Measuring Instruments_domastic wiring

7/29/2019 Measuring Instruments_domastic wiring

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CHAPTER 3

MEASURING INSTRUMENTS

3.0 Introduction:The measurement of a given quantity is the result of comparison between the

quantity to be measured and a definite standard. The instruments which are used for such

measurements are called Measuring Instruments.

The necessary requirements for any measuring instrument are:

1. The circuit conditions should not be altered with the introduction of the

measuring instrument.2. The power consumed by the instruments for their operation should be as

small as possible.

The instrument used for measuring current flow is called Ammeter, and that for the

measurement of voltage across any two points is called Voltmeter. The instruments usedfor the measurement of power are called Power meters or Wattmeters.

Classification of Measuring Instruments:

1. Indicating instruments.

2. Recording instruments.

3. Integrating instruments.

Essential Requirements of an Instrument:

For satisfactory operation of any indicating instrument, the following systems mustbe present in an instrument.

1. Deflecting system producing deflecting torque Td.2. Controlling system producing controlling torque Tc.3. Damping system producing damping torque.

Deflecting System:

The force or torque, which is proportional to the quantity to be measured, deflects the

pointer. The system, which produces such a deflecting torque, is called Deflecting

System. The deflection system uses one of the following effects produced by current orvoltage, to produce deflecting torque.

1. Magnetic effect

2. Thermal effect

3. Electrostatic effects4. Induction effects

5. Hall effect

Controlling System:

The functions of this system are:1. It produces a force equal and opposite to the deflecting force in order tomake

the deflection of pointer at a definite magnitude.


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2. It brings the moving system back to zero position when the force whichcauses the movement of the moving system is removed.

The types of control are

1. Gravity control2. Spring control

Gravity Control:

A weight, called Control weight, is used to produce the controlling torque duetogravity. It can be shown that the deflection is proportional to current i.e. quantity to be

measured.

I sin ()But as it is a function of sin (), the scale for the instrument using gravity control is not

uniform.

Advantages:

1. Its performance is not time dependent.

2. It is simple and cheap.3. The controlling torque can be varied by adjusting the position of the control

weight.

4. Its performance is not temperature dependent.

Disadvantages:

1. The scale is non-uniform, causing problems to record accurate readings.

2. The system must be used in a vertical position only and must be properly leveled.3. As delicate and proper leveling is required, in general, it is not used forindicating

instruments and portable instruments.


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Spring Control:

Two hairsprings are attached to the moving system, which exerts controllingtorque. To employ spring control to an instrument, the following requirements are

essential. The spring should be non-magnetic. The spring should be free from mechanical

stress. The spring should have a small resistance, sufficient cross-sectional area. It should

have low resistance temperature co-efficient.It can be shown that the deflection produced is proportional to the current.

i.e. I

The spring control is very popular and is used in almost all indicatinginstruments.

Damping System:

The deflecting torque provides some deflection and controlling torque acts in theopposite direction to that of deflecting torque. So, before coming to rest, the pointer

always oscillates due to inertia, about the equilibrium position. When the pointer attains

the final position, the reading can be taken. Thus, to bring the pointer to rest within ashort time, damping system is required.

If the moving system reaches its final position rapidly but smoothly without

oscillations, the instrument is said to be critically damped. If the moving system oscillatesabout the final position with decreasing amplitude and then settles to its final position,the

instrument is said to be under damped. If the moving system moves slowly to its finalsteady position, then the instrument is said to be over damped.

The following methods are used for producing the damping torque.1. Air friction damping

2. Fluid friction damping

3. Eddy current damping


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Air friction damping

Fluid friction damping

Eddy current damping

Permanent Magnet Moving Coil (PMMC) Instruments:

Construction:

The permanent magnet moving coil instruments are most accurate for d.c.

measurements. The action of these instruments is based on the motoring principle. Whena current carrying coil is placed in the magnetic field produced by the permanent magnet,

the coil experiences a force and moves. As the coil is moving and the magnet is


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permanent, the instrument is called permanent magnet moving coil instrument. This basicprinciple is called DArsonval principle. The amount of force experienced by the coil isproportional to the current passing through the coil.

Torque Equation:

Let I = Current through the coil

Ks = Spring constant = deflection of the pointer

PMMC Instrument

The deflection of the pointer is directly proportional to the current passing

through the coil. The deflection o the pointer can be used to measure the current. The

equation is given by,Ks

I =

G

As the direction of current through the coil changes, the direction of thedeflectionof the pointer also changes. Hence such instruments are well suited for d. c.

measurements.

Advantages:

1. It has uniform scale.

2. With a powerful magnet, its torque-to-weight ratio is very high. So,operating current is small.

3. The sensitivity is high.

4. The eddy currents induced in the metallic former over which coil is woundprovides effective damping.

5. It consumes low power. (25 W to 200 W)

6. It has high accuracy.

7. Instrument is free from hysteresis error.8. Extension of instrument range is possible.


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Disadvantages:

1. The cost is very high. This is due to delicate construction accurate

machining.

2. Suitable for d. c. measurements only.3. Aging of permanent magnet and control springs introduces errors.

4. Friction due to jewel-pivot suspension may cause errors.

Extension of Ranges:

Extension of Current Range:

Consider a basic meter having resistance Rm and full-scale deflection current as

Im. The range of this instrument can be extended using a shunt resistance Rsh connected

in parallel with the basic meter.

Basic Ammeter

Let Rm = internal resistance of coilRsh = shunt resistance

Im = Full-scale deflection current required by the meter

I = total current

It can be shown that the value of shunt resistance is given by,Rm

Rsh =

m 1

where, Im =

Im

The shunt resistance may consist of a constant temperature resistance wire within

the case of the meter or it may be external shunt having low resistance.

Extension of Voltage Range

The basic DArsonval meter can be modified to increase its range by adding a

series resistance Rs, called Multiplier. The main function of the multiplier is to limit thecurrent through the basic meter so that the meter current does not exceed its full-scale

deflection value, on high ranges.

Basic Voltmeter


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Let Rm = internal resistance of coil i.e. meterRs = series resistance

Im = full-scale deflection current required by the meter

V = full range voltage to be measuredIt can be shown that the multiplier value is given by,

Rs = Rm (m 1)

Where, m = Multiplying Factor, defined as the ratio of full rangevoltage to be measured and the drop across the basic meter.

Requirements of Shunts:

1. It should have low temperature co-efficient.

2. Its resistance should be stable with time.

3. They should carry current without excessive temperature rise.4. They should have a low thermal electromotive force with copper.

5. Since Manganin satisfies these requirements, it is widely used to construct

shunts.

6. Requirements of Multipliers:7. Their resistance should not change with time.

8. Change in the resistance with temperature should be small.

9. They should be non-inductively wound for a. c. meters.Commonly used materials for construction are Manganin and Constantan.

3.1 Dynamometer type Wattmeter:

It is used for the measurement of DC as well as AC power. These instruments are

based on the principle that mechanical force exists between the current carrying

conductors.

Dynamometer type wattmeter

It consists of a fixed coil, which forms the current coil and a moving coil, which

forms the potential coil of the wattmeter as shown in the figure. Fixed coils are air coredto avoid hysteresis loss when used for measurement of AC power. Fixed coils are

connected in series with the load and carry load current. The moving coil is pivoted on

the spindle between the two fixed coils. A pointer is attached to the spindle between thetwo fixed coils. A pointer is attached to the spindle, which moves over the graduated


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scale. The moving coil is connected across the supply and carries a current, which isproportional to the applied voltage. A high resistance is connected in series with the

moving coil to limit the current flowing through it. Two phosphor bronze hairsprings

provide the controlling torque. Air friction damping is employed.

3.11 Principle:

When a current carrying moving coil is placed in the magnetic field produced by

another current carrying fixed coil, a force is exerted on the coil sides of the moving coildue to which a deflection torque is produced and the moving coil deflects. The

controlling torque produced by the springs controls the deflecting torque. When the

moving coil deflects, the pointer attached to the spindle moves over a graduated scale and

comes to rest at a point where the deflecting torque is equal to the controlling torque.

D.C. Operation:

The deflecting torque produced is given by,Td V I1

Where,Td = Deflection torqueV = Supply voltageI1 = Load current = Current through fixed coil

A.C. Operation:

The deflecting torque is given by V I1 cos ()

where,

= Deflecting torqueV = Supply voltage

I1 = r.m.s. Value of load current

cos () = power factor of load

Electrodynamometer wattmeter

Advantages:

1. It can be used for both DC and AC power measurement.

2. As the coils are air cores, these instruments are free from Hysteresis and

eddy current losses.3. They have a uniform scale.

4. They are accurate and reliable.


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Disadvantages:

1. The instrument has a low sensitivity due to low torque to weight ratio.

Also it introduces increased frictional losses.

2. At low power factors, errors are more.3. The reading of the meter is affected by stray magnetic fields acting on the

moving coil.

4. They are expensive when compared to other types of instruments.

3.2 Induction type single phase energy meter:

Induction type instruments are most commonly used as energy meters, whichmeasure quantity of electricity.

Induction type single phase energy meter

It mainly consists four main parts.

1. A driving system.

2. A moving system.3. A braking system.

4. A recording mechanism.

Driving System:The driving system consists of two electromagnets (1) A series magnet and (2) A

shunt magnet. A series magnet consists of u-shaped laminations of silicon steel .A coilthick wire is wound on it. It is connected in series with the load. This is known as the

current coil. The load current flows through this coil, and hence produces flux 1 which

is in phase with the load current I1. The shunt magnet consists of a number of E-shaped

silicon steel laminations. A coil of thin wire is wound on the central limb of the shuntmagnet. This coil is known as voltage coil or potential coil, and is connected across the

supply. It is excited by a current proportional to the voltage. In order to obtain the

deflecting torque, the current in the pressure coil must lag behind applied voltage by


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90 .In orders to achieve this condition a copper shading band is provided on the central

limb of the shunt magnet.

Moving System:It consists of a light aluminium disc mounted on the spindle. The aluminium disc

is positioned in the air gap between the series magnet and shunt magnet. The spindle is

supported on jewel bearings. The disc rotates under the action of the deflecting torqueproduced due to the resultant magnetic field.

Braking System:A permanent magnet, known as break magnet is positioned near the edge of

aluminium disc. When the aluminum disc rotates under the influence of the magnetic

field produced by the magnet, an e.m.f is induced in the disc, due to which eddy currents

circulate in the disc in such a direction as to produce a torque opposing the rotation of thedisc. Thus a braking torque is exerted on the disc.

Recording Mechanism:

The function of the recording mechanism is to record continuously a number,which is proportional to the number of revolutions made by the disc. Rack and pinion

arrangement is made on the disc for recording mechanism.

3.21 Working Principle:

When the energy meter is connected to the supply and load is applied, the current

coil carries the load current I1, and the flux 1 produced by this current is in phase with

it. The potential coil carries current I2 which is proportional to the applied voltage and

flux 2 produced by this current is in quadrature with the applied voltage. Thus a phase

difference of 90 exist between two fluxes 1 and 2. The resulting flux produced is arotating magnetic field. The rotating field interacts with the disc and produces a driving

torque and hence the disc starts rotating. The number of revolutions made by the disc isproportional to the energy consumed by the load. The spindle is connected to the

recording mechanism, which records the electrical consumed by the load in kWhrs. The

speed of the disc is adjusted by varying the position of the brake magnet. The number of

revolutions in time t is proportional to the energy supplied.

Advantages:

1. Its construction is simple and strong.2. It is cheap in cost.

3. It has high torque-to-weight ratio, so frictional errors are less.4. It is more accurate.5. It requires less maintenance.

6. Its range can be extended using instrument transformers.

Disadvantage:

It is used only for A.C. circuits.


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3.3 Megger:

Megger is an instrument used to measure very high resistance of the order of

mega ohms such as the insulation resistance of cables.

Megger

3.31 Construction:

It consists of a hand driven DC generator and a direct reading ohmmeter. There is

a potential coil PC and a current coil CC. Which are fixed to the spindle which is free torotate between the poles N and S of a permanent magnet. The coils will not exert torque

on the moving system. The current coil is connected in series with a resistance R1

connected between the generator terminal and the supply line L. Resistance R1 limits the

current through current coil and controls the range of the instrument. The potential coil isconnected in series with a compensating coil and protection resistance R2 and across the

generator terminals. A guard ring is provided to shunt leakage current over the test

terminals .The terminal G is known as the guard terminal, which is used to connect guardring to the insulation under test.

3.32 Working principle:

The high resistance, which is required to be measured, is connected between the test

terminals L and G. The generator handle is then steadily rotated at a uniform speed tillthe pointer gives a steady reading. This reading gives the value of resistance. The test

terminals are kept open .Now the resistance across L and G is infinite .The generatorhandle is rotated due to which current flows to the potential coil. The resultant torqueproduced deflects the pointer in the opposite direction and makes the pointer read 0

marking on the scale.

After checking, the high resistance to be measured is connected between testterminals. The generator handle is rotated. Now the torque produced deflects the pointer

on the scale, which gives the value of the resistance that is being measured.


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Instrument Transformers:

In heavy currents and high voltage a.c. applications, the measurement can not be

done by using the method of extension of ranges of low range meters by providing

suitable shunts. In such conditions, specially constructed accurate ratio transformerscalled instrument transformers are used. These can be used, irrespective of the voltage

and current ratings of the a.c. circuits. These transformers not only extend the range of

low range instruments but also isolate them from the high current and high voltage a.c.circuits. This makes their handling very safe. The instrument transformers are classified

as,

Current Transformers (C.T)Potential Transformers (P.T)

Current Transformers:

The large alternating currents which can not be sensed or passed through normal

ammeters and current coils of watt meters, energy meters can easily measured by use ofcurrent transformers along with normal low range instruments.These transformers are

basically step up transformers i.e. stepping up a voltage from primary to secondary.Hence obviously the current gets stepped down from primary to secondary.

Current Transformer

The current and number of turns relationship of C.T. is given by,I1 N1

=

I2 N2It is very important that the secondary of the C.T. should not be kept open. It

should either be shorted or must be connected in series with a low resistance coil such as

current coil of wattmeter, coil of ammeter etc.

Potential Transformers:

The basic principle of these transformers is same as that of Current Transformers.

The high alternating voltages are reduced in a fixed proportion for the measurement with

the help of P.T.


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Potential Transformer

These are extremely accurate ratio step down transformers. The voltages and

number of turns ratio is given by,

V1 N1=

V2 N2Thus, if the voltage ratio of P.T. is known and the voltmeter reading is known,then the high voltage to be measured can be determined.

Advantages:

1. The normal range ammeter and voltmeters can be used along with these

transformers to measure high voltages and currents.2. The rating of low range meter can be fixed irrespective of the value of high

voltage or current to be measured.

3. These transformers isolate the measurement from high voltage and current

circuits. This ensures the safety of the operator and makes the handling of the

equipments very easy and safe.4. These can be used for operating many types of protecting devices such as relays

or pilot lights.5. Single transformer can feed several instruments economically.

Disadvantage:

They can be used only for a.c. circuits.


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Solved Problems:

1. A PMMC instrument has a coil of dimensions 10mm8mm. The flux density inthe air gap is 0.15 Wb/m2. If the coil is wound for 100 turns, carrying a current of 5 mA

then calculate the deflecting torque. Calculate the deflection if the spring constant is

0.210-6 Nm/degree.

Solution:The deflection torque is given by,

Td = NBAI NmNow,

A = area = 108 = 8010-6 m2

Therefore,

Td = 1000.158010-6510-3= 610-6 Nm

Now, Td = Tc = K

Therefore,610-6 = 0.210-6

6 10-6

= = 300.2 10-6

2. A moving iron ammeter deflects through 50 for a current of 2A flowingthrough it. Find the deflection if a current of 1A is flowing through it if the instrument

uses i) Gravity control system and ii) Spring

Control system.Solution:

i)Gravity Control System:

For gravity control system, the controlling torque is proportional to sin () ratherthan . And the deflecting torque is proportional to the square of the current for moving

iron instrument.

Therefore, Tc sin ()And Td I2

At steady state,

Td = Tc

So, I2 sin ()

i.e. I2

= constantsin ()

Therefore,I12 sin (1) = I22 sin (2)

Now, I1 = 2A,

1 = 50

I2 = 1ATherefore,

22 12


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=sin (50) sin (2)

sin (2) = 0.19152 = 11.04 for a current of 1A.

ii)Spring Control System:

In this system, the controlling torque is directly proportional to .Therefore, Td I2

And Tc

At steady state,Td = Tc

Therefore, I2

I2

= constant

Therefore,

I12 I22=

1 2

5012

2 =

22

= 12.5 for a current of 1A.

3. The inductance of a moving iron instrument is given by,L = (12 + 6 2) H

Where is the deflection in radians from zero position. The spring constant is

1210-6 Nm/radian. Calculate the deflection for a current of 8A.

Solution:

The rate of change of inductance with the deflection is,dL d(12 + 6 2)

= = (6 - 2) H / radian

d dFrom the torque equation,

I2 dL

=

2Ks d

= [82(6-2) 10-6] / [21210-6]

0.375 = 6 2

= 2.526 radians = 144.74


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4. While calibrating a single phase energy meter it is observed that disc makes 25

revolutions in 70 seconds. The energy meter constant is 1200 revolutions / kWh. The

supply voltage is 230 V and the load current is 5A. Find,i) true energy

ii) recorded energy

iii) percentage error of energy meterSolution:

The energy meter constant is 1200 revolutions/kWh. So, energy meter will record

the energy,25 1kWh

Er = in 25 revolutions.

1200

= 20.833 Wh = 75000 JNow, true energy consumed by the load is,

Et = VIt where t = time for 25 rev. in sec.

= 230570= 80500 J

true energy recorded energy% Error = 100

true energy

= [(80500 75000) 80500] 100= +6.8322 %

5. A moving coil instrument gives a full-scale deflection for a current of 20mAwith a potential difference of 200mV across it. Calculate i) Shunt required to use it as an

ammeter to get a range of 0-200A ii) multiplier required to use it as voltmeter of range 0-

500 V.Solution:

For the given meter,

Im = 20 mA

Vm = 200 mVNow,

Vm = Im Rm

Vm 200Rm = = = 10

Im 20

i) As an ammeter,Rm

Rsh =m 1

where,

I 200

m = =Im 2010-3


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10

Rsh = = 0.001

10000 1ii) As an voltmeter,

V 500

Rs = - Rm = - 10 =24990

Im 20 10-3


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Chapter 4

DOMESTIC WIRING

4.0 Introduction

Wiring done in domestic premises for providing electrical power for lighting, fans

and other domestic appliances is called domestic wiring.The method of wiring to be adopted depends on,

1. Durability: The wiring scheme must be durable. It should be according to the

specifications. It should fulfill the consumer requirements.2. Electrical safety: Wiring should be shockproof and leakage-proof.

3. Appearance: Beauty of the house should not get spoiled due to wiring.

In the case of factory wiring, appearance apart from neatness is not usuallyimportant.

4. Cost: Wiring scheme should be selected depending on type of building. Before

recommending a particular wiring scheme, the funds madeavailable by consumermust be taken into consideration.5. Maintenance: Minimum maintenance is desired. There should be scope for

further extension. Easy renewal of wiring is desired.

4.1 Different types of wiring:

i. Cleat wiringii. Casing capping

iii. Batten wiring

iv. Conduit wiring

(i) Cleat wiring:

In this system, the vulcanized India Rubber (V.I.R) conductors are run betweentwo porcelain cleats. The cleats are made into two halves, one of which is grooved to

receive the wire and other half is placed over it and both of them are secured on the wall

by means of screws. The cleats may have one, two or three grooves depending on thenumber of wires required to run in the wiring system. This system is used for temporary

installations.

Advantages:1. Materials use can be easily retrieved.2. Easy inspections and alterations can be done.

3. Accidents through fire are rare.

4. Conductors are insulated from each other thus reducing the risk of short circuit.5. Cheaper when compared to other wiring systems.

6. Most suitable for temporary work.


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Disadvantages:

1. Since the conductors are exposed, there is a risk of mechanical injury.

2. . Regular cleaning is required.

3. It does not have a good appearance. (Due to sagging)4. High maintenance costs.

5. Not suitable for permanent jobs.

(ii) Casing / Capping

The casing is a rectangular strip made from teakwood.(or P.V.C) It hastwo grooves into which the wires are laid. The casing is covered with a rectangular strip

of wood (or P.V.C.) of the same width, called capping. Insulated conductors are laid

inside rectangular teak wood boxes having grooves inside it. A rectangular strip of wood

called capping is fixed over it. The casing is attached to the wall or ceiling. Now a daysPVC is used instead of wood.

Advantages

1. Cost is low.2. It is used for low voltage installations.

3. It is accessible for inspections, alterations and repairs.

4. Wires are separated in respective grooves of the casing.5. Good protection to conductors from dangerous atmospheric conditions.

6. Neat and clean appearance.

Disadvantages1. Wood used should be properly seasoned to avoid damage from white ants etc.

2. Skilled carpenters are required to make proper casings and cappings.

3. Fire will cause damage to the casings and cappings.4. They must be used only in dry areas.

5. Casing cannot be buried in plaster, nor fixed in contact with gas pipes, water pipes

or immediately below the latter.

(iii) Batten wiring:

In this wiring system, insulated conductors are laid on teak wood batten and

clipped. The batten is fixed on the wall or ceiling. It is also known as tough rubber

sheathed system (TRS). The conductors are completely covered over by a thick coatingof tough rubber.


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Advantages

1. It is cheaper than wooden casing and capping.

2. No risk of fire and mechanical injury.3. It is suitably insulated against atmospheric conditions like smoke, fume etc.

4. It has reduced risk of short circuit.

Disadvantages

1. Wiring should not be exposed to sunlight, rain etc.

2. Sharp bends must also be avoided.3. Skilled wiremen are required to do the wiring.

(iv) Conduit wiring

Here mild steel tube or PVC tube is run on wall or ceiling to carry insulatedconductors. The steel tube or PVC tube is called conduit. The conduits are either laid

over the surface or enclosed in ceiling or wall.

Depending on whether the conduits are laid inside the walls or supported on thewalls, there are two types of conduit wiring, which are:

1. Surface conduit wiring:

In this method, conduits are mounted or supported on the walls with the

help of pipe books or saddles. In damp situations, the conduits are spaced apart from the

wall by means of wooden blocks.

2. Concealed conduit wiring:In this method, the conduits are buried under the wall at the time of

plastering. This is also called Recessed Conduit Wiring.

Advantages

1. Risk of fire is completely eliminated.

2. Protection against mechanical injury is ensured.3. Earthing and electrical continuity is assured.

4. The same tube carries the lead and return wires.

5. The wiring scheme is durable.

6. Very less maintenance cost.7. The beauty of the premises is maintained.

Disadvantages

1. It is costlier than other systems.

2. Under wet conditions, there is a risk of short circuit.

3. Skilled workers are required for erection.4. Repairs are very difficult.

5. In concealed conduit wiring, keeping the conduit at earth potential is a must.


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4.2 Specifications of wires

The conductor wires may be specified according to either type of

conductor or type of insulation. Since copper is costly, aluminum is used as conductor

material. The current carrying capacity depends on its size. The current flowing throughthe wire must be always under safe limits. Conductors of larger areas of cross section are

required to carry larger currents.

Types of wires generally used are,

1. Vulcanized India Rubber (V.I.R) insulated

2. Tough Rubber Sheathed (T.R.S).(a) Poly Vinyl chloride (P.V.C) insulated.

(b) PVC insulated and PVC sheathed.

3. Cab Tyre Sheathed wires (C.T.S)

4. Flexible wires.

Vulcanized India Rubber (V.I.R) Wire:

This type of wire consists of tinned conductor coated with rubber insulation. This

is further covered with protective cotton and bitumen compounded finally finished with

wax. This makes it moisture and heat resistant. These are always single-core wires.Though are covered with a cotton layer it has tendency to absorb moisture and hence

rarely used now a days.

Poly Vinyl Chloride (P.V.C) Wire:

These are most commonly used wires. These have conductors with P.V.Cinsulation. P.V.C has following characteristics.

It is non-hygroscopic and moisture-proof.

It is tough and hence durable.

Resistant to corrosion.

It is chemically inert.

As it is tough, additional covering is not required.

The only disadvantage is that it softens at a high temperature and hence it is

avoided where extreme of temperatures may occur. e.g. in earthing appliances.

Cab Tyre Sheathed wires (C.T.S) Wire:

In this type, ordinary rubber insulated conductors are provided with an additional

through rubber sheath. The wire is also known as through rubber sheathed (T.R.S) wire.


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It provides additional insulation and along with that protection against moisture, chemicalfumes and wear-and-tear.

Flexible Wires:

Twisted twin flexible wire

These are used very commonly in domestic wiring or for wiring of temporary

nature. It consists of two separately insulated standard conductors. The insulation is

mostly rubber. More commonly available in parallel to twisted twins. Due to its flexiblenature the handling of these wires become very easy.

4.3 Control of One Lamp from One Switch:

For a lamp, one live and one neutral is necessary. To control the supply to the

lamp, switch is introduced in the live wire and neutral is directly connected to the lamp.

When the switch is ON, a full voltage gets applied to the lamp and it glows. When

the switch is turned OFF, the circuit gets opened and the lamp gets switched off.

4.4 Two way control of lamps:

In big halls, corridors, bed rooms and stair case, it is necessary to control

the lamp from two points. In such cases, a two way control lamp circuit is use for wiring.

This is also called Staircase wiring.Consider the figure, which shows the connections made to control the lamp from

two points.

Control of one lamp from two points


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It consists of two-way switches. A two-way switch operates in one of the twopossible positions. Assume that the lamp is on first floor. Switch A is on first floor and

switch B is on second floor. In the position shown, the lamp is OFF. When person

changes the position of the switch A from 1 to 2, then the lamp gets through switches Aand B, and it gets switched ON.

ON position of lamp

When person reaches on second floor, the lamp must be OFF. The switch position

is changed from 2 to 1 by the person, due to which the phase connection reaching thelamp gets opened and the lamp will be switched OFF

OFF position of lamp

4.5 Three way control of lamps:

In very big corridors, go downs or workshops, it is necessary to control from three points.

The circuit is as shown in figure. This is also a staircase wiring. It consists of two-way

switches A and B, and one intermediate switch C.


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The intermediate switch can have positions to connect points 1-4, 3-2 as shown or1-2 and 3-4 shown dotted. The switch A is on the first floor and switch B is on the third

floor say. In the position shown, the lamp is ON. When the person from floor 2 changes

switch C position to have connections 1-2, and 3-4 then it can be seen that there is opencircuit in the connection and the lamp gets turned OFF

Now, if the person from 3rd floor changes position of switch B from 1 to 2, then

again lamp gets supply through position 2 of A, 3-4 of C and 2 of B as shown down. The

lamp gets switched ON. Again if switch A position is changed, the lamp gets switchedOFF. Thus the lamp can be controlled from three different positions.

4.6 Earthing:

Earthing or grounding is to connect the body of electrical equipment to the earthby a wire of negligible resistance. Earthing brings the body of the equipment to zero

potential, and thus avoids shocks to the personnel, in case the body of the equipment

comes in contact with a live wire. The neutral of the supply system is solidly earthed.

There are mainly two methods of earthing.

4.6.1 Pipe Earthing:In this method of pipe earthing in which a Galvanized Iron (G.I) pipe of approved

length and diameter is used. The size of the pipe depends on the current to be carried and

type of the coil. It should be placed to a depth of 4.75 m it must be placed permanently ina wet ground. The length of the pipe is 2m and diameter is 38mm. The pipe at the bottom

should be surrounded by broken pieces of coke and charcoal. Charcoal if mixed with salt,

reduces the resistance. The usual practice is to put alternate layers of salt and coal.During summer, in order to have effective earthing, the funnel should be filled with 3 to 4

buckets of water. The leading wire from the body of the apparatus to the earthing pit must

be made of GI wire or GI strip of sufficient cross sectional area to carry the fault currentsafely. When the ground wire is run from one machine to another machine it should be

well protected against mechanical injury.


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Advantages:

The earth wires can be physically inspected from time to time.

These connections can be checked for performing continuity checks.

Disadvantage:

The embedded pipe length has to be increased sufficiently in case the soil specificresistivity is of high order. This increases the excavation work and hence increased cost.

4.6.2 Plate Earthing:

In this method of plate earthing, a plate of size 60603.18 mm or GI plate of

size not less than 60606.35 mm is used for the purpose of earthing. This plate is

connected at the bottom of the pit and is covered with thick alternate layers of salt and

powdered charcoal for improving the soil condition and efficiency of the earthing system.

The earthing wire is securely bolted to the earth plate with the help of bolt, nut and

washer. Another GI pipe with the funnel and wire mesh at the top is provided to pourwater in the pit to ensure better earthing.

Advantage:

The earthing efficiency increases with the increase in the plate area and depth of

embedding. If the resistivity of the soil is high, then it is necessary to embed the plate

vertically at a greater depth into the ground.


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Disadvantage:

Discontinuity of the earth wire from the earthing plate below the earth can not be

observed physically. This may cause misleading and may result into heavy losses underfault conditions.

Uses of Earthing:

1. To maintain the line voltage constant.

2. To protect tall buildings and structures from atmospheric lighteningstrikes.

3. To protect all the machines, fed from overhead lines, from atmospheric

lightening.4. To serve as return conductor for telephone and traction work.

5. To protect the humans from any unwanted electrical accidents.

Date post:	14-Apr-2018
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