Distribution Transformer

A distribution transformer is used to step down the 11 kv a.c. supply to 230/440 volts a.c. and are available in different KVA rating depending upon the requirement of the consumers. The main parts of a distribution transformer are explained

a) Primary Winding: The primary winding of a distribution transformer is connected in delta as shown in figure. There are three different windings and is circular wound. These are placed on the three different limbs of the transformer core. The size of primary winding wire used is thinner than secondary winding but having larger number of turns.

b) Secondary Winding: The secondary winding of a distribution transformer is connected in star as shown in figure. There are three different windings and is circular wound. These are placed on the three different limbs of the transformercore. The size of winding wire used is thicker than primary winding but having lesser number of turns.

c) Transformer Oil: The transformer oil is used to transfer the heat generated due to copper losses in the winding as well as in the transformer core due to eddy currents and hysteresis. Due to passage of time, the transformer oil loses its dielectric strength due to absorption of moisture through the breather. The solid impurities present in the oil also reduce its dielectric strength. A good
remedy is to filter and dehydrate the transformer oil with a special technique and machine, known as “Transformer oil Filtration Machine” to increase its dielectric strength.

d) Conservator: It is used as reservoir mounted on the top of the transformer.It is connected to the transformer through the buchhol’s relay.This relay is only used in large capacity transformers. An oil level indicator is provided to observe the oil level in the reservoir. The oil level can be maintained by adding more dry oil in the conservator. It provides a reserve space for the expended oil due to increase in temperature of the winding and core. When the temperature is reduced the expended oil starts contracting and goes back to the transformer tank and hence it acts as a reservoir.

e ) Breather: As the name indicates, it is used for providing breathing passage for the transformer. The air present in the empty conservator is forced to escape to the atmosphere through the breather, when the oil level in the conservator increases due to increase in temperature and vice versa. This air is escaped to atmosphere through a box containing silica gel in the form of crystals. The colour of fresh or fully activated gel gets changed from blue to bright pink as it absorbs the moisture. It absorbs the moisture from the atmosphere, when the transformer oil starts contracting or during inward rush of air. This pink deactivated silica gel is removed from the breather and baked in a baking oven for reactivation. After reactivation its colour will again converted into blue.

f) Oil Drain Point: It is used to drain the transformer oil in a container when it becomes necessary to drain all the oil from the tank for visual inspection or repair and maintenance of winding. Sometimes we have to replace the transformer oil when the dielectric strength gets reduced. A screwed cap and valve is also provided on the out let, for opening and closing the drain point.

g) Cooling Tubes: As the name indicates, these tubes are used to cool down the temperature of the transformer oil. The shape, size and number of tubes depend upon the KVA rating of the transformer. In small capacity transformers cooling is provided by natural convention of heat generated, through the cooling tubes but in large capacity transformers the fans are provided for forced air-cooling.

h) Oil Level Indicator: The oil level indicator is used to indicate the minimum oil level in the reservoir. The level may be top up by adding new or dehydrated transformer oil.

i) Earth point: The earth point is used to connect the neutral wire and body of the transformer to ground potential. It is mandatory from safety point of view.

j) Explosion Vent: When there is sudden increase in oil level due to any external or internal faults, the oil level in the reservoir will increase sharply. Although the buchhol’s relay is provided to meet such type of eventualities or emergencies but in the event of failure of all other protections the safety plug will burst and oil is allowed to flow out through the explosion vent.

k) Tap Changer: The tap changer is used to increase or decrease the a.c. out put voltage of the transformer depending upon the requirement and conditions.It is always connected in the low current winding so that minimum arc is produced, while changing the voltage tapings. We can change the tapings with or without load. In small rating transformer it can be operated manually where as the large capacity transformers are motor operated through remote instrumentation.

Power Factor

“Cosine of the angle between applied voltage and current drawn in any electrical circuit is known as power factor” and it is denoted by “Cos F”.When a purely resistive circuit is connected across the a.c. supply, the current drawn from the main supply will be in phase with applied voltage and hence the power factor will be unity. But in case of purely inductive and capacitive circuit,the current drawn will lag behind and lead the applied voltage, by an angle of 90 0 respectively. Now, to understand the power factor in detail, let us take an example of inductive circuit

When alternating current (a.c.) supply is connected across the purely inductive circuit as shown above in figure (a). The alternating current will flow in the circuit through the inductance coil. The resistance represents the internal resistance of the inductive coil, as resistance is directly proportional to length of coil, which may be very less. The current drawn (I) by the coil will lag behind the applied voltage by an angle F. This lagging current can be resolved into two components i.e. one I Cos F along the X-axis and I Sin F along the Y-axis, as shown in figure (b). If we multiply these values by KV, as given in figure in (c) then we have

KW = KVI Cos F = KVA Cos F = Active Power or True Power

KVA = Apparent Power

KVAR = KVI Sin F = KVA Sin F = Reactive Power or Watt-less Power

So from the triangle OAB (Figure d)

Cos F = OA/AB = KW/KVA = Active Power/Apparent Power

So, we can say the power factor is ratio of active power to the apparent power.

We can also explain the power factor by an impedance triangle OAB

Cos F = OA/AB = R/Z

So, we can also say that power factor is the ratio of resistance to impedance.

Where z 2 = R 2 + Z 2

The inductive reactance (X L ) and capacitive reactance (X C ) of an electric circuit depends upon supply frequency, because X L = 2pfL and X C = 1/2pfc.So, the inductive reactance increases with the increase in supply frequency where as capacitive reactance decreases and vice versa. With the change in the value of X L and X C , the value of Z changes. Because in this case

Z 2 = R2 + (X L2 ± X C 2 )

So, they create a phase difference between applied voltage and current drawn from the main circuit. In other words for same power more current is needed in ordered to overcome the inductive and capacitive effect. Mathematically we can say cosine of the angle of lag or lead of current with applied voltage is known as power factor.

i. Disadvantage of Low Power Factor
The disadvantages of low power factor are given here.

a) We know that P = VI Cos F In case of single-phase supply.

Also P = Ö3 V L I L Cos F In case of three-phase supply.

The current drawn from the supply mains will be

I = P/V Cos F

I = P/Ö3 V L Cos F

In case of single-phase supply. ———— equation no. 1

In case of three-phase supply. ————— equation no. 2

It is evident from equation 1 and 2, that the current drawn is inversely proportional to the power factor. If the values of power factor increase the value of current decreases and vices versa and hence the cost per unit will increase.

b) Due to increase in current drawn from the supply mains due to decrease in power factor, the rating of transformer, alternator, all the transmission and distribution system will be over load. So, all these equipment need to be upgraded and hence increase in cost per unit.

c) The efficiency of transmission lines, alternator and transformer reduces with increase in current drawn due to low power factor because the copper losses (I 2 R).

d) The voltage drop in transmission line increases due to increase in current and hence the voltage regulation will be poor. To improve the regulations either reduces the current drawn from the main supply or increase the diameter of transmission line conductor. The cost per unit will again increased.Thus we can say, due to decrease in power factor, the cost of transformer, alternator, transmission lines and its accessories will increase and hence costs per unit will increases.

ii. Causes of Low Power Factor

The main causes of low power factor are given here.

a) All the induction motors running on lesser load than the full load is responsible for poor power factor. Approximately, the power factor of motor running on full load is 0.85, at half load, 0.5 and at 25 % of full load, is too low as on no load approximately. Arc lamps and arc furnaces are highly responsible for reducing the power factor drastically.

b) Some time due to defective bearings, the rotor may touch the stator. To overcome such problem, some material removed from the outer surface of the rotor by turning on lathe, which leads to increase in air gap between stator and rotor. So the power factor of the motor will increase due to increase in magnetizing current.

iii. Methods of Improving Power Factor

The main methods of improving the power factor are given here.

a) Static Capacitor: The power factor is generally improved by installing static capacitor bank in parallel with the application having poor power factor

When the supply is switched on the capacitor bank, which is permanently connected in parallel with the application responsible for low power factor starts drawing leading current and hence the phase angle of the resultant current is improved  So, the power factor will increase. In case of three- phase supply, the capacitor is connected either in star or delta

b) Synchronous Condenser: - Synchronous motor with over excited can be used to improve the overall power factor of the complete installation by compensating the lagging components of the current drawn due to inductive loading

iv. Advantages of improved power factor

The advantages of improved power factor are as follows.

a) Voltage regulation is improved.

b) More output power can be taken from the same plant and equipment associated with generation, transmission and distribution of energy.

c) Efficiency of alternator, transformer and transmission lines is improved due to reduction in copper losses.

Distribution System

The system used to distribute the electrical energy to various consumers safely is known as distribution system. Generally 3-phase 4-wire a.c. system is adopted for distribution of electrical energy in industries where as single-phase two-wire system is used in home, offices and commercial purpose. There are three different systems of power distribution detailed below.

i. Single Phase Two wire System

This system of distribution is widely used in homes, offices and commercial installation having total connected load less than 5 kilowatts. Single-phase two-wire system consists of one phase and one neutral wire. The potential difference between the phase and neutral wire is 230 volts. To understand this system of distribution of electrical energy, kindly refer to circuit

Twin core PVC cable is used to energise the single-phase energy meter. The cross-sectional area of the cables depends upon the load requirement of the consumer and it varies from 4 to 10 mm 2 . This cable is supported by galvanised steel wire.The output of energy meter is controlled by I.C.D.P. (Iron Clad Double Pole) main switch. The phase wire is connected with fuse link and the neutral with neutral link,made of thick copper strip. The output of this I.C.D.P. main switch is connected to main distribution board, which may be fixed on the same iron sheet or wooden board. Then the out put of I.C.T.P. switch is sub-divided into different sub circuits and connected to different small capacity double pole switches depending upon the number of rooms and power points. It should be ensured that all the phase wires are controlled through the fuse links.

ii. Three Phase Three wire System

This distribution system is used to transfer the large quantum of electrical energy from the generating station to different sub-stations by using three-phase wire only.The generated voltage of an alternator may be 6.6, 11 and 15.7 kilovolts. This generated voltage is generally step up to 220 kv depending on distance between generating station and ultimate consumer. If the distance is too more, this generated voltage can be step-up to 440 kv. To understand three phase three wires systemkindly refers to the circuit

The working of isolators and circuit

breakers has already been described in the working of indoor and outdoor sub-station. The generated voltage is stepped-up from 11 kv to 220 kv at the generating station and then this voltage is transferred to distribution sub-station. Here, it is step down to 11 kv with the help of 220/11 kv sub-station.

iii. Three Phase Four wire System:

This system of distribution uses three Phase and one neutral wire. The out put of three phase three wire distribution system as explained above, is the input of this system. This system is most widely used for industrial and medium consumer having total connected load of less than 5 KW. However this system is explained

In this system of distribution we have only one source of electrical energy. 11/0.440 KV step-down transformer is used to step down the voltage. The voltage between any two-phase wires is 440 volts approximately whereas between any phase and neutral wire it is 230 volts. The primary winding of the transformer is delta connected whereas the secondary is star connected, to form the three phases and one neutral wire and hence the system’s name. Three phase and neutral wire is connected to three-phase energy meter to record the total energy consumed. The output of energy meter is connected to distribution panel through air circuit breaker, for effective distribution. Small domestic consumer having total connected load less than 5 KW can use one phase and neutral wire.

Transformer

A transformer is a static electrical device used to transfer electrical power from one circuit to another keeping the supply frequency constant. We can increase or decrease the voltage in the secondary of the transformer with the corresponding changes in current and hence the power transfer will be almost constant if we neglect the losses. This transfer of power and change in secondary voltage will take place due to linkage of common magnetic flux and mutual induction between the two coils as explained below in working principle of transformer.

Working Principle: A transformer works on the principle of Faraday’s laws of electro magnetic induction, which states that “whenever a closed circuit coil is placed in an alternating magnetic field, an electro motive force (e.m.f.) will induced in that coil and magnitude of this induced e.m.f. will depends upon the number of turns in the coil.” To understand the working principle

When the primary winding of the transformer is connected to a.c. supply, the magnetic flux (F) will be set up and circulate in the laminated magnetic core. This magnetic flux will be alternating in nature as the supply frequency and links with both the windings (Primary and secondary) of the transformer. Now according to faraday’s laws of electromagnetic induction an e.m.f. will be induced in the secondary winding due to mutual induction. This induced e = n dF/dt in the secondary winding will depend upon the rate of change of flux and number of turns. If the number of turns in the secondary winding is greater than primary, the transformer will act as step up transformer and vice-versa. Whether a transformer is step up or step down depends upon the transformation ratio (K), which is explained below.

Transformation Ratio (K): As discussed above, when the primary winding of the transformer is connected across the a.c. supply, an alternating current will flow in the primary winding. And as a result of this alternating current, an alternating magnetic field (F) will be set up and circulate in the laminated core

This circulating alternating magnetic flux will link with both the primary and secondary windings of the transformer and as a result of it; an alternating electromotive force(e.m.f.) will be induced in these windings, due to self and mutual induction respectively. Induced e.m.f. (E 1 ) in the primary winding will be almost equal and opposes the applied voltage and hence known as back e.m.f. This induced e.m.f. (E 2 ) in the secondary winding depends upon the rate of change of flux and number of turns

(e = n dF/dt).

Let N 1----> Number of turns in the primary winding.

N 2----> Number of turns in the secondary winding.

E 1 --->Induced e.m.f. in the primary winding.

E 2 --->Induced e.m.f. in the secondary winding.

V 1--->In put a.c. voltage to the primary winding.

V 2---> Out put a.c. voltage across the load.

Induced voltage in the primary wi nding per turn =(E 1/N 1)volts

Since the same flux will link with both the primary and secondary windings.

So, this induced e.m.f. in the secondary winding will be

Where K is called the transformation ratio.

If K is greater than 1, the transformer is step-up transformer and

If K is less than 1, the transformer is step-down transformer.

For an ideal transformer input power = out put power

Or V 1 I 1 = V 2 I 2

(V 2/ V 1)= (I 2/I 1)=  K

Where K is known as transformation ratio of the transformer.

Sub-station

We know that the electrical energy is generated in different types of generating stations located at distant places from the consumer’s premises. For example,nuclear and coal fired based thermal power generating stations are commissioned far away from the populated area from safety point of view. So, to transfer such a huge quantum of electrical energy from generating stations to the ultimate consumer,number of transformers and other switching equipments are required to step up or step down the voltage. Any station comprising of transformers and switching equipment is known as sub-station. In general the sub-station are of following three types: -

i. Pole Mounted Type Sub-station

As the name indicates, this type of sub-stations is mounted on Reinforced Cement Concrete (R.C.C.) pole structure consisting of H-type (Double Pole) or square- type (Four Pole). A platform is prepared at suitable height by rolled steel joist, for placing the 11/0.440 kilovolts distribution transformer

A gang-operated switch (G.O. switch) is used to energise the transformer. These types of sub-station are commissioned on roadsides and hence do not require any building structure and sub-station attendant for its operation. So, these are cheapest and most economical as compared to other types of sub-station. These sub-stations are most widely used for rural electrification and small colonies or medium consumers in urban areas. The input voltage to this type of sub-station is 11 kilovolts and output is 440 volts between any two-phase wires and 230 volts between any phase and neutral wires.

ii. Outdoor Type Sub-station

These types of substations are commissioned away from the populated areas from safety point of view and is used to control the 11kv to 132 kv supply. Transformers,circuit breakers and other accessories are commissioned out side the building but are controlled from a building through wires by using remote instrumentation

We have two 132 kv incoming feeders from two different sub- stations or generating stations. Both the incoming feeders terminate on separate 11 kv bus bars through isolators, main oil circuit breakers (MOCB) current transformer(CT), potential transformer (PT) and 132/11 kv step down transformers, etc. All these equipment are commissioned in a closed boundary. The current transformer and potential transformers are used to monitor the line potential and current and hence the units supplied to the sub-station. We can shift the entire load on either feeder by selecting the proper bus bar and bus-coupler. Both the source can also be used independently. This bus coupler is used to couple both the bus bars to use single incoming feeder. Lightning arrester (LA) is used to discharge the excessively induced voltage in the line conductors to ground, induced due to the lightning effect.

Earth switch is used to connect all the conductors to ground, to avoid accidents when the line is under permit to work (PTW). The output of this sub-station is connected to 11/0.440 kv distribution transformer. Sub-station attendants are required for the operation of the outdoor type sub-stations.

iii. Indoor Type Sub-station

In this type of sub-station, the 11/0.440 kv distribution transformer, main oil circuit breaker (MOCB), L.T. air circuit breaker (ACB) and distribution panels are commissioned in a building whereas the isolators and gang operated switches are commissioned outside the building. Transformer may be located outside the building in shed duly covered boundaries. The circuit diagram of the indoor type sub-station

We have two incoming feeders from different outdoor sub- stations. Gang operated. switches are used to select the incoming source of supply.Isolators are used to physically isolate the transformer from incoming or outgoing feeders/circuits. Main oil circuit breaker is used to switch on or off the 11/0.440 kv distribution transformer whereas the air circuit breaker is used to operate the low-tension distribution panel. We can operate both the circuit breakers under fully loaded conditions whereas the isolators should be operated when the load is switched off. Oil circuit breaker 1 and 2 are used to energise the two different distribution transformers. Lightning arrester (LA) is used to arrest and discharge excessively induced voltage in the line due to lightning effect.

Sub-Station, Transformer, Distribution System and Power Factor

We will study the various sub-stations used for transmission and distribution of electrical energy. Use and working of isolators, circuit breakers,lightning arresters, earth switches, bus-bars, bus-couplers, step-up and step-down transformers etc. we will also be have an exposure about different types of distribution systems used for distribution of electrical energy. This includes the single-phase two wires system, three-phase three wires system and three-phase four wires system, used to transfer and distribute the electrical energy from generating station to the ultimate consumer. Transformers play an important role to transfer and distribute large quantum of electricity and we can step up and step down the voltage with the help of transformer. We will also see the internal construction of transformer and various safety devices used to protect the transformer from increase in temperature and moisture. Power factor is an important factor, which should be monitored for the safe and efficient operation of any electrical installation. In this unit we will study, how power factor effects the efficiency of generating station, transformer and substation etc. Different methods to improve the low power factor by installing static capacitor bank and synchronous condenser (Synchronous motor) will be also discussed in detail.

Diesel Generating Set.

Diesel generating sets are used to generate electrical energy in emergency when the main supply is suddenly interrupted. In other words, we can say that these are used as standby source of electrical energy. These are available in different sizes varying from 5 to 500 Kilo-Volt-Ampere (KVA). As compared to another sources of electrical energy, these are cheap in initial cost, less commissioning time, easy to handle and very quick to start generation, as no warming time is required.

Requirement of large quantity of cooling water is the main problem in all other generating sources of electrical energy but these plants require very less quantity of water. Nowadays, radiator type fan cooled diesel generating sets are available in the market, which needs too less quantity of cooling water. Although we have used the spray pond for cooling purpose. In-spite of several advantages the diesel generating sets cannot be used as continuous source of electrical energy. Because the high-speed diesel is used as fuel in internal combustion engine (I. C. Engine), which is costlier than the fuel used by other generating sources and hence, there is increase in cost of generation. Although the working of a diesel generating set will be slightly differs depending upon the KVA rating and method of starting the diesel engine. However, the working of a diesel generating set in general is explained here.

Working: When the engine is started, the alternator starts moving. The magnetic field coils are mounted on rotor, which changes its position as the rotor moves, and three-phase winding is placed on the stator. So, three-phase supply will generate in the stator winding. Initially the supply will be generated due to residual magnetism of the pole shoes. For continuous and constant voltage generation, the pole shoe requires fixed quantity of external excitation for producing alternate north and south poles. So, the initially generated a.c. supply is converted into d.c. supply by metal rectifiers and feedback to armature through slip rings and carbon brush assembly.The change over switch should be operated before starting the engine. When full voltage builds up, the air circuit breaker is switched on. Different parts of a diesel generating set
 

i. Engine

Engine is the most important part of a diesel generating set and which acts as a prime mover. The type of engine used is internal combustion engine (I.C. Enginer) and works on two stroke or four stroke cycle principle. High-speed diesel is used as fuel. The capacity of engine depends upon the K.V.A. rating of alternator and it should be capable of taking over loads for at least two hours, keeping the temperature with in safe limit. Generally high-speed diesel engines are preferred because the over all generation cost is reduced due to reduction in cost of engine and alternator, etc. Multi cylinder engines are also preferred to reduce the noise level and vibrations produced in the engine while working.

ii. Alternator

Alternator also known as “A.C. Generator” operates on the same principle of Faraday’s law of electro magnetic induction as d.c. generator. In these a. c. generators salient poles are keyed on rotor and field windings are mounted over it to produce permanent magnetic field with alternate north or south. The stator also consists of circular silicon stampings, having slots on its inner periphery for accommodating the stator winding may be single phase or three phase depending upon requirement. Slip rings are mounted on the rotor shaft to feed d.c voltage for excitation to produce magnetic field. This excitation may be supplied from small d.c. generator coupled to the same shaft or from metal rectifier. The alternator is used to generate a.c. voltage.

iii. Air Circuit Breaker and Bus Bar

The air circuit breaker is used to switch on or off the main supply generated by the alternator. The circuit breaker should be switched on when the alternator builds up full voltage. When it is switched on, the supply will energise the bus bar. A busbar is a bundle of conductors or thick copper strips, used to distribute the generated voltage with the help of change over switch. The change over switch is used to select the source of generation i.e. either generated or external source of supply.

iv. Engine Cooling System

This includes jacket water tank, used to make up the quantity of cooling water, jacket water pump for forced circulation of cooling water. Heat exchanger to exchange the heat of cooling water with circulating water, circulating water pump to spray the water in the spray pond and spray pond to cool down the temperature of circulating water.

v. Engine Fuel System

This system includes diesel tank to store the fuel and fuel transfer pump to transfer the fuel from fuel tank to engine. The strainer is used to restrict the flow of highly viscous liquid; fuel filter is used to filter the fuel and heater to main viscosity of fuel especially in winter season within limit.

vi. Engine intake and exhaust system

To ensure fresh and dust free air for the combustion of fuel, air filters are provided which may be cleaned by air pressure or replaced depending upon the conditions.To exhaust the burn out gases from the engine, exhaust pipe is used and muffler is used to reduce the noise level of exhaust gases. The exhaust pipe should be discharge to the atmosphere outside the room.

vii. Engine starting system

Depending upon the rated capacity of diesel generating sets, different types of starting systems are used. Small capacity D.G. sets (5KVA) are started manually with handle and above 5 KVA are started with 24 volts D.C motors, which are generally know as “Self-starter”. For this purpose higher A.H. capacity batteries are used to supply the d.c. voltage and are recharged with dynamo coupled with same shaft of alternator. Big capacity diesel generating sets also uses compressed air motor to start.

Direct On Line and Star Delta Starter

We will come to know about the circuit diagram, main parts,working and necessity, of “Direct On Line (D.O.L.) and Star Delta starters”.As the name indicates “Starter” is used to start and stop the motor. The starter is also used to protect the motor and other connected equipment from sustained overload, under voltage, single phasing, etc., and also provide automatic control whenever required. Now the question arises why we need starter for the safe operation of the motor. As already explained that the motor acts as a transformer with secondary winding (Rotor winding) short-circuited. When the motor is started from standstill position, high starting current will circulate in the rotor winding and simultaneously the stator will draw heavy current from the supply mains. As a result of this heavy current, the system or line voltage will be drastically reduced, which is objectionable from safety point of view. The other sensitive equipments connected with same line may trip on low voltage. Therefore large capacity motors(Above 5 H.P.) should be connected through such a mechanism, which can reduce the starting voltage. If the starting voltage is reduced, the starting current drawn by the motor will be reduced automatically and thus the line voltage will remains almost constant.

Several types of starters are available in the market depending upon the type and rating of the motor. For example, Direct On Line (D.O.L.) starter is used up-to 5 H.P. motors; Star Delta starter is used from 7.5 to 20 H.P. motors and Auto- transformer/Resistance types of starter is used above 20 H.P. rating motors. Except D.O.L. starter, these starters are used to reduce the starting voltage and hence to reduce the starting current. Direct on line (D.O.L.) and Star Delta starters are discussed below.

i. Direct On Line Starter (D.O.L.)

As the name indicates “Direct On Line Starter” a device used to connect a motor directly on line voltage. As already discussed these starters are used up to 5 H.P. capacity motors as full load current of these motors is less and hence lesser starting current. Figure given below shows the internal connections and main parts.

a) No-Volt-coil (N.V.C.) : This coil is connected across two phases in series with interlock switch, overload relay, red button (off switch), green button (on switch) etc. Green button is connected in parallel with interlock switch. When green button is pressed for a few seconds the N.V.C. will magnetized and pulls the plunger down ward against the spring tension. Four copper strips will make the contacts with lower contact points and hence the motor will continue to work even after releasing the green button. When the red button is pressed for a second the N.V.C. will demagnetised and the plunger will go back to its original position. So, the motor will switch off.

b) Over Load Relay: This relay is used to protect the motor from overload which may be due to actual overload, defective bearings and single phasing etc. Thick heating strips are connected in series with the motor. A bimetallic strip is also fixed near these strips. When higher current (more than normal) starts flowing through these heating strips, bimetallic strip will start bending due to heating and opens the over load switch contacts. Hence the electric supply to N.V.C. will interrupt and motor will stop.

c) On Switch (Green button): This switch is used to switch on or start the motor and is normally open (Push to on) type  When this switch is pressed, N.V.C. will magnetized and motor will start running.

d) Off Switch (Red button): This switch is used to stop the motor and is normally close (Push to off) type as shown in the figure above. When this switch is pressed for a second, electric supply to N.V.C. will interrupt and motor will stop running.

ii. Star Delta Starter
These types of starters are used for motors having rated capacities from 7.5 H.P. to 20 H.P. Like direct on line starter it has also off switch, over load relay, no volt coil etc.
 

Star delta starter is used to reduce the starting current drawn by the motor when it is switched on. When the handle is pressed to start position (Star), the motor’s out put terminals A 2 , B 2 and C 2 gets short circuited with a thick copper strip on the handle while the other terminals A 1 , B 1 and C 1 gets connected across the supply. The applied voltage across the motor gets reduced to 58 % of the full voltage and hence the starting current drawn by the motor will also reduce to 58 % of the total starting current drawn by the motor if full voltage is applied. Although the starting torque will also reduced to same proportions. When the motor picks up more than 75 % of the rated speed, the handle is switched over to run position(Delta). Now the motor terminals C 1 A 2 , A 1 B 2 , and B 1 C 2 will get short circuit and the motor will get the full voltage. So, the motor will take full load current and develops full torque. In delta position handle is magnetically picked up and motor will continue to run on full load. An off switch is connected in series with overload relay and no volt coil. This switch is used to switch off the motor. Over load relaymmis used to switch off the motor by disconnecting the supply to the no volt coil.

Single Phase Induction Motors

We will study the different types of single-phase squirrel cage induction motors, working principle and their different parts. Single-phase motors also works on the same principle on which the polyphase (Two or three phase) induction motor works, i.e. “whenever a short circuited conductor or coil is placed in a rotating magnetic field, the conductor tends to move.” In construction, these motors are similar to polyphase induction motors with the exception that the stator has single-phase winding 

The magnetic field produced by the stator current is fixed in space instead of rotating, but its magnitude is changing sinusoidally. Such a field is equivalent to two fields of equal magnitude rotating in opposite directions at equal speed, each being half of the maximum value of the alternating field. So the single-phase motors are not self-start in the first instance. However, if the rotor of such motor is given a rotation in any direction by switching on the supply to the stator, a starting torque will be produced and motor pick up full speed in the direction in which the rotation was given.

As discussed above, these motors are not self-start. Splitting the stator field into two fields having an angle between them can solve this problem. The phase splitting up can be achieved by either providing an auxiliary winding in addition to main windings or dividing the stator field into two fields. Provision of an auxiliary or starting winding in addition to the main or running winding is easier and is spaced 90 0 electrical apart. Since the phase difference between the starting and running winding is large and hence behaves like a two-phase winding. These windings are connected in parallel and across the supply. So, according to the starting devices, the single-phase motors may be split phase motors, repulsion types motors and universal motors. And according to construction, these motors may be induction type motor, repulsion type motors and A.C. series type motors etc. Here we will discuss the different types of single-phase induction motors only, which are:

i. Split Phase Induction Motors

As the name indicates, the stator is temporarily converted into two-phase stator by providing an extra winding i.e. starting or auxiliary winding for the starting purpose only. The main winding has low resistance and high reactance whereas the starting winding has high resistance and low reactance, in order to produce phase difference between the magnetic field produced by these windings. Sometimes an extra resistance is added in series with the starting winding to increase the starting torque. These windings are connected in parallel and across the supply

When the supply is switched on, two magnetic fields will be produced having some phase difference and hence the combined effect of these magnetic fields will produce a starting torque in the rotor. When the motor attains 75 to 80 percent of full load speed, the centrifugal switch cut off starting winding from the main supply. This switch is mounted on the shaft of the motor and connected in series with the starting winding. In hermetically sealed units, the centrifugal switch is replaced by an electromagnetic relay. The relay is connected in series with the main winding and contact pair of the switch are connected in series with starting or auxiliary winding, which are normally open (NO type). When the supply is switched on, the main winding will draw excessively higher current and the relay contacts are closed. So, the motor starts working as split phase motor. The starting winding will be cut off from the supply when the motor attains more than 80 percent of the full load speed. Because the current drawn by the main winding will be reduced to too low enough to cause the electromagnetic relay’s contact open and the motor will continue to run on single winding as single phase induction motor. Generally these are available in fractional H.P. Reversing the direction of current either in starting or running winding, the direction of rotation can be reversed.

ii. Capacitor Start Induction-run Motors

Construction wise these motors are almost similar to split phase induction motors except the starting resistance is replaced by a static capacitor to produce starting torque

A centrifugal switch is connected in series with starting winding. As already explained,its function is to connect the starting winding across the supply and running winding,for producing starting torque and cut off the same when the motor picks up 75 % of the full load speed. If this winding remained in circuit it will damage due to heating. Because this winding is made of lesser number of turns of very fine copper or aluminium wire as compared to main winding. Starting and running procedure is same as split phase induction motor. In these motors also, reversing the direction of current either in starting or running winding the direction of rotation can be reversed. Generally all the small capacity motors up to 3 H.P. are widely used in domestic and industrial applications.

iii. Capacitor-start Capacitor-run Motor
This motor is also known as permanent capacitor motor. In these motors, the centrifugal switch is removed and the starting winding is so designed that it can work permanently without overheating .The resistance, inductive reactance and size of winding wires of both the windings are not exactly the same but it is almost same. So, the starting winding will remain in the circuit permanently without overheating and drawing excessive current from the supply. The overall starting and running torque is increased because the capacitor remains permanently in the circuit and hence the name.

iv. Shaded Pole Induction Motor

In such motors the rotor is same as in all the previously discussed induction motor except the stator has salient poles to accommodate the winding. The stator of the motor has four poles. The field coils connected in series for alternate polarity (North and South Pole). These poles are made of laminated silicon stampings. These stampings are clamped together to form stator poles. Approximately 1/3 rd portion of the pole is shaded to accommodate a thick copper ring, which acts as auxiliary or starting winding to produce starting torque.This thick copper ring is known as copper shading ring. When the supply is switched on, the alternating magnetic field is produced in each pole. This alternating flux will divide into two-portion i.e. shaded portion flux and un-shaded portion flux. The shaded portion flux will link with the shading ring and an electro-motive-force will be induced in the shading ring. This induced e.m.f. will circulate its eddy currents in the shading rings and opposes the shaded portion flux and hence the phase difference between the two fields will be produced. The combined effect of these fields will produce starting torque in the rotor and the motor will continue to run.These motors are available in very low capacity varying from 3 to 125 watts. The use of this type of motor is restricted due to poor starting as well as running torque, very less overload capacity and poor efficiency, etc. So, generally these motors are used for small fans, toys, ventilators, hair drier and small instruments, etc.

Three Phase Induction Motors

“A machine which converts three-phase electrical energy into mechanical energy based on principle of electro-magnetic induction is known as three-phase induction motor”. These motors are quite different from D.C. motors,because the three-phase electric supply is given to its stator only. There is no electrical connection to the rotor from the main supply and also no other source of supply is required to run the induction motors. Whereas in case of D.C. motor, both the stators as well rotors (Armature) are connected across the supply to run themotor. These motors are mostly used in factories for different applications such as for lathe, drill, blower, printing machine, etc. The speed of an induction motors is almost constant but slightly falls with the increase in load. The starting current is 5 times higher than the full load current. Interchanging any two-phase connection to the motor or starter can reverse the direction of rotation of these motors. The working principles and different parts of induction motors are given below.

i. Working Principle

When stator winding of a three-phase induction motor is connected across the three phase electric supply, a rotating magnetic field will be produced. This rotating magnetic field, when links with the short-circuited rotor bars, an electro-motive-force (E.M.F.) will be induced in the rotor bars. This induced electro-motive-force will circulate a heavy eddy current in the rotor bars and produced its own magnetic field. By Lenz’s law this induced current will oppose the cause producing it i.e. relative motion of the stator field. The combined interaction of both the magnetic fields (Stator and Rotor) will produce a net resultant moving force (Torque) in the rotor. As the current in the rotor bars is set up entirely by effect of electromagnetic induction from the stator, hence the name “Induction Motor”.

a) Stator: As the name indicates, it is the stationary part of the induction motor and made of silicon steel strips of thickness, varying from 0.3 to 1.35 mm.These strips are combined together, which are called laminated strips and the combination is known as laminated core. These laminated stampings/strips are slotted to receive the winding. These slots may be of open or semi-closed types, to facilitate the winding. The core and end covers are fitted in the cast iron frame, so that external mechanical injury may not take place to stator winding.

The same stator can be used for single-phase induction motors. The stator carries three-phase winding and is fed from three-phase electric supply. The stator is wound for a definite number of poles and depends upon the speed of motor. Greater the number of poles lesser will be the rotor speed and vice-versa. This speed can be calculated by the formulae, N s = 120f/p. Where N s, is the synchronous speed of the stator field, f, is the supply frequency, which is normally 50 cycles per second, and P is the number of poles. The synchronous speed for a 2-pole motor is 3000 r.p.m. and for a 4-pole motor it is 1500 r.p.m.Generally the speed of rotor is slightly less (up-to 5% of synchronous speed) than the speed of stator field. The difference between the speed of stator field and rotor is known as slip and it should be as low as possible.

b) Rotor: The rotor is the rotating part of the motor and made of silicon steel strips. The thickness of these strips varies from 0.3 to 1.35 mm, as in case of stator. These strips are clamped together to form rotor core, called as laminated core. This laminated core is slotted to totally closed type, to receive rotor winding. In large capacity motors thick aluminium bars are inserted and are short-circuited with end rings. Nowadays, melted aluminium is filled in these slots, which works as short-circuited winding. This winding is known as squirrel cage winding. As we know plain induction motor is having poor starting torque and draws heavy starting current. So, such types of problems can be over come by modifying the rotor slightly by providing double cage in the rotor

The rotor has been provided with double slots to accommodate two rotor windings. A low resistance copper bars are deeply embedded in the slots and brass or aluminium bars are inserted near the surface. This type of winding is known as double cage winding. The whole assembly is keyed on the shaft of the motor.

c) End Covers: As the name indicates these covers are used to cover the ends of the motor and are made of cast iron. These end covers are fitted with the stator frame with the help of nuts and bolts. The ball bearings are fitted in the end covers to keep the rotor exactly in the centre of the stator, so that it can move freely.

d) Shaft and Bearings: Mostly ball bearings are used in large capacity motors where as the bush bearings is used in small capacity motors because the noise level is high in ball bearing as compared to bush bearings. The main purpose of bearings is to keep the rotor exactly in centre and ensure free movement for the rotor. The shaft is a long circular bar, made of mild steel. The rotor assembly and cooling fan is securely keyed to the shaft of the motor.

e ) Fan: In general, the fan is used to cool down the temperature. When the motor runs on load, the heat is produced in the motor winding as well as in the core due to copper and iron losses respectively. So, the fan serves the purpose of transferring the heat from inside to outside of the motor by forced air circulation.It sucks the air from the atmosphere through the air ducts and discharge back to atmosphere after cooling the winding and core of the motor. In large capacity motors these fans may be fitted on the rotor shaft near the winding whereas in small capacity motors these may be fitted outside the end covers. A mild steel or cast iron cover is used to cover the fan to avoid chances of accidents.

A.C Motors Starters and D.G. set

We will study about internal construction, and working principles of single-phase and three-phase squirrel cage induction motors. The single-phase motors are not self-start. How a single-phase motor can be made self-start will be discussed in detail. Single phase induction motors such as split phase, capacitor start induction run, capacitor start capacitor run and shaded pole, etc. also will be discussed in detail along with figures. We will also have an exposure about direct on line (DOL) and star delta starters used to start and reverse the direction of rotation of squirrel cage induction motors. We know that in industries certain equipment need to be run continuously. In case of emergency breakdown of electric supply or power cut off, the production is seriously effected. So, the only and instant power of source, which could serve the purpose, is “Diesel Generator set”. This alternate source of power will be discussed in detail in this current unit.

Layout of Wiring System

We will study how the electricity comes to your house, offices and factories, starting from nearby service pole. Generally, the houses, offices and shops, etc., are connected with single-phase supply, if the total connected load is less than 5 kilowatt. If the load is more than 5 kilowatt, the three-phase four-wiresystem is recommended. In factories and plants, we need to connect lighting as well as three phase induction motors, hence three-phase four-wire system is used.

i. Domestic wiring diagram
A four-core weatherproof, 660 volts, aluminium cable is used to connect the three phase four wires (Three phase and a neutral wire) energy meter, from the service pole. This cable is supported by an 8 S.W.G. galvanised steel wire, from service pole to consumer premises

G.I. pipe is used to enclose the service cable. This pipe is fixed on the wall, with the help of saddles, gutties and wooden screws and it should be so selected that the weatherproof cable is easily pulled into the pipe without damage to the insulation of the cable. The service cable is made to enter the building through the pipe and connects the main board for connecting the three-phase energy meter. The output of the energy meter is controlled through the I.C.T.P. main switch. The phase wires are connected the fuse links whereas the neutral wire is directly connected to the neutral link. The output of this I.C.T.P. main switch (one phase and neutral) is distributed through the single-phase I.C.D.P. main switches, which acts as main control switch for a room or for a power point.

ii. Factory wiring diagram

The wiring diagram for a factory or for a dairy plant is almost similar to three-phase four wire domestic wiring diagram, up-to I.C.T.P. main switch. The only difference is the output of this I.C.T.P. main switch is connected to different small capacity I.C.T.P. main switches . Similar to the previous case, a four-core weatherproof, 660 volts, aluminium cable is used to connect the three-phase four wires (Three phase and a neutral wire) energy meter, from the service pole. The size of this cable depends upon the load of the entire factory. An 8 S.W.G. galvanised steel wire, from service pole to consumer premises supports this cable. G.I. pipe is used to enclose the service cable. This pipe fixed on the wall, with the help of saddles, gutties and wooden screws and it should be so selected that the weatherproof cable is easily pulled into the pipe without damage to the insulation of the cable. The service cable is made to enter the building through the pipe and connects the main board for energizing the three-phase energy meter. The output of energy meter is controlled through the I.C.T.P. main switch. The phase wires are connected to the fuse links whereas the neutral wire is directly connected to the neutral link. The output of this I.C.T.P. main switch (Three phase and neutral) is distributed through small capacity I.C.T.P. main switches, which acts as main control switch for a section

Domestic Wiring System

In this sub-unit, we will come to know the various important factors, which are to be taken into considerations, while selecting type of wiring system to be used.These factors are durability, safety, appearance, cost, accessibility and maintenance cost, etc. Apart from above, the environmental conditions, such as temperature,humidity, indoor or outdoor, direct sunlight and corrosive acid fumes, etc., are also be taken into consideration. The following are the types of internal wiring usually employed in houses and industries.

i. Cleat Wiring

The wooden plugs (gutties) are fixed in the wall at regular interval of 30 - 60 cm apart. This distance should not increase in any case; otherwise there will be risk of wire/cable touching the wall. The cables/wires are supported and gripped between the porcelain cleat approximately 10 mm above the wall. Cleats are made in two halves, the base, which is grooved to accommodate the wire and the cap is put over it


ii. Wooden Casing Capping Wiring

The casing is the base unit in which U shaped grooves are made usually two in number, into which wires are laid in such a way that the wires of opposite polarity are laid in different groove. The second part is “capping” which is a rectangular strip of same size as casing

Usually these are made from seasoned teakwood free from knots and other defects. All the sides of casing and capping should be well varnished with a good quality varnish. The size of the casing capping to be used depends upon the number and size of the wires to be accommodated. Nowadays, wooden casing capping are being replaced very fast by plastic made casing capping (Channel), as the cost of wood is too high as compared to plastic. Secondly the outer look is very good and does not require any maintenance.

iii. Tough Rubber Sheathed Wiring (TRS or CTS) or Batten Wiring

This wiring also known as batten wiring system, in which Tough Rubber sheathed or Cab Type sheathed wires are used. The batten is prepared from seasoned teakwood. The minimum thickness of batten should not be less than 13 mm. In domestic wiring normally used width is 13, 19, 25, 31, 44 and 50 mm and is suitable for 2, 3, 4, 5, 6 and 7 wires, respectively.Joint link clips are fixed on batten with the help of nails. Wooden gutties are fixed in the walls at an interval of 75 cm. The batten is fixed on wall with the help of wooden screws and gutties. The wires are fixed on batten by means of joint link clips, already fixed on batten. This method is mostly used for all ordinary residential and non-residential installation, being cheap in cost, quick installation, fault finding and repair, etc. But it has a great disadvantage that this wiring is unsuitable where it is exposed to direct sunlight and rainfall. It is suitable where acidic and alkalis firms are present.

iv. Lead Sheathed or metal wiring
This system of wiring is also similar to that of T.R.S or C.T.S sheathed wiring system as in previous case. Here the wires are replaced by lead sheathed or metal sheathed wires/cables. Rest of the procedure such as fixing of batten on wall, fixing of lead sheathed wire in joint clips, etc is also the same. As these wires are covered, with extra coating of lead, not exceeding 15-mm thickness, this type of wiring is suitable on places exposed to direct sunlight, rain and damp situation. This type of wiring gives fairly good-looking and comparatively longer life.

v. Conduit Wiring
As the name indicates “Conduit Wiring” mean the wires are enclosed in steel pipe known as “ Conduit.” When this conduit is coated with enamel is known as black conduit and when galvanized it is known as galvanised conduit, to protect the pipe from corrosion and insulation damage to wires. There are three types of conduit wiring mostly in use.

a) Concealed Wiring: In this wiring system, the conduits are embedded into the walls and ceiling, with the plaster at the time of construction. These conduits should be mechanically as well as electrically continuous and connected to earth wire at some suitable places. The P.V.C wires are drawn into the conduit, by means of steel wire not less than 18 S.W.G. Number of inspection boxes are provided, in order to facilitate the drawing of wires in the conduits. Nowadays,P.V.C. conduit pipe is also available in the market, which do not require any threading for jointing the pipe. The joints are made with special solution. This type of wiring is not suitable for damp situation.

b) Surface Conduit Wiring: In this wiring the conduits are fixed on the surface of wall and ceiling with the help of conduit saddles, wooden screw and gutties.In damp situation, the conduit can be spaced from walls by means of wooden spacers, below the conduit along its length at regular intervals. The wires are drawn in the conduit with the help of steel wire. Junction boxes are provided to facilitate the drawing of wires in the conduit. This system of wiring is commonly used for industrial wiring, especially in three-phase wiring. As the,fault finding and repair is quick, easy in wiring, cheap in cost but not suitable for domestic wiring, as it does not give fairly good outlook.

c) Flexible Conduit Wiring: This wiring is carried out to provide mechanical protection to cable between conduit and machine. Generally this wiring is not used for wiring purposes. It is generally used for connecting conduit with machine terminal box in case of motor wiring, energy meter and main switches.

Electrical Wiring Accessories

After studying the different electrician tools, we will study about the different electrical accessories, which are being used in domestic as well as commercial wiring. Although there are hundreds of electrical accessories available in the market but some important electrical accessories

i. Switch

A switch is used to make or break the electric circuit. Or it is used to switch “ON or OFF” the electric supply. When we switch off the electric supply, the arc is produced between the switch blades and contacts terminals. This arc may burn or damages the contacts of the switch. Formation of this arc should be avoided.Usually, this arc is avoided by means of providing a spring action movement to these switching blades to have a quick switching on or off the supply. In this way,the chances of formation of arc are drastically reduced depending upon the circuit load. The switches may be one-way switch and two-way switch. The one-way switch is used to control single circuit or lamp, where as the two-way switch isused to divert the flow of current to either of two directions. The two-way switch can also be used to control one lamp from two different places (As in case of stair case wiring). The different types of switches are available in the market, depending upon the requirement, few important are detailed below

a) Surface Switch: The switches are mounted on wooden board, fixed over the surface of the wall. These switches are also known as “Tumbler switches”.

b) Flush/Piano type switch: The switches are used where good appearance is required. The switches are fixed in flush with the wall and do not project out.These switches are also known as “Piano Type Switches”.

c) Bed switch: As the name indicates, it is used to switch “ON or OFF” the light from the place, other than switch-board or from near the bed, while going to sleep or getting up. This switch is connected through flexible wire.

d) Rotary switch: This switch is used to control different lamps from one places one by one or as selector switch, to select different voltage tapping of transformer in voltage stabilizer.

e ) Push Button switch: These switches are used to control the electric bell and indicating lamps etc. When the push button is pressed, the circuit is completed and the bell or lamp is switched on. The supply to bell or lamp is switched off as the push button is released.

f) Pull switch: These switches are operated with a single pull of the cord, for the on and off position. These are also used in bedroom and bathroom. These switches are fixed near the ceiling and hence these are also known as “Ceiling Switches”.

g) Intermediate switch: This switch has four terminals and four different connection position. The main function of this switch is to control a lamp from three or more different places, along with ordinary two-way switch. Generally this switch is used in double stair case wiring or corridor wiring. This switch is also known as four-way switch.

h) Lamp Holder: As the name indicates, a lamp holder is used to hold the lamp,and connect it electrically to supply terminals, required for lighting purposes.The lamp holders may be bayonet cap and screw type. Depending upon our requirement, these lamp holders are also manufactured in different designs.

Some important types of lamp holders are detailed below:-

ii. Lamp Holder

a) Batten Lamp Holder: These lamp holders may be of brass or bakelite, with brass plunger. The holders are fixed on either on round block or wooden board with the help of wooden screws.

b) Pendant Lamp Holder: This lamp holder is used to hang the lamp from ceiling rose, with flexible wire. Some time these holders are provided with lampshades,to divert the upward light to down ward. These may be of brass or bakelite,with brass plunger.

c) Angle Lamp Holder: The angle holder is used to focus the light at an angle and is fixed directly on wall or round block with the help of gutties and wooden screws. These may be of brass or bakelite with brass plunger.

d) Swivel Lamp Holder: These lamp holders are used for lighting of shop windows,show case etc. It consists of ball and socket joint fitted between back plate and lamp holder, for the purpose to move the light to a wide angle.

e ) Bracket Lamp Holder: These lamp holders are used to focus the light on the floor or at some angle, slightly away from the walls. Light shades can also be used for diverting all light on floor. Such lamps are provided with such fittings which make them water tight so that these can be used out side the houses or for street lighting. The bracket may be of wood, aluminium and brass. The lamp holder is simply a pendant holder made of brass or bakelite.

iii. Fluorescent Lamp (Tube) Holder

These are used to hold the fluorescent tube and pin type holders are generally used.

iv. Ceiling Rose

These are fixed on walls near the ceiling. Normally these are used to provide tapping to the pendent lamp holder, ceiling fan and fluorescent tube etc through the flexible wire.

v. Socket

The sockets have insulated base with moulded base having three terminal sleeves.The two terminal sleeves having same cross-section are used to connect phase and neural wire where as the third having greater cross-section is used to connect with earth wire. These are made for 5 amps and 15 amps load. Two-pin sockets are also available, in which only phase and neutral wires are connected. These are available in 5-amp capacity. These may be surface type or piano-types. The surface type accessories are obsolete now days.

vi. Plug

The plugs are also having moulded three pins of brass or any electrically conducting material. These are also made of 5 amp and 15-amp rating. These are used for taking power from socket.

vii. Main Switch

As the name indicates that this switch is used to switch “on or off” the main supply. In other words these switches are used to control the whole supply for a house, office and machine. In single-phase circuit I.C.D.P. main switches are used,whereas in three-phase circuits I.C.T.P. main switches are used to control the supply. The main switches are of the following type:

a) I.C.D.P. Switch: Iron Clad Double Pole main switch is used in single-phase supply circuits. These are available in 15 Amp, 30 Amp, 60 Amps and 100 amperes current rating. In these switches, either two numbers of fuse links are provided or a fuse link and a neutral link is provided. The neutral wire is directly connected with the neutral link and phase wire is connected with the fuse link.Normally 15 Amp current rating main switches are made of plastic moulding instead of iron.

b) I.C.T.P Switch: Iron Clad Triple Pole main switches are used to control the three phase supply circuit. These are available in 15 Amp, 30 Amp, 60 Amp,100 Amp, 150 Amp and 250 Amps current rating. Generally these switches are also known as 3 phase 4 wires main switches. In these switches, three fuses and a neutral link is available. The neutral wire is directly connected with the neutral link and phase wires are connected with these fuse links.

viii. Fuse

It is the most common and important type of safety device used for domestic and commercial installations. These fuses are of kit-Kat type and are also known as cut-out. These cut-outs are made of porcelain in current rating of 15 to 300 amperes. The material used as a fuse wire is tin, lead, silver, antimony, copper and aluminium etc. Copper or lead, tin alloy is mostly used in ordinary fuse wire.

ix. Miniature Circuit Breaker (M.C.B.)

A miniature circuit breaker is an electro-mechanical safety device which operate and disconnect the circuit supply, when the current drawn from the circuit increases to a pre-determined value. These are used in lieu of fuses and can be fitted in consumer’s distribution board. An MCB will normally operates at 1.25 times itsrated current value. It can be reset, by simply lifting its operational knob, where as the fuse has to replace its fuse wire. Hence the miniature circuit breaker are replacing the rewirable fuses now a days.

Electrician Tools and their Handling

Now, we will study the different types of tools, which are required for the repair and maintenance work at our home, office, plant, etc. These tools should be handled carefully, as mishandling of tool may be the cause of serious injury to the person working with these tools. The proper size of the tool should be used for aparticular type of job. So, we should be familiar with the different type of tools, their size availability, use and precautions to be taken while using. The electrician tools are generally made of iron and these should be provided with a lubricant layer, so that in rainy season or humid atmosphere, these should not get corroded. The various electrician tools, which are required for the repair and maintenance of any electrical installation along with precautions to be observed and their use are given below.

i. Different Electrician Tools and their Use

Different electrician tools, which are necessary for the repair and maintenance of electrical installations, are detailed below along with figure.

Different Electrical Tools

ii. Precautions while Handling the Electrician Tools

Great caution and care is required in handling the electrician’s tools during working and handling. A worker is liable to injure himself in addition to the damage caused to the tools, if he at any time is slacked in handling them. The following are the safety precautions, which should be observed while handling these tools.

1. The sharp edged tools, such as pocketknife, chisels etc., should never be put in pocket without shield. While working with such tools, care should be taken not to place hand or finger in the path of motion of the cutting tools. Also, care should be taken not to hand over the sharp edge tool with its sharp edge side.

2. When cutting with a chisel, always cut away from yourself, rather than towards yourself.

3. Before using hammer, its handle must be examined carefully, whether it is properly secured or not. It should not be oily or greasy.

4. When marking a cut with hacksaw, the blade must be guided with a first finger and thumb of one hand, otherwise the blade is liable to break which may cause serious injuries.

5. After using tools, they should never be left at the top of the ladder or any other high level places since they may fall accidentally and can cause injury.

6. Only a suitable tool should be used for the proper purpose. If a particular tool does not suit the requirement, do not try to use the tool. Always go for a proper size tool matching with the requirement.

7. All injuries must be attended to immediately, since delay may cause infection.