds) Neutral Black Earth Green-and-Yellow Colour code for cores of all cables in other European countries: Phase Brown or Black Neutral Blue Earth Green-and-Yellow In 1999 CENELEC accepted a formal international standard on conductor identification by colour. At this point the United Kingdom had to agree to amend IEE Regulations (BS 7671:1992) or withdraw it as it is a requirement to withdraw any conflicting standards in CENELEC member countries. The United Kingdom did not like to change to a system with only two colours for phase conductor and to resolve this, European countries agreed to introduce a third colour (grey) for their phase conductors. In order to facilitate exhausting cables manufactured up to then, it was agreed to allow use of old cables up to 31st March 2006. Accordingly at sites where wiring work starts after 1st April 2006 it is required to follow the new colour code. Where the wiring work was started after 1st April 2004, it is allowed to use cables with core colours of either old or new codes, but not both. IEE / BSI issued an amendment to BS 7671 incorporating necessary changes in 2004 and a complete book (with brown cover) incorporating all the amendments up to 2004 has been issued as BS 7671: 2001 (2004). 105
In existing installations there is no intention of changing the core colours to those with new colour code. In an extension to existing installation, cables with old core colours may be used up to 31st March 2006. Alternatively cables with new core colours could be used with markings at the changing point. Thus in domestic installations the change would not cause much of a problem as all new cables (rigid and flexible) will have the same core colours. But as long as the old cables are available in the market there is a possibility of confusion of core colours of blue and black with the old and new colour codes. (The Regulations allow the use of yellow and blue single core cables for the phase conductor for single phase work in place of red colour in large installations in the old code. Similarly black or grey colour could be used in place of brown as phase conductor in the new code. Therefore there is a possibility, of black colour single core cables remaining in future also, continuing the confusion). Where multi (four or more) core cables are used, cores with blue and black colours will be available on both sides of the installation at the changing point (referred to as interface), where an extension is added to an existing installation, with new core colours. It is important to mark the connection with the correct core colours at this interface. To deal with this BS 7671: 2001 (2004) contains a new regulation requiring to provide a warning notice and also contains an appendix called “Harmonized cable core colours” to provide guidance on marking at the interface between old and harmonized (new) colours, and general guidance on the colours to be used for conductors. The following are some extracts: Regulation on “Warning Notice – non standard colours” If wiring alterations or additions are made to an installation such that some of the wiring to previous version of these Regulations, a warning notice shall be affixed at or near the appropriate distribution board with the following warning: This installation has wiring colours to two versions of BS 7671. Great care should be taken before undertaking extension, alteration or repair that all conductors are correctly identified.
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From the Appendix: Harmonized cable core colours 1.
Introduction
BS 7671 has been harmonized with the technical intent of CENELEC Standard “Identification of cores in cables and flexible cores”. BS 7671 has been modified to align with these cable core colours, but also allows other suitable methods of marking connections by colours (tapes, sleeves or discs), or by alphanumerics (letters and/or numbers). Methods may be mixed within an installation. 2.
Alteration or addition to an existing installation
2.1
Single-phase
An alteration or addition made to a single-phase installation need not be marked at the interface provided that: i. the old cables are correctly identified by the colour red for phase and black for neutral, and ii. the new cables are correctly identified by the colour brown for phase and blue for neutral. 2.2
Two- or three-phase installation
Where an alteration or an addition made to a two- or three-phase installation wired in the old core colours with cable to the new core colours, unambiguous identification is required at the interface. Cores shall be marked as follows: Neutral conductors Old and new conductors: N
Phase conductors Old and new conductors: L1, L2, L3.
TABLE 14 Example of conductor marking at the interface for additions and alterations to an a. c. installation identified with the old cable colours Function Old conductor New conductor Colour Marking Marking Colour Phase 1 of a. c. Red L1 L1 Brown (1) Phase 2 of a. c. Yellow L2 L2 Black (1) Phase 3 of a. c. Blue L3 L3 Grey (1) Neutral of a. c. Black N N Blue Protective conductor (earth) Green-and-Yellow Green-and-Yellow (1) Three single-core cables with insulation of the same colour may be used if identified at the terminations. (Source: Table 7A, Appendix 7, BS 7671:2001(2004))
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3. Switch wires in a new installation or an alteration or addition to an existing installation Where a two-core cable with cores coloured brown and blue is used as a switch wire, both conductors being phase conductors, the blue shall be marked brown or L at its terminations. 4.
Intermediate and two-way wires in a new installation or an alteration or addition to an existing installation
Where a three-core cable with cores coloured brown, black and grey is used as a switch wire, all three conductors being phase conductors, the black and grey conductors shall be marked brown or L at their terminations. 5.
Phase conductors in a new installation or an alteration or addition to an existing installation
In a two- or three-phase power circuit the phase conductors may all be one of the permitted colours either identified L1, L2, L3 or marked brown, black, grey at their terminations to show the phase. 6.
Changes to cable core colour identification TABLE 15. Rigid cables
Cable type Single-core Single-core Two-core Three-core Four-core Five-core
Old core colours Red Black Red, Black Red, Yellow, Blue Red, Yellow, Blue, Black Red, Yellow, Blue, Black, Green-and-Yellow
New core colours Brown Blue Brown, Blue Brown, Black, Grey Brown, Black, Grey, Blue Brown, Black, Grey, Blue, Green-and-Yellow
(Source: Table 7C BS 7671:2001(2004))
TABLE 16. Flexible cables Cable type Two-core Three-core Four-core Five-core
Old core colours Brown, Blue Brown, Blue, Green-and -Yellow Black, Blue, Brown, Green-and-Yellow Black, Blue, Brown, Black, Green-and-Yellow
(Source: Table 7D BS 7671:2001(2004))
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New core colours No change No change Brown, Black, Grey, Green-and-Yellow Brown, Black, Grey, Blue, Green-and-Yellow
The Sri Lanka Standards Institution has amended the relevant standards on cables to suit these changes and the manufacturers have made necessary changes to produce cables abiding by these standards. Therefore cables with new core colours are available now and there could be both sets of colours available in the market for some time. Therefore it is essential to take necessary care in following the correct colour code to avoid confusion. From the IEE publication referred to above it shall be noted that: ii. There is no need to change the colour code of the existing installation iii. Where both types of cables are used a warning notice shall be provided at the distribution board a. In single phase installations there is no need to mark at the interface, if the colour code has been correctly followed in both existing and new installations b. In two- or three-phase installations cores shall be marked with alphaneumeric characters (L1, L2, L3 and N) (Two diagrams provided by IEE to illustrate these are shown in Fig. 47 and Fig. 48.)
Addition
New Circuit
Fig. 47: Addition and alterations using cables with new core colours to an existing installation 109
Addition
Fig. 48: Addition of a socket-outlet to an existing installation using new cables. It should be further noted that the Regulations permit use of differently coloured cable cores by marking connections with correct colour tapes, sleeves or discs etc. Therefore cables with old core colours (red, yellow, blue, black) could be used an installation starting work on-site after 1st April 2006 with correctly coloured sleeves (tapes or discs etc.) of brown, black, grey and blue at their terminations. The only prohibition is the use of any different colour marking on green-and-yellow coloured cable. (i.e. Earth cable shall not be used as a phase or neutral conductor) The Regulations also permit the use of two- or three- cables with different core colours to be used at switching points where only one phase is used. This is applicable when the wiring is done in multi-core cables with intermediate junction boxes. In such a situation a two-core, a three-core, or a four-core cable could be drawn, to a switch, a two-way switch or an intermediate switch respectively, from the nearest junction box. Only one phase will be connected to each of these switches and therefore each core shall be marked “brown” colour. On the other hand when wiring three-phase installation with single core cables, normally only one colour (old code – red; new code – brown) will be used. It is necessary to correctly identify the different phases throughout the installation. Therefore coloured sleeves or similar means shall be provided to identify the brown cables used as L2 (black) and L3 (grey) phase conductors. 110
With this change the traditional R, Y, B notation used to mark phases will loose its significance. The symbols L1, L2 and L3 will be used instead. (Use of first letter of the chosen core colours may lead to confusion). An interesting situation will arise with regard to the use of red, yellow and blue colours for phase indicator lamps. These colours will have no significance with the new core colours, but it is very unlikely they could be replaced with brown, black and grey lamps. There is no Regulation to govern the colours of indicator lamps and most probably use of the colours red, yellow and blue would be continued.
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References: 1. “Regulations for the Electrical Equipment of Buildings”, 14th Edition, Reprinted in Metric Units Incorporating Amendments, The Institution of Electrical Engineers, 1970 2. “Regulations for Electrical Installations”, 15th Edition, The Institution of Electrical Engineers, 1981 3. BS 7671:1992 “Requirements for Electrical Installations, IEE Wiring Regulations”, 16th Edition, BSI and The Institution of Electrical Engineers, 1992 4. BS 7671:2001(2004) “Requirements for Electrical Installations, IEE Wiring Regulations”, 16th Edition, Reprinted incorporated amendments up to 2004 BSI and The Institution of Electrical Engineers, 2004 5. K. A. Miller, “Guide to the IEE Wiring Regulations”, Second Edition, Peter Peregrinus Ltd., 1970 6. J.F. Whitfield, “A Guide to the 15th Edition of the IEE Wiring Regulations”, 3rd impression with minor corrections, Peter Peregrinus Ltd.(on the behalf of IEE), 1982 7. “On-Site Guide to the 16th Edition Wiring Regulations”, The Institution of Electrical Engineers, 1992 8. “The Crabtree Electrical Handbook”, 4th Edition, J. A. Crabtree & Co. Ltd., 1968 9. J. O. Paddock and R. A. W. Galvin, “Electrical Installation, Technology and Practice”, ELBS Edition, English Universities Press, 1970 10. W.E.Steward, “Modern Wiring practice”, 7th (Metric) Edition, Newnes-Butterworths, 1971
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