Motor tip: The effect of surge arrestors on motor RTG

Many large critical motors have surge arrestors installed to prevent voltage transients from damaging the motor insulation. Some of these arrestors will have a breakdown voltage within the 5-kilovolt (kV) range of the MCEMAX test equipment. An example is a 4kV motor with surge arrestors that start conducting at a ground wall voltage of 3.7kV. If you have the MCE test voltage set at 5kV for a standard test or polarization index (PI), you will push the surge arrestor beyond its breakdown voltage and see a very low resistance-to-ground (RTG) value. If you want to verify the breakdown voltage, run a step voltage test. Once the breakdown voltage is reached you will see a dramatic drop in RTG or an increase in current flow through the insulation. All tests after that should be performed at a voltage less than the breakdown voltage, which will allow valid insulation testing of the ground wall without showing the arrestors as a parallel resistance.