Surge & voltage
Surge Protective Device (SPD) Selector
Get the SPD type, minimum Uc, target Up, and discharge rating for a low-voltage installation from the system earthing and the point of installation, to IEC 61643-11 / IEC 60364-5-534. Confirm the final device against the equipment withstand voltage and the manufacturer’s data.
How the SPD is selected
Four properties define the device. The type follows the point of installation and the lightning exposure. The Uc (maximum continuous operating voltage) must sit above the system voltage in the relevant connection mode: at least 275 V for line-to-neutral modes on 230/400 V systems, and 440 V for line-to-line modes such as IT. The Up (voltage protection level) must be below the impulse withstand of the protected equipment — 2.5 kV for standard category II equipment, 1.5 kV for sensitive category I. The discharge rating is Iimp for Type 1 and In / Imax for Type 2.
Frequently asked questions
Type 1, 2 or 3 — which do I need?
Type 1 at the origin where direct lightning is a risk (external LPS, overhead lines); Type 2 at main and sub-distribution boards for most installations; Type 3 at sensitive equipment for fine protection, always with a Type 2 upstream.
How is Uc chosen?
From the system earthing and connection mode. For 230/400 V TN and TT, line-to-neutral Uc is at least 275 V; IT and line-to-line modes need at least 440 V. Always confirm against the manufacturer’s rated Uc.
Does the SPD need a backup fuse?
Yes — an SPD needs an overcurrent disconnector (fuse or breaker) sized per the manufacturer’s data, unless one is integrated. Keep connecting leads short to keep the effective Up low.
Specify the SPD
Match the type, Uc, Up, and discharge rating to a device, and add the correct backup protection.
Guidance for a starting specification only. Final selection, coordination, backup protection, and installation must be verified by a qualified person against the equipment withstand voltage and the manufacturer’s datasheet.