Booklet on Domestic Wiring

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DOMESTIC WIRING

THE INSTITUTION OF ENGINEERS

SRI LANKA

MAY 2009

Technical Information Series Publication

No: TIS001, Edition 1

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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.

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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

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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

16

th

Edition of the IEE Wiring Regulations which was the standard

that was being followed at the time of writing it although it is the

17

th

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

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CONTENTS

Scope 11

Introduction 12

Basics of electrical wiring 13

Control of supply 13

Protection 13

Conductor 13

Insulator 13

Cables 13

Additional protection for cables 13 Stranding of conductors 14

Flexible cords 14

Electric circuits 14

Relationship between voltage and current 15

The basic circuit 15

Waveform 16

Types of available a. c. supplies 16 Current carrying capacity 16

Overcurrent 17

Faults – short circuits 17

Earth faults 17

Direct and indirect contacts 18 Prevention of fires and accidents 18

Switching 19 Switches 19 Isolator 19 Fuse 19 Circuit breaker 19 Emergency disconnection 20 Voltage and current rating of equipment 20 Earth fault protection 20

Resistance area of earth electrode 21 Earth loop impedance 22 Earth leakage circuit breakers 22 Voltage operated earth leakage circuit breaker 22 Residual current circuit breaker 23 Residual current device 24 Distribution inside the house 24

Distribution board 24

Final circuit 24

Fixed wiring 24

Socket outlet 25

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Different types of final circuits 25 Sequence of supply controls 26 Declared or nominal voltage 27

Accessory 27

Diversity 27

The consumer unit 27

Safety precautions 27 Types of switches 29 Wiring systems – additional protection for cables 30

Looping in system 30

Identification (Colour code) 31 Single line diagram 31

IEE Regulations 33

Summary of Regulations relevant to domestic wiring 35

Fundamental Principles 35

Protection for safety 35 Protection against electric shock 35 Protection against thermal effects 35 Requirements for safety 35 Conductors and equipment 35 Fuses and circuit breakers 36 Connection of switches 36 Emergency disconnection 36 General provision for earthing 36 Installation of equipment 36 Special conditions 36 Addition to an installation 36 Testing and inspection 36 Control distribution and over-current protection 37

Protective equipment 37 Control of supply 37

Isolation 37

Overcurrent protection 38 Protection against electric shock 39 Protection against direct contact 39 Protection against indirect contact 39 Earthed equipotential bonding 39 Circuit protective conductors 40 Connection with earth 41

Labelling 41

Maximum demand and diversity 41

Nature of demand 41

Final circuits 42

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Voltage rating 42

Current rating 42

Voltage drop 42

Cross-sectional area of conductors 42 Protection against mechanical damage 42 Identification of conductors 43 Terminations 43 Installation of equipment 43 Lamp holders 43 Lighting points 44 Ceiling roses 44 Lighting fittings 44 Plugs and socket-outlets 44

Switching for safety 46

EXCERPTS FROM PREVIOUS EDITIONS 47 Earth fault currents 47 Detached buildings 47

Diversity 47

Overcurrent protection 47 Provision of protective devices 47

Final circuits 48

Final circuits of rating exceeding 15A 48 Domestic ring and radial circuits 48 Bending radius of cables 49

Terminations 49

Conduit systems 49

Metal conduit systems 50 Flexible conduit systems 50 Suspension from non-metallic boxes 51 Equipment liable to cause overheating 51

Ceiling roses 51

Isolated metal 51

Electrical discharge circuits 51 Earth fault protection 51 Room containing a bath 52 From the “ON-SITE GUIDE” 53

Maximum demand and diversity 53 Conventional circuit arrangements 55 Final circuits using 13A socket-outlets 55 Circuit protection 55

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Cooker circuits in household premises 56 Correction factors for current carrying capacity 56

Selection of a System 59 Conductors 59 Insulation 59 Wiring systems 60 Overcurrent Protection 60 Means of isolation 61 Earth fault protection 62 Distribution boards 62

Layout 62

Type of final circuits 65 Selection of cables 65 Voltage drop calculation 67 Lighting circuits 68 Socket outlet circuits 68 Conduit capacities 69 Planning the installation 71

Example 75

Installation 83

Conduits on walls and ceilings 84 Conduits for switches 84 Conduits for socket outlets 85 Conduits for distribution boards 86

Wiring 86

Example 88

Single line diagram 89

Conduit layout 89

Wiring procedure 90

Testing 95

Tests to be carried out 95 Continuity of earth conductors 97 Continuity of ring final circuits 98 Insulation resistance 100 Earth electrode resistance 102

Operation of residual current devices 103 New colour code for cable cores 105 Regulation on “Warning notice – Non standard colours” 106

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FROM THE APPENDIX: “HARMONIZED CABLE CORE COLOURS” 107

Introduction 107

Alteration or addition to an existing installation 107

Single phase 107

Two- or three-phase installation 107

Switch wires 108

Intermediate and two-way switches 108

Phase conductors 108

Changes 108

References 112

List of Diagrams

Fig.1 Components of a cable 13 Fig.2 Stranding of conductors 14 Fig.3 Series and parallel circuits 15 Fig.4 Basic domestic circuit 15 Fig.5 A.C. Voltage Wave 16 Fig.6 Electrical faults 17 Fig.7 Direct and indirect contacts 18 Fig.8 Multi pole switches 19 Fig.9 Earth conductor 21 Fig.10 Resistance area 22 Fig.11 Voltage operated earth leakage circuit breaker 23 Fig.12 Residual current circuit breaker 23 Fig.13 Ring and radial circuits 25 Fig.14 Sequence of supply controls 26 Fig.15 Polarity: position of fuses (mcbs) and switches 28 Fig.16 Polarity: connections to screw type lamp holders 28 Fig.17 Polarity: connections to socket outlets 29 Fig.18 Circuits with two-way and intermediate switches 29 Fig.19 Looping in wiring system 30 Fig.20 Single line diagram 31 Fig.21 House – Electrical points 73, 74 Fig.22 Upper floor distribution board 79 Fig.23 Ground floor distribution board 80 Fig.24 House – Electrical layout 81, 82 Fig.25 Conduit junction boxes 83

Fig.26 Sunk box 83

Fig.27 Conduit connections to lamps and a switch 84 Fig.28 Termination for a switch on wall 84 Fig.29 Conduit connection to socket outlets at skirting level 85 Fig.30 Conduit entries to surface mounted distribution boards 86 Fig.31 Method of connecting cable to draw wire 87 Fig.32 Cables entangling in a junction box 87

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Fig.33 Wiring points in a house 88 Fig.34 Single line diagram 89

Fig.35 Wiring layout 89

Fig.36 Conduit layout 90 Fig.37 Example wiring using looping in method 91 Fig.38 Connections to switches and ceiling roses 93 Fig.39 Typical layout for 13A socket outlets 94 Fig.40 Continuity test for earth conductor (Method 1) 97 Fig.41 Continuity test for earth conductor (Method 2) 97 Fig.42 Continuity test for ring final circuits (Method 1) 98 Fig.43 Continuity test for ring final circuits (Method 2) 99 Fig.44 Insulation resistance between phase and neutral conductors 101 Fig.45 Measurement of earth electrode resistance 102 Fig.46 Testing RCCB with a lamp 104 Fig.47 Addition and alteration to an existing installation 109 Fig.48 Addition of a socket-outlet using new cables 110 List of Tables

Table 1 Identification of conductors 43 Table 2 Overcurrent protection of lampholders 44 Table 3 Plugs and socket-outlets 45 Table 4 Minimum internal radii of bends in fixed wiring 49 Table 5 Current demand to be assumed for points and equipment 53 Table 6 Allowances for diversity 54 Table 7 Final circuits using 13A socket-outlets 55 Table 8 Correction factors for groups of more than one circuit 57 Table 9 Current ratings applicable when protected by circuit breakers 66 Table 10 Current ratings applicable when protected by rewirable fuses 67 Table 11 Data on Flexible Cords 67 Table 12 Conventional final circuits 69 Table 13 Conduit capacities for PVC / PVC cables 70 Table 14 Example of conductor marking at the interface 107 Table 15 Changes to cable core colour identification - rigid cables 108 Table 16 Changes to cable core colour identification - flexible cables 108

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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.

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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.

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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.

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.

Sheath for

mechanical protection

Insulation Conductor(Solid or stranded)

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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.

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.

1 3 1+6 1+6+12 1+6+12+18

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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.

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).

Voltage source

Flg.4: Basic Domestic Circuit Fig.3 Series and Parallel Circuits

Voltage source Voltage 1 Voltage 2 Voltage 3

Lamp 1 Lamp 2 Lamp 3 Total Voltage Current Current 1 Current 2 Current 3 Total Curr ent Total Voltage Lamp 1 Lamp 2 Lamp 3 Supply Transformer Line I = Current

Phase (Line) Conductor

V = Voltage Neutral Neutral Conductor Z = Impedance Transformer neutral earth point

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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.

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.

Fig. 5: A.C. Voltage Wave

1 Cycle

TIME

VOL

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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.

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.

Fig. 6: Electrical Faults

(a) Short circuit (b) Earth fault fault

fault Earthed metal parts

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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.

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.

Fig. 7: Direct and Indirect Contacts

(a) Direct Contact (b) Indirect Contact

Switch Phase Neutral Earth (High Impedance) Earth Fault conductor Phase To lamp

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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.

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

Fig. 8: Multi-Pole Switches

Phase Single Pole Switch Phase Neutral Double Pole Switch Phase 1 Phase 2 Phase 3 Neutral Four Pole Switch

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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.

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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.

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.

Fig. 9: Earth Conductor

Phase R Supply Transformer Phase B Phase Y Transformer neutral earth point

Earth return path Main Earthing Terminal

Phase (Line) Conductor

Neutral Conductor Circuit Protective Conductor

(cpc) Protected Metal Work Earthing Conductor CONSUMER SUPPLY AUTHORITY L O A D L

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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.

Ground Fault Current Soil Layers Soil Layers Earth Electrode Paths of Fault Current (b) Plan View (a) Sectional Elevation

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(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.)

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.). Supply Test Resistance Trip Coil Insulated Earth Lead Consumer’s Earth Electrode Auxiliary Earth Electrode Protected Metal work

Fig. 11: Voltage Operated Earth Leakage Circuit Breaker

Supply Trip Coil Multiplier Search Coil Test Button Test Resistance

Fig. 12: Residual Current Circuit Breaker

Protected Metal Work Earth Electrode L O A D L O A D

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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.

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Socket-outlet

The part of the facility to connect and disconnect the movable equipment connected to the fixed wiring portion is known as a socket-outlet. 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 socket-outlets 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-using-appliance 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.

Fig. 13: Ring and Radial Circuits Distribution Board Ring Circuit Radial Circuit Spur

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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.

Final Circuits Sub Distribution Board Final Circuits Main Distribution Board Sub-main Cables to another Sub-distribution board Service Main Service Fuse Meter Supply Authority’s Terminals Consumer’s Terminals

Residual Current Device Main Switchfuse or DP MCB ORIGIN OF THE INSTALLATION Consumer’s Earth Electrode Load Cables Consumer’s Terminals Supply Authority’s Equipment

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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

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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.

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.

Fig. 15: Polarity: Position of Fuses (mcbs) and Switches MCB on Neutral side

Incorrect Correct

Switch on Neutral side Incorrect

Fig. 16: Polarity: Connections to screw type lamp holders Phase to centre contact Neutral to

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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.

Fig. 17: Polarity: Connections to socket outlets

Fig. 18: Circuits with two way and intermediate switches Two way switch Lamp Two way switch Lamp Lamp Lamp Intermediate switch Two way switch SWITCHING POSITIONS

(b) Two way and Intermediate swiches (a) Two way swiches

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0

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.)

Fig. 19: Looping in Wiring System

To Upper Floor Circuit

Distribution Board

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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-and-Yellow (Earth) is adopted. (The latest amendments to IEE Regulations have changed the colour coding as Brown (phase), Blue (neutral) and Green-and-Yellow (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.

Fig. 20: Single Line Diagram 30 A DP MCB 40 A DP 30 mA RCCB Socket Outlets Lamps 2 x 1 2 x 2.5 Ring 2x6

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Symbol Description Switch (General) Triple Pole Switch Circuit Breaker Single Pole MCB Fuse

Link ( Neutral)

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

Electric Bell Two way Switch

Bell Push

Earth Connection

STANDARD SYMBOLS

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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”.

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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.

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Summary of Regulations Relevant to Domestic Wiring Fundamental Principles

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

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

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

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. Protection for safety Protection against electric shock Protection against thermal effects Conductors and equipment

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Good workmanship and proper materials shall be used. All equipment shall be constructed, installed and protected to prevent danger.

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.

All fuses and single pole control devices such as switches, circuit-breakers, thermostats etc. shall be connected in the phase conductor only.

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.

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.

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. All equipment likely to be exposed to weather shall be so constructed and protected as may be necessary to prevent danger from such exposure.

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.

Fuses and circuit breakers Connection of switches Emergency disconnection General provision for earthing Installation of equipment Special conditions Additions to an installation Testing and Inspection

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Control, Distribution and Overcurrent Protection

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.

Every consumer’s installation should have switchgear at a readily accessible position with means of isolation, overcurrent protection and protection against electric shock. 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

Protective equipment

Control of supply Isolation

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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.

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 Overcurrent

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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 shall be provided by appropriate measures providing protection against direct contact and indirect 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. 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

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. Protection against electric shock Protection against indirect contact Earthed equipotential bonding Protection against direct contact

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0

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.

Cross sectional area of every circuit protective conductor not contained in a multi core cable shall have the cross section as follows:

Circuit protective conductors

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Cross sectional area Cross sectional area of of phase conductor circuit protective conductor

(S) (Sp)

mm2 mm2

S ≤ 16 S 16 < S < 35 16 S ≥ 35 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.)

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.

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. The maximum demand of an installation shall be assessed. Diversity may be taken into account for the purpose.

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. Connection with earth Labelling Maximum demand and diversity Nature of demand

Figure

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References

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