Surge protective device (SPD)

An SPD protects equipment from short, high-voltage spikes — transients from lightning activity or switching events — by clamping them before they reach sensitive gear.

Normally the SPD does nothing. When a transient appears, its internal components (typically metal-oxide varistors) become low-resistance for an instant and divert the surge energy to earth, holding the voltage down to a safe level.

Each surge degrades the component slightly. After enough events (or one big one) the SPD reaches end of life and indicates a fault — at which point it's no longer protecting and must be replaced.

At switchboards and distribution boards to protect downstream equipment, and on sensitive electronics. A status window or flag shows when the device has expired.

SPDs fail by design — sacrificially. The indicator window changes colour or a flag pops, and from that moment everything downstream is unprotected while working perfectly. Nobody notices until the next storm takes out the gear the SPD used to be protecting.

The louder failure: an MOV fails short and takes out the SPD's dedicated fuse or breaker — or trips upstream protection during a storm.

Field testing is a visual job: check the status window/flag on every module, and check the SPD's dedicated fuse or MCB hasn't operated. Where the SPD has a remote-signal contact, confirm it's actually wired to something that someone watches.

Proper electrical verification of clamping performance needs specialist gear — in practice, an indicated or suspect module gets replaced, not tested.

Check the SPD first after any storm damage — if it expired quietly last season, that's why this strike got through. Make the status window part of any board inspection; it's a two-second check nobody does.

Lead length matters more than brand: long, looped connection leads add inductance that lets the spike ride over the top of the protection. Short and straight, per the instructions, or the SPD is decorative.