Basic insulation level

BIL or basic impulse insulation level is the dielectric insulation gradient of a material tested to withstand the voltage stress at a voltage impressed between the material and a conductive surface beyond the BIL rating, an electric tracking starts to occur which will then result into an arcing flashover to the conductive surface.

In addition it is the capacity of an equipment to withstand mechanical stress.. like lightning strike without causing any damage to the equipment...

Insulation Coordination is the process of determining the proper insulation levels of various components in a power system as well as their arrangements. It is the selection of an insulation structure that will withstand voltage stresses to which the system, or equipment will be subjected to, together with the proper surge arrester. The process is determined from the known characteristics of voltage surges and the characteristics of surge arresters.