Humidity and Insulation Resistance

Humidity, like temperature can affect insulation resistance-to-ground measurements. Depending on the humidity level and ambient temperature, moisture may settle onto the cables, windings, motor frame, etc. This will affect the surface leakage component of the total current flowing to ground from the MCE® test equipment during Resistance-To-Ground (RTG) and Polarization Index (PI) tests. The higher humidity causes a higher leakage current and therefore a lower insulation resistance value. This is similar to changes in temperature, where higher motor temperatures cause a decrease in insulation resistance.

Can we correct for humidity like we correct for temperature? The short answer is “No”. Too many variables exist from motor to motor and even day to day to be able to correct for humidity. However, historical trending of insulation resistance and humidity could give insight into changes in insulation resistance that do not appear to be consistent with temperature changes. Keep in mind that humidity alone is not the variable but rather temperature and dew point. Based on the humidity, when the surface temperature drops below the dew point moisture in the form of condensation will settle onto these cooler surfaces. It is this condensation that affects the insulation resistance of your motor. Some guidelines exist in the industry that suggest a high limit of 50% – 65% humidity to test insulation systems. This might be good for acceptance testing but when it comes to field testing or troubleshooting, and especially in the state of Florida, these guidelines will not apply. Beyond historical trending of humidity and resistance, space heaters are always recommended, which keep the motor temperature above the dew point and prevent condensation from forming on your insulation system.

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