Also known as basic insulation level (BIL): A specific insulation level, expressed in kilovolts, in accordance with which the complete system (including the line pole insulation to earth at every pole, the switchgear, the line isolators and reclosers, and the substations connected to the system, including their Transformers) is designed and constructed. (NRS 039:1995)
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BIL is basic insulation level as per you or in other words it would be "basic impulse level" which implies the limit up to which an insulator could withstand impulse due to lightning strikes. Impulse is generated on the insulation due to the high voltage surges and spikes due to lightning strikes. BIL is generally much higher as compared to power frequency withstand voltage. In other words BIL is lightning Impulse withstand voltage. Impulse withstand voltage is the voltage that is generated due to switching operation and it's much higher than power frequency withstand, but BIL or the lightning withstand voltage is a the voltage level that an insulation can withstand during surge or lightning strikes.
Lightning impulse withstand voltage
That is the voltage rating the product can stand up to. If the lightning withstand voltage was 100,000,000V, that means the item could get struck by lightning that is up to 100,000,000V without damaging it.
Sudden rise in Voltage is called Impulse voltage.
It is the rated test voltage that will not cause insulation breakdown.