Type 1, Type 2 and Type 3 SPDs explained, with what to put where
What surge protective device types mean, the ratings that matter (Iimp, In, Imax, Up, Uc), and how to coordinate SPDs from the main incomer to sensitive equipment and solar inverters.
Ashima Electrotech technical team · 3 min read
Short answer: Type 1 SPDs handle partial direct lightning current at the service entrance, where there is an external lightning protection system or an overhead supply. Type 2 SPDs handle induced and switching surges at distribution boards. Type 3 SPDs give fine protection close to sensitive equipment. They work as a coordinated cascade. Solar plants also need SPDs rated for PV DC voltage on the DC side.
What the types mean
| Type (IEC 61643-11) | Tested with | Key rating | Where it goes |
|---|---|---|---|
| Type 1 | 10/350 µs impulse (lightning-like) | Iimp | Main incomer, where there is an external LPS or an overhead supply line |
| Type 2 | 8/20 µs current impulse | In, Imax | Main and sub-distribution boards, inverter AC output |
| Type 3 | Combination wave | Uoc | Close to sensitive equipment: IT, controls, instruments |
Combined devices such as Type 1+2 or Type 2+3 exist to save space and simplify coordination.
The ratings that matter
- Uc, maximum continuous operating voltage: must exceed the highest voltage the SPD will see continuously, with margin for the system's earthing arrangement.
- Up, voltage protection level: the voltage let through to the equipment. It must be lower than what the equipment can withstand. Lower is better.
- Iimp, impulse current: for Type 1 devices, the 10/350 µs lightning current each pole can handle.
- In and Imax: nominal and maximum 8/20 µs discharge current for Type 2 devices.
- Short-circuit rating and the backup fuse or MCB the manufacturer specifies.
Where to put them: a practical cascade
- Main incomer. Install a Type 1 (or 1+2) if the building has an external LPS or an overhead supply. Otherwise a Type 2 is usually enough.
- Sub-distribution boards more than about 10 m of cable from the main board: Type 2.
- Sensitive equipment more than about 10 m from its board: Type 3 at the equipment.
Distances matter because the cable between an SPD and the equipment it protects adds voltage during a surge. Follow the coordination guidance from the SPD manufacturer and IEC 62305-4 for the exact arrangement.
Installation mistakes that defeat SPDs
- Long connecting leads. Keep the total lead length (phase to SPD plus SPD to earth bar) as short as possible. Every extra metre adds hundreds of volts during a fast surge.
- A poor earth connection, or an SPD earthed to a different point from the equipment it protects.
- No backup protection, or the wrong backup fuse rating.
- Ignoring status indicators. A pluggable SPD with a spent cartridge offers no protection until it is replaced.
Solar PV: the DC side is different
PV arrays produce DC at high voltage with no current zero, so AC SPDs cannot be used on the DC side. Use SPDs designed and tested for PV, to IEC 61643-31 (formerly EN 50539-11), with Ucpv above the array's maximum open-circuit voltage at the lowest site temperature. Place them at the inverter DC input. Add them at the array or combiner too if the DC cable run is long. See the solar earthing checklist.
From Ashima
We supply Mersen Surge-Trap pluggable SPDs:
- Type 2, in AC 320 V,
- Type 2, in PV DC 600 V and 1000 V models.
All have replaceable cartridges and status indication. We also supply Mersen gPV fuses and fuse holders, and DC distribution boxes built to your single-line diagram. See SPD & DC fuses.
Need a quotation?
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Products mentioned
General technical guidance, not a design for a specific site. Follow your project specification, the current editions of the applicable standards and the requirements of the inspecting authority.