What Is BIL (Basic Insulation Level)?
Quick answer — BIL (basic insulation level) is the crest value of the standard lightning impulse voltage (a 1.2/50 microsecond wave) the transformer insulation is designed to withstand — a key measure of its insulation strength. In IEC it is often called the rated lightning impulse withstand level (LI); in IEEE it is given in kV at the system voltage.
What is BIL / basic impulse level?
What it is
BIL is the peak impulse voltage the insulation must survive. It sets the insulation margin above the normal operating and switching voltage, protecting against lightning and switching surges.
How it is tested
A standard 1.2/50 us lightning impulse is applied, and the insulation must withstand it without breakdown; the full-wave and chopped-wave impulse tests are part of the type test.
Why it matters
A higher BIL means a larger insulation margin and heavier, more costly insulation. The BIL is chosen for the system voltage and the required protection coordination with surge arresters.
References & standards
- IEC 60076-3 (insulation levels)
- IEEE C57.12.00
Related
Related manufacturers
Prolec GE (GE Vernova) · Prolec GE Waukesha · Hitachi Energy USA · Siemens Energy · Eaton · Virginia Transformer Corp
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Technical content is prepared from engineering references, standards frameworks, manufacturer documentation and industry sources. Contractual or compliance decisions should refer to current official standards. TransformerPath Technical Editorial.
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