Basic Impulse Insulation Levels (BIL)

The Basic Impulse Insulation Levels refers to the ability of electric equipment such as transformers to withstand lightning surges.  When lightning strikes a transmission line, a traveling wave is created.  This traveling wave travels along the line and damages the transformer winding.

Surge arresters mounted on the line can mitigate the surge, however, they cannot totally eliminate it.  Voltage surges can also be created by the switching of circuit breakers and switches. 

The BIL or the Basic Impulse Insulation level indicates the ability of the transformer to withstand these heavy surges.  Transformers with a rating of 600 volts and below are designed to have a BIL of 10 kV.