Voltage Regulator - Insulation resistance testing

[Brad1234]

Hello,

I had to perform a Megger insulation resistance test recently on a single phase step voltage regulator and the first part of the procedure had us bond all three bushings together. I can’t find a clear explanation as to why all three bushing (Source, Load, and Source Load bushing) need to be bonded together. Red lead (hot) connected to the source bushing and black lead connected to ground. The Megger is set to PI and we run the test for 10 min to saturate the winding fully.

[backpack96]

In a single phase regulator the shunt winding and series windings cannot be isolated so when you energize one of the three bushings, SL, S and L you are energzing the other 2 bushings. Normally you would like to have all of the bushings at the same potential. Since there are no windings to guard or measure between there is one test to ground.