I was wondering where could i find answer to this question.
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A measurement of the impulse withstand capability of a piece of electric power equipment based on its ability to withstand 50% of impulses applied at the BIL voltage.
"BIL is Basic Insulation Level and NOT basic impulse level, as mentioned by you incorrectly. The BIL includes the power frequency overvoltage withstand capacity and the impulse (both Lightning Impulse and p the Switching Impulse) withstand capacity"