Insulation coordination is the engineering trade-off between how much dielectric strength to buy and how much surge protection to fit, judged against the lightning, switching and temporary overvoltages a system will see. Engineers compare arrester protective levels with withstand levels such as BIL and keep a margin, typically at least 20 percent for lightning impulses. Done well, any flashover happens at a cheap, self-restoring point such as a line insulator, not inside a transformer.
With the arrester 15 metres from the bushing, the insulation coordination margin drops under 20 percent, move it closer.
Also heard as
- insulation co-ordination
Related terms
BIL
Basic lightning impulse insulation level: the peak impulse voltage equipment must withstand without breakdown.
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.
Overvoltage category
Class from I to IV ranking equipment by its position in an installation and the surges it must survive.