Transformer Insulation Resistance Testing

[dinocu]

Just wanted to get an opinion on whether the windings of a transformer should be shorted together when performing an insualtion resistance test on a transformer. I was taught to always short them, however some people believe it does not matter.

I know when you perform a capacitance and dissipation factor test the bushings should be shorted. Does anyone have an explanation for why this is. Thanks.

 

[electricpete]

First, you already understand that you need to ground the windings not under test. But the question is whether they are shorted together.

One possible factor - if for some reason there is a rapid transient during the testing, any traveling wave can nearly double at an open-circuit. That condition is to be avoided if possible. Source of transient might be tripping of a test set, inadvertant rapid grounding (without grounding resistor), etc.

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