Engineers test solid insulation using two important test methods, and both use the Direct Voltage. One of the tests involves checking the resistance of the insulation, while the other tests the leakage current through the insulation at high voltages.
Insulation Resistance Testing
The instrument used for conducting this test is a megohmmeter. The instrument can be hand-cranked, motor driven, or electronic. Regardless of its principle of operation, a megohmmeter generates a direct voltage in the range of 100-15,000 V, and when applied to the insulation, indicates the material’s resistance in megohms.
As the resistance of any insulation material is temperature dependent, all readings need correction to the standard temperature for the class of the equipment under test. Usually, engineers refer to a table for the temperature correction factors for various electrical apparatus.
The value of the resistance of an insulation material is inversely proportional to the volume of insulation under test. For instance, a material a hundred feet long would have one-tenth the insulation resistance of another material a thousand feet long, provided all other conditions remained identical.
Engineers use this test typically to obtain an indication of deterioration trends of the insulation they are testing. However, the value of the insulation resistance by itself is no indication of the weakness of the insulation or the total dielectric strength of the material.
However, if the value of the insulation resistance showed a continuous downward trend, it usually points towards a contamination of the insulation, along with deterioration ahead. Therefore, engineers measure the insulation resistance in four common methods— short-time readings, time-resistance readings, polarization index test, and step-voltage test—to check for deterioration in the insulation system.
This reading is only a rough check of the condition of the insulation. It measures the value of the insulation resistance for a short period, through a spot reading. The reading usually lies on the curve of increasing value of the insulation resistance. When comparing this value with the previous values, if a downward trend is indicated, then the insulation is deteriorating.
All good insulation materials show a continued increase in the value of their resistance over the period when the voltage is applied. However, contamination with moisture and dirt decreases the resistance value of an insulation system, indicating contamination.
Indicating conclusive results of the state of the insulation, the time-resistance method is independent of the equipment size and its temperature. The state of the insulation system is derived from a ratio of the time-resistance readings.
This is a specialized application of the dielectric absorption test. Polarization Index is the ratio of the insulation test at 10 minutes to the insulation test at 1 minute. A ratio of less than one indicates the insulation is deteriorating and needs immediate maintenance. Usually, this test is reserved for dry insulation systems such as for cables, dry type transformers, rotating machines, and so on.
A controlled voltage method is used to apply voltage in steps to the insulation under test. If the insulation is weak, it will show a reduction in resistance that would not be apparent under lower voltage levels.