Transformer Insulation Resistance Test Voltages

[michaellabeit]

I've got a question regarding which DC test voltages to use for dry type power transformers. I do the standard IR tests for transformers, i.e., high to low, high to ground with low guarded, and low to ground with high guarded. NETA MTS-2019 Table 100.5 prescribes a DC test voltage based off of the transformer rating, e.g., 5kV transformer means a 2.5kV test voltage. The trouble is that the high and low side windings have different ratings, do they not? A step-down transformer may be rated 4160V on the high side but 480/277V on the low side. So for a high to low test, which DC test voltage is the appropriate one since windings of different ratings are being energized? Table 100.5 says to apply 2.5kV to a 5kV rated transformer and 1kV to a 600V rated transformer. But if the high side hypothetically was 5kV and the low side was 600V would the appropriate test voltage for a high to low test be 2.5kV or 1kV? I've searched high and low for a definitive answer.

My understanding is that it must be 2.5kV because testing high to low is akin to testing multiple resistances in series, i.e., the resin that covers the windings and the paper. If the insulation covering the 5kV rated high side windings can withstand a test at 2.5kV, then surely the insulation of the 5kV high side AND the insulation of the 600V low side, in series, can withstand a test at 2.5kV.

[nwilkening]

At the company I work for, when we test from high to low and high to ground we use the voltage associated with the high side of the transformer, and when we test from low to ground we use the voltage associated with the low side of the transformer.

[mjamesvaughan]

The insulation system between the high and low voltage windings should be tested at the value of the higher voltage winding.

[Pommes]

When you do high to low, test at the high side rating (same with high to ground), and when you do low to ground, use the low side rating.