Surge Protection Devices (SPDs): How They Work, Types & How to Choose

Direct answer: a surge protection device (SPD) is a protective component that clamps dangerous voltage spikes and redirects them to earth before they can damage electronics. You choose it by Type (1, 2 or 3), by rated voltage (e.g. 230 V AC or 1000 V DC for solar), and by discharge capacity (kA).

Lightning doesn't need to hit your building to destroy your equipment. A nearby strike — or even a big motor switching on in the same grid — can send a surge down the line. Without an SPD, that spike lands on the circuit board of your meter, inverter, controller or appliance. With one, it's clamped in microseconds and dumped into earth.

How an SPD Actually Works

An SPD sits between the power source and the equipment it protects. Under normal voltage it's a high-impedance bystander. When the voltage jumps, the MOV (metal-oxide varistor) or gas tube inside conducts, clamping the spike and sending the excess energy to the ground conductor. When the voltage returns to normal, the SPD goes back to standby.

SPD Types: What the Numbers Mean

Most commercial and residential installs combine Type 2 at the panel with Type 3 at the socket. For solar arrays, the DC side needs its own SPD rated for the array voltage (1000 V or 1500 V DC is common today).

Key SPD Ratings to Compare

• Uc (max continuous voltage): must match your system voltage — 275 V AC for 230 V mains, 1000 V DC for solar.
• In (nominal discharge current): typically 20 kA for Type 2; what the SPD can handle repeatedly.
• Imax (max discharge current): the one-off limit, often 40 kA — this is the headline number to compare.
• Up (voltage protection level): the lower, the better — the clamp voltage your equipment actually sees.
• Mode: 2P for single phase, 3P/4P for three-phase systems.

Surge Protection for Solar Systems

PV arrays are exposed, rooftop, and full of long DC cabling — a lightning magnet. A certified solar SPD (e.g. our T2 2P PV 1000 V 40 kA solar surge protector) protects the DC side before the inverter. It installs in the combiner box or the inverter's DC input, and a TUV-certified unit gives you the compliance documentation many utilities and insurers now ask for.

How to Choose an SPD (Quick Checklist)

1. Is it AC mains or DC solar? → pick 230/400 V AC or 1000/1500 V DC
2. Where does it go? → Type 1 at the entrance, Type 2 in the panel, Type 3 at the device
3. How many phases? → 2P, 3P or 4P
4. What's the surge risk? → 20 kA nominal / 40 kA max is the common industrial spec
5. Certification? → look for IEC 61643 compliance and third-party marks (TUV, CE)

FAQ

Do I need an SPD if I have a surge-protection power strip?

Power strips are Type 3 devices — useful, but they can't absorb a large surge alone. A Type 2 SPD at the panel is the primary protection; the strip is a fine second layer.

How long does an SPD last?

Most have a service-life indicator and can handle many small surges. After a major event, check the indicator and replace if depleted.

Can one SPD protect the whole house?

One Type 2 SPD at the main panel protects everything downstream from grid surges. For direct-strike areas, add a Type 1 at the service entrance.

Buy Factory-Direct

Yomin Electric manufactures SPDs, DC isolators, fuses and the rest of the PV protection chain in Zhejiang, China, and ships to 95+ countries. Every unit is tested before dispatch — send us your specs for a quote.