Lightning Surgers

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BIL (or Basic Impulse Insulation Level) is a withstand voltage rating derived from + and - polarity impulse tests using a standard 1.2/50 voltage waveform. For most insulation, + and - polarity flashover will not be the same so BIL is based on the lower of the two. Thus, an impulse voltage of a magnitude below the BIL should not result in flashover of the insulation.

Although there are other impulse withstand ratings on insulation (steep wave, chopped wave, switching) BIL is one point of coordination with lightning protective devices such as surge arresters. BIL is normally coordinated with the surge arrester's discharge voltage, which is simply the voltage that appears across the arrester during discharge of a surge current of a given magnitude through the internal structure of the arrester (typically non-linear metal-oxide varistors). Since that voltage will also appear across the terminals of the insulation it is protecting, the insulation BIL must be greater than the arrester IxR discharge voltage to prevent flashover of the insulation.

Practically all insulation has a volt-time flashover characteristic. The shorter the time, the higher the flashover voltage. The operating characteristics of a modern surge arrester are much more consistent. On the volt-time curve, BIL is normally coordinated with the arrester at 10 microseconds and the insulation switching surge withstand level is often assumed to be 83% of the BIL, with a time to crest between 200 and 2,000 microseconds.

Details of specific tests can be found in ANSI/IEEE-4, "Techniques for High-Voltage Testing".

Surge arresters would normally be used for lightning and switching surge protection for current and voltage transformers. The specific class of arrester would depend upon the transformer and application.