Performing insulation resistance test on potential transformers

[antigfk]

What is the proper voltage to test High-side to ground for an insulation resistance test on a set of 4200-120V potential transformers? Specifically, a "maintenance" test as this is older used equipment. I have the NETA MTS table that says for "electrical apparatus" up to 5000V, the MEGGER test voltage should be 2,500VDC minimum. Does this apply to PT's? Does the NETA and the ANSI recommended test voltages differ?

Thank you in advance.

 

[dpc]

NETA Maintenance Test Standards specifies the same test voltage for instrument voltage transformers as for any other transformer. It's Table 10.5 in my version, which is probably out of date.

 

[esee135]

The trick with any transformer insulation tests is to ensure that the winding that is part of the test specimen is tested at the correct voltage. ie: do not do a 5kV megger on a 120V secondary and hope that it survives.

Other techniques such as grounding the non tested winding are also employed to test primary to core + secondary, or secondary to core + primary.

Be aware of what you are actually testing. In other words, if you are testing the insulation to the core, ensure that one of your test leads is actually on the core.

As with any winding, insulation resistance tests may not test all the portions of a winding (eg: turn to turn, strand to strand).