Course contents

1. Electrical basics -1-

2. Concepts of Phase and line quantities -5-

3. Control panel (Switch gear) -7-

4. Electrical Diagram -11-

5. Transformer basics -12-

6. TIC -13-

7. Sequence controller -15-

8. Motors -18-

Appendix I- Table showing SWG, selection of wires

Appendix II- Current rating for A.C. Motors

Appendix III - Selection Table for Starter / Relay / Fuse / Cable for DOL Motors

- 1 -

Terms in Electrical-

1. Voltage

Force behind the flow of electrons in conductors

2. Current

Rate of flow of electrons

3. Resistance It is the property of a substance due to which it opposes the flow of electricity

(i.e. electrons) through it.

Practical unit of Resistance is ohm

A conductor is said to have a resistance of one ohm if it permits one ampere

current to flow through it when one volt is impressed across its terminals.

Resistance offered by a conductor :-

varies Directly as its length L.

varies Inversely as the cross section A.

Depends on nature of material

Depends on temperature of conductorOHM’S LAWRatio of

potential difference (V) between any two points on a conductor to

the current (I) flowing between them is constant, provided the temp.

of the conductor does not change.PowerPower in DC circuit, Power

P = Voltage X Current = V X I Watt

= I2 X RA. C. CIRCUITA.C. CIRCUITS - SINGLE

PHASEReal Power P = Voltage X Current X Power factor

= V X I X Cos WattReactive Power Q = Voltage X

Current X Sin = V X I X Sin Volt amperes

reactive ( Wattless Component of

- 2 -

Power )Apparent power ( S ) = Voltage x current

= V X I Volt amperesAlternating Current or Voltage : Direction

reverses at regular intervals.Sinusoidal waveformEm - Maximum

emf.One cycle gets completed in 2p ( 360O )PHASEIt is the fraction of

time period of that alternating current, which has elapsed since the

current last passed through zero position of reference.

- 3 -

Few Important terms associated with single phase or three phase

systems.Line Phase Live Neutral Earth Ground Balanced circuit

Un-balanced circuitConcept of Star and Delta Connection

- 4 -

Star Connection

Delta connection

Concept of line

voltage and Phase voltage in star connection

Neutral

- 5 -

Line Voltage

Phase Voltage

- 6 -

What is switchgear ?Switchgear is a general term covering a wide range of equipment concerned with switching and protection of electrical circuitSwitchgear is an essential part of power system and also that of any electrical circuit.The switching devices are used to make or break the circuits in healthy or in abnormal conditionsFaultA fault in an electrical equipment is defined as a defect in its electrical circuit due to which the current is diverted from the intended path. Generally caused by breaking of conductors or failure of insulation Other causes•Mechanical failure•Accidents•Excessive stressesFault classification

Single line to ground fault Open circuit

Line to line fault

Double line to ground fault

Three phase fault

Faults cannot be eliminated but can be minimized

- 7 -

Switchgear components Switches

Fuses

Circuit breakers

Relays

Isolators

Lightening arresters

Current transformers

Potential transformers

What is a circuit Breaker?

A circuit Breaker is a switching and current interrupting device in switchgear.

The circuit breaker serves two basic purposes

A] Switching during normal operating conditions for the purpose of

operations and maintenance

B] Switching during abnormal conditions such as short circuits and

interrupting fault currents

Different types of circuit breakers

1.Miniature circuit breaker MCB

2.Molded case circuit breaker MCCB

3.Minimum oil CB MOCB

4.Bulk oil CB BOCB

5.Air circuit breaker ACB and ABCB

6.Earth leakage CB ELCB

7.SF6 circuit breaker SF6CB

- 8 -

Other switchgear components

Isolators- Isolators are disconnecting switches that are used for

disconnecting the circuit in no load (no current) condition. They are installed

near circuit breakers

Lightning arrestors- They divert the over voltages to earth and protect the

equipment from over voltages

CT s and PT s- These are used for transforming the voltage and currents

to a lower value for the purpose of measurement , protection and control

Electrical interlocks- They are provided to ensure the following sequencing

of operation

While opening * Circuit breaker

* Isolator

* Earthing switch

While closing * Earthing switch

* Isolator

* Circuit breaker

Indicators- Indications are provides on panels by

means of panel mountable lamps.

- 9 -

Relays

Definition: - Relay is a device, which detects an abnormal fault condition by constantly

measuring the electrical quantities, which are different under normal and faulty

conditions.

Properties of a RELAY system

1) Speed :-

Relay system should be as fast as possible

1 to 2 cycles for operating system.

2.5 to 3 cycles for interrupting system.

2) Selectivity :-

Only nearest Circuit Breaker should operate such that minimum part of

the system should get disconnected.

3) Sensitivity :-

The relay should operate for a low value of actuating quantity.

4) Reliability :- Reliable.

5) Simplicity :- The system should be simple in construction and design. Easy for

maintenance. Minimum no. of relays and contacts should be used.

6) Economy :- Too much of protection is also bad.

- 10 -

Typical wiring diagram of relay operation

- 11 -

BatteryCircuitBreaker

C.T.Relay Coil

Trip Coil

Relay operating circuit

Tripping circuit

Sensing circuit

TRANSFORMER

What is a Transformer ?

It is a device that

Transfers electric power from one circuit to another

(Electrical isolation).

It does so without change in frequency.

It accomplishes this by electromagnetic induction.

The different types of

transformers arePower transformers

1. Instrument transformersPotential transformers

o Current transformers

- 12 -

Temperature indicator and controller (TIC)Basic block diagram

For

temperature sensing we can use-

1. RTD Resistance temperature detectors

2. Thermistors

3. Thermocouples

Different ways for temp indicating are

1. Thermometers

2. Electronic displays

The controller-

This is a device that takes action according to the signals sent by the sensors. If the temp

is sufficient, accordingly signals should go to the switching circuitry for taking necessary

actions (Extinguishing the flame)

The controllers can be simple logic controllers using discrete components or using the

advanced microprocessor technologies. They can simultaneously control the sequence

controller.

- 13 -

Water storage

Temp sensor

Controller for taking decisions

To ignition transformer

Flame extinguishing mechanism

RTD- Resistance Temperature Detectors (Thermoresistive transducers)

For measurement of temperature the basis is that when the temperature of a material changes, its resistance also changes.Two categories of such materials are1. Conductors i.e. metals- In this case the resistance is directly proportional to

temperature. e.g. Nickel, Copper, Platinum, Tungsten, Silver etc. Devices in which such materials are used are called as RTDs.

2. Semiconductors- In this case the resistance is inversely proportional to temperature. The devices are called as thermistors.

Thermistor

Platinum (RTD)

Temperature

It is found that compared to other metals, platinum is a standard material used as a resistance elements as RTD.Properties of resistance elements: - High electrical resistivity High thermal coefficient of resistivity Linear resistance temperature relationship Corrosion resistant No change in state within reasonable temperature range

- 14 -

Spe

cifi

c re

sist

ance

- oh

m-

cm

Terminals

Electrical Leads Metal sheath Protective cement

Platinum Resistance coil Hollow Ceramic Former

Schematic Diagram of Platinum Resistance ThermometerRange is – 900C to + 6000C

Sequence Controller

The sequence controller controls the complete operating cycle for the boiler, Sequence

starts at the ignition of the flame, up to extinguishing flame. Sequence controller works on

220 Volts AC, 50 Hz.

The ignition transformer is important device in the sequence. The rating is 230V-12.5 KV.

VA rating is up to 500 VA to 750 VA. The high voltage creates spark at the contacts to

generate a flame.

The sequence of the operation can be explained through the following steps-

1. When the line voltage is available at terminals L! of sequence controller supply goes

through the NC (Normally closed) contactsS2 to terminal T

2. Through external circuit it returns to terminal TH of the sequence controller. Terminal

TH is normally used for connecting blower pump motor and fuel pump motor.

3. From terminal TH supply goes through fixed resistance R1, relay coil L1 and

condenser C1 to neutral terminal LN.

4. Photocell connected to C & C terminals is in dark and hence does not conduct

resulting in voltage across L1. L1 energises and operates contacts K1S1 and K2S2.

5. Supply is now available at terminal A, for ignition transformer.

6. L2 energises and operates contacts K2S1 K2S2 and K2S3. Contact K2S1 is hold on

contact for coil L2.

7. Supply from terminal A goes to the ignition transformer through the external electrical

circuit and comes back to V. Neutral is connected through K2S3, L2 and LN giving

spark.

8. Supply is also now available at terminal U of the sequence controller which is fed to

the burner solenoid valve through the external circuit..

9. When the flame is established, the photocell senses the light and conducts to de-

energise the relay coil L1. Therefore contacts K1S1 and K1S2 open.

10.Supply to terminal is cut off and ignition transformer is cut off.

11.When the supply to the terminal L1 cuts off due to external control devices, the

sequence controller stops and there will not be any alarm under this condition.

- 15 -

12.When the supply to terminal L1 is restored, the sequence controller conducts a fresh

firing cycle.

13. If the flame extinguishes due to taking over by some safety devices or due to some

unhealthy conditions, the photocell goes in dark and relay coil gets energised. In turn,

ignition transformer is energised and the sequence controller tries to restore the

flame.

14. If flame is not restored within 10 seconds the photocell remains in dark for that much

time.

15.Under such condition, current continues to flow from terminal TH, contact K1S1,

bimetallic strip heats up and due to its action contact S2 changes over.

16.Supply from terminal L1 of the sequence controller now goes to terminal S. An audio

visual alarm connected to this terminal gets supply and sounds.

17.The firing cycle can be started only after manually pressing the resetting button

provided on the cover of the sequence controller.

- 16 -

R1 L1 Photocell

K1S1 R2

K2S1

L2 K2S2

K2S3

K2S2 TR

S2

- 17 -

13

1

LN

U

L2

A

V

1H

C

S

C

L1

Sequence controllerPETERCEM MA 810

How Motor works?It converts electrical energy into mechanical energy.Conversion is based on principle that when Current carrying conductor is placed in magnetic field experiences a mechanical force whose direction is given by FLEMING's Left hand rule whose magnitude is given by

F = BIL Newton.This force on each conductor collectively produce driving torque rotate armature.

- 18 -

Appendix ITable showing conversion of SWG of wires to its m.m.

equivalent

SWG m.m. SWG m.m. SWG m.m.1 7.62 18 1.219 35 0.21342 7.01 19 1.016 36 0.19303 6.40 20 0.914 37 0.17274 5.89 21 0.813 38 0.15245 5.39 22 0.711 39 0.13126 4.88 23 0.610 40 0.12197 4.47 24 0.569 41 0.11188 4.06 25 0.509 42 0.10169 3.66 26 0.457 43 0.0914

10 3.25 27 0.4166 44 0.081311 2.96 28 0.3759 45 0.071112 2.64 29 0.3454 46 0.061013 2.34 30 0.3150 47 0.050614 2.03 31 0.2946 48 0.046815 1.83 32 0.2743 49 0.030516 1.626 33 0.254 50 0.025417 1.422 34 0.23

- 19 -

Current rating in Amperes as per ISS 692-1965 for Aluminium conductor 1100 V underground cable

Nom

inal

are

a in

sq

.mm

.

Num

ber

and

size

of

wir

es in

mm

Current rating for cables laid in ground

Current rating for cables laid in ducts

Current rating for cables laid in air

Single core un-

armoured

Twin core

Three and

multi core

Single core un-

armoured

Twin core

Three and

multi core

Single core un-

armoured

Twin core

Three and

multi core

6 1/2.80 50 57 48 42 44 40 56 48 4010 1/3.55 70 74 62 56 60 51 72 66 5616 7/1.70 90 96 81 76 80 68 94 88 7225 7/2.24 115 122 107 98 108 90 124 117 9735 7/5.50 138 147 128 116 130 105 151 141 11950 19/1.80 172 180 158 140 159 128 184 177 15070 19/2.24 208 219 192 1700 190 156 227 220 18295 19/2.50 244 262 224 198 224 184 272 258 224120 37/2.06 278 302 257 222 254 211 312 298 258150 37/2.24 316 346 296 249 287 243 358 339 300185 37/2.50 359 398 336 279 323 278 412 387 348240 37/3.00 430 485 413 335 397 340 520 492 437300 61/2.50 466 536 438 358 422 364 570 524 475400 61/3.00 553 618 513 412 515 425 680 635 545500 81/2.80 595 - - 445 - - 760 - -625 91/3.00 670 - - 490 - - 895 - -

- 20 -

Appendix IICurrent rating for A.C. Motors

B.H.P. of motor

Current rating per phase of Induction motor

Single phase 230 V

Three phase 400 V

Three phase 440 V

0.125 1.00 0.30 0.300.25 1.80 0.70 0.600.50 3.50 1.20 1.000.75 4.80 1.70 1.401.00 6.20 2.00 1.701.25 7.40 2.50 2.201.50 8.70 2.80 2.501.75 10.00 3.20 2.802.00 11.80 3.50 3.202.50 14.00 4.30 4.003.00 17.50 5.00 4.504.00 20.00 6.50 6.005.00 24.00 8.00 7.507.50 36.00 12.00 11.00

10.00 47.00 15.00 14.0012.50 59.00 19.00 18.0015.00 70.00 22.00 21.0020.00 91.00 29.00 28.0030.00 135.00 42.00 39.0040.00 183.00 56.00 56.0050.00 227.00 71.00 66.00

- 21 -

Appendix IIISelection Table for Starter / Relay / Fuse / Cable for DOL

Motors

H.P. with 3 phase

415 V, 50 Hz

Full load current appro.

DOL starter LT/LK make

Relay scale

Back up HRC fuse in Amps.

Typical cable size in sq.mm.

0.5 1.0 MK1 / ML0 1 – 1.6 6 1.5 / 2.50.75 1.5 MK1 / ML0 1.5 – 2.5 6 1.5 / 2.51.0 2.0 MK1 / ML0 1.5 – 2.5 10 1.5 / 2.51.5 2.6 MK1 / ML0 2.5 – 4.0 10 1.5 / 2.52.0 3.5 MK1 / ML0 2.5 – 4.0 15 1.5 / 2.53.0 5.0 MK1 / ML0 4.0 – 6.5 20 1.5 / 2.54.0 6.2 MK1 / ML0 4.0 – 6.5 20 1.5 / 2.55.0 7.5 MK1 / ML0 6.0 – 10 25 2.56.0 9 MK1 / ML0 6.0 – 10 25 2.57.5 11 MK1 / ML1 9 – 14 25 4.010.0 14 ML1 10 – 16 35 4.012.5 18 ML2 13 – 21 50 6.015 21 ML2 20 - 32 50 6.0

17.5 24 ML2 20 – 32 63 10.020 28 ML2 20 – 32 63 10.025 35 ML3 28 - 42 80 1630 40 ML3 28 – 42 100 2535 47 ML3 / ML4 45 – 70 125 2540 55 ML3 / ML4 45 – 70 125 2545 60 ML3 / ML4 45 – 70 160 3550 66 ML8 60 - 100 160 3560 80 ML8 60 – 100 160 5075 100 ML8 60 – 100 200 7090 120 ML8 90 – 100 250 95100 135 ML 12 120 – 200 250 95150 200 ML 12 180 - 300 350 185200 275 ML 12 180 - 300 500 300/400225 300 ML 12 180 - 300 600 300/400

- 22 -

Tinned Copper Fuse wire Table

S.W.G. Current rating of Fuse in Amps.

Fusing Current Approx.

40 1.5 339 2.5 438 3.0 537 3.5 636 4.5 735 5.0 834 5.5 933 6.0 1032 7.0 1131 8.0 1330 8.5 1329 10 1628 12 1827 13 2326 14 2825 15 3024 17 3323 20 3822 24 4821 29 5820 34 7019 38 8118 45 10617 65 13516 73 16615 78 19714 102 23013 130 295

- 23 -