delabs Circuits

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. - Ananth

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