BIL, basic lightning impulse insulation level, is the crest value of a standard 1.2/50 microsecond impulse that insulation must survive in a type test. It is quoted in kV peak, for example 95 kV for many 15 kV class distribution transformers and up to 1050 kV for 245 kV equipment, and steps through standard values for each voltage class. BIL anchors insulation coordination studies and surge arrester selection.
Spec calls for 125 kV BIL on the 24 kV switchgear, make sure the cable terminations match or beat it.
Also heard as
- basic insulation level
- LIWL
- lightning impulse withstand level
Related terms
Insulation coordination
Sizing insulation and surge arresters together so any flashover lands at a cheap, self-restoring point, not inside plant.
Surge arrester
Device that clamps transient overvoltages by diverting surge current to earth, then returns to insulating.
Flashover
Disruptive discharge over or around insulation that creates a short circuit through the air.