2007 Electrical Workshop Manual

A HAND BOOK

FOR

ELECTRICAL WORKSHOP

Electrical & Electronics Engineering

Amrita School of Engineering

Amrita Vishwa Vidya Peetham

Amritapuri Campus

Electrical Workshop

Department of ECE, 2 |

ASE, Amritapuri Campus

CONTENTS

Part 1 General

1. Syllabus

2. Lab rules 3. Safety precautions

4. Electrical engineering - An overview 5. Electric power supply system

Part 2 Study of safety devices

1. Importance of safety devices

2. Circuit breakers – MCB, MCCB & RCBO (ELCB) etc 3. Earthing systems

Part 3 House wiring 1. Introduction 2. Systems of distribution of electrical energy

3. Systems of wiring 4. Selection of wiring system

5. Electrical wiring materials

Part 4 Experiments 1. One lamp controlled by one switch

2. Series connection 3. Parallel connection 4. Staircase wiring 5. Hospital wiring 6. Godown wiring 7. Fluorescent lamp wiring

Part 5 Domestic appliances

1. Fan

2. Electric Mixer

3. Electric Iron 4. Refrigerator

5. Air conditioner 6. Electric lamps

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Course Contents

List of study and practical exercises for Electrical Workshop: Ex. No. 1: Study of power supplies and safety devices

� Introduction to electrical supply system � Importance of Safety device in domestic installation

� Study of safety devices such as Fuses, MCB, MCCB, ELCB & Earthing.

Ex. No. 2: Electrical wiring practices (House wiring)

� Distribution of electrical energy in a domestic

electrical installation � Study of wiring tools & accessories � Various types of domestic wiring

Exercise in wiring practice

� One lamp controlled by one switch � Series and parallel connection � Staircase wiring

� Hospital wiring � Godown wiring

� Plug socket connection � Fluorescent lamp wiring

Ex. No. 3: Study of domestic appliances

� Study of different types of electric Lamps – Incandescent lamp,

Fluorescent, CFL, Metal halide, Mercury vapour, Sodium vapour and

halogen lamp.

� Study of home appliances – Mixie, Fan, Refrigerator, Air Conditioner, Iron box, Water heater & Energy meter.

Ex. No. 4: � Mini Project

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List of study and practical exercises for Electronics

Workshop Ex. No. 1: Familiarization of electronic components and study of measuring devices.

� Ohms law verification

� Half wave and full wave rectifier

Ex. No. 2: PCB fabrication and soldering practice

� Assembling and soldering of Astable multivibrator circuit on a copper clad sheet.

Ex. No. 3: Study of Intel 8085 microprocessor trainer kit concepts

� Familiarization of trainer kit

� Writing an assembly language program using Intel 8085

Ex. No. 4: Personal computer hardware workshop

� General introduction to the PC

� Familiarization of main components of a typical desktop computer

� PC power supply

� Role of a processor in a computer system

� Memory

� Essential primary components of a motherboard

� BIOS

� Commonly seen Ports and slots in a general PC

� Essential components of a computer network

Ex. No. 5: Mini Project

� Project – Assemble and solder any electronic circuit and demonstrate its operation.

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General Workshop Rules

All students in the workshop are expected to adhere to the following guidelines.

• The students are supposed to come in proper workshop uniform dress. Wearing

shoes in the workshop is compulsory.

• Do not fool around in the lab: Take your lab work seriously and behave

appropriately in the laboratory. Be aware of your classmates’ safety as well as your own at all times.

• To successfully complete the experiments in one lab period, you must come

prepared to the laboratory. You must read the experiment in advance and answer

the pre-lab questions.

• Please treat the instruments with care, as they are very expensive.

• Return the components to the correct bins when you are finished with them.

• Before leaving the lab, place the stools under the lab bench.

• Before leaving the lab, turn off the main power switch to the lab bench.

• Keep your work area neat and uncluttered- Have only books and other materials that are needed to conduct the experiment in the laboratory.

• Experiment: The student works with a partner and they both take the data on

separate notebooks. The lab instructor will look at the data and sign on your notebook at the end of the experiment.

• Any student missing a lab (not present in the lab) with no proper or reasonable excuse will get a “0” grade on that specific lab and will have his/her final letter

grade reduced. Any student missing two labs with no proper excuse will automatically get a failing grade (F).

• This laboratory can be used by students during laboratory hours only.

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Electrical Safety Principles

When planning and performing work on electrical systems and equipment, keep

these principles in mind:

� Understand the procedure completely before starting the work.

� Use good quality footwear/shoes in order to provide maximum resistance.

� Never energize any circuit unless you are sure that no one is working on the circuit. Give electric supply to the wiring system only after thorough verification.

� Before replacing a blown fuse always remember to put the switch off.

� Do not touch switch boards, main switches, holder points etc with wet hands.

� Do not use broken switches, sockets or plug.

� Use non-conductive tools whenever possible.

� Before putting the plug pins in socket put off the plug switch and disconnect the plug by pulling the plug pin and not by pulling cable.

� Take utmost care while handling lamps, lamp holders, switches etc, because these

materials are brittle.

� Never drape electrical cords over heat sources

� Before beginning work, tie back long hair, and roll up loose sleeves.

� Know the location and how to operate shut-off switches and/or circuit breaker panels. Use these devices to shut off equipment in the event of a fire or

electrocution.

� Don’t over bend cables when pulling them through a bend in a raceway, often a pressure or squeezing develops causing insulation damage.

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Electrical engineering an over view

Some definitions

Electric current: - Electric currant can be termed as a continuous flow of electrons through a conductor. One ampere is the current produced when a pressure of one volt is applied across a

circuit having one ohm resistance.

EMF: - EMF is electro motive force. Potential difference between two points in a circuit is the

electrical pressure difference required to drive a current between them. Potential difference may be termed as voltage.

Voltage of a torch battery is 1.5 V and that of automobile battery is 12V. KSEB supply voltage for domestic installation is 240 V.

Electric power (watt):- Electric power, P = Voltage * current * Power factor Unit of electric

power is watt (W) Electric energy:-Unit of electric energy is KWh (Kilo Watt hour) 1 unit energy = 1 KWh KSEB

provides one KWh meter at every Installation for measuring consumed energy.

Resistance is the property of a substance due to which it opposes the flow of current through it. Unit of resistance is ohm Resistance, R = Specific resistance * I / A

Where I is the length of material & A is the area of cross section

Effect of temperature on resistance:-When temperature increases resistance of pure metals and Alloys increases when temperature increases resistance of electrolyte, insulators etc

decrease. Resistance in series:-Consider three resistors connected in series, and then the total

resistance of the circuit will be the sum of the three resistors.

Ohms law:-Ohms law states that, the ratio of potential difference between any two points in a conductor to the current flowing between them is constant. R = V /1 Keeping temperature constant.

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Study of Electric Power supply Eelectricity

Electricity is a form of energy. Electricity is the flow of electrons. We get electricity, which is a secondary energy source, from the conversion of other sources of

energy, like coal, natural gas, oil, nuclear power, Hydel power and other natural sources, which are called primary sources.

Electric power supply system

AC&DC: DC or direct current is steady current. It never changes its direction, and AC is alternating in nature. AC voltage can be increased or decreased with the help of transformers. By using high voltage AC, we can drastically reduce the transmission losses. AC can be converted

into DC easily but reverse is not so easy.

In India state electricity boards are the authorities to generate and distribute electric energy. KSEB generates electric power at a voltage of 11 KV. This power is transmitted by increasing the voltage at different levels as 33 KV, 66KV, 110 KV, 220KVor 400 KV from different

substations. At load centers this voltage again stepped down as 11 KV and a feeder network is created. This feeder line energizes the 11KV/415V step down transformer, and from these

transformers electric supply can be given to consumers at 240V and 415 V as single phase or three phases.

All domestic and commercial consumers get electric energy from the distribution network of concerned electricity boards. Based on the power requirements of

consumers Electricity Boards may give 3-phase connection (for high power) or single phase connection (for low power). In the three phase connections 4 wires are provided, where as in single-phase connection one phase and a neutral connection are provided

to the consumers. Phase to neutral voltage in our country is 230 V and phase-to-phase voltage is 400 V of frequency 50 Hz. Most of the appliances work on single-phase

supply. There are some motors, which requires three phase supply. A KWh meter is provided at the consumer end for measuring the electrical energy consumed.

KSEB introduces different tariffs for different consumers, as per their connected load and nature of connection.

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Study of safety devices

Importance of safety devices

The safety features are inbuilt with electric power distribution. The current is to flow

through the path it is expected to pass and should not take another path through which

it is not expected to pass. Conductors made of copper or aluminium are provided across

the path for carrying the current and insulators like PVC, paper or rubber are provided

across the path through which the current is not expected to flow.

Under abnormal condition there can be failure of insulations and current will flow

through the undesired path which can cause damage to equipments and more

important the safety of the user. Sometimes the user may inadvertently touch a live

conductor and cause electric shock. The circuit may also carry under short circuit

condition much more than normal value of the current. The inbuilt safety features will

isolate the faulty circuit from the rest of the supply.

The very high currents caused by short circuit situation can cause lots of damage to

electrical installation. Protective devices are needed to break short-circuit and

overload currents.

Circuit breakers and fuses are protective devices that control the power going to a

particular route of wiring. In case of an overload or a short on that circuit, the

breaker or fuse trips and automatically shuts off power to that circuit. Fuses are the

commonly used protection devices to protect components like wires, transformers

electronics circuit modules against overload. The general idea of the fuse is that it

"burns fuse link" when current gets higher than it's rating and thus stops the

current flowing.

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Types of safety devices

� Fuse

� Circuit breakers( MCB, MCCB & ELCB)

� Earthing.

Basically two types of protections are provided in the power supply system of domestic consumers.

a. Protection from over current.

b. Protection from leakage current due to failure of insulation or inadvertent contact with live conductors by the user.

Over current and Short circuit

One type of situation that wiring needs to be protected against is over current. The electrical wiring is rated for certain maximum current. If you try to pull more current through it, the wiring will heat considerably. When the wiring heats too

much, it will cause the melting of cable insulation, cause fire if there is something flammable near cable and even melt the copper conductors in the cable. So

protection is needed to guarantee that in case of something tries to pull too much current through mains wiring, this cannot happen for any long time until the fuse blows and stops the current.

Many people are familiar with a "short circuit", which is a type of fault that occurs when two conductors of an electric circuit touch each other. The current flow

caused by a short circuit is usually high and rapid and is quickly detected and halted by conventional circuit protective devices, such as fuses or circuit breakers. Ground faults are one type of problem when the insulation fails.

Protection against over current

Every electrical circuit shall be protected against over current by suitable over current devices. These devices could be

a. Miniature Circuit Breaker (MCB) b. Moulded Case Circuit Breaker

c. Semi enclosed rewirable fuses d. High Rupturing Capacity (HRC) fuses

Typical breaking capacities of protective devices are as follows:

HRC fuses - 80 kA MCB - 16 kA

Rewirable fuses - 1 to 4 kA Protection against electrocution

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The use of exposed, substandard, badly wired, wrongly connected or damaged

equipment as well as frayed or badly repaired cables reduces the safety of an

installation and increases the risk of person receiving an electric shock.

Electrocution is a passage of current through human body, which is dangerous. The

flow of current through human body effects vital functions.

a. Breathing

b. Heartbeat

A correctly chosen RCCB can detect small currents flowing to earth and reduces the

risk of electrocution. Effect of electric current through human body has been well

researched and following chart summarizes the results:

Human sensitivity to electricity

500mA

Immediate cardiac arrest resulting in

death.

70-100mA

Cardiac fibillarillation; the heart begins

beats at a steady

20-30 mA

Muscle contraction can cause respiratory

paralysis

10mA

Muscle contraction : the person remains

stuck, to the conductor

1-10 mA

Prickling sensations

However, electrocution should not be viewed in terms of current alone but in terms

of contact voltage. A person gets electrocuted by coming in contact with an object

that has a different potential from his/her own. The difference in potential causes

the current to flow through the body.

The human body has known limits:

- Under normal dry conditions, voltage limit = 50V.

- In damp surroundings, voltage limit = 25V.

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FUSES

Fuse is a wire of short length having low melting point which gives protection against excessive

current. This excessive current may be due to over load or short circuit. Under normal working

condition the current flowing through the circuit is within safe limit. But when some faults such

as short circuit occurs the current exceeds the safe limit value, the fuse wire gets heated and

melts. This will cause breaking of the circuit. After one fusing operation, fuse wire must be

rewired with the same size wire.

This basic guide will help you decide which fuse to fit to ensure the safe use of your household appliances.

• Appliances up to 700 Watts = 3 Amp fuse • Appliances between 700 and 1000 Watts = 5 Amp fuse • Appliances over 1000 Watts = 13 Amp fuse

COMMON FUSE TYPES

1. Rewirable fuse

2. Cartridge fuse 3. HRC fuse

1. Rewirable fuse:

This is the cheapest method for protecting a circuit from short circuit. Wires of

different diameters made of lead and tin are used in the circuit. When large current

flows these wires melts and disconnects the faulty circuit from the rest of the supply.

There are different types of fuses. The usual type is the rewirable type in which the

fuse wire is carried in a removable fuse link (Fig. a). The fuse link is made of

porcelain or other suitable insulating material. The fuse carrier is push-fitted to the

fuse base to make the connection through. An advantage of this type is that the

blown fuse wire can be replaced with negligible cost. But there is a chance of

selecting a wrong size of fuse wire. Another disadvantage with rewirable fuse is that

it may sometimes lead to fire hazards, when the fuse wire blows.

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Fig. (a) Rewirable fuse

The semi enclosed rewirable fuses has the following drawbacks:

• It normally melts on 50 % to 100 % excessive overload. The melting

current cannot be accurately predicted.

• It takes time to rewire the fuse. • Standard fuse wire should be always made available.

However it is the cheapest mode of protection from short circuit.

2. Cartridge fuse

Cartridge fuse consists of a tube with metal end caps at both ends (Fig. b). The tube

is usually made of glass with no filling material. The fuse wire is placed inside the

tube, connected between the end caps. Since the tube is made of glass, the fuse

element can be easily inspected for breakage. When the fuse is blown, the whole

cartridge has to be replaced. The advantages of cartridge fuses are, quick and easy

replacement and the fuse rating is marked on the end cap of the cartridge itself.

Cartridge fuses are mainly: used in various electrical and electronic equipment.

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3. High Rupturing Capacity Fuse (HRC):

This is a completely enclosed cartridge type of fuse. These fuses are screwed or linked in the circuit. Generally it is used in the high power circuits. High Rupturing

Capacity (HRC) fuse consists of a porcelain tube! with metal end caps and fixing tags (Fig. c). The fuse element is held inside the tube between the end caps and the tube is filled with silica sand or granulated quartz. When the fuse element blows, the

silica inside the tube prevent the formation of an arc, and thus avoids the possibility of fire hazards. HRC fuse links are available in a range of 10A to 800A.

The HRC fuse has the following advantages:

• It is very reliable. • It has an enclosed fuse wire, therefore no chance its arc doing any

damage to the surroundings. • It has low temperature rise at rated load. • Maintenance free.

The drawbacks are:

• It is costly.

• Take time to replace the fuse.

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Circuit breakers MCB and ELCB

MCB is miniature circuit breaker. It is automatic in action. When excessive current passes

through the circuit, handle of MCB will moves down and thus trips the circuit. After one such an

operation we can manually reset the supply by solving the fault in that circuit. Thus rewiring fault

size fuse wire in the case of fuse can be avoided by using MCBs.

ELCB is earth leakage circuit breaker. It protects the circuit from any leakage of current. It

protects the circuit from lightning and thunder.

Miniature Circuit Breaker (MCB)

Miniature circuit breakers are compact devices used in distribution boards for

protection against overload and short circuit. The overload protection is achieved by a

thermal trip mechanism using a bimetallic strip. An electromagnetic trip mechanism

is also incorporated for instantaneous tripping in the event of a short circuit.

When there is a sudden increase in current due to a short circuit, the circuit should

open immediately, but the bimetallic strip does not respond quickly. In this case, the

solenoid attracts the plunger and thus triggers the trip mechanism. After clearing

the fault, the MCB can be switched on manually.

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Fig. below shows the current path in a typical miniature circuit breaker when it is in

the 'on' position. The current passes through a solenoid coil and a bimetallic strip.

When an overload condition persists for a few seconds, the bimetallic strip bends and

triggers the trip mechanism.

The principle of operation of an MCB is based on the following two principles.

a. Thermal operation

b. Magnetic operation

a. Thermal operation

In thermal operation, the extra heat produced by the high current warms the

bimetal strip. This results in bending the bimetallic strip and trips the operating

contacts. The thermal operation is slow. Hence, it is not suitable for speedy

disconnection required to clear fault currents. However, it is ideal for operation in

the event of small but prolonged overload currents. Thus, in general the thermal

operation is suitable for opening the circuit in the event of excessive current due to

the overloaded machines.

b. Magnetic operation

The magnetic operation, on the other hand is suitable for protection against high

short circuit currents. This magnetic operation is due to the magnetic field set up by

a coil carrying the current, which attracts an iron part to trip the breaker when the

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current becomes large enough. The magnetic operation is very fast and is used for

braking fault currents.

In most cases of MCB' s, both types are provided so that overload currents and

short circuit currents are handled with the same degree. It should however be

remembered that the mechanical operation of opening the contacts takes a definite

minimum time, typically 20ms, so that there can never be the possibility of truly

instantaneous operation.

In many installations, MCBs are preferred over fuses mainly because there is no need

of rewiring the fuse wire or replacing the cartridge. MCBs are available in a range of

0.5A to 63A normal operating current and for the entire range, the, physical

dimensions are almost identical.

The major advantages of MCB’s are

• Instantaneous opening of the contact on short circuit faults

• Can be designed to operate even for very small overload currents

• They can be quickly reset by hand

• They cannot be reclosed if fault persist

• In many cases they preferred over fuses as there is no need to rewire

it.

Earth Leakage Circuit Breaker

The earth leakage circuit breaker (ELCB) is a protective device, which will

automatically trip, when there is an earth leakage within the installation. It is also

known as residual current circuit breaker (RCCB). It works on the current balance

principle. The main part is a core consisting of three windings. Here one winding

carries the phase current, the other winding carries the neutral current and the

third winding to the tripping circuit. Under normal operating conditions the net flux

in the core is zero as such no emf induced in the trip coil. However, when earth

fault occurs, the phase and neutral current varies, the net flux in the core will be

different and as such, emf is induced in the trip coil and it is energized. It then

opens the circuit. The functioning of the ELCB can be checked using a switch.

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RCD - Residual Current Device. This is a generic term for the entire range of

RCDs.

RCCB - Residual Current Circuit Breaker. This is basically a mechanical switch with an RCD function added to it. Its sole function is to provide protection against earth fault currents.

RCBO- Residual Current Breaker with Over current Protection. This is basically an

over current circuit breaker (such as an MCB) with an RCD function added to it. It has two functions,

Types of RCD

RCDs can be divided into two categories based on the means by which they detect and respond to earth fault currents. The two types are Voltage Independent (VI)

and Voltage Dependent (VD). These are sometimes also referred to as electromechanical and electronic types respectively. The VI type uses the output

energy from the CT to activate a relay which in turn activates a tripping mechanism causing the RCD to trip. The VD type uses electronic circuitry to detect the earth fault current and to activate a tripping mechanism causing the RCD to trip. The VI

device derives its operating energy from the earth fault current whereas the VD device derives its operating energy from the mains supply.

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Earthing

What is earthing /grounding? Earthing or grounding is the term used for electrical connection to the general mass of

earth. Equipment or a system is said to be 'earthed' when it is effectively connected to the ground with a conducting object. Earthing provides protection to personal

and equipment by ensuring operation of the protective gear and isolation of faulty circuit during:

• Insulation failure • Accidental contact

• Lightning strike Importance of earthing

Earthing is necessary for proper functioning of certain equipments. Earthing is done

also for preventing the operating personal from hazardous shocks caused by the damage of the heating appliances. Consider an electric heater connected to the supply using two-pin plug and socket. If by some chance the heating element

comes in contact with the metallic body of the heater, the body of the heater being a conducting material will be at the same potential as the heating coil. If a person

comes and touches the body of the heater, current will flow through his body, which will result in an electric shock.

To avoid unnecessary accident, it is recommended that electric heater be connected to a 3-pin socket using a 3-core cable. (Note: To see a three-core cable, open a

plug of an electric iron. There will be three wires, red, blue and green. The green wire connected to the body of the iron is the earth wire) In this case the body of the

electric heater is connected to the green wire of the cable, which is connected to the earth through the earth terminal. Besides the body of the electric heater, bodies of hot plates, kettles, toasters, heaters, ovens, refrigerators, air conditioners, coolers,

electric irons etc could be earthed using three pin plugs. The resistance of the path to the earth terminal through the earth wire is very low. Hence, even if the heating

element comes in contact with the metallic body and a human being comes in contact with the metallic body, major part of the current will flow only through the earth wire (usually the green wire in a 3 core cable). Moreover because of the low

resistance path, a large current will flow through the phase wire and the fuse will blow off. For large current to flow, earth resistance should be low. To achieve this

proper earthing has to be done. Earthing is classified as:

a. System earthing

b. Equipment earthing

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System earthing: It is the earthing of neutrals of generating stations and substations. It is employed to limit the voltage of live conductors with respect to

potential of general mass of earth. This is necessary to prevent failure of insulation.

Equipment earthing: Is earthing of non current carrying metal parts of electrical equipments. As per Rules 33 and 61 of Indian Electricity Rule 1956 non-current carrying metal parts must be earthed with two separate and distinct earth continuity

conductors to an efficient earth electrode. However equipments with double insulation need not be earthed.

Some Definitions:

Earthing: A tower/ equipments connecting to the general mass of earth by means of an electrical conductor.

Earth Electrode: Connection to earth is achieved by electrically connecting a metal plate, rod or other conductors or an array of conductors to the general mass of

earth. This metal plate or rod or conductor is called as "Earth electrode".

Earth lead: The conductor by which connection to earth is made. Earth loop impedance: The total resistance of earth path including that of

conductors, earth wire, earth leads and earth electrodes at consumer end and substation end.

Factors affecting the value of earth electrode resistance

• Electrode material.

• Electrode size. • Material and size of earth wire. • Moisture content of soil. • Depth of electrode of underground. • Quantity of dust and charcoal in earth pit.

Earth resistance consists of following components • Resistance of metal electrode

• Contact resistance between electrode and soil • Resistance of soil away from electrode surface.

The resistance decreases with the presence of moisture and salt in soil. To increase the effectiveness of earth, the total earth resistance should be reduced. Efforts

should be made to reduce the resistance contributed by each of above three components.

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Earth Electrodes

Earth electrodes can be following shapes

• Driven Rods or pipes • Horizontal Wires • Four Pointed Stars • Conductive Plates

o Round Vertical Plates o Square Vertical Plates

• Buried Radial Wires • Spheres made of metal • Water Pipes

Water pipe as earth electrode As water pipes exist extensively and these are most of the time embedded in earth,

they can make a good earth electrode. Such earthing is not objectionable with alternating currents. But with direct currents, the flow of fault currents in pipes

produces electrolysis and results in heavy corrosion of pipes. This electrolysis process makes the water also harmful to certain extent. If water pipes are proposed to be used as earth electrode, then only main water supply pipe should be used as

an electrode. The water supply main pipe should have metal-to-metal joints between its segments. A perfect electrical connection should be made between

water pipe & earth conductor. Pipe should be cleaned thoroughly with emery paper. Earth conductor also should be cleaned thoroughly. The cleaned conductor should be wrapped 4 to 5 times and ends clamped by nuts & bolts. The earth resistance

achieved by such an arrangement is usually a fraction of an ohm. Low resistance of such system is due to long length of water pipe and the fact that it is mostly

embedded below earth. This method is mostly used for grounding in telephone services. Electrodes should be made of a metal, which has a high conductivity. Normally copper is used. The size of the electrode should be such, that it is able to

conduct the expected value of stray equipments. For example a 3 phase star wound generator must have its neutral point at earth potential.

The salts commonly used for chemical treatment of soil are

• Sodium Chloride • Calcium Chloride

• Sodium Nitrate • Magnesium Sulphate

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Other factors, which affect the soil resistivity, are

1. Temperature of soil: the resistivity increases when temperature falls below the freezing point. If the temperature falls from 20degrees C to O degree C, soil

resistivity goes up from 7200-ohm cm to 14000-ohm cm.

2. Moisture Content of Soil: Small changes in moisture content seriously affect the resistivity. For example if the moisture content changes from 25% to 30%, soil resistivity drops from 250000-ohm cms to 6400-ohm cm. It is important that earth

electrodes should be in contact with moist soil. It should be ensured that the electrodes are deep in soil and if possible below the permanent water level.

3. Mechanical Composition of soil: finer the grading, lower the resistance.

Methods of placing earth electrodes in soil

1. Pipe Earthing:

Fig. E (1) Cross section of pipe earthing

Pipe earthing is done by permanently placing a pipe in wet ground. The pipe can be made of steel, galvanized iron or cast iron. Usually GI pipes having a length of 2.5m and an internal diameter of 38mm are used. The pipe should not be painted or

coated with any non-conducting material. Fig. E (1) shows an illustration of a typical pipe electrode. The pipe should be placed

atleast 1.25m below the ground level and it should be surrounded by alternate layers of charcoal and salt for a distance of around 15 cm. This is to maintain the moisture

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level and to obtain lower earth resistance. The earth lead of sufficient gauge should

be firmly connected to the electrode and it should be carried in a Gl pipe at a depth of 60cm below the ground level. A funnel with a wire mesh should be provided to pour water into the sump. Three or four bucket of water should be poured in a few days

particularly during summer season. This is to keep the surroundings of the electrode permanently moist.

2. Plate earthing

Fig. E (2) Plate earthing

A typical illustration of plate earthing is shown in Fig. E (2). The plate electrode should have a minimum dimension of 600x600x3.15mm for copper plate or 600x600x6.3mm

for Gl plates. The plate electrode should be placed atleast 1.5m below the ground level. The earth conductor is to be securely connected to the plate by means of bolts and nuts. The bolts and nuts should be of the same material as that of the plate. The

earth conductor should be carried in a Gl pipe buried 60 cm below the ground level. The plate electrode should be surrounded by a layer of charcoal to reduce the earth

resistance. A separate Gl pipe with funnel and wire mesh attached is provided to pour water into the sump.

3. Strip earthing

For all places having a rocky soil bed, this type of earthing is suitable. On this system, wires or strips made of GI of size 25 mm x 4 mm or made of copper of size 25 mm x 1.6 mm are embedded 0.5 m, below the soil in the form of a network. The

length should not be less than 1.5 m as per ISI specification. Detail are given in figure below.

Ground level Cast iron cover

Wire mesh

Cement concrete

19mm dlaGI pipe Charcoal

600x600x6.3mm Gl

plate

or

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Effect of Soil Properties in Earthing

While it is not possible to change the fundamental nature /properties of soil at a given location, but local variations of soil conditions do occur even in a small area.

When a location for making earthing pit has to be selected, preference should be given to location, which is likely to give minimum electrical resistance. In the list

below, soils have been arranged in ascending order with regard to their electrical resistance.

• Wet marshy lands, or lands containing ashes (Avg Resistivity 2400 ohm cms)

• Clay, loamy soil, arable land clay • Clay & loam mixed with varying proportion of gravel & sand (Avg Resistivity 15,800 ohm cms)

• Damp & wet sands • Dry sand

• Gravel & Stones

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House Wiring

Introduction

A network of wires connecting various accessories for distribution of electrical energy

from the supplier’s meter board to the numerous electrical energy consuming devices such as lamps, fans and other domestic appliances through controlling and safety

devices is known as wiring system. The supplier’s service cable feeding an installation terminates in what is usually called

the service fuses. In an ordinary house the service fuse is called as service cutout. Such cutouts including service meters remain the property of the supplier and represent the furthest point of the supplier responsibility. The point at which the consumer's wiring is

connected into cutout is known as point of commencement of supply or consumer's terminals. From consumer terminals onwards the supply cables are entirely under the

control of consumer's and so laid out as per his selection. A typical house wiring circuit is shown in fig. a

fig (a)

Systems of distribution of electrical energy

Since as per recommendations of ISI the maximum number of points of lights, fans and socket-outlet that can be connected in one circuit is 10 and the maximum load that can

be connected in such a circuit is 800 watts, hence in case more load or more points are required to be connected to the supply system, then it is to be done by having more than one circuit.

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Distribution Board System

In distribution board system, which is most commonly adopted for distribution of electrical energy in a building, the fuses of various circuits are grouped together on

a distribution board, some times simply known as fuse board. The two copper strips, known as bus-bars, fixed in a distribution board of hard wood or

metal case are connected to the supply main through a linked switch so that the installation can be switched off as a whole from both the poles of supply if required. A

fuse is inserted in the + ve or phase pole of each circuit so that each circuit is connected up through its own particular fuse.

In large buildings, however, if only one distribution board were used, some of the points

would be at a considerable distance from it and in such cases it is advisable to employ sub-distribution boards either to save cable or to prevent too great voltage drop at

the more distant points (lamps or fans or other appliances). In such cases main distribution board controls the circuit to each sub-distribution board from which the sub-circuits are taken, as shown in fig. a

The number of circuits and sub-circuits are decided as per number of points to be wired and load to be connected to the supply system. For determination of load of an

installation the following ratings maybe assumed unless the values are known or specified.

a) Fluorescent lamps — 40 watts. b) Incandescent lamps, fans, and socket outlets — 60 watts.

c) Power socket-outlets — 1,000 watts. d) Exhaust fans — as per capacity of exhaust fans.

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The Tree System

Another system of distribution of electrical energy in a building is the tree system. In this system smaller branches are taken from the main branch, as shown in fig. b and the wiring system resembles a tree. As each branch is taken off, a fuse is inserted. This

system used to be employed in early days. Now-a-days it is no more adopted due to the following draw-backs in this system.

a) The voltage across all the lamps does not remain the same. The lamps in

the last branch will have least voltage across them on account of voltage

drop in leads,

b) A number of joints are involved in each circuit.

c) Fuses are scattered.

d) In case of occurrence of fault all the joints have to be located and if some

of these joints are concealed beneath floors or roof spaces, a lot of

difficulties are to be faced. Sometimes a number of such joints are required

to be opened for testing purposes, so damage is caused to installation,

conductors and building.

Methods of wiring

There are two methods of wiring known as

a) joint box system (or Tee system) and

b) Loop-in system

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1. Joint Box or Tee System:

In joint box system the connections to the lamps are made through joints made in joint boxes by means of suitable connectors or joint cutouts. In this method though there is a saving in the quantity of wire or cable required but the same is offset by the extra

cost of joint boxes. The other disadvantage of T-connections is that the number of T-connections made in a wiring system results in weakness if not properly made. Now-a-days the use of this system is limited to temporary installations only, as its cost is low.

2. Loop- in- system:

This system is universally employed for connections of various lamps or other appliances in parallel. In this system when a connection is required at a light or switch, the feed conductor is looped-in by bringing it direct to the terminal and then carrying it forward

again to the next point to be fed, as shown in fig. d. The switch and light feeds are carried round the circuit in a series of loops from one point to another until the last point on the circuit is reached.

The phase or line conductors are looped either in switch board or box and neutral conductors are looped either in switch board or from light or fan. Line or phase should

never be looped from light or fan.

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The advantages and disadvantages of loop-in system are as follows; Advantages

a) Joint boxes are not required.

b) In loop-in system no joint is concealed beneath floor or in roof spaces. As they are made only at outlets so they are accessible for inspection and opening out merely by removing the fitments

concerned. Hence fault location is easy.

Disadvantages:

a) Length of wire or cable required is more and voltage drops and copper losses are, therefore, more.

b) Looping-in switches and lamp holders is usually difficult.

SYSTEMS OF WIRING

The types of internal wiring usually employed in our country are:

1. Cleat wiring:

In this system of internal wiring the cables used are either VIR or PVC type. The cables are held by porcelain cleats about 6 mm above the walls or ceiling. The cleats are made in two halves, one base and the other cap. The base is grooved to accommodate the

cables and the cap is put over it and whole of it is then screwed on wooden plugs (gutties) previously cemented into the wall or ceiling. Thus the cables are firmly griped between the two halves of the cleats and secured to the supporting wall or ceiling. The

cleats used are of different sizes and different types in order to accommodate cables of various sizes and different numbers of cables respectively. The cleats are of three

types—one groove, two grooves and three grooves to accommodate one, two, and three cables respectively.

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Advantages:

a) It is the cheapest system of internal wiring. b) Its installation and dismantlement is easy and quick.

c) Material is recoverable after dismantlement. d) Inspection, alterations and additions can be easily made.

e) Skill required is little.

Disadvantages:

a) It is not good looking.

b) It is quite temporary and perishes quickly.

c) The wires are exposed to mechanical injury.

d) The insulation catches dampness from the atmosphere and common salt

like substance appears on the insulation which lowers the insulation

resistance and Causes leakage. Hence this system of wiring cannot be

used in damp places.

e) Oil and smoke are injurious to VIR insulation.

Fields of Application:

The wiring of this type is very suitable for temporary installations in dry places. This is also acceptable where appearance is not so important and cheapness is the main

consideration. This system is not suitable for use in domestic premises.

2. Wooden Casing and Capping Wiring:

The cable used in this type of wiring is either VIR or PVC or any other approved

insulated cables. The cables are carried through the wooden casing enclosures. The casing consists of V-shaped grooves (usually two to hold the cables of opposite polarity in

different groves) and is covered at the top by means of rectangular strip of wood, known as capping, of same width as that of casing. The capping is screwed to the casing by means of wooden screws fixed at every 15 cm on the centre fillet. To protect

the casing against white ants first class seasoned teak wood, varnished by shellac varnish is employed. Two or three cables of same polarity (either all phases or all

neutrals) may be run in one groove and in no case the cables of opposite polarity should be run in the same groove. The casing ia usually placed 3.2 mm apart from the wall or ceiling by means of porcelain distance pieces of thickness not less than 6.5 mm in order

to keep the casing dry at the back.

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3. CTS or TRS Wiring.

In this type of wiring the cables used may be single core, twin core or three core TRS cables with a circular oval shape. Usually single core cables are preferred. TRS cables

are sufficiently chemical proof, water proof, steam proof but are slightly affected by lubricating oils. TRS eaoles are run on well seasoned, perfectly straight and well

varnished (on all four sides) teak wood batten of thickness 10 mm at least. The width of the batten depends upon the number and size of cables to be carried by it. The

battens are available in width of 13,19,25,31,38,44,50,56,63,69 and 75 mm. The wooden battens are secured to the walls or ceiling by flat head wood screws to wood or other approved plugs at an interval not exceeding 75 cm. The cables are held on the

wooden batten by means of tinned brass link clips already fixed on the batten with brass pins and spaced at an interval of 10 cm in case of horizontal runs and 15 cm in case of

vertical runs. The wiring after erection is neatly painted with two coats of oil-less non-cracking paint as specified in IS 732 and so on.

Advantages

a) Its installation is easy and quick and saving in labor largely compensate

for the extra cost of the cable.

b) Its life is long. c) Within certain limits it is fire proof.

d) It can withstand the action of most chemicals such as acids and alkalies. e) It is cheaper than other types of wiring except cleat wiring.

f) If the job is carried out with proper attention, it gives a nice appearance.

Disadvantages

a) Good workmanship is required to make a sound job in TRS wiring.

b) This type of wiring cannot be recommended for use in situations open

to sun or rain unless preventive steps are taken to preserve the

insulation of cables.

Fields of Application

The TRS wiring is suitable for low voltage installations and is extensively used for lighting purposes everywhere i.e. in domestic, commercial or industrial buildings except workshop where it is liable to mechanical injury.

This type of wiring is suitable in situations where acids and alkalies are likely to be present.

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4. Lead Sheathed Wiring

This type of wiring employs conductors insulated with VIR and is covered with an outer sheath of lead aluminum alloy containing about 95% lead. This metal sheath gives protection to the cable from mechanical injury, dampness and

atmospheric corrosion. The whole lead covering is made electrically continuous and is connected to earth at the point of entry to protect against electrolytic action due to leaking current and to provide safety against the sheath becoming

a live. The cables are run on wooden batten and fixed by means of link clips as in TRS wiring. The great part of the cable employed is flat twin (the cable

having two insulated conductors side by side covered with red and black tape respectively and under one flat covering of lead alloy). Three-core flat type cable is also used in certain cases as well as single core cables under a circular sheath

of lead alloy.

Advantages

a) It provides protection against mechanical injury better than provided by

TRS wiring.

b) It is easy to fix and looks nice as it can be run in building without

damaging decoration and can be painted to suit colour scheme of the

surroundings.

c) Its life is long if proper earth continuity is maintained throughout.

d) It can be use din damp situations provided protection against moisture

effect on the ends of the cable is given.

e) It can be used in situations exposed to rain and sun provided no joint is

exposed.

Disadvantages

a) It is costlier than TRS wiring.

b) It is not suitable for places where chemical corrosion may occur.

c) In case of damage to insulation the metal sheath becomes alive and

gives shock, so as to provide safety against electrical shock it is necessary

that the sheath is properly earthed and an earth wire is run side by side

with it and all pieces are properly bounded or joined together so that not

a single cover is left unearthed.

d) Skilled labour and proper supervision is required.

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This wiring system is suitable for low voltage (up to 250 volts) installations. It may be used in places exposed to sun and rain provided no joint is exposed. It may also be used in damp places with a suitable protective covering. It should not be used in places where

chemical corrosion may occur.. This type of wiring is not very common in use these days except for some small installations and distribution boards etc.

5. Conduit Wiring

In this system of wiring steel tubes, known as conduits, are installed on the surface of

walls by means of saddles or pipe hooks or buried under plaster and VIR or PVC cables are drawn into afterwards by means of a GI wire of size of about 18 SWG. In damp situations the conduits can be spaced from the walls by means of small wooden blocks

fixed below the pipes at regular intervals. In order to facilitate drawing of wires numbers of inspection fittings are provided along its length. The conduits should be

electrically and mechanically continuous and connected to earth at some suitable point. The conduits used for this purpose are of two types namely (i) light gauge (or split type)

conduit and heavy gauge (or screwed type) conduit. Light gauge or split conduit with a seam along its length is used for cheap work. It is not water tight or even damp proof and is not permitted on medium voltage (i.e. on voltages higher than 250V). Screwed conduit

(solid drawn or with welded seam) is used for all medium voltage (250 V or 600 V) circuits and in places where good mechanical protection and absolute protection from

moisture is desired. In general the finish of the conduit is black stove-enamelled, there being a smooth coating of enamel both on the inside and outside surface of the tube. Galvanized conduit is also employed, especially in damp situation when the conduit is

on the surface but under ordinary conditions buried in walls it offers little, if any, advantage over good enamelled conduits.

Advantages

a) It provides protection against mechanical damage.

b) It provides complete protection against fire due to short-circuits etc. c) The whole system is water proof. d) Replacement and alteration of defective wiring is easy. e) Its life is long if the work is properly executed. f) It is shock proof also if earthing and bonding is properly done.

Disadvantages

a) It is very costly system of wiring.

b) Its erection is not so easy and requires time.

c) Experienced and highly skilled labour is required for carrying out the job.

d) Internal condensation of moisture may cause damage to the insulation

unless the system outlets are properly drained and ventilated.

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As this system of wiring provides protection against fire, mechanical damage and dampness so this is the only approved system of wiring for:

a) Places where considerable dust or puff is present such as in textile mills, saw mills, flour mills etc.

b) Damp situations. c) In workshops for lighting and motor wiring. d) Places, where there is a possibility of fire hazards such as in oil mills, varnish

factories etc. e) Places, where important documents are kept such as a record room.

f) Residential and public buildings, where the appearance is the prime thing. The recessed type conduit wiring is preferred for residential and public buildings.

CHOICE OF WIRING

The following factors should be considered before selecting a particular type of wiring.

a. Safety: The first and foremost consideration is safety to a person using electricity against leakage or shock. Where there is a possibility of fire hazard, conduit wiring is used.

b. Mechanical Protection: The wiring must be protected from mechanical damage during use.

c. Permanency: The wiring must not deteriorate unduly by action of weather, fumes, dampness etc.

d. Appearance: The wiring should he good looking. e. Durability: The wiring must be durable.

f. Accessibility: In wiring system there should be facilities for extension, renewal or alterations.

g. Initial Cost: The wiring selected should suit the pocket of the owner of the building.

h. Maintenance Cost: The wiring should have, as far as possible, the lowest maintenance cost.

The other factors, in addition to above, to be kept in view while making the choice of wiring

is load voltage to be employed, type of building etc.

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Tools used in Electric Wiring

Some of the most commonly used tools are described below:

Sl # Tool Size Uses

1. Screw Driver(Smaller

size screw driver is

called Connector

10,15,20,30 cms

Used for loosing or tightening or to keep screws in

position.

2. Combination Pliers 15,20,25 cms

For holding, twisting or cutting wires.

3. Round Nose Pliers or

Flat Nose Pliers

10 cms

For holding, twisting or joining the wires at narrow

places.

4. Side Cutting

Pliers(side cutter)

20 cms

For cutting wire at narrow or ordinary

places and for removing insulation.

5. Electrician Knife

10 cms

It has two blades, one for removing

insulation of wires and other for cleaning the

wires.

6. Electric Soldering iron 25,40,65,125

W To solder the joints of wires and winding

wires.

7. Cross peen Hammer

!/4 kg to 2 kg Used for fixing clip and making gitties hole in

wall.

8. Ball peen Hammer

'/* kg to 2 kg

Best suited for chipping on teak wood

batten, and riveting purpose in sheet metal

works.

9. Tenon saw or Hand

saw 30.5 cm &

40.5 cm

Used for cutting wooden boards, block

casings etc.

10. Poker

10, 15 cm

Used for making pilot holes for fixing wood

screw.

11. Hand drill

3,6,12mm Used for making holes in wooden blocks and

boards.

12. Hacksaw

16,20,25,30

cms

Used for cutting conduit G.I. pipes or mild steel.

13. Measuring Tape

10,20mm

Used for measuring the dimension of the

wiring. It is made of steel or cotton cloth.

14. Wire Stripper & Cutter

Used for removing insulation of PVC wires

and available with adjustable 22 SWG and

onwards.

15. Files (Flat, round half) 3" to 4" To smooth the surface or corners of any iron

board etc.

16. Crimping Tool

1.5,2.5,6mm

As soldering on Aluminium is difficult, this

plier is used to crimp the joint or lugs.

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STUDY OF WIRING ACCESSORIES

Any device, associated with the wiring and electrical appliance of an installation, such

as a switch, a fuse, a plug, a socket-outlet etc. is called the wiring accessory. The cables, flexible cords and various wiring accessories in common use are briefly

described below.

Cables:

The cable or wire used in internal wiring is covered with insulation. The conductor is covered with insulation so that it may prevent leakage of current

from the conductor and thus minimize the risk of fire and shock.

The wire employed for internal wiring of buildings may be divided into different groups according to

a. Conductor used

b. number of cores used

c. voltage grading and

d. type of insulation used

According to the conductor material used in cables, these may be divided into two classes

known as copper cables and aluminum cables.

According to the number of cores, the cable consists of, the cables maybe divided into the classes known as single core cables; twin core cables; three core cables; two core with

ECC (earth continuity conductor) cables etc.

According to voltage grading the cables may be divided into two classes:

(i) 250/440 volt cables

(ii) 650/1,100 volt cables.

According to type of insulation the cables are of the following types:

1. Vulcanized Indian Rubber (VIR) Cables:

VIR cables are available in 250/440 volt as well as in 650/ 1100 volt grades and

are used for general electrical wiring in cleat, casing-capping and conduit wirings.

VIR cable consists of either tinned copper conductor or aluminum conductor covered with a layer of vulcanized Indian rubber insulation. Over the rubber insulation cotton tape sheathed covering is provided with moisture resistant compound bitumen wax or

some other insulating material for making the cables moisture proof. The thickness of

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rubber insulation depends upon the voltage grade for which the cable is required.

The copper conductor is tinned to provide protection against corrosion due to presence of

traces of sulphur, zinc oxide and other mineral ingredients in the VIR.

2. Tough Rubber Sheathed (TRS) or Cab Type Sheathed (CTS) Cables:

These cables are available in 250/440 volt grades and used in CTS'(or TRS) wiring. TRS cable is nothing but a vulcanized rubber insulated conductor with an outer

protective covering of tough rubber. These cables are water proof, hence can be used in wet conditions. These cables are available as single core, circular twin core, circular three

core, flat three cores, twin core with an earth continuity conductor etc. In wiring of a three pin plug separate earth wire may be used, as it will be cheaper in cost and easy in installation.

These cables are cheaper in cost and lighter in weight than lead alloy sheathed cables, described later and have the properties similar to those, of lead sheathed

cables.

3. Lead Sheathed Cables:

These cables are also available in 250/440 volt grades and are used for internal

wiring where climatic condition is not dry and has a little bit moisture. The lead sheathed cable is a vulcanized rubber insulated conductor covered with a continuous sheath of lead. The lead sheath provides very good protection against the absorption of

moisture and sufficient protection against mechanical injury and so can be used without casing or conduit system. It is available as a single core, twin core, flat three core and

flat twin core with an earth continuity conductor.

4. Polyvinyl Chloride (PVC) Insulated Cables:

These cables are available in 250/440 and 650/1,100 volt grades and are used in concealed wiring system. In this type of cable conductor is insulated with PVC insulation. Since PVC is harder than rubber so PVC cable does not require cotton tapping

and braiding over it for mechanical and moisture protection. Since the PVC is thermo-plastic insulation, so it is affected at high temperatures and it

may soften and flow down. These cables cannot be used for giving connections to the heating appliances, pendant lighting etc. Though the insulation resistance of PVC is lower than that of VIR but its effect is negligible for low and medium voltages below 600 volts,

5. Weather Proof/Cables

These cables are used for outdoor wiring and for power supply or industrial supply. These cables are either PVC insulated or vulcanized rubber insulated conductors being

suitably taped (only in case of vulcanized rubber insulated cable) braided and then compounded with weather resisting material. These cables are available in 250/440 volt and 650/1100 volt grades. These cables are not affected by heat or sun or rain.

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Although TRS cables can be used for outdoor purposes but due to their higher cost, weather proof cables are generally used for outdoor services.

6. Flexible Cords

A cable containing one or more cores, each formed of a group of wires, the diameter of

cores and of the wires being very small to afford flexibility, is known as flexible cord. These are used as connecting wires for such purposes as from ceiling rose to lamp

holder, or from socket-outlet to portable apparatus such as radios, fans, lamps, heaters etc. The flexibility of such wires facilitate in handling the appliances and prevent the wires from breakage. The flexible cords used for house hold appliances

are available in various pleasing colours, sizes and of various thickness of insulation. These wires should never be used for fixed wiring.

Switches

A manually operated device used for closing and opening or for changing the

connections of a circuit is known as a switch.

The switches used in internal wiring may be classified in various ways. According to the type of base material they are classified as porcelain or bakelite switches. According to

colour of base they are either white or black or brown coloured switches. According to operation required, they are classified as one way, two-way, centre off, double pole etc.

switches.

1. One-way Switch

This type of switch consists of two terminals which can be easily seen from the

back side of the switch as well, without removing the cover. The switch is always connect* din series with the point (lamp, fan or socket-outlet) to be controlled.

2. Two-way Switch

The switch of this type consists of four terminals, two of them being short-circuited

inside the switch. The switch of this type is usually used for the stair-case wiring or circuits where one point is to be controlled from two different places.

3. Two-way Centre off Switch

The switch of this type is just like a two-way switch but having three operations. In the centre it becomes off. Such switches are used when two lamps are to be operated alternately.

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4. Double Pole Switch

This is a combination of two one-way switches, which can be operated simultaneously as ON-OFF terminals of both the switches, are connected together by

a handle made of bakelite. Such switches are used as interlinked switches when the load current is less than 5A and supply voltage is under250V.Incaseeither of the voltage

or current exceeds the limits mentioned above DPI C switch is used.

5. Push-button Switches

Such switches are used for controlling the electric bells. When the knob is pressed, the

circuit is completed and the bell rings and as soon as the knob is left, the circuit becomes open.

6. Table Lamp Switch

This is a small on-off switch which is commonly used in table lamps.

7. Bed Switch

Such switches are used to switch off the table lamps or other lamps while going to sleep or making the lamp on while getting up at night. It is connected in aeries with one of the

two flexible wires. The specialty with this switch is that fluorescent material is applied to its knob so that it may glow at night and can easily be seen in darkness. This is a

pendant type switch.

The switches are of two types known as surface switches (or tumbler switches) and flush

switches (or concealed switches).

i. Tumbler or Surface Switches

Tumbler switches are those which are fixed on the mounting blocks directly fixed on the surface of the wall. Such switches project out the surface of the wall

and are in common use. Surface switches are available in round and oblong base. Round base switches are cheap and in common use. Oblong surface switches are good in appearance, but being costly, are rarely used.

ii. Flush Switches

Flush switches, as obvious from their name, are fixed in flush with the wall

and do not project out. These switches are used where high quality performance and appearance are required.

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Ceiling Rose

The ceiling rose is used to connect the pendant lamps, fans or fluorescent tubes to the installation through flexible or silk covered wires. These are not used on a circuit, the voltage of which normally exceeds 250 volts.

Fig26.8 shows a modern form of moulded ceiling rose which includes the earth terminal and a shrouded terminal for looping in live wire.

Socket-Outlets

The socket-outlets are used to supply electrical connections whenever required for

electrical appliances such as radios, table fans, table lamps, iron, stoves etc. Socket-outlets are of two types— two pin type and three pin type. Two pin socket-outlets have

become obsolete now-a-days. The three pin type socket-outlet has got three hollow terminals in which three pin plugs can easily be inserted but not loosely. Two holes being of same size, are meant for making connections to the flexible wire of the appliance

and the third hole, which is bigger comparatively, is meant for earth connections. Thus three holes or sleeves are for live, neutral and earth connections. The three pin socket-

outlets are also of two types:

(i) 5 A for table fans, table lamps, radios etc, and

(ii) 15 A for power circuits as heater, stove, iron etc.

Three Pin Socket-Outlet Flush Mounting

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Plugs

Plugs are used to take the supply from the socket-outlets for electrical appliances

such as table lamp, table fan, heater etc. Similar to socket-outlets plugs are also of two types namely two pin and three pin. Two pin type plugs have become obsolete

now-a-days. Three pin type plugs consist of three pins usually made from brass. To the two pins which are thin and of same size, flexible wires are connected and then covered up. To the third pin, which is thicker comparatively, earth wire from the

electrical appliance is connected. Similar to 3 pin-socket outlets 3 pin plugs are also of two types—5 A and 15 A. (see fig. 26.10)

Lamp Holders

As the name indicates the function of lamp holder is to support the lamp and also to connect it electrically. These are designed for quick removal and replacement of the lamp. Lamp holders are of many types. A few will be described here.

Lamp-holders may be either of brass or bakelite type with porcelain interior. Brass holders are more durable but may give shock if connections are poor. Though bakelite

holders are not durable, but do not give shock.

The following are the different types of lamp holders

1. Batten Holders

Such lamp holders are used where the lamp is to he fitted to the roof or to the wall i.e. it is directly fitted either to batten or to wooden boards. Such lamp holders are

bayonet type i.e. in such a lamp holder the lamp is forced in. turned slightly and left in position.

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2. Pendant or Cored Grip Holders

Such a lamp holder is used when the lamp is to be suspended from the flexible cord. Such a lamp holder is hanged vertically downward from the ceiling with flu flexible

cord, one end of which makes electrical connections with the ceiling rose and other with the lamp holder and thus with the lamp.

Pendant Lamp Holder

3. Angle Holders

Such lamp holders are used when the lamps are to be fixed directly on the walls and to give light at an angle. Such lamp holders are available in various fancy designs and colors.

4. Slanting Holders

Such lamp holders are used for lamps to be fixed on advertising boards, for flood lights and for stage lights. Such lamp holders are used along with shades (hand shape shades)

so that light is concentrated on the material displayed and does not trouble the viewer.

5. Bracket Holders

Such lamp holders are used to give direct light in the room or above a particular place. These cannot be fixed on the roof or made to hang. Usually these are fixed on the wall.

These may also be used in table lamps.

6. Water Tight Bracket Holders

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Such lamp holders are provided with tubular glasses fixed with water tight cover. Such lamp holders are used outside the houses and for street lighting where

there is no cover to save the bulb from falling of water over it.

Junction Box

In joint box system of wiring all joints in conductors are made by means of suitable connectors or joint cutouts in junction boxes. In looping back system of wiring, which is widely used now-a-days, junction boxes are not required.

Domestic appliances

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Some of the commonly used domestic appliances are

� Fan

� Food mixer

� Iron box

� Refrigerator

� Air Conditioner

Fan

Aim: - To study the working and maintenance of table fan ceiling

fan

Theory:-

Fan is an essential home appliance nowadays and is available in different style and facili-ties. Generally used types are table fan and ceiling fan. We can mount the ceiling fan on

the ceiling for providing wind to whole the room. As per IE rule the minimum height from floor to fan must be2.5 meter. Table fan can be places on tabletop or any flat surface. But

it has minimum space limit compared to ceiling fan.

Construction

Main parts of a ceiling fan are (a) Winding

(b) Capacitor & (c) Regulator

Winding of the motor can be done manually or by automated machine. Regulator may be electronic type or resistance type. Electronic type regulator has negligible power loss and

compact size. But in the case of resistance type, resistances are connected in series with the circuit; this may cause power loss as heat.

In table fan one permanent split capacitor run (PSC) motor is the heart of a fan. This

motor consists of two windings one as starting winding and other as running winding.

Starting winding of this motor has high resistance and low reactance but running winding

has low resistance and high reactance. One capacitor is connected in series with the

starting winding and whole of this circuit is put in parallel across running winding. In the

case of ceiling fan these two windings are placed in stator in the inner side of the fan.

Rotor has no winding; it is the outer body of the fan. Ceiling fan motor operates just in

opposite manner as compared to general motor. That is actual rotor of the motor is

blocked and the stator is free to rotate. So ceiling fan runs in anticlockwise direction. At

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the same time table fan motor is operated as normal case and so it runs in clockwise

direction. Capacitor connected in series with the starting winding should be value 2.5

micro farad. Pyranel insulated foil paper capacitor is using for this purpose. It helps to

provide a split phase effect from single phase AC supply.

SERVICING

Problems and solutions normally occurring in fans are as follows

1. Fan is not working when supply is given

• Check the supply at the consuming end.

• Dismantle the fan from ceiling and remove the cover. Check the windings, if it is burnt rewind it with proper gauge copper wire.

• Number of turns must be equal to the previous winding, because it may affect the speed of the fan. If starting winding is burnt, it alone can be replaced but in

the case of running winding we want change these two sets of windings.

3. Fan is not starting and will work when push to start

• Check the voltage at the consuming end • Dismantle the capacitor from fan and connect it to AC supply for 30 sec. Then

disconnect and short circuit the capacitor terminals. At that time we can hear one spot sound if it working, otherwise it can be replaced by new one.

• Check the bearing of the motor; if it is dirty grease may be applied.

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Electric food Mixer

Aim:-

To study the principle of working and servicing of mixer (Mixy)

Working principle:-

Mixer is an essential home appliance used for dry grinding, wet grinding mixing and

other purposes. Different types of blades are used for these purposes. These blades

can be replaced for specific work on the same shaft itself. The power rating of the

mixer is varies from 500W to 600W according to various manufacturers. The speed

of this motor is around 1800 rpm. The motor used in mixer is universal type. So it

can be operated on AC and DC. In this type motor stator winding and rotor winding

are connected in series through two carbon brushes which is in contact with the

segments of the commutator. One over load relay is put in series with this circuit for

providing overload protection. Generally used relay has current rating of 2 A. This

relay get tripped when the load become more than the specified. When mixer

become off while using (due to tripping of relay) we can reset it by pressing the

button provided at the bottom of the mixer. One speed controlling knob is provided

for controlling the speed of the motor. This can be done by adjusting the number of

field winding tappings.

Universal motors The Universal motor is the most common type of high speed motor found in appliances and portable line operated power tools. Typical uses include vacuum

cleaners, floor polishers, electric drills, routers, and sewing machines. They are likely to be found anywhere medium power, high speed, and/or variable speed

controls are required capabilities. Construction consists of a stationary set of coils and magnetic core called the 'stator' and a rotating set of coils and magnetic core called the 'armature'.

Incorporated on the armature is a rotating switch called a 'commutator'. Connection to the armature is via carbon (or metal) contacts called 'brushes' which are

mounted on the frame of the motor and press against the commutator. Technically, these are actually series wound DC motors but through the use of steel laminated magnetic core material, will run on AC or DC - thus the name universal.

Changing direction requires interchanging the two connections between the stator and the armature.

This type of motor is found in blenders, food mixers, vacuum cleaners, sewing machines, and many portable power tools. Speed control of DC motors: The speed of a dc motor is given by the

relation N α Eb/Ф

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When Eb = V-IaRa

N α V-IaRa/ Ф From the above equation it is clear that the speed of Dc motors can be controlled;

� By varying flux per pole. This is known as flux or field control method.

� By varying the armature drop, i.e by varying the resistance of the armature circuit. This is known as armature control method.

� By varying the applied voltage. This is known as voltage control method.

Field control method: The speed of series motor can be controlled by

varying the flux produced by the series field winding. The variation flux can be brought about by tapping the field winding.

Tapped field control:

In food Mixers tapped field control method is used for varying the speed. In this method the number of turns of the series field winding can be changed by short circuiting a part of it as shown in figure.

We know that the flux produced by the winding depends upon the ampere turns (i.e. Ф α Ise x No. of turns). As the number of turns is

reduced, the speed of the motor increased (N α1/ Ф). Servicing

1. Mixer produces spark and smoke

• Check wires and connections • Check the brushes and replace it, if it gets damaged. We can by the same

brushes from the market for replacing

2. Mixer is not working.

• Check the cable and winding

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• Check the overload relay. Replace it if it gets damaged. This over load relay can be buy from market.

4. Jar of the mixer becomes loose.

• Dismantle the coupler shaft and check the washer.

If it gets damaged replace or add two or three washers.,

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3. ELECTRIC IRON BOX

Aim: - To study the principle of working and servicing of electric iron box

Working principle:- Electric iron boxes are very essential home appliance used for ironing garments.

Nowadays this iron boxes are available at different forms with types of facilities. Generally used two types are as follows. A. Ordinary B. Non automatic type

The parts of an ordinary non automatic type iron boxes are chromium plated sole plate, Nichrome heating element with mica covering, One single plate (Pressure plate) to cover the heating

element, Chromium plated case and ebonite handle. When supply is given to nichrome heating element it will heated up and the sole plate

touching with it also gets heated. This nichrome element is mica sheet in order to insulate electricity and conduct heat easily. The iron box body should be earthed well, since there is a usual

chance for electric shock. While using this ordinary iron box we have to switch off it frequently in order to prevent over heating; because there is no control for heating of element. The heating element is fitted between sole plate and pressure plate tightly, to prevent air contact. Ebonite

handle of the iron box is used to move it without heating or electric shock to body. Power rating of this type iron box is around 450 W.

B. Automatic iron box

This type of iron box has a thermostat switch and is connected in series with line. Thermostat controls the temperature, prevent over heating of Iron box and avoid damage of heating element. The required temperature for different types of garments can be obtained by adjusting the

thermostat knob provided on the iron box body. Thermostat is a thermal switch which operates automatically due to the variation of heat

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produced around it. Thermostat consists of two contacts, one as fixed contact and other as moving contact. The movable contact of thermostat consists of a bimetal which is normally in

contact with another strip of metal having fixed contact. When supply is given to iron box the current starts to flow through the contacts and the tempera-

ture rises. This increment in temperature causes the contacts of bimetal strip to bent and opens. It will cause to stop the flow of current through heating element. While ironing when temperature reduces than pre fixed value bimetal strip regains its normal position and the circuit again com-

pleted. An indicator lamp of 3.8 V is connected in the circuit on the handle of iron box to indicate the working. Automatic iron boxes are now available in light weight form with power

rating 750 W.

Servicing

1. Iron box is not working

• Check the supply at consuming end

• Check the thermostat for open circuit • Check the heating element continuity

2. Iron box has no enough temperature when knob is placed at one position.

• Check the heating element. • Adjust the screw below the knob to produce enough temperature.

3. Shock on body

• Check the continuity of earth wire to body. If it does not get continuity dismantle the cover and connect earth wire properly.

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Refrigerator

Introduction:

Refrigeration systems are used for maintaining low temperature say from 20 degree Celsius down to very low temperature, as those required for food prseveration, industrial applications, laboratories etc.

The Purpose of Refrigeration: The fundamental reason for having a refrigerator is

to keep food cold. Cold temperatures help food to stay fresh longer. The basic idea behind refrigeration is to slow down the activity of bacteria (which all food contains) so that it takes longer for the bacteria to spoil the food.

Refrigeration: It is the process of removing heat at a low temperature level and

rejecting it at a relatively higher temperature level. By its nature heat flows from a body at a higher temperature to another at a lower temperature. Refrigeration is accomplished by various methods such as – vapour compression

system, absorption system, steam jet refrigeration cycle. The vapour compression cycle is used in most house hold refrigerators.

Refrigerant: Refrigerants are heat carrying medium, which during their cycle in the

refrigeration system absorb heat at a low temperature level and discard the heat so absorbed at a higher level.

The various components of a vapour compression refrigeration system are:

� Evaporator

� Refrigerant

� Compressor

� Condenser and

� Throttling device(capillary tube)

� Thermostat

� Fan

� Motor

Evaporator or Heat-exchanging pipes:- Coiled set of pipes inside the unit.The process of heat removal from the substance to be cooled or refrigeration is done in the evaporator.

Refrigerant:- Refrigerants are heat carrying medium, which during their cycle in

the refrigeration system absorbs heat at a low temperature level and reject it at a higher temperature level.

Freon: Which is commonly used refrigerant in vapour compression refrigerator system. Freon (F) is trade name given to gas, which is mixture of chlorine, fluorine

and carbon. Freon-12, Freon-22, and Freon-114 are extensively used for domestic

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and commercial refrigeration. A majority of these refrigerants have boiling points much below ordinary room

temperature. So they exist as gases and are only held in liquid state by keeping them under pressure, such as in refrigerant cylinder.

Compressor: It is a form of pump for compressing gases in order to increase its pressure.This device is used for raising the low pressure refrigerant vapour to a high

pressure. Rotary compressors are mainly used in domestic refrigerators; these are most suited for pumping refrigerants having moderate or low condensing pressures.

If the pressure of the refrigerant is raised (2.6 to11.25 kg/cm sq ) then the high pressure vapour can be condensed in to a liquid by cooling it with the atmospheric air available say at temperature about 32.2 degree Celsius.

In domestic refrigerator compressor and motor are assembled in a single unit and is called “hermetically sealed unit”. The unit is installed at the bottom of the

refrigerator and the advantage is that the problem of gas leakage is minimized as there is no extension of moving parts through the sealed housing.

Condenser or Heat-exchanging pipes: Coiled set of pipes outside the unit Apparatus used for condensing vapours in to liquid, it consists of a condenser tube

which is freely exposed to air through which refrigerant vapour is circulating for condensing.

There are three kinds of condensers, namely Air cooled condenser use air as the cooling medium, water cooled condenser use water as the cooling medium and the evaporative condenser is a combination of

both water and air. In domestic refrigerator natural air cooling is done by providing metallic fins with condenser coil.

Expansion valve or throttling device: By the use of throttling device the pressure of the liquid can be reduced to the pressure needed in the evaporator. To lower the

pressure of a refrigerant vapour capillary tube is used. Thermostat: An instrument which measures changes in temperature, and directly

or indirectly controls sources of heating and cooling to maintain a desired temperature.

Fan: Fan for circulating air over the evaporator or cooling coil.

Motor: Single phase induction motor of rating 1 hp is used in the rotary compressor.

There are two things that need to be known for refrigeration.

� A gas cools on expansion.

� When you have two things that are different temperatures that touch or are near each other, the hotter surface cools and the colder surface warms up. This is a law of physics called the Second Law of Thermodynamics.

The basic mechanism of a refrigerator works like this:

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� The compressor compresses the refrigerant gas. This raises the

refrigerant's pressure and temperature (orange), so the heat-exchanging coils outside the refrigerator allow the refrigerant to dissipate the heat of pressurization.

� As it cools, the refrigerant condenses into liquid form and flows through the expansion valve.

� When it flows through the expansion valve, the liquid refrigerant is Allowed to move from a high-pressure zone to a low-pressure zone, so it

expands and evaporates. In evaporating, it absorbs heat, making it cold.

� The coils inside the refrigerator allow the refrigerant to absorb heat, making the inside of the refrigerator cold. The cycle then repeats.

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Air Conditioner

An air conditioner removes heat and moisture from the air by passing it over a cold

surface. When warm, moist "inside" air is blown across the surface of the unit's cooling coil, the air temperature drops and the water vapor in it condenses making the air cooler and drier and therefore more "comfortable."

Room air conditions are installed on windows or wall openings. The assembly

incorporates a refrigeration unit, and double shaft fan motor with fans mounted on both sides of the motor, one on the evaporator side and other for the air cooled

condenser. The room (or cooling) side and the outdoor (or heat rejection) side of the unit are separated by an insulated patrician within the casing.

How Does an Air Conditioner Work?

Air conditioners and refrigerators work the same way. Instead of cooling just the small, insulated space inside of a refrigerator, an air conditioner cools a room, a whole house, or an entire business.

Air conditioners use chemicals that easily convert from a gas to a liquid and back again. This chemical is used to transfer heat from the air inside of a home to the

outside air. The machine has three main parts. They are a compressor, a condenser and an evaporator. The compressor and condenser are usually located on the outside air

portion of the air conditioner. The evaporator is located on the inside the house. The working fluid arrives at the compressor as a cool, low-pressure gas. The

compressor squeezes the fluid. This packs the molecule of the fluid closer together. The closer the molecules are together, the high its energy and its temperature. The working fluid leaves the compressor as a hot, high pressure gas and flows into the

condenser. If you looked at the air conditioner part outside a house, look for the part that has metal fins all around. The fins act just like a radiator in a car and help the

heat go away, or dissipate, more quickly. When the working fluid leaves the condenser, its temperature is much cooler and it has changed from a gas to a liquid under high pressure. The liquid goes into the

evaporator through a very tiny, narrow hole. On the other side, the liquid's pressure drops. When it does it begins to evaporate into a gas. As the liquid changes to gas

and evaporates, it extracts heat from the air around it. The evaporator also has metal fins to help in exchange the thermal energy with the surrounding air. By the time the working fluid leaves the evaporator, it is a cool, low pressure gas. It

then returns to the compressor to begin its trip all over again. A fan is connected to the evaporator that circulates the air inside the house to blow

across the evaporator fins. Hot air is lighter than cold air, so the hot air in the room rises to the top of a room. There is a vent there where air is sucked into the air

conditioner and goes down ducts. The heat exchanging takes place at the evaporator as the heat is removed from the air, the air is cooled. It is then blown into the house through other ducts usually at the floor level.

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This continues over and over and over until the room reaches the temperature you want the room cooled to. The thermostat senses that the temperature has reached

the right setting and turns off the air conditioner. As the room warms up, the thermostat turns the air conditioner back on until the room reaches the comfortable

temperature.

The refrigeration cycle in an air conditioner is :

1. The compressor compresses cool Freon gas, causing it to become hot, high-pressure Freon gas (red in the diagram above).

2. This hot gas runs through a set of coils so it can dissipate its heat, and it condenses into a liquid.

3. The Freon liquid runs through an expansion valve, and in the process it evaporates to become cold, low-pressure Freon gas (light blue in the diagram above).

4. This cold gas runs through a set of coils that allow the gas to absorb heat and cool down the air inside the building.

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Induction type single phase energy meter

Induction type instruments are used only for a.c measurements. These instruments

can be used either as ammeter, voltmeter or wattmeter. However, the induction principle

finds its widest application as an energy meter. Induction type single-phase energy meter is

used invariably to measure the energy consumption in any a.c circuit in a prescribed period

where supply voltage and frequency are constant. Energy meter is an integrating

instrument which measures the total quantity of electrical energy supplied to the circuit in

a given period.

Principle:

The basic principle of induction type energy meter is electromagnetic induction.

When alternating current flows through two suitably located coils (current coil and potential

coil) produces rotating magnetic field which is cut by the metallic disc suspended near to

the coils, Thus an e.m.f is induced in the disc which circulates eddy currents in it. By the

interaction of rotating magnetic field and eddy currents, torque is developed and causes

the disc to rotate.

Construction

An induction type single phase energy meters, as shown in fig. has the

following main parts of the operating mechanism.

� Driving system

� Moving System

� Braking System

� Recording mechanism

Driving System:-

The driving system of the meter consists of two electromagnets,

a. Series magnet

b. shunt magnet

a. Series magnet:

It consists of a number of U- shaped laminations of silicon steel together to form a

core. A core of thick wire having a few turns is wound on both the legs of U-shaped

magnet as shown in fig. This coil is connected in series with load. Thus it is excited

by the circuit current I and is known as current coil. This magnet is placed below

the aluminium disc and produces the magnetic field φse proportional to and in

phase with the line current I.

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b. Shunt Magnet

It consist of a number of M-shaped laminations of sil icon steel assembled

together to form a core. A coil of thin wire having large number of turns is wounded

on the central limb of the magnet as shown m above: Fig. The coil is connected

across the load. Thus it is excited by the current proportional to the supply voltage

and is known as, potential or pressure coil. This magnet is placed above the

aluminium disc.

In order to obtain deflecting torque, current in the pressure coil must lag behind the

supply voltage by 90°. For this the copper shading band (Short circuiting copper ring) is

provided on the central limb of the shunt magnet. The phase difference of 90° is obtained

by adjusting the position of this shading band. The shading band acts as Short circuited

transformer secondary. Since its resistance Is negligibly small as compared to its inductance,

therefore current circulating in the shading band lags behind the supply voltage nearly by

90°. Thus the shunt magnet produced a field φsh proportional to applied voltage. This field is in

phase with the current flowing through the pressure coil Ip but is in quadrature with the

applied voltage.

Moving System:

It consists of a light aluminium disc mounted on a vertical spindle. The aluminium

disc is positioned in the air gap between series and shunt magnets, The spindle is supported

by a cup shaped jewelled bearing at the bottom end and has a spring jewel bearing at the

top end. Since there is no control spring the disc makes continous rotation under the

action of deflecting torque.

Braking system:

A permanent magnet positioned near the edge of the aluminium disc as showin the Fig.

forms the braking system. When the aluminium disc moves in the field of the braking

magnet, flux is cat and currents are induced in the disc. The direction of induced current is

such that it opposes the rotation (lenz's law). Thus braking torque is produced. Since

the induced current is proportional to the speed of the disc (N) therefore braking torque (T ) is

proportional to the disc speed (ie) T. x N.

The position of braking magnet is adjustable and therefore, braking torque can be adjusted by

shifting the magnet to different radial positions. If the braking magnet is moved towards the

centre of the disc, flux cut the disc is less which reduces the induced current and thus the

braking torque is reduced. Hence by the inward movement of the magnet, braking torque

decreases but the speed of the disc increases and vice-versa.

Recording mechanism

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The function of recording or registering mechanism is to record continuosly a number on

the dial which is proportional to the revolutions made by the moving system. The number

of revolutions of the disc is a measure of the electrical energy passing through the

meter.

Working:

When the energy meter is connected in the circuit, the current coil carries the load

current and pressure coil carries the current proportional to the supply voltage. The

magnetic field produced by the series magnet (series coil) is in phase with the line

current and magnetic field produced by the shunt magnet (pressure coil) is in

quadrature with the applied voltage (since the coil is highly inductive). Thus a phase

difference exist between the fluxes produced by the two coils. This set up a rotating

field which interacts with the disc and produces a driving torque and thus, disc starts

rotating. The number of revolutions made by the disc depends upon the energy

passing through the meter. The spindle is geared to the recording mechanism so

that electrical energy consumed in the circuit is directly registered in kWh.

The speed of the disc is adjusted by adjusting the position of the braking

magnet. For example, if the energy meter registers less energy than the energy

actually consumed in the circuit. Then, the speed of the disc has to be increased

which is obtained by shifting the braking magnet nearer to the centre of the disc and

vice-versa.

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Electric lamps

A brief on electric lamps

The first ever practical model of the incandescent lamp was invented in 1879. Since

then, there has been series of developments in the area of light source and lighting

technology. The first incandescent lamp was made with a carbon filament with a useful

light output of only three lumens per watt. A long time has gone by since then and today

there are about 200 thousand different types of demands keeping in view the wattage,

size, applications, etc. It includes about 40,000 types of incandescent lamps alone.

Lamp differ from each other in terms of luminous flux, light, the colour of the light, their

colour rendering characteristics, size and energy consumption. Broadly, different types

of lamps can be classified as follows:

1. Incandescent lamp (GLS)

2. Fluorescent lamps (FTL)

3. High Pressure Mercury Vapour lamps (HPMVL)

4. Halogen lamps

5. High Pressure Sodium Vapour Lamps (HPSVL)

6. Low Pressure Sodium Vapour lamps (LPSVL)

7. Metal Halide lamps

8. Mercury Blended lamps

9. Compact Fluorescent lamps

Incandescent Lamps

Incandescent lamp has a history of over a century. The design of the lamp has changed

many times, but still it remains to be the most popular type due to its simple construction,

easy replacement and cheap cost. Incandescent lamps are available in wattage rating upto

1500W.

Construction

Fig.a illustrates the construction of a general lighting service (GLS) lamp. Incandescent

lamps work on the principle that visible light and infrared radiation are emitted as a result

of heating of the filament wire by a current passing through it. These emissions become

noticeable above 500°C. Tungsten is usually used as the filament material since it has

high melting point. The diameter of the filament wire is determined by the operating

current and the length of the filament by the operating voltage. For normal voltages, the

length will be too much and a coiled coil arrangement is adopted to accommodate the

long filament wire. The filament is mounted on leads that carry the current.

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Fig.a Incandescent lamp construction

To reduce the effect of vibration, additional filament supports are also employed. The

filament assembly is usually housed in a pear shaped glass bulb. The bulb diameter

for 25W, 40W, 60W and 100W coiled coil type lamps is 60 mm. The size of the bulb

is decided by the maximum limit for cap temperature. For the same wattage if the

bulb size is reduced, the cap temperature will increase and eventually result in failure

of mounting accessories.

Inside the lamp it may be vacuum or a filling of inert gas like argon or krypton with a

small percentage of nitrogen. The choice between vacuum and gas filling is made

after considering the following factors. Vacuum reduces heat loss and therefore helps to

get the highest temperature of the filament and hence more light output. But it also

increases the evaporation rate of tungsten, reducing its life. Filling with an inert gas

reduces evaporation of the filament material, but conducts heat away from the

filament, reducing the temperature and the light output. Generally vacuum is used for

low wattage lamps (15W or below) and gas filling for higher power lamps.

Performance

Incandescent lamp is the most widely used lamp because of its cheap cost and simple

usage. It is available in a wide range of voltage and wattage ratings and do not

require any additional accessories for starting or normal operation. They have

excellent colour rendering index and are used as automobile lamps, panel lamps etc.

in addition to general lighting purposes.

The major disadvantage of the incandescent lamp is its low efficacy. A typical 40W

lamp may have an efficacy of around 10 lumens/watt only. Compared to other types

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of lamps, the life is also less - around 1000 hours.

Tungsten Halogen Lamps

In incandescent lamps, the filament material (tungsten) evaporates and gets deposited

on the inner surface of the bulb. This affects the performance of the incandescent

lamp. First it erodes the filament material resulting in reduction of current and hence

the light produced. It also reduces the life of the lamp. Also, tungsten deposited on

the glass bulb reduces its transparency resulting in lower light output. These

problems are solved to a great extent in tungsten halogen lamps in which some

halogen is also introduced into the bulb along with the filling gas.

When a halogen is added to the filling gas, the tungsten vapour and halogen

molecules combine to form tungsten-halogen molecules. These molecules diffuse

back to the filament and dissociate; tungsten gets deposited on the filament and the

halogen will be available for further reaction. Commonly used halogens are iodine and

bromine. To ensure the combination of tungsten vapour and the halogen, the lamp

envelope also should be at a higher temperature than in ordinary tungsten lamps. For

that, the envelope is made smaller and is usually made of quartz or high melting point

glass to withstand the higher temperature.

The tungsten halogen lamps have longer life compared to incandescent lamps. For

the same wattage, light output is also higher compared to an incandescent lamp.

These lamps are commonly used as projector lamps, photographic lamps, automobile

lamps etc. and are also used for flood lighting.

Fluorescent Lamps

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Fluorescent lamp is the most widely used discharge lamp. It is an energy

efficient lamp available in low and medium wattage range making it

suitable for domestic and commercial lighting purposes.

Construction

The construction of a standard fluorescent lamp is shown in Fig. c. It consists of a

glass tube of around 36 mm diameter and a length of 1200 mm. The inner surface of

the tube is coated with a fluorescent powder - usually phosphor coating. Tungsten

wire electrodes with bi-pin cap are provided at both ends. There is an electrode

shield around each electrode to reduce the blackening of the tubes due to deposition

of evaporated tungsten. The tube is filled with an inert gas such as argon to a

pressure of 1.5 to 5 mm of mercury. A small drop-let of mercury is also introduced into

the tube. During normal operation this mercury vaporizes and helps to maintain the

discharge.

Operation

Fluorescent lamps are designed for switch start operation. A typical switch start

circuit is shown in Fig. c. The starter consists of two bimetallic contacts, housed in a

small glass bulb filled with a noble gas at low pressure. The contacts are positioned

with a narrow separation between them. When the normal voltage is applied, it

creates a glow discharge between the bimetallic contacts and due to heating they

bend towards each other. The contacts touch each other for one or two seconds and

the current path is completed through the inductive ballast and the filament

electrodes. This current results in preheating the electrodes. As the bimetallic contacts

touch, the glow discharge stops and now the contacts cool down and leave apart to

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open the circuit. The sudden break of current will induce a high voltage (600-1500V)

in the ballast and is applied across -the tube, which in turn trigger the discharge

through the tube. The capacitor, which is connected across the starter contact, is

provided to reduce the radio interference due to switching operations. The starter has

no function once the lamp is started.

Like other discharge lamps, fluorescent lamps are also having a negative temperature

coefficient of resistance. This means the resistance of the tube decreases when

temperature is increased, resulting in increase of current. Therefore the ballast is

essential during normal operation also to regulate the lamp current. When the ballast

is connected in series with the circuit, it regulates the lamp current. The capacitor

across the supply line is for power factor improvement.

When there is a discharge through the lamp, it produces radiations mainly in the

ultraviolet region. This radiation is converted to visible radiation by the phosphor

coating on the inner side of the glass tube.

Performance

The luminous efficiency of the fluorescent lamp is around 75 lumens/watt, which is

much higher than incandescent lamps. The colour rendering index of this lamp is in

the rage of 50-60 and this is sufficient for normal domestic or commercial lighting.

Fluorescent tamps have an expected life varying from 6000 to 20000 hours. One

disadvantage with this lamp is that the power factor of the circuit is low (around

0.5), but this problem can be solved to some extent by connecting a capacitor across

the supply.

Compact Fluorescent Lamps

Compact Fluorescent Lamps (CFLs) are now becoming very popular. It is a smaller

type fluorescent lamp that is even more energy efficient. CFLs are available in wattage

ratings of 5W, 7W, 9W, 11W, 13W, 18W, 23W etc. and usually they come with an

adapter having a cap similar to the incandescent lamp cap so that the lamp can

directly be fixed into an ordinary lamp holder. The adapter contains necessary circuits

for startup and normal operation of the lamp. The principle of operation is very similar

to ordinary fluorescent lamps, but uses a thin tube. The tube may be U shaped or

having multiple folds. The light output of an 11W CFL is equivalent to that of a 60W

incandescent lamp. This means the energy saving by the use of CFLs is enormous.

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1. Mercury Vapour Lamps

Mercury vapour lamp is a discharge lamp, available in a wattage range of 50 to 1OOO

W. It is commonly used for street lighting, yard lighting etc.

Construction and operation

The lamp has an arc tube inside and an outer envelope as shown in Fig. d The inner

side of the envelope has a phosphor coating. The actual discharge takes place inside

the arc tube.

Fig. d Mercury vapour lamp construction

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The arc tube is made of quartz and is capable of withstanding high temperature and

pressure. Inside the discharge tube there are two main electrodes placed at both

ends and an auxiliary electrode near to one of the main electrodes. The tube

contains a mixture of mercury vapour and an inert gas - usually argon. Initially,

when the voltage is applied there will be no discharge between the main electrodes,

but the same voltage is available between the auxiliary electrode and the main

electrode next to it. Since the gap between these two electrodes is less, it triggers a

discharge between them. A series resistance limits this discharge current. The heat

generated by this discharge evaporates more mercury and within a few minutes,

the discharge starts between the main electrodes, giving its full brightness.

The radiations produced by the tube include ultraviolet and infrared rays in nearly

equal proportions. The ultraviolet rays are converted to visible radiation by the

phosphor coating on the outer envelope.

Being discharge type, mercury vapour lamp also has a negative temperature

coefficient of resistance. Therefore an equipment to regulate the current is necessary.

A ballast connected in series with the lamp serves the purpose.

Performance

There are different types of mercury lamps available and the efficacy and colour

rendering index vary from type to type. The highest efficacy is around 60

lumens/watt, but with colour rendering index of around 20 only. Certain types of MV

lamps have higher colour index at the expense of efficacy.MV lamps are generally

used for street lighting and some commercial exteriors where colour rendering is

not important. These lamps have long life (upto 20000 hours).

4. Metal Halide Lamps

Metal halide lamp is a modified form of mercury vapour lamp. In this lamp, metals other than mercury are also added to the discharge tube. Due to the

spectral contribution of these metals, light emission occurs over a wide range of visible spectrum and hence the colour rendering index and efficacy are considerably increased. Commonly used metals are sodium, scandium, gallium thallium, indium

etc. The selection of metals and the composition are dependant on the type of emission required.

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The metals are usually used in the form of their halide salts - hence the name metal

halide. The halide group includes fluorine, chlorine, bromine and iodine but in most commercially available lamps, only iodine compounds are used, like sodium iodide,

scandium iodide etc. Metal halide lamps are available in a wide variety of sizes and shapes. The power rating

is in a range of 40 - 1800W. One typical construction is very similar to mercury

vapour lamps as shown in Fig (d).

Another commonly used type of metal halide lamp has both the leads connected to a

bi-pin cap as shown in Fig (e). The arc tube is made from high quality silica. The

electrodes used are of tungsten and the filling for the arc tube consists of argon,

mercury and metal halides appropriate for the emission required.

Performance

Metal halide lamps have efficacy in the range of 60-80 Im/W and a colour rendering

index of 70-85. That means both the efficacy and colour rendering are better than

mercury vapour lamps. Due to this reason, many users now prefer metal halide

lamps over mercury vapour lamps. Metal halide lamps are widely used as projector

lamps, for television and film shooting etc.

Sodium Vapour Lamps

Sodium vapour lamp is also.a discharge lamp. The discharge tube of this lamp

contains a mixture of sodium vapour and an inert gas — usually neon. At normal temperature, the sodium inside the tube shall be in solid state and therefore do not contribute to discharge. The inert gas is added as a 'starting gas' and the initial

discharge shall be due to the presence of this gas. The radiation produced by sodium lamp is predominantly a monochromatic yellow coloured radiation at 589 nm

wavelength. This wavelength has a speciality that it is very near to the peak of the eye sensitivity curve.

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Construction and operation

There are two variants of sodium vapor lamps - low pressure and high pressure

types. There is slight difference in the construction of these two types.

Fig. (f) Low pressure sodium vapour lamp

Fig. (g) High pressure sodium vapour lamp

Above figure shows the construction of a low pressure sodium vapour lamp. The

vapour pressure of sodium is about 0.1 Pa for this type. The discharge tube is a U shaped glass tube. The inside of the arc tube is coated with a sodium resistant glass

layer. At the two ends of the tube, there are coiled electrodes. When the lamp is not burning, the sodium will be in solid state deposited on the inner side of the tube. At startup the lamp functions like a neon lamp with a characteristic pink colour. As the

temperature builds up, the sodium vapourises and starts radiating yellow light. The lamp shall give it full brightness within a few minutes.

There is an outer envelope for the lamp and the space between the discharge tube and the envelope is vacuum. Vacuum is necessary to reduce the heat loss from the

discharge tube. It is also important to maintain the temperature of the discharge

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tube at around 260° C for generating proper radiation. In high pressure sodium vapor lamp, the vapour pressure is much higher (about

7000 Pa). At this pressure, the radiation from the discharge covers a good part of the visible spectrum and therefore the colour rendering properties improves. The

temperature of the discharge tube is around 1300° C. Fig 17.10 shows a typical construction of a high pressure sodium vapour lamp. The operating temperature is much higher compared to the low pressure type. The arc tube is made of a

translucent ceramic material. This material is particularly selected because it does not react with sodium or loose its shape even at higher temperatures. There is an

outer envelope for the lamp and the space between the arc tube and the envelope is vacuum or an inert gas filling. Sodium vapour lamps are also having a negative temperature coefficient of resistance

and therefore requires a ballast to control the current during normal operation. The lamp has another requirement that for starting, it requires a higher voltage than the

normal operating voltage. For low pressure sodium vapour lamp, the starting voltage is around 450V and for high pressure type the striking voltage is between 1000 and 4000 V. Fig. 17.11 shows a typical circuit suitable for low pressure sodium vapour

lamp. An autotransformer with high leakage reactance is used for starting and to control the current during normal operation. High leakage reactance results in

higher voltage regulation. At start the lamp current will be low and the high voltage required for starting will be available from the auto transformer. As the sodium

vapourise, the lamp current increases and due to the high regulation of the autotransformer, the voltage falls to the normal operating voltage. These circuits have low lagging power factor between 0.3 and 0.4 and to improve the power factor, a

capacitor is connected across the input. Electronic igniters are available now for all types of discharge lamps. These igniters are

designed to give the high startup voltage required by discharge lamps.

Performance

Low pressure sodium vapour lamps have very high efficacy of 100 to 180

lumens/watt. This is mainly because the radiation produced is in the visible

spectrum. On the other hand these lamps have extremely low colour rendering index

since the light output is monochromatic. Low pressure sodium vapour lamps are

available in the range of 18-180 W.

The colour rendering index of high pressure sodium lamps is better than low

pressure type (about 25%), but the efficacy is slightly less (65 to 140 lumens/watt).

High pressure sodium vapour lamps are available in the range of 35 - 1000 W.

Due to the low colour rendering properties, the application for sodium vapour lamps is

limited to street lighting, security lighting etc where the colour discrimination is not

important. The life of these lamps is about 20000 hours.

2007 Electrical Workshop Manual

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