Transients, surges & overvoltage

A transient is a very brief overvoltage — microseconds to milliseconds — that rides on top of the normal supply, often many times the nominal voltage. It's far too quick for normal protection to react to, but more than enough to puncture insulation or destroy electronics.

They come from two broad sources: external, mainly lightning activity coupling onto the supply network, and internal, from switching — large loads, capacitor banks, or inductive loads being switched produce their own surges, including the inductive kick from coils.

Surge protective devices divert the spike to earth before it reaches sensitive equipment. They sit across the supply doing nothing at normal voltage, then conduct hard the instant the voltage exceeds a threshold, clamping it to a survivable level — the let-through voltage.

Protection is usually staged: a robust device at the origin handles the big external surges, and finer devices nearer sensitive equipment clamp what's left. Because they take the hits, surge devices wear out — many have an end-of-life indicator, and one showing failed has stopped protecting whatever is downstream.