Surge protective device (SPD)
Diverts transient over-voltages (from lightning or switching) to earth to protect equipment.
What it is
An SPD protects equipment from short, high-voltage spikes — transients from lightning activity or switching events — by clamping them before they reach sensitive gear.
How it works
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.
Where it's used
At switchboards and distribution boards to protect downstream equipment, and on sensitive electronics. A status window or flag shows when the device has expired.
Safety first
A faulted/expired SPD leaves downstream equipment unprotected. Isolate before replacing the module; this is licensed work.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Related faults
Surge protection device (SPD) showing a fault / end of life
A board-mounted surge protective device shows a fault indicator (window changed colour / flag) — it has likely reached end of life after absorbing surges and no longer protects.
Safety switch trips during storms / lightning activity
A safety switch trips during thunderstorms/lightning — usually transient surges or moisture, sometimes a marginal install — and it's about telling a transient nuisance from a real fault.
Related definitions
Circuit breaker (MCB)
Automatically disconnects a circuit on overload or short circuit, and can be reset rather than replaced.
Earthing & bonding
Connecting exposed metal to earth so a fault blows protection fast and metalwork can't become live.
Distribution & sub-mains
How power is split from the main board into final circuits and sub-boards, with protection at each level.