Electrical Hipot Insulation Breakdown

Please provide more detail of what specifically you were testing that failed, plus test voltage and connections used (DC or AC?). You mention switchgear, cables, and (I presume instrument) transformers. Was it the instrument transformers that failed? Were you hipotting the primary winding or the secondary? Or from pri to sec?

You should check the manufacturer's specifications for acceptable test values. You may be exceeding allowed value, and causing failure. Also check the InterNational Electrical Test Association ANSI/NETA 2009 Standard for Acceptance Testing Specifications:

http://www.netaworld.org/

Minimum insulation thicknesses are determined by the manufacturer from standards (ANSI, IEC, etc.) to get minimum creepage distances, etc. However, I suspect creepage is not your problem, but overvoltage stress of the insulation.

Dear S P Rajah,

I presume you are using a DC high potential test set which step-up with the epoxy and gas or oil immersed insulation testing transformer.The input voltage,say 240v 50Hz AC is step-up to the required test voltage and rectified.This arrangement allows smaller and lighter test transformer.For the tesing of long and large cables,the charging current is large in the initial stage even there is no fault in the cable.For switchgears,the test current or leakage current increases with the magnitute of partial discharge/ionisation of leakage paths/medium.

You should check/change the output current limiter (in mA) CB on your test set.Over loading the test transformer will burnt it and it is very costly.Another type of test set uses ultra low frequency ac for cable testing.

Large magnetising current may be due to shorting of turns in the winding or loss of magnetising property of the core material/silicon steel of the testing transformer which is less likely.

Regards,

Khor.