By Sensitive electronics fail not only from direct lightning strikes but from transient voltage surges — brief spikes on power and data lines caused by nearby lightning, switching operations or grid faults. A surge protection device (SPD) clamps these spikes to a safe level, diverting the excess energy to earth before it reaches your equipment. This guide explains SPD types, classes, and how to choose one.
What does a surge protection device do?
An SPD sits between the live conductors and earth. Under normal voltage it does nothing. When a surge appears, it becomes conductive within nanoseconds, shunts the surge current to earth, and then resets — protecting downstream equipment. It is the internal counterpart to an external lightning protection system: the LPS handles the direct strike, the SPD handles the induced surge.
SPD classes explained
SPDs are graded by where they sit and how much energy they handle (per EN 61643-11 / IEC standards):
- Class B (Type 1) — installed at the service entrance / main panel; handles the highest-energy surges, including partial direct-strike current.
- Class C (Type 2) — installed at sub-distribution boards; protects against residual and switching surges.
- Class D (Type 3) — installed close to sensitive equipment for fine protection.
For full protection, these are used in a coordinated cascade — a Class B at the incomer, Class C at sub-boards, and Type 3 at the point of use.
See Ashlok’s SPD range (AC, solar and street-light) → Surge Protection Devices
Specialised SPDs
- Solar PV (DC) SPD — protects solar inverters and DC strings, a common failure point in rooftop and utility solar.
- Street-light SPD — protects LED street-light drivers from surges on long feeder lines.
- AC SPD (3+1) — standard configuration for three-phase supplies.
How to choose a surge protection device
Consider: the point of installation (incomer vs sub-board vs equipment), the system (AC three-phase, solar DC, street lighting), and coordination with any existing SPDs. Matching the class to the location is what makes the protection effective. If you’re unsure, ask Ashlok for a recommendation.
Frequently asked questions
Is a surge protection device the same as a lightning arrester? No. A lightning arrester (air terminal) intercepts a direct strike on the structure; an SPD protects electrical lines from voltage surges. Complete protection uses both.
Do I need an SPD if I already have lightning protection? Yes — the external system protects the building; the SPD protects the wiring and electronics from induced surges.
Where should the main SPD be installed? A Class B (Type 1) SPD goes at the service entrance / main distribution board, ideally with Class C at sub-boards.
Do solar systems need special SPDs? Yes — solar arrays need a DC-rated PV SPD to protect the inverter and strings.
How do I know if an SPD has done its job? Most SPDs have a status window/indicator that changes when the module has degraded and needs replacement.
Ashlok manufactures AC, solar and street-light surge protection devices to EN 61643-11. Explore surge protection devices →