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.


 
Insulation Resistance Testing - AEMC

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.

Megger - MIT200 Insulation Tester

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.

Here is a Graph from a Scientific Journal which shows relationship between Temperature and Insulation Resistance or High Resistance of Substances.

Another graph plotting the relationship of Humidity and Temperature from the same American journal. I will add the name of the Author and Document when i locate it in my files. Note that the graphs are from experts. I was focused on creating the electronics. I did verify some of the parameter and its variations later.

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 10^{6 -} 10⁸ 10^{10 -} 10¹²  10^{12 -} 10¹⁴ Wood 10^{6 -} 10⁷ 10^{10 -} 10¹² 10^{11 -} 10¹³ Vulcanized fibers   fibers 10^{6 -} 10⁹ 10^{8 -} 10⁹ 10^{11 -} 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. - delabs

Errors may exist due to scanning and ocr

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