DC SPDs are a specialist product that many installers encounter for the first time on their first solar PV job. They look similar to AC SPDs but are designed for a completely different electrical environment — high-voltage DC rather than 230V or 400V AC. Specifying the wrong device, or installing an AC SPD in a DC circuit, can result in the device failing to clear a fault, potentially causing a fire.

In an AC circuit, the voltage crosses zero 100 times per second (50Hz × 2). When a surge event causes an SPD's protective element (typically a Metal Oxide Varistor or spark gap) to conduct, the arc extinguishes naturally at the next zero crossing. In a DC circuit, there is no zero crossing — if the SPD element begins to conduct, it must be capable of interrupting the arc itself. This requires a fundamentally different internal design with DC-rated arc interruption capability.

Using an AC SPD in a DC circuit risks the device failing to interrupt the arc, leading to a sustained short circuit, overheating, and potentially a fire. Always use DC-rated SPDs in DC circuits, regardless of whether the voltage rating appears similar.

Both devices are 3-module width and designed for DIN rail installation in a string combiner box or at the inverter DC input. They protect against surges induced in exposed DC string cables — the long cable runs from roof-mounted panels that act as antennas for nearby lightning activity.

DC SPDs are installed at the DC input of the solar inverter, or within a dedicated string combiner box upstream of the inverter. The WCED WMPV and WPPV solar combiner boxes include integrated DC SPDs as standard — the 1-string and 2-string models provide complete DC surge protection alongside string fusing in a single weatherproof enclosure. See: Solar combiner boxes explained

Further reading:
Solar combiner boxes explained
SPD for solar and heat pumps
← Back to Electrical Safety Knowledge Base