SPD Selection Guide
Seven steps from the protected equipment to type, parameters, back-up protection and earthing
This sequence applies to building power distribution, telecom and data, industrial control, video surveillance, PV and EV charging installations. Define what is being protected and where it is installed first, check the electrical parameters second, and deal with back-up protection and earthing last — that order avoids most cases where equipment is still damaged although an SPD was fitted.
Selection sequence
Establish whether the circuit is AC power, signal and data, antenna/feeder, or direct-strike and earthing, and which system it belongs to (distribution, telecom, industrial control, CCTV, PV, EV charging). Devices for different circuits are not interchangeable.
For power circuits confirm the earthing system (TT / TN-S / TN-C-S / IT), rated voltage and maximum continuous operating voltage Uc. For signal circuits confirm the interface (RJ45, RS485, BNC, antenna), operating voltage and data rate.
Use Class I devices (10/350 µs, rated by Iimp) at the LPZ0→LPZ1 boundary, Class II devices (8/20 µs, rated by In / Imax) in sub-distribution boards at LPZ1→LPZ2, and Class III or socket-type devices at the equipment.
The voltage protection level Up must be below the impulse withstand voltage Uw of the equipment — in practice Up ≤ 0.8×Uw. Check In, Imax, Uc and response time as well, and confirm the mounting method (35 mm DIN rail, cabinet or wall).
Install a back-up protector (SCB) or a fuse / circuit breaker matched to the SPD upstream of it. The device must clear power-frequency short-circuit current when an SPD fails short-circuit, yet not trip on lightning impulses — a standard breaker does not always satisfy both.
Keep SPD earthing conductors short, straight and thick — a total length below 0.5 m and copper cross-section of at least 6 mm² is common practice — and bond them to the equipotential bar. Earthing resistance follows the applicable standard and local soil conditions.
Leave sufficient cable length between two SPD stages (typically at least 10 m), or add a decoupling element as described by the manufacturer. Without coordination the downstream device may see a share of the current beyond its rating.
Quick selection table
| Location / circuit | Recommended type | Key parameters | Product range |
|---|---|---|---|
Connection by earthing system
| System | Typical locations | SPD connection |
|---|---|---|
- The number of poles must match the earthing system — a mismatch can make the SPD inoperative or cause nuisance operation.
- Single-phase and three-phase SPDs are not interchangeable; confirm the system and number of phases before ordering.
Common mistakes
- Buying on price and ignoring Up — if Up is above the impulse withstand voltage of the equipment, the SPD can operate and the equipment can still be damaged.
- Using an ordinary breaker instead of a back-up protector, although the two duty cycles differ and the breaker may trip or fail to clear follow current.
- Long or thin earthing leads, whose inductance raises the residual voltage actually applied to the equipment.
- Protecting only one stage — remote equipment remains exposed to reflected and induced overvoltages.
- Protecting the power supply but leaving the signal interfaces unprotected, which is a frequent path of damage.
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