SOLUTION

The loss of charge method has certain serious drawbacks and these are as follows:

(i) The current actually flowing, does not depend upon the insulation resistance alone. Due to the absorption effect in the dielectric of the capacitor, a small absorption current also flows through the insulation. This absorption current decays very rapidly at first but afterwards, this decay is very slow, therefore, observations must be continued for a longer period in order to attain high accuracy in measurement.

(ii) In the rubber-covered cables this absorption current is large, about 5-6 times the leakage current and is dependant only on the resistance after the application of the input voltage for 1 min and is equal in value to the true leakage current even after 7 min. It has been observed that the absorption current is about 5% to 10% of the total current flowing through the insulation after the voltage has been applied for 6 to 7 hours. Therefore, the insulation resistance depends to a greater extent on the time interval of application of the voltage. In commercial testing, the time of application of the voltage is normally not more than 1-2 min and therefore, from the foregoing discussion it is clear that the insulation resistance so obtained will be much less than the actual value.

(iii) The resistance of the insulating materials decreases with the increasing temperature and in some materials, this drop is very rapid. Therefore, it is important that while stating the results of insulation resistance measurements, the temperature must be specified at which the measurements have been carried out.