This document discusses techniques for measuring the DC resistivity of insulation materials through non-destructive testing. It describes three main methods: using a Wheatstone bridge arrangement to measure resistivity of solid insulation; measuring resistivity of liquid insulators with a Wheatstone bridge; and measuring resistivity with a simple DC galvanometer. The document provides examples of calculations for determining volume resistivity from experimental measurements of resistance, thickness, and electrode geometry.

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Subject :- High Voltage Engineering
Topic :- Non – destructive insulation test
techniques:
Measurement of DC resistivity
Guided by:-
Pr. Brijal Y. Panchal

2. Presented by :-
Hitesh patel
Arpit Brahmabhatt
Nikita Chauhan
Rajiv Jha
Jainil Joshi
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3. Introduction.
Aim of Testing the Insulation Properties.
Measurement of DC Resistivity.
Methods of Measurement.
Specimen and Electrode.
Materials Used for Electrode.
Practical Fillings of Specimen.
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4. Wheatstone Bridge Arrangements For Winding Resistivity
Of The Specimen (For Solid Insulation).
Measurement of resistivity by Wheatstone Bridge Method
(For Liquid Insulators).
Measurement of resistivity by simple DC Galvanometer.
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5. • Various electrical devices, insulation is vital part to
safeguard the operation and life of equipment.
• Used as a various natures, shapes and sizes
• Types of insulation are solid, liquid, gaseous and
combinations
• Should have wide range of operating temperatures, to
withstand the varying climatic condition and wide range
of frequencies even up to several mega hertz
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6. • It should be tested a sample of it so that it cannot create
problem in operation.
• A properly shaped specimen of insulation is taken for
these tests and tests are conducted at the desired
temperature
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7. • Using Galvanometer
• Using DC Amplifier
• Using Wheatstone bridge
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9. • Lead of aluminum foil of thickness 10 to 50 µm thickness,
should be pressed and rolled on the specimen by
suitable roller.
• Silver paint which is a very good conductor can also be
used.
• Mercury can be used on which a specimen can be made
to float on it.
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11. • Rs = standard resistance
• RT = resistance of specimen
• Wheatstone bridge is formed by four resistance say
RA and RB = resistance of two arms.
• One of the resistance say RA is made variable to get the
null point on the galvanometer.
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12. • By adjusting the variable resistance RA of the bridge arm,
the bridge is balanced. This is indicated by ammeter
reading of zero.
• The DC HV stable supply is generally 500 volts to 2 KV.
This is measured by a suitable voltmeter V.
• Two way centre off switch B is used. Initially for charging
the circuit by supply voltage it is thrown to position 1.
• When the necessary observation are noted, then for
discharging the circuit the switch B is Thrown to position 2.
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13. • At balance condition R or Rp = Rs* Ra/Rb Wheatstone
bridge principle.
• With the specimen arrangement used
𝜌 =
𝑅∗𝜋
𝑡
r2 Ω m
Where 𝜌= volume resistivity in Ω m
t = thickness in m
r = effective radius in meters
Rp= measured resistance in Ω by bridge
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15. • The experimental set up is the same as in solid insulation
specimen.
• In place of solid insulation here it is liquid solution.
• The three electrode are connected to the bridge circuit
and by varying RA a nut point is obtained by balancing
the bridge.
• In this case following calculation are done.
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16. 𝜌 =
2𝜋𝑅𝑙
loge 𝑑2/𝑑1
Ω cm
Where 𝜌 = volume resistivity in Ω cm.
R = measured resistance in Ω.
l = effective length of the guarded electrode in meter.
d1 = outer diameter of guarded electrode.
d2 = inner diameter of unguarded electrode.
Know
n
value
s
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• Galvanometer is first calibrated by use of standard
resistance of the values 10 M Ω with tolerance ± 0.5 to ±
1%.

18. • The resistances of the specimens (Rp) is given by Rp =
𝑉
𝐷∗𝐺
where D = deflection in cm with specimen
V = Galvanometer sensitivity A/c
G = (V/Rs)*
1
𝑛
1
𝐷𝑠
Here Rs = standard resistance used for calibration
V = Supply voltage
Ds= Deflection in cm with standard resistance in position
n = The universal shunt ratio,
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19. • Volume resistivity 𝜌 = 𝜋r2R/t
where t = thickness of specimen meter
R = Specimen resistance
r = radius of electrode
• The ideal will be cleared by taking actual observations of
the experimental in the following case.
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20. Stable
voltage (V)
Std
resistance
(Rs)
Universal
shunt ratio
Radius of
electrode
deflectio
n
Galvanomet
er deflection
with
specimen
1 kV 10 MΩ n= 3000 r=5 cm Ds=33cm D=3.3cm
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Specimen thickness was 1.95 mm= 1.95 *10-1 cm. From these test result
calculate the volume resistivity in ohm-meter.
Solution :
From the data given we can find galvanometer sensitivity in A/cm.
∴ G =
𝑉
Rs
*
1
Ds
*
1
n
=
1∗103
10∗106 *
1
3000 *
1
33
= 0.010101 ∗ 10-7 A/c
Resistance R =
𝑉
𝐷∗𝐺
=
1000
3.3∗0.010101∗10−7 =30* 1010 Ω

21. • Volume resistivity 𝜌 =
𝜋r2R
𝑡
=
𝜋∗(5)2
1.95 ∗10−1*30*1010
= 1.2083*1014 Ωcm
∴ 𝜌 = 1.2083*1014 Ωcm
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22. • In this figure when B is taken on position 1 ,the voltage is
applied and circuit is charged .
• But after this if the switch is taken to position 2, it is
disconnected to the supply and the capacitance of the
specimen is discharged through it's own volume
resistance.
• Discharging current is taken in finite time interval and we
can obtain current time characteristic.
• The slop of this curve gives time constant . 22

23. Time constant τ =CR ;
C=capacitance of the specimen.
R=volume resistance.
R =
𝑡𝑖𝑚𝑒(𝑡)
𝐶 loge
Va
V
• Where t = time in second when voltage from normal voltage to
some volume value V and C is capacitance in farad.
• From this , volume resistivity can be calculated
𝜌 =
𝜋r2R
𝑡
Ωcm

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