Download electrical instrumentation final lab manuals for EX

ST.ALOYSIUS INSTITUTE OF TECHNOLOGY, JABALPUR (M.P)
2015-2016
SAIT, JABALPUR
DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGG.
ELECTRICAL INSTRUMENTATION LAB
SUB.CODE-EX-303
BRANCH-EX, SEM-III (GRADING)
LIST OF EXPERIMENTS
1. Study of various types of Indicating Instruments.
2. To measure the low resistance using Kelvin’s Double Bridge.
3. To measure the medium resistance using Wheatstone Bridge.
4. To measure the insulation resistance using Megger.
5. To measure the earth resistance by fall of potential method and verification by using
Earth Tester.
6. To measure the power in a single phase ac circuit by 3 voltmeter / 3 ammeter methods.
7. To measure the power in three phase circuit by two wattmeters.
8. To test the Current Transformer.
9. To test the Potential Transformer.
DEPARTMENT OF ELECTRICAL & ELECTRONICS
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INDEX
S.
NO.
1.
NAME OF EXPERIMENT
PAGE
NO.
DATE
SIGN
REMARK
Study of various types of Indicating
Instruments.
2.
To measure the low resistance using
Kelvin’s Double Bridge.
3.
4.
5.
6.
7.
8.
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EXPERIMENT NO.-01
AIM:-
Measurement of low resistance using Kelvin’s Double bridge.
APPARATUS REQUIRED:- Rx1=1Ω, Rx2=1Ω, Rx3=1Ω, Rx3=1Ω, Rx1=1Ω, Rx1=1Ω, Rx1=1Ω,
CIRCUIT DIAGRAM
THEORY:Bridges are divided into two categories:
1. AC Bridges – Inductance , Capacitance
2. DC Bridges – Resistance
Some bridges are used for measuring inductance, capacitances, like that for measuring resistances Kelvin’s
Bridge is used.
Here we are using a circuit for measuring low resistances. Measurement of low resistances is not suitable for
high resistance measurement, i.e., it will give more errors.
In order to avoid this serious error measurement of low resistance technique is used. There are some methods
for measurement of low resistance
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Ammeter Voltmeter Method
1. Kelvin’s Double Bridge
2. Potentiometer method
3. Ducter
Kelvin’s bridge is the modification of Wheatstone bridge and provides greatly increased accuracy in
measurement of low value resistances.
Consider the bridge circuit in fig. 1 shown below.
Where
r = resistance of the lead connected between R (unknown resistances)
s = Standard Resistances
When the G connected to point m, lead resistance is added to the standard resistances, resulting in very low
value of R.
When G connected to point n, lead resistance is added to the unknown resistance, resulting very high value
for R.
P/Q = r1/r2
Suppose that instead of using point m or n, we make G connection to any intermediate point c . So because of
this point c at the potential between point m & n to eliminate the effect of connecting lead of resistance R and
the standard S.It is shown in the above figure.
The ratio P/Q is made equal to the p/q under balanced condition. There is no current through the
galvanometer i.e, Ead=Eamc.
Now
Ead=p/(p+Eab)
Eab=1(R+S+(P+Q)R/p+q+r)
Eac=1(R+P/(p+q){(p+q)r/p+q+r})
For zero galvanometer deflection.
Ead=Eamc
If IF, = (P/Q).S
This is the usual working equation of Kelvin’s Bridge.
PROCEDURE:1. Switch ‘ON’ the power supply of the kit.
2. Connect unknown resistor ‘Rx’ in the circuit at proper place using patch chords.
3. Now measure the supply the voltage E using DC voltmeter.
4. Change the range selector switch for each of resistance from Rx1 to Rx2.
5. By varying the Pot S make bridge balanced i.e, galvanometer to shows zero deflection.
6. Disconnect the supply and measure the value of pot S.
7. Find thee value of unknown resistor Rx using formula Rx=SP/Q.
8. Repeat the same procedure for the other values of Rx.
Calculation:The unknown resistor R can be found out by
R=SP/Q
OBSERVATION:Ratio Arm Resistor
P
Std. Arm Resistor S
Calculated Rx
Q
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RESULT:Using Kelvin’s Double Bridge the value of unknown resistor is found and the measured value and calculated
value is nearly same.
EXPERIMENT NO.- 02
AIM: - Measurement of medium resistance using Wheatstone’s bridge
THEORY:A very important device used in the measurement of resistances is the bridge. A Wheatstone Bridge has been
is use longer than almost any electrical measuring instrument. It is still an accurate and reliable instrument
and is extensively used in industry. The Bridge is an instrument for making caparison measurements and
operates upon a null indication principle. This means the indication is independent of calibration of the null
indicating instrument or any of its characteristics. For this reason, very high degrees of accuracy can be
achieved using bridge. Accuracy of 0.1% is quite common with a Wheatstone bridge as opposed to
accuracies of 3% to 5% with ordinary ohmmeter for measurement of medium resistances. Fig. (1) Shows the
basic circuit of a Wheatstone bridge. It has four resistive arms , consisting of resistances P , Q , R & S
together with a source of emf (A Battery) and a null detector , usually a Galvanometer (G) or other sensitive
current meter. The current through the galvanometer depends on the potential difference between points ‘C’
& ‘D’ .The Bridge is said to be balanced when there is no current through the galvanometer or when the
potential difference across the galvanometer is Zero. This occurs when the voltage from point ‘B’to point ‘A’
equals the voltage from point ‘D’ to point ‘B’: or, referring to the other battery terminal, when the voltage
from point ‘D’ to point ‘C’ equals the voltage from point ‘B’ to point ‘C’.
For bridge balance, we can write:
I1P = I2P
------------------------------------- (1)
For the galvanometer current to be zero, the following conditions also exist:
I1 = I3 = E/P +Q
------------------------------------- (2)
And
Where
------------------------------------- (3)
I2 = I4 = E/R +S
E
= emf of the battery
Combining Eqns. (1), (2) & (3) and simplifying, we obtain:
P/P+Q = R/R +S
----------------------------------- (4)
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From which QR
= PS
------------------------------------ (5)
Eqn. (5) is the well known expression for the balance of Wheatstone bridge. If three of the resistances are
known, the fourth may be determined from Eqn. (5) and thus obtaining:
R
= S (P/Q)
-------------------------------------- (6)
Where R is the unknown resistance S is called the ‘Standard Arm’ of the bridge and ‘P’ & ‘Q’ are called the
‘Ratio Arms’.
PROCEDURE:1. Connect the unknown resistance across terminals marked X on the front panel.
2. Keep the function band switch at VR and toggle switch at R position.
3. Keep the Galvanometer switches at ‘Int’ position, battery switch at ‘Int’.Keep the ratio dial at ‘1’ &
all other knobs at ‘0’.Simultanously press G & B switches. Note the deflection in the galvanometer.
i. Generally galvanometer deflection should be in between.20 to 30 division (in
maximum case).
ii. If Galvanometer showing deflection out of scale then change ratio dial knob to higher
range.
4. When the deflection comes into the scale then change the series arms knobs to take the null point.
5. Note down the readings
i. Let the reading of Ratio arm (Multiply By) = P/Q
ii. Let the reading of Series arm
= SΩ
iii. Then the resistance of sample
= S (P/Q) Ω
6. For external Galvanometer and battery change the toggle switch on (EXT) external point.
7. Now repeat the experiment with different ratio arm readings and take the mean of all the readings.
a. External Galvanometer like spot deflection Galvanometer can be connected to increase the
sensitivity.
PRECAUTIONS:_
1. Galvanometer button should be just pressed to see wheather there is any deflection in galvanometer.
The button should be immediately released if reading going over scale.
2. If dead null point is not obtained take the mean reading for each side deflection of the Galvanometer.
FOR RESISTANCE MEASUREMENT
Formula Used R = S (P/Q) Ω
S.No.
1.
Ratio Arm P/Q
Series Arm S
Calculated Value of
unknown Resistance
1
100 Ω
100 Ω
DEPARTMENT OF ELECTRICAL & ELECTRONICS
Actual Value of
unknown
Resistance
100 Ω
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2.
1
1K Ω
1K Ω
1.02 KΩ
3.
10
1K Ω
10 KΩ
10.02 KΩ
EXPERIMENT NO.-03
AIM: - Measurement of insulation resistance using megger.
APPARATUS REQUIRED:S. NO.
NAME OF EQUIPMENT
RANGE
QUANTITY
01.
01
02.
01
03.
01
THEORY:Megger is the portable insulation tester. It is used to remove very high resistance of the order of mega ohms.
The megger is an indicating instrument, which is generally used to measure very high resistance such as
insulation resistance.
Construction of the megger has been shown in fig. As shown, it consists a hand driven d.c.generator, which
can generate 500 V or 1000 V dc.It also consists two coils namely current coil (or deflecting coil) and
pressure coil (or control coil).Both coils are fixed together at some angle in between the poles of permanent
magnet and are free to rotate, about a common axis, along with the pointer, which moves from ∞ (extreme
left) depending upon the resistance between the two terminals.
The resistance under test is connected between L and E and the generator handle is then rotated at uniform
speed of about 180 to 300 rpm. The generator will generate 500V/1000V (however a 500 V megger is used
for testing the domestic installation).The current coil is connected in series with the test resistance and the
combination in series with the current limiting resistance R1 connected across the generator terminal this
produce reflecting Torque. The pressure coil (or control coil), P1 is connected across the generator terminal
in series with the current limiting resistance R2 and compensating winding. The compensating coil, P2 is
used to obtain better scale proportions.
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The movement of the pointer on the scale depends upon the value of test resistance, which may be as
follows:
1. When the test terminal is open, the resistance will be measured if infinite and pointer will be on the
extreme left of the scale. As due to absence in current coil no reflecting torque is produced and due to
torque is produce by the control circuit coil pointer will remain at ∞.
2. If the test terminal are joined together by a piece of wire, the resistance is zero ,so the pointer will
move to extreme right under the influence of the torque produce on account of f s d current flowing in
the current coil.
3. If the resistance will be measured is less then ∞ but more than zero(as megger is used to measure in
mega ohm) then an appreciable amount of current flow in both the coils, producing torque in opposite
direction and the pointer will indicate the value of the test resistance on the scale.
PROCEDURE:
1. Connect the insulation resistance between the two terminals of megger.
2. Rotate the generator handle with the speed of 160 rpm and maintain it for a period of time.
3. Note down the reading of angle scale. This will be the value of insulation resistance.
PREACUTIONS:
1. Do not touch the terminal during rotation of handle.
2. Connection should be made carefully.
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ST.ALOYSIUS INSTITUTE OF TECHNOLOGY, JABALPUR (M.P)
EXPERIMENT NO. – 04
AIM: - Measurement of Earth Resistance with Earth Tester.
APPARATUS REQUIRED:S. NO.
01.
02.
03.
NAME
Earth Tester
Steel Rod(Electrode)
Wire
RANGE
0-100 Ω
1 meter
QUANTITY
01
02
THEORY: - The resistance of earth electrode or earth path is measured by a special Megger called Earth
Tester. An earth tester essentially is a direct reading Ohmmeter which can read low value resistance. The
tester essentially consists of two coils, one is current coil and another is pressure coil. The figure given below
shows the internal construction of such tester. As shown in figure P1C1 are connected together and forming
earth terminal, P2 and C2 are forming other two terminals. So an earth tester has three terminals instead of
two terminals as in the megger. Some manufacturers of these instruments indicate these terminals as ‘E’
(Earth), ‘H’ (High) and ‘U’ (Under) . Here these terminals are E, P and C.
Terminal E is connected to earth electrode / earth conductor or earthed metallic parts. C and P
are connected to auxiliary electrode buried in soil at a suitable distance as per requirements. Three readings
are generally taken by buying the electrode, a metal spike. Firstly ‘P’ electrode will be buried between E and
C, secondly, 3 meters away from ‘E’ on left side of ‘C’. Thirdly, 3 meters away from ‘C’ on its right side.
The man of the three readings is the earth resistance.
CIRCUIT DIAGRAME:-
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PROCEDURE :1.
2.
3.
4.
Two terminals P1 and C1 are sorted to form a common point to be connected to the earth tester.
The other two terminals P2 and c2 are to auxiliary electrode P and C respectively.
Rotate the spindle with a minimum speed of 60 r.p.m.
Note the meter reading (in Ω).
RESULT: -
The earth tester has been studied and the observed value of earth resistance is …………Ω.
PRECAUTIONS:1.
2.
3.
4.
Speed should be maintained at 60 r.p.m.
Earth electrodes should be inserted properly under the ground.
If the resistance observed is maximum, check the connection.
All connections should be right and tight.
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EXPERIMENT NO. – 06
AIM:- To perform load test on a 3-ф SQCG IM and plot its performance characteristics.
APPARATUS REQUIRED:S.NO
.
NAME
TYPE
RANGE
01.
Squirrel Cage Induction
Motor
3-ф
RPM-1410
QUANTITY
VOLTS-415
With
Mechanical
Loading
AMPS-8.1
CONNECTION- Δ
01
PHASE-3
TEFC
HZ-50
RATING-CONT.
INSULATION-B CLASS
KW/HP-3.7/5.0
Mechanical
Loading
02.
Wattmeter
(a) UPF
*
0-1500W/150-300-600V/10-20A
DEPARTMENT OF ELECTRICAL & ELECTRONICS
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03.
Voltmeter
MI
300-600V
01
04.
Ammeter
MI
5-10A
01
05.
(a) Auto-
3-ф
12.211KVA,0-415-470V,15A/Ph
01
(b) D.O.L. Starter
3-ф
3.7 KW/5.0 HP,415V,50Hz
01
06.
Connecting Wires
Multi-Strand
2.5 sq. mm.
As Per
Requirement
07.
Tachometer
(a)Digital
Contact
60-50,000 RPM
01
60-1,00,000 RPM
01
Transformer
(b)Digital Photo
THEORY:By conducting the load test on three phase induction motor, the performance of the motor viz. slip, power
factor, input, efficiency etc. at various loads can be studied.
The induction motor is located by any of the following methods :
1. Brake test
2. By connecting a d.c. generator
On induction motor side, ammeter reads line current and voltmeter reads line voltage VL. The two wattmeters
are connected as per the two wattmeter method hence,
Pin = Power input = W1 + W2
Pin of induction motor = W1 + W2 W
cos Φ = Pin/(√3VL IL )= (W1 + W2)/(√3VL IL) = power factor
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where
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Ns = 120f / P for a given motor
For various loads above parameters are obtained
As the load on the induction motor increases,
1. The output of motor increases.
2. The power factor increases.
3. The efficiency increase up to certain load and then decreases.
4. The speed decreases marginally.
5. The slip increases.
6. The input current increases.
The various performance characteristics can be obtained as shown in the Fig.2.
Fig. 2
The graphs indicate the behaviour of various performance parameters against output of the induction motor
and not shown to the scale.
CIRCUIT DIAGRAM FOR LOAD TEST ON 3-ф SQUIRREL CAGE INDUCTION MOTOR
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PROCEDURE:1. Connect ckt. as shown in diagram.
2. Adjust ‘Zero set’ for Balances.
3. Switch on Mains supply.
4. Start AC Motor Pressing START Push button of DOL Starter.
5. Note down readings of voltmeter, Ammeter, Wattmeters & load on balances W1 & W2 Kg.
6. Using hand wheel of Brake drum arrangement load the motor in steps from no
load to rated torque.
7 Rated torque T = (W1-W2)*A
A = Break drum Constant = Radius of Pulley (Meter) * 9.81
8 At each step repeat step 5.
9. Calculate power output P = 2ЛNT.
10. Calculate efficiency = output / input.
OBSERVATION TABLE:-
DEPARTMENT OF ELECTRICAL & ELECTRONICS
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ST.ALOYSIUS INSTITUTE OF TECHNOLOGY, JABALPUR (M.P)
S.No.
Line
Line
Wattmeters
I/P
Load
Load
Volts
Amps.
watts
W1
W2
VL
IL
Power
P1 + P2
Kg.
Kg.
P1
P2
W1-W2
Speed
RPM
1
2
3
4
CALCULATIONS:-
RESULTS:-
PRECAUTIONS:1. All connections should be right and tight and as per the circuit diagram .
2. Ammeter should always be connected in series and Voltmeter should always be connected in parallel with
the
circuit .
3. The Current Coil of Wattmeter should always be connected in series and the Potential Coil of Wattmeter
should always be connected in parallel with the circuit .
4. Uninsulated parts should never be touched.
DEPARTMENT OF ELECTRICAL & ELECTRONICS
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5. Before switching ON the circuit, check whether the pointer of the auto-transformer and the pointers of all
the
meters is at zero mark or not, if not bring them to zero.
6. Check for parallax, remove if it exists.
7. Depending upon the circuit, proper range of meters should be selected.
EXPERIMENT NO. :-07
AIM: - Study of testing of Current Transformer.
APPARATUS REQUIRED:S.
NO.
NAMEOF EQUIPMENT
RANGE
01.
Current Transformer
Primary C - 10/30/50/100 01
Secondary C - 5A
02.
Single
Phase
Transformer
03.
04.
QUANTITY
Auto- 0-270 V , 10 A
01
Voltmeter
0-300 V
01
Ammeter
0-10/20 A
02
THEORY:The current transformer is used with its primary winding connected in series with line carrying the current to
be measured and, therefore the primary current is dependent upon the load connected to the system and is not
determined by the load(burden)connected to the secondary winding of the current transformer. The primary
winding consists of few turns only and therefore there is no appreciable voltage drop across it. The secondary
winding of current transformer has more number of turns, the exact number being determined by the Turns
Ratio. The ammeter is connected directly across the secondary winding terminals. Thus a current transformer
operates its secondary winding nearly under short circuit conditions. One of the terminals of the secondary
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winding is earthed so as to protect the equipments and personnel in the vicinity in the event of a breakdown
in the current transformer.
The above fig shows current being measured by act. The primary winding is so connected that the current
being measured passes through it and the secondary winding is connected to an ammeter. The C.T. steps
down the current to the level of ammeter.
CIRCUIT DIAGRAM FOR CURRENT TRANSFORMER
PROCEDURE: –
1. Connect auto-transformer, ammeters, current transformer and resistive load in series as shown in the
fig.
2. Gently increase the Auto-Transformer upto 220 V.
3. Switch “ON” the Resistive Load circuit. Now switch “ON” The knob of 1 A (installed at RL) in steps
so that eventually 10A current flows through the circuit.
4. Note the readings of two Ammeters (i.e. one on the primary side of C.T. that reads 10A and the other
on secondary side of C.T. that reads 5 A, confirming the C.T. Action).
5.
Now bring the jockey of Auto-Transformer to zero position and switch “OFF” the AC supply.
6.
Now reduce the 10A Resistive Load to 5 A by switching “OFF” the 5 knobs out of 10.
7.
Now interchange the connection of the two Ammeters so that the Ammeter which is connected to the
primary of C.T. and is in series is ready to read 10 A (which is the incremented value).
8. Note the readings of the two Ammeters i.e. one on the primary side of C.T. that reads 5 A and the
other on the secondary side of C.T. that reads 10 A , again confirming C.T. action either ways.
OBSERVATION TABLE:Case I: S.
NO.
C.T. as Step Down Transformer
Input
supply Applied Resistive Load Current
voltage(AC) in volts to in parallel i.e. 10 primary
the primary winding of Resistances of
220 Ω winding
DEPARTMENT OF ELECTRICAL & ELECTRONICS
in Current
in C.T.Ratio
secondary
winding side i.e. PW/SW
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each in parallel and side of C.T. of C.T. in A
controlled by ON-OFF in A
knob in Ω
C.T.
1.
2.
Case II: S. No.
C.T. as Step UP Transformer
Input
supply
voltage(AC) in volts to
the primary winding of
C.T.
Applied Resistive Load in
parallel i.e. 5 Resistance
of
220 Ω each in
parallel and controlled by
ON-OFF knob in Ω
Current
in
primary
winding side
of C.T. in A
Current
in C.T.Ratio
secondary
winding side i.e. PW/SW
of C.T. in A
1.
2.
RESULT:The result obtained from the above experimental setup confirms the C.T. action either ways.
PRECAUTIONS:1. All connections should be tight and right and as per the circuit diagram.
2. Proper range of meters should be selected.
3. Uninsulated parts should never be touched.
4. Ammeter should always be connected in series and Voltmeter in parallel with the circuit.
5. Gently vary the jockey of the Auto-Transformer to bring the pointer to the desired value of voltage
(input).
6. Before switching “ON” the circuit. Check whether the position of pointer in the meters is at zero or
not, if not bring it to zero.
7. Check the parallax, and remove if it exists.
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EXPERIMENT NO.- 08
AIM: - Study of testing of Potential Transformer.
APPARATUS REQUIRED:S. NO.
01.
NAME OF
EQUIPMENT
Potential Transformer
RANGE
Primary C - 250 V
QUANTITY
01
SecondaryC-75/150/300/600V
02.
Single
Phase Auto- 0-270 V , 10 A
Transformer
01
03.
Voltmeter
01
0-300 V
THEORY:Potential Transformers are used to operate Voltmeters, the potential coils of Wattmeters and relays from high
voltage lines. The primary winding of the transformer is connected across the line carrying the voltage to be
measured and the volt circuit is connected across the secondary winding. The design of the potential
transformer is quite similar to that of a power transformer but the load of a potential transformer is always
small, sometimes only a few volt amperes. The secondary winding is designed so that a voltage of 100-120
volt is delivered to the instrument load. The normal secondary voltage rating is 110 volt.
The adjoin fig. shows voltage measurement with a P.T. The primary winding is connected to the voltage
being measured and the secondary winding to the voltmeter. The P.T. steps down the voltage to the level of
voltmeter.
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CIRCUIT DIAGRAM
PROCEDURE:
1. Connect Auto -Transformer, Voltmeters and Potential Transformer in parallel to each other as shown in
the fig.
2.
Now switch “ON” the power supply.
3.
Gently increase the Auto-Transformer up to 220V.
4. Note the readings of the two Voltmeters i.e. one on the primary side of P.T. that reads 220V.
5. Now gently bring the jockey of Auto-Transformer at zero position and switch “OFF” the AC supply.
6.
Now reverse the position of supply so that the previous primary now becomes secondary and viceversa. This implies that AC supply is now given between C and 75/150/300V and the secondary reads
250V, confirming P.T. action.
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RESULT:
The result obtained from the above experimental setup confirms the P.T. action either ways.
OBSERVATION TABLE:
Case I: P.T. as step Down Transformer
S.No.
Applied Input Voltage to Measured Output Voltage Ratio of P.T.
Primary Winding of P.T. in at Secondary Winding of
i.e.PW/SW
volts
P.T. in Volts
1.
2.
3.
4.
5.
Case II: P.T. as step Down Transformer
S.No.
Applied Input Voltage to Measured Output Voltage Ratio of P.T.
Primary Winding of P.T. in at Secondary Winding of
i.e.PW/SW
volts
P.T. in Volts
1.
2.
3.
4.
5.
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DEPARTMENT OF ELECTRICAL & ELECTRONICS
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