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Insulation Resistance Theory - doc00030

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 DRT101 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 DRT101 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 flows 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.

Resistivity-Temperature



Resistivity-Humidity

High Resistance Materials Tables

Liquids

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

70% RH CONTAMINATED CLEAN
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
MATERIALS   SOLIDS              OHM -M  

Solids

MATERIALS OHM -M
Polyethylene 1017
Poly propylene 1017
PVC 1018
Polyurethane 1015
Asbestos fiber + phenol 108
Mica + phenol 1012
Glass-fiber + polyester 1012
Glass-fiber + epoxy 1014
 
Pages 1, 2, 3, 41, 42, 43 of DRT101 Insulation Tester and Tera Ohm Meter Operating Manual, A product that i used to make-n-sell. - Ananth
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