BIL is the crest value of the impulse
voltage that the transformer is required to withstand without failure. The
transformer BIL impulse duration is 1.2 × 50 μs. That is, the impulse reaches
its peak value in 1.2 μs and then decays to 50% of its peak value in 50 μs.
In addition to full BIL value, transformers are tested for chopped-wave withstand
(115% of BIL) and front-of-the-wave withstand (160% of BIL). These
tests are intended to simulate conditions that can occur when transformers
are subjected to lightning surges.
5.7 Preventive