What is the basic insulation level of various electrical equipments?

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...