Tech Tip High Voltage Testing

Tech Tip: Motor Testing

Caution – High voltage testing existing machines!

I have been tasked with investigating a number of unexpected machine failures in the field. In each case the failures relate to dead grounds on large, form wound machines. When investigating the cause we discovered that the machines were operational until testing. At this point, high potential testing was discovered to have been used on machines that have high levels of contamination.

Were there ways to deal with avoiding the failures? There are some steps that can be taken to avoid failures when performing high voltage testing, but these just reduce the high risk slightly.

For high voltage insulation to ground tests, the first thing to consider is evaluating the insulation system. This can be accomplished by performing a polarization index test in which the polarization curve is observed. Deviations to the PI curve will indicate dry and brittle insulation systems or ground-wall contamination. With these conditions, the risk of failure during a high voltage insulation test is extreme.

The next decision is to then determine: the type of high potential test to perform; how it is performed; and, the value to be applied. In an existing winding, the safest method is the DC high potential test at a value of twice the voltage plus 1000 Volts times 1.7 and then select 60% to 70% of the value. The test should be performed in increments in which it should be expected that the current (leakage currents) will jump and then settle down to a lower value. This value should be noted and trended. If there is a sharp uptick, then it should be considered that the insulation system has ‘failed’ the test. The extreme risk in this test is if the technician applies the full voltage immediately.

Before considering any of these situations, however, you need to ask yourself: “is this test necessary considering the risk?”