Transient overvoltages from lightning and switchings destroy sensitive electronics in microseconds. Surge protection devices (SPD) have become a normative requirement in many installations. Here are the SPD types, placement, the critical 0.5 m rule and the interplay with lightning protection.
Introduction
An overvoltage is a short-term (transient) voltage spike that is many times higher than the normal mains voltage and lasts microseconds. It comes from two sources: lightning strikes (direct or induced in the network) and switching in the network (switching on and off large loads). The spike is short, but energy-rich enough to break through insulation and damage electronics. An overvoltage protection (SPD — Surge Protective Device) leads the spike to earth before it reaches the sensitive equipment.
When is SPD required?
The need is assessed according to DS/HD 60364-4-44 (section 443) based on the consequence of an overvoltage: if there is a risk to human life, public services, cultural heritage or large commercial/industrial activity, protection is required. In practice, the newer requirements mean that SPD is now normally required in the vast majority of new installations — including residential — unless a risk assessment shows that it can be omitted. The choice and installation of SPD itself is covered by DS/HD 60364-5-534.
The three SPD types
SPDs are divided into three types based on how much energy they can discharge and where in the system they are located. They work in cascade — the heavy discharge first, the fine protection closest to the equipment.
| Type | Tested with | Placement | Protects against |
|---|---|---|---|
| Type 1 | Lightning impulse current Iimp (10/350 µs) | At the installation's origin (main panel) — required for direct lightning protection | Direct/partial lightning currents |
| Type 2 | Nominal discharge current In / Imax (8/20 µs) | Main and sub-panels | Switching overvoltages and the rest of the lightning energy |
| Type 3 | Combination wave (1.2/50 + 8/20 µs) | Close to sensitive equipment | Remaining tips locally |
The key parameters
- 01Uc — højeste kontinuerlige driftsspænding SPD'en tåler uden at lede. Skal ligge over netspændingen med margen (typisk Uc ≥ 1,1 · U0).
- 02Up — the protection level: the voltage that the SPD lets through at a surge. Up must be below the protected equipment's surge withstand voltage (overvoltage category).
- 03Iimp (Type 1) — impulse current in 10/350 µs waveform, stated in kA per pole.
- 04In / Imax (Type 2) — nominal and maximum discharge current in 8/20 µs waveform.
Up is the figure that determines whether the protection works: equipment in a 230/400 V installation typically has a surge voltage category (e.g. category II with 2.5 kV withstand voltage for socket outlet devices). Choose the SPD so Up with good margin lies below that value.
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