Classification, measurement of low, medium and high resistance

1. MEASUREMENTS AND
INSTRUMENTATION
SARUN KUMAR K P
Assistant Professor
IESCE
21-Oct-22 Sarun Kumar K P EEE Dept., IESCE 1

2. Measurement of resistance
• Choice of suitable method depends on several
factors like range of resistance to be
measured, accuracy of measurement, working
conditions etc.
• Range is the most important factor
• Basic classification:
1. Laboratory Measurement: More accurate
2. Workshop Measurements: Less accurate
21-Oct-22 Sarun Kumar K P EEE Dept., IESCE 2

3. Classification of Resistances
1. Low Resistances:
 When the value of resistance is below one ohm (R<1 Ω)
then it is called as low resistance
 e.g. resistance of armature winding of generator,
resistance of series field winding of DC series generator,
resistance of transformer winding, bus bar resistance,
earth wire resistance etc.
21-Oct-22 Sarun Kumar K P EEE Dept., IESCE 3

4. Classification of Resistances
2. Medium Resistances:
 When the value of resistance lies between 1Ω to 0.1 mega
ohm (i.e. 1 Ω<R<0.1 M Ω) then it is called as medium
resistance.
 e.g. resistance of field winding of DC shunt generator,
resistance of long transmission line etc.
21-Oct-22 Sarun Kumar K P EEE Dept., IESCE 4

5. Classification of Resistances
3. High Resistances:
 When the value of resistance is greater than 0.1 mega
ohm (i.e. R>0.1 MΩ) then it is called as high resistance.
 e.g. resistance of cable insulation, resistance of insulator
disc of transmission line etc.
 Generally the resistance of conductor is under the
category of low range and that of insulator is treated
as high range.
21-Oct-22 Sarun Kumar K P EEE Dept., IESCE 5

6. Methods for Low & Medium
Resistances
Measurement of low resistances:
1. Ammeter-Voltmeter Method
2. Potentio-meter Method
3. Kelvin Double Bridgde Method
4. Ohm-meter Method
Measurement of medium resistances:
1. Ammeter-Voltmeter Method
2. Substitution Method
3. Wheatstone Bridge Method
4. Carey-Foster Bridge Method
5. Ohm-meter Method
21-Oct-22 Sarun Kumar K P EEE Dept., IESCE 6

7. Measurement of High Resistances
Problems associated with measurement of high resistances
 Leakage resistance that occur over & around the component
or specimen under test, or over the binding posts by which
the component is attached to the instrument or within the
instrument itself – Avoided by use of Guard Circuit
 Due to electrostatic effect, stray charges may appear in the
measuring circuit & so cause errors. Alternating fields can also
effect – Screening the critical points of measuring ckt
 Resistance of insulating materials, generally, falls rapidly with
temperature
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8. Measurement of Insulation Resistance
1. Direct Deflection Method
2. Loss of Charge Method
3. Megger Method
4. Megohm Bridge Method
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9. Loss of Charge Method
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 C is capacitor of known capacitance
 V is electrostatic voltmeter
 R1 is the total leakage resistance of the
capacitor & Voltmeter
 R is the resistance to be measured
 1st the capacitor is charged by means of
a battery to some suitable voltage by
putting the switch S on stud1
 Then allowed to discharge through the
resistance R & R1 by throwing switch S
to stud 2
 The time(t) taken for the potential
difference to fall from V1 to V2 during
discharge is observed by a stop watch
 Test then repeated with unknown R
disconnected

10. OHM METERS
• Direct reading device for the measurement of resistance
• Simple, convenient & fast method but accuracy is low
• Has a large field of application & gives an approximate value
by direct meter reading with practically no adjustments
required by the operator
• Used for approximate measurement & sorting of resistances
used in electronic circuits, measurement of resistance of
heater elements, field coils of motor etc
21-Oct-22 Sarun Kumar K P EEE Dept., IESCE 10

11. OHM METERS
1. Shunt-Type Ohmmeter : For low value resistors.
2. Series-Type Ohmmeter : For medium-value
resistors.
3. Megger-Type Ohmmeter: For high-value
resistances, such as the insulation of a cable.
21-Oct-22 Sarun Kumar K P EEE Dept., IESCE 11

12. MEGGER
• This meter is used for measurement of high resistance & insulation
resistance
• The device enable us to measure electrical leakage in wire, results
are very reliable as we shall be passing electric current through
device while we are testing.
• The equipment is basically used for verifying the electrical
insulation level of any device such as motor, cable, generator
winding, etc.
• This is a very poplar test being carried out since very long back.
• Not necessary it shows us exact area of electrical puncture but
shows the amount of leakage current and level of moisture within
electrical equipment/winding/system.
21-Oct-22 Sarun Kumar K P EEE Dept., IESCE 12

13. MEGGER
Types of Megger
1. Electronic Type (Battery Operated)
2. Manual Type (Hand Operated)
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14. MEGGER- MANUAL TYPE
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15. MEGGER- MANUAL TYPE
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16. MEGGER- MANUAL TYPE
• Consists of a hand-driven DC generator and a direct
reading true ohm-meter
• Permanent magnets provide the field for both the
generator & the ohm-meter
• Moving element of ohm-meter consists of 3 coils: current
coil, pressure coil & compensating coil, which are
mounted rigidly to a pivoted central shaft & are free to
rotate over a stationary C-shaped iron core
• Coils are connected to the circuit by means of flexible
leads that exert no restoring torque on the moving
element
21-Oct-22 Sarun Kumar K P EEE Dept., IESCE 16

17. MEGGER- MANUAL TYPE
• When a resistance under test is connected between test
terminals L & E, the opposing torques of the coils balance
each other so that the pointer comes to rest at some
intermediate point on the scale
• Scale is calibrated in mega ohms or thousands of ohms
• Guard ring is provided to shunt the leakage current over the
test terminals or within the tester itself to the negative
terminal of the generator without passing through the
current coil of the instrument
• Test voltage is generated by the hand driven generator, G
21-Oct-22 Sarun Kumar K P EEE Dept., IESCE 17

18. MEGGER- MANUAL TYPE
21-Oct-22 Sarun Kumar K P EEE Dept., IESCE 18
Analog display:- Analog display
provided on front face of tester
for IR value recording.
Hand Crank:- Hand crank used
to rotate helps to achieve
desired RPM required generate
voltage which runs through
electrical system.
Wire Leads:- Used same as in
electronic tester i.e. For
connecting tester with electrical
system

19. MEGGER- MANUAL TYPE
 Advantages
1. No external source required to operate.
2. Cheaper available in market
 Disadvantages
1. At least 2 person required to operate i.e. one for rotation of crank
other to connect megger with electrical system to be tested.
2. Accuracy is not up to the level as it’s varies with rotation of crank.
3. Require very stable placement for operation which is a little hard to
find at working sites.
4. Unstable placement of tester may impact the result of tester.
5. Provides an analog display result.
6. Require very high care and safety during use of the same.
21-Oct-22 Sarun Kumar K P EEE Dept., IESCE 19

20. MEGGER - ELECTRONIC
21-Oct-22 Sarun Kumar K P EEE Dept., IESCE 22
Digital Display :- A digital display to
show IR value in digital form.
Wire Leads :- Two nos of wire leads
for connecting megger with
electrical external system to be
tested.
Selection Switches :- Switches use
to select electrical parameters
ranges.
Indicators :- To indicates various
parameters status i.e. On-Off. For
Example Power, hold, Warning, etc.

21. MEGGER - ELECTRONIC
Advantages
1. Level of accuracy is very high.
2. IR value is digital type, easy to read.
3. One person can operate very easily.
4. Works perfectly even at very congested
space.
5. Very handy and safe to use.
21-Oct-22 Sarun Kumar K P EEE Dept., IESCE 23

22. WHY DO WE NEED TO HAVE GOOD EARTHING?
 The grounding system is an essential element for the
electrical system security and it is required to:
1. Allow for protective devices activation when there is
an insulation fault.
2. Equalize the potential of conductive parts that can be
accessed simultaneously, with the potential in the
surrounding soil in order to prevent people from
being exposed to hazardous voltages.
3. Allow the lightning strike energy to be safely
dissipated.
4. To Reduce electromagnetic interferences.
21-Oct-22 Sarun Kumar K P EEE Dept., IESCE 24

23. Measurement of Earth Resistance
• Resistance between the earthing plate & the
surrounding ground
• Earth Resistance is the resistance existing between the
electrically accessible part of a buried electrode and
another point of the earth, which is far away
• The earth resistance should be as minimum as possible
• Fall of potential method & Earth tester
• The resistance area of an earth electrode is the area of
the soil around the electrode within which a voltage
gradient measurable with commercial instruments
exists
21-Oct-22 Sarun Kumar K P EEE Dept., IESCE 25

24. Measurement of Earth Resistance
• E is the earth electrode under test & B is an
auxiliary electrode positioned so that the two
resistance area do not overlap
• One more auxiliary electrode is placed half
way between E & B
• A current is passed through the earth path
from E to B and the voltage drop across E and
the 2nd auxiliary electrode is measured
• Earth resistance, Re =voltage drop between E &A
Current through the earth path
• To ensure that the resistance areas do not overlap, the
auxiliary electrode A is moved to positions C & D
• If Re measured are of same value, the mean is taken
• Otherwise B is driven in at a point further away from E &
test is repeated
21-Oct-22 Sarun Kumar K P EEE Dept., IESCE 26

25. Measurement of Earth Resistance
• To eliminate electrolytic effect, it is
necessary to use an alternating current
source
• Can be performed, with current at
power frequency from a double wound
transformer, by means of a voltmeter &
an ammeter
21-Oct-22 Sarun Kumar K P EEE Dept., IESCE 27

26. Measurement of Earth Resistance
21-Oct-22 Sarun Kumar K P EEE Dept., IESCE 28
Earth Tester
Special type of Megger which sends
AC through earth & DC through the
measuring instrument
Four terminals: P1,C1,P2 & C2
Value of resistance is indicated by
the instrument directly when its
handle is turned at uniform speed