simplify the correction of resistance values. Table 7-3 gives such factors for rotating equipment, transformers, and cable. You multiply the reading you get by the factor corresponding to the temperature (which you need to measure).
For example, assume you have a motor with Class A insulation and you get a reading of 3.0 megohms at a temperature (in the windings) of 131F (55C). From table 7-3 you read across at 131°F to the next column (for Class A) and obtain the factor 15.50. Your correct value of resistance is then 3.0 megohms x 15.50 = 46.5 megohms (reading at
(Correction ( Corrected 131F) factor for reading for 68F Class A in- or 20C) sulation at 131F) Note that the resistance is 14.5 times greater at 68F (20C) than the reading taken at 131F. The reference temperature for cable is given as 60F (15.6C), but the important point is to be consistent-correcting to the same base before making comparisons between readings.
We mentioned in this chapter about the presence of moisture in insulation and its marked effect upon resistance values. You might expect that increasing humidity (moisture content) in the surrounding (ambient) air could affect insulation resistance. And it can, to varying degrees.
If your equipment operates regularly above what is called the "dew-point" temperature (that is, the temperature at which the moisture vapor in air condenses as a liquid), the test reading normally will not be affected much by the humidity. This stability is true even if the equipment to be tested is idle, so long as its temperature is kept above the dew point. In making this point, we are assuming that the insulation surfaces are free of contaminants, such as certain lints and acids or
Table 7-3. - Temperature Correction Factors (Corrected to 20C for Rotating Equipment and Transformers; 15.6 for Cable)