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Monday, August 07, 2006

Insulation Resistance Theory

Insulation resistance is a very complex parameter, but it is very important to study it in the field of electrical & electronic engineering. The I.R. value, of a specimen changes with time on continuous application of D.C.voltage, the IR value changes with change in magnitude of the applied DC voltage and IR value also varies with Temperature, Humidity and contamination. When measurement is made, the IR value slightly increases after each measurement cycle. This is due to residual polarization of the previous application of voltage due to all the above factors, IR measurement becomes both difficult and complicated. The DRT 101 overcomes most of the conventional problems and makes Insulation resistance analysis more scientific & practical.

The problem of residual polarization can be overcome by :
  1. The terminals of the insulation shorted for a few minutes.
  2. The terminals are reversed after each measurement.
The DRT1C11 shorts the specimen terminals automatically when it does not apply voltage. But most accurate results are obtained only on, the 1st measurement on a specimen or by giving a gap of 30 mts for depolarization between each measurement.

When a steady DC voltage is applied to an insulation, the observed behavior is a analogous to that of changing a leaky capacitance through a series high resistance. The resultant current comprises of these components.
  1. A charging current due to the natural capacitance. This is large in the beginning and becomes negligible in a few seconds.
  2. The absorption current flaws in the body of the dielectric and decays with time. The time may run to many minutes, this absorption current gives rise to polarization residue effect.
  3. The actual conduction current through the insulation. This is constant with time. But can vary with voltage & temperature.
  4. The surface leakage current depends much on Humidity contamination.
Insulation may fail due to the following reasons:
  1. Mechanical vibrations may rupture insulation.
  2. Temperature decreases thermal strength.
  3. Dust & moisture cause contamination & decay.
  4. Corrosive material & electrochemical reactions.
Mode of insulation failure :
  1. Intrinsic breakdown.
  2. Thermal effects.
  3. Ionization.
Methods of measuring high resistance and IR :
  1. High voltage bridge.
  2. Leakage current measurement method.
  3. Loss of charge method.

Pages 1, 2, 3 of DRT101 Insulation Tester and Tera Ohm Meter Operating Manual, A product that i used to make-n-sell. - delabs

Friday, August 04, 2006

High Resistance Materials Tables


20 C OHM -M
min max
Benzene 1011 1012
Transformer oil 1010 1013
Gasoline 1010 1013
Castor oil 108 1010
Acetone 104 105
Ethanol 104 105
Distilled water 103 104

Glass At High Temperature

200 C OHM -M
Sodium Pyrex 2 * 106
Potassium Pyrex 8 * 109
Lead glass 2 * 1011

Effect Of Contamination On Surface Resistivity

Window glass 2 * 108 1012
Fused quartz 2 * 108 1013

Relation Of Resistivity With RH % And Temp

MATERIAL 70% RH 20 deg C 0% RH 100 deg C 0% RH 20 deg C
Marble 106 - 108 1010 - 1012 1012 - 1014
Wood 106 - 107 1010 - 1012 1011 - 1013
Vulcanized fibers fibers 106 - 109 108 - 109 1011 - 1012


Polyethylene 1017
Poly propylene 1017
PVC 1018
Polyurethane 1015
Asbestos fiber + phenol 108
Mica + phenol 1012
Glass-fiber + polyester 1012
Glass-fiber + epoxy 1014