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MODULE 6

TARIFF, WIRING AND LIGHTING SYSTEMS

In this module, we will be learning about the different types of tariff schemes, basic concept

of wiring, devices used in wiring and protection and various types of light and its working.

The following topics will be discussed.

Different Types of Tariffs

Classification of Consumers.

Conduit Wiring

Service Mains

Switch Board

Distribution Board

Fuses and Other Protective Devices

Earthing and Methods

Different Types of Lamps

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1. TARIFF

The rate at which electrical energy is supplied to a consumer is known as tariff. Although

tariff should include the total cost of producing and supplying electrical energy plus the

profit, yet it cannot be the same for all types of consumers..

A tariff should include the following items :

Recovery of cost of producing electrical energy at the power station.

Recovery of cost on the capital investment in transmission and distribution

systems.

Recovery of cost of operation and maintenance of supply of electrical energy e.g.,

metering equipment, billing etc.

A suitable profit on the capital investment.

1.1 Desirable Characteristics of a Tariff

A tariff must have the following desirable characteristics :

Proper return : The tariff should be such that it ensures the proper return from

each consumer..

Fairness : The tariff must be fair so that different types of consumers are satisfied with the

rate of charge of electrical energy. Thus a big consumer should be charged at a lower

rate than a small consumer. Similarly, a consumer whose load conditions do not deviate

much from the ideal (i.e., non variable) should be charged at a lower rate than the one

whose load conditions change appreciably from the ideal.

Simplicity : The tariff should be simple so that an ordinary consumer can easily understand

it. A complicated tariff may cause an opposition from the public which is generally

distrustful of supply companies.

Reasonable profit : The profit element in the tariff should be reasonable.

Attractive : The tariff should be attractive so that a large number of consumers are

encouraged to use electrical energy.

1.2 Types of Tariff

There are several types of tariff. However, the following are the commonly used types

of tariff :

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a) Simple tariff. When there is a fixed rate per unit of energy consumed, it is called a

simple tariff or uniform rate tariff. In this type of tariff, the price charged per unit is constant

i.e., it does not vary with increase or decrease in number of units consumed.

Disadvantages

There is no discrimination between different types of consumers since

every consumer has to pay equitably for the fixed charges.

The cost per unit delivered is high.

It does not encourage the use of electricity.

b) Flat rate tariff. When different types of consumers are charged at different uniform per

unit rates, it is called a flat rate tariff. In this type of tariff, the consumers are grouped into

different classes and each class of consumers is charged at a different uniform rate. The

advantage of such a tariff is that it is more fair to different types of consumers and is

quite simple in calculations.

Disadvantages

Since the flat rate tariff varies according to the way the supply is used, separate

meters are required for lighting load, power load etc. This makes the application

of such a tariff expensive and complicated.

A particular class of consumers is charged at the same rate irrespective of the

magnitude of energy consumed. However, a big consumer should be charged at a

lower rate as in his case the fixed charges per unit are reduced.

c) Block rate tariff. Here energy consumption id divide into blocks.The price per unit is

fixed in each block. The price per unit in the first block is the highest and it is progressively

reduced for the succeeding blocks of energy. This type of tariff is being used for majority

of residential and small commercial consumers.

d) Two-part tariff. In two-part tariff, the total charge to be made from the consumer is

split into two components viz., fixed charges and running charges. The fixed charges

depend upon the electrical load (maximum demand) of the consumer while the running

charges depend upon the number of units consumed by the consumer. Thus, the consumer is

charged at a certain amount per kW of maximum demand plus a certain amount per kWh of

energy consumed i.e.,

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Total charges = Rs (b × kW + c × kWh)

where,

b = charge per kW of maximum demand

c = charge per kWh of energy consumed

This type of tariff is mostly applicable to industrial consumers who have appreciable

maximum demand.

Advantages

It is easily understood by the consumers.

It recovers the fixed charges which depend upon the maximum demand of the

consumer but are independent of the units consumed.

Disadvantages

The consumer has to pay the fixed charges irrespective of the fact whether he has

consumed or not consumed the electrical energy.

There is always error in assessing the maximum demand of the consumer.

e) Maximum demand tariff. It is similar to two-part tariff with the only difference that

the maximum demand is actually measured by installing maximum demand meter in the

premises of the consumer.

f) Power factor tariff. The tariff in which power factor of the consumer’s load is taken into

consideration is known as power factor tariff. In an AC system, power factor plays an

important role. A low power factor increases the rating of station equipment and line losses.

Therefore, a consumer having low power factor must be penalised.

g) Three-part tariff. This type of tariff is generally applied to big consumers. When the

total charge to be made from the consumer is split into three parts viz., fixed charge, semi-

fixed charge and running charge, it is known as a three-part tariff. i.e.,

Total charge = Rs (a + b × kW + c × kWh)

Where,

a = fixed charge made during each billing period.

b = charge per kW of maximum demand,

c = charge per kWh of energy consumed.

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1.3 Classification of Consumers

The consumers are classified into following categories.

Domestic consumer : Small consumer with single phase supply. e.g.: houses.

Commercial consumer : Mainly shops and small offices.

Industrial consumer : Big factories and industries with three phase supply.

Public utility : Street lights and public amenities.

Public institutions : schools, colleges, hospitals etc.

The consumers are also classified as

Low tension (LT) consumer : Consumers who are supplied with 230V single phase or

415V three phase supply.

High Tension (HT) consumer : Consumers who are supplied with 11kV or 22kV

sypply.

Extra High Tension (EHT) consumer : Consumers who are supplied with 33kV and

above.

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2. WIRING

A network of wire connecting various accessories for distribution of electrical energy from

supplier 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. There are different types of wiring

1. Cleat wiring.

2. Wooden casing and capping wiring.

3. CTS or TRS wiring

4. Lead sheathed wiring

5. Conduit wiring

2.1 Conduit Wiring

Out of the above different types, we will be learning about conduit wiring in detail. There are

two types of conduit wiring

(i) Open type

(ii) Concealed type.

In open type conduit wiring steel tubes known as conduits are installed on the surface of the

walls by means of saddles or pipe hooks.

In concealed type the conduits are buried inside the walls of building. The wire are then

drawn into the conduits. PVC conduits are commonly used for residential building. The

concealed type conduit wiring is preferred for residential and public buildings. The conduit

used for this purpose is of two types namely

(1) Light gauge

(2) Heavy gauge.

Heavy gauge conduits are used for all medium voltage (250 V to 600 V) circuits and in places

where good mechanical protection and absolute protection from moisture is desired.

Conduit wiring are used in places where

considerable dust is present,

damp situations

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in workshops for lighting and motor wiring

residential and public buildings where the appearance is the prime thing

2.2 Service Mains

These are the service wires drawn from the electric post to home and other buildings. A

service wire has two separate and insulated conductors inside it. For a single phase supply

only once service wire will be drawn. For a three phase supply 2 service wire will be drawn.

2.3 Meter Board.

These are installed on the consumer premises. The service wire from the post terminated at

meter board. In the meter board, there will be an energy meter for measuring the electricity, a

main switch and a fuse for protection.

2.4 Distribution Board.

These are installed inside the consumer building. It can be either concealed or open type. The

connection from the meter board terminates at the distribution board. From distribution board,

the electricity is distributed to different floor and room. Each room/floor is provided with a

separate circuit. Each circuit from the distribution board is protected with MCB’s

2.5 Main Switch

The main switch is used to protect the circuit against excessive current. It is also known as

switch fuse unit. Switch fuse unit is made of iron, so known as iron clad switch. It may be

double pole for controlling single phase two wire circuits or triple pole for controlling three

phase, 3-wire circuits or triple pole with neutral link for controlling 3-phase, 4-wire circuits.

The respective switches are known as double pole iron clad (DPIC), triple pole iron clad

(TPIC) and triple pole with neutral link iron clad (TPNIC) switches.

2.6 Fuse

Fuse is a protective device. It is a thin wire having low melting point. Under normal working

condition the current flowing through the circuit is within safe limits. When a fault occurs,

the current exceeds the limiting value, the fuse wire gets heated, melts and breaks the circuit.

So fuse protects the machine or apparatus from being damaged due to excessive current.

Larger the current, the more rapidly the fuse will blow.

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The materials used for fuse wires must be of low ohmic loss and high conductivity. The

materials commonly used for this purpose are tin, lead, copper, zinc, aluminum and alloy of

tin and lead. An alloy of tin and lead (37% lead and 64% tin) is used for small current rating

fuses (below 10 amperes.) Beyond 20 amperes copper wire fuse is employed.

Minimum fusing current. It is defined as the minimum value of current at which the

fuse element or fuse wire melts.

Current Rating of Fuse element. It is defined as the current which the fuse wire can

normally carry without overheating or melting. Its value is always less than the value

of minimum fusing current.

Fusing Factor. The ratio of minimum fusing 'current to the current rating of fuse

element is known as fusing factor.

Fusing factor =

Different Types of Fuse are :

Kit-Kat type fuse

It is made of porcelain. It has two parts: a porcelain base and a porcelain fuse carrier.

The fuse base holds the fixed contact. The fuse wire is connected on the fuse carrier.

When the carrier is inserted to the base, the fixed contacts gets connected together

through the fuse wire. If excess current flows through the circuit, the fuse wire blows

and protects the circuit. The fuse wire can be easily replaced by removing the carrier

from the base.

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Cartridge type Fuse.

This is a totally enclosed type fuse unit. It essentially consists of an insulating

container of bulb or tube shape and sealed at its ends with metallic cap. This container

encloses the fuse element. For fuses with high current rating, the container is filled up

with powder or granular material known as filler. These fillers may be sand, calcium

carbonate, quartz etc. During short circuit, the fuse blows. The spark thus produced is

eliminated by the filler inside. Since it is totally enclosed it will not be possible to

rewire and therefore the whole unit will have to be replaced, once it blows out. This

type of fuse is available up to 660 V and the current rating up to 800A.

High Rupturing Capacity (HRC) Fuses.

It is same as cartridge fuse except that it is used for very high current rating and in the

case of heavy fault current. The main advantages of HRC fuses are

They are cheap as compared with other types circuit interrupter of same

breaking capacity.

No maintenance is required

The operation is quick and sure.

They are capable of clearing high as well as low currents.

2.7 Miniature Circuit Breaker (MCB)

These are small-capacity automatic circuit-breakers used for protection from short circuits

and overloads. They automatically detects the faults and opens the circuit. This is called

tripping. MCB has the advantage that it has no fuse wire. So that we need not rewire it when

it has tripped. Once the MCB has been tripped, the circuit can be turned ON by closing the

switch present in it. The MCB can also be turned ON and OFF manually by the operating this

switch. MCB doesn’t respond to over voltage and earth fault.

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2.8 Earth Leakage Circuit Breaker (ELCB)

These are small-capacity automatic circuit-breakers used for protection from earth faults and

electric shocks. When an earth fault or electric shock occurs, the current flows to earth

through the fault. They automatically detects the current flowing to earth and trips circuit.

Once the ELCB has been tripped, the circuit can be turned ON by closing the switch present

in it. It can also be turned ON and OFF manually by the operating this switch.

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3. EARTHING OR GROUNDING

The potential of the earth is considered to be ZERO. For all practical purposes, the

transformer neutral and the generator neutral is always connected to earth. Also the body of

all electrical equipment is connected to the earth by means of a wire of negligible resistance.

This is called earthing or grounding. Earthing brings the potential of the body of the

equipment to ZERO i.e. to the earth’s potential. If a fault occurs in an equipment and a line

comes in contact with the body of the equipment, the current in the body of the equipment is

safely discharged to earth, thus protecting the operating personnel against electrical shock. If

the body of the electrical equipment is not connected to earth, the current will discharge

through the body of operating personnel causing electrical shock.

Thus earthing is to connect any electrical equipment to earth with a very low resistance wire,

making it to attain earth’s potential. The wire is usually connected to a copper plate or a GI

pipe placed at a depth of 2.5 to 3 meters from the ground level.

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3.1 Necessity of Earthing

To protect the operating personnel from danger of shock in case they come in contact

with the charged frame due to defective insulation.

To maintain the line voltage constant under unbalanced load condition.

Protection of the equipments

Protection of large buildings and all machines fed from overhead lines against

lightning

The resistance of earth must be made minimum so that the earthed apparatus attains earth

potential. The resistance of earth is affected by the following factors

1. Material properties of the earth wire and the electrode

2. Temperature and moisture content of the soil

3. Depth of the pit

4. Quantity of the charcoal used

3.2 Methods of Earthing

The important methods of earthing are the plate earthing, pipe earthing, rod earthing and strip

earthing. The earth resistance for copper wire is 1 ohm and that of G I wire less than 3 ohms.

The earth resistance should be kept as low as possible so that the neutral of any electrical

system, which is earthed, is maintained almost at the earth potential. The typical value of the

earth resistance at powerhouse is 0. 5 ohm and that at substation is 1 ohm.

1. Plate earthing

2. Pipe earthing

Plate Earthing

In this method a copper plate of 60cm x 60cm x 3.18cm or a GI plate of the size 60cm x

60cm x 6.35cm is used for earthing. The plate is placed vertically down inside the ground at a

depth of 3m and is embedded in alternate layers of coal and salt for a thickness of 15 cm. In

addition, water is poured for keeping the earth electrode resistance value below 5 ohms. The

earth wire is securely bolted to the earth plate. A cement masonry chamber is built with a cast

iron cover for easy regular maintenance.

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Pipe Earthing

Earth electrode made of a GI (galvanized) iron pipe of 38mm in diameter and length of 2m

(depending on the current) with 12mm holes on the surface is placed upright at a depth of

4.75m in a permanently wet ground. To keep the value of the earth resistance at the desired

level, the area (15 cms) surrounding the GI pipe is filled with a mixture of salt and coal.. The

efficiency of the earthing system is improved by pouring water through the funnel

periodically. The GI earth wires of sufficient cross- sectional area are run through a 12.7mm

diameter pipe (at 60cms below) from the 19mm diameter pipe and secured tightly at the top.

When compared to the plate earth system the pipe earth system can carry larger leakage

currents as a much larger surface area is in contact with the soil for a given electrode size.

The system also enables easy maintenance as the earth wire connection is housed at the

ground level. Pipe earthing is as shown in the following figure

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4. LAMPS

Lamps are devices which convert electrical energy to light energy. Different types of lamps

are available in market which has different working principles. Here we will learn in details

some of the lamps which we come across in our day to day life. Before we go into detail, we

can familiarize some of the terms related to lamp and light.

Luminous Flux : It is the total amount of light energy emitted per second by a source.

Unit is Lumens.

Luminous Intensity : It is the luminous flux emitted by the source per solid angle.

Unit is Candela.

Illumination : When light falls on a body, it is called illumination. It is measured in

Lumens per square meter or Lux.

Candle Power : It is the light radiating capacity of a source in a given direction.

Denoted by C.P.

4.1 Sodium Vapour Lamp

Sodium vapour lamps are mainly used for street lighting. They have low luminosity hence

require glass tubes of large lengths, which makes them quiet bulky.

Construction:

The lamp consists of a U shaped inner glass tube filled with neon gas at a pressure of 10mm.

It also contains a small quantity of sodium and argon gas. The initial ionization voltage is

reduced, as the ionization potential of argon is low. Two oxide coated tungsten electrodes are

sealed into the tube at the ends. This tube is enclosed in an outer double walled vacuum

enclosure to maintain the required temperature.

Working:

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A voltage of the order of 380- 450 volts (depending on the wattage) is necessary to start the

discharge, which is obtained from a high reactance transformer or an autotransformer.

Initially the sodium vapour lamp operates as a low-pressure neon lamp emitting pink colour.

As the lamp gets heated and reaches a temperature of 200° C the sodium deposited on the

sides of the tube walls vaporizes and radiates yellow light. It has a maximum efficiency at

220° C. Proper mounting of the lamp is to be ensured to prevent the sodium blackening the

inner walls of the tube .A capacitor C is used to improve the power factor.

4.2 Metal Halide Lamp

A metal-halide lamp is an electric lamp that produces light by an electric arc through a

gaseous mixture of vaporized mercury and metal halides (compounds of metals with bromine

or iodine). It is a type of high-intensity discharge (HID) gas discharge lamp. They are similar

to mercury vapour lamps, but contain additional metal halide compounds in the quartz arc

tube, which improve the efficiency and colour rendition of the light. The most common metal

halide compound used is sodium iodide. Metal-halide lamps have high luminous efficiency

and produce an intense white light. Lamp life is 6,000 to 15,000 hours.

The lamps consist of a small fused quartz or ceramic arc tube which contains the gases and

the arc, enclosed inside a larger glass bulb which has a coating to filter out the ultraviolet

light produced.

Metal-halide lamps produce light by making an electric arc in a mixture of gases. In a metal-

halide lamp, the compact arc tube contains a high-pressure mixture of argon or xenon,

mercury, and a variety of metal halides. The argon gas in the lamp is easily ionized when

supply is given, and initiates the arc across the two electrodes. The heat generated by the arc

then vaporizes the mercury and metal halides, which produce light as the temperature and

pressure increases.

4.3 Mercury Vapour Lamp

A mercury-vapour lamp is a gas discharge lamp that uses an electric arc through vaporized

mercury to produce light. The arc discharge is generally confined to a small fused quartz arc

tube mounted within a larger borosilicate glass bulb. The outer bulb may be clear or coated

with a phosphor; in either case, the outer bulb provides thermal insulation, protection from

the ultraviolet radiation the light produces, and a convenient mounting for the fused quartz

arc tube.

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The mercury in the tube is a liquid at normal temperatures. It needs to be vaporized and

ionized before the tube will conduct electricity and the arc can start. So, like fluorescent

tubes, mercury vapour lamps require a starter, which is usually contained within the mercury

vapour lamp itself. A third electrode is mounted near one of the main electrodes and

connected through a resistor to the other main electrode. In addition to the mercury, the tube

is filled with argon gas at low pressure. When power is applied, there is sufficient voltage to

ionize the argon and strike a small arc between the starting electrode and the adjacent main

electrode. This starting arc discharge heats the mercury and eventually provides enough

ionized mercury to strike an arc between the main electrodes. This process takes from 4 to 7

minutes, so mercury lamps are slow starting.

The mercury vapour lamp is a negative resistance device. This means its resistance decreases

as the current through the tube increases. So if the lamp is connected directly to a constant-

voltage source like the power lines, the current through it will increase until it destroys itself.

Therefore, it requires a ballast to limit the current through it

4.4 Fluorescent Tube Light

A fluorescent lamp tube is filled with a gas containing low pressure mercury vapour and

argon, xenon, neon, or krypton. The inner surface of the lamp is coated with a phosphor

coating. The lamp's electrodes are typically made of coiled tungsten and usually referred to as

cathodes. They are coated with a mixture of barium, strontium and calcium oxides.

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When power is first applied to the circuit the starter contacts connects the two filaments of the

fluorescent lamp and the ballast in series to the supply voltage. The current through the

filaments causes the argon gas to heat up. This heat vaporizes the mercury in the tube. When

the starter contacts opens the current through the filaments and the inductive ballast is

abruptly interrupted. This will create a very high voltage between the filaments. This high

voltage will initialize the discharge through the vaporized mercury gas. The mercury will

emit UV radiations which is absorbed by the phosphor coating and emits visible light.

4.5 Compact Fluorescent Lamp

The principle of operation in a CFL is the same as in fluorescent tube light except that it is

compact in size. The tube of the CFL consist low pressure mercury vapour and argon gas.

When supply is turned ON, the argon gas heats the mercury vapour and discharge occurs. The

UV rays emitted by the mercury vapour is absorbed by the phosphor coating and converts it

to visible light.

4.6 Light Emitting Diode

It is a P-N junction diode which emits light when energy is applied on it. This phenomenon is

generally called electroluminescence, which can be defined as the emission of light from a

semi-conductor under the influence of an electric field. The charge carriers recombine in a

forward P-N junction as the electrons cross from the N-region and recombine with the holes

existing in the P-region. The energy dissipated during recombination of the electrons and the

holes is in the form of heat and light.

The electrons dissipate energy in the form of heat for silicon and germanium diodes but in

Gallium arsenide phosphide (GaAsP) and Gallium phosphide (GaP) semiconductors, the

electrons dissipate energy by emitting photons. If the semiconductor is translucent, the

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junction becomes the source of light as it is emitted, thus becoming a light emitting diode

(LED) but when the junction is reverse biased no light will be produced by the LED and, on

the contrary, the device may also get damaged.