BIL (Basic Insulation Level) — Transformer Glossary

BIL is expressed in kV peak for a standard 1.2/50 μs lightning impulse waveform. It is the primary parameter for specifying the insulation coordination of a transformer. For example, a 33 kV class transformer typically has a BIL of 170 kV. Higher BIL values provide greater margins against transient overvoltages but increase insulation cost and physical size. BIL is tested during type testing per IEC 60076-3 or IEEE C57.12.90, using a standard lightning impulse generator. The test waveform front time is 1.2 μs (±30%) and tail time is 50 μs (±20%).