LV Surge Arrester (SPD) Selection: Type 1, Type 2 and Type 3 Wiring and Cable Sizing
Type 1, Type 2 and Type 3 SPDs per IEC 61643-11, reading Uc, Up, In and Iimp, 3+1 and 4+0 connections in TT and TN boards and the 0.5 m lead length rule.
A low-voltage surge arrester (SPD, surge protective device) diverts lightning and switching overvoltages to earth within microseconds and so protects equipment in the panel. IEC 61643-11 sorts SPDs by test class into Type 1, Type 2 and Type 3 (formerly classes B, C, D); protection only works when each stage sits in the right board with short connections.
> Related Category: Browse models by class and pole configuration in Low Voltage Surge Arresters.
1. Type 1 vs Type 2 vs Type 3
| Class | Former name | Test waveform | Typical rating | Location |
|---|---|---|---|---|
| Type 1 | B | 10/350 µs lightning impulse current (`Iimp`) | 12.5–25 kA per pole | Main distribution board; buildings with external lightning protection or overhead supply |
| Type 2 | C | 8/20 µs (`In`, `Imax`) | In 20 kA, Imax 40 kA | Main board (no lightning protection system) and every sub-board |
| Type 3 | D | Combination wave (1.2/50 µs voltage + 8/20 µs current) | Uoc 6–10 kV | Directly ahead of sensitive equipment, socket or machine panel |
At the same peak, a 10/350 µs impulse carries roughly 20 times the energy of an 8/20 µs impulse, so only Type 1 can handle partial direct lightning current. Combined Type 1 + 2 devices merge both stages in one body for the main board.
2. Reading the nameplate
- Uc (maximum continuous operating voltage): at least 253 V line-to-neutral on 230/400 V systems; 275 V or 320 V is common for tolerance to mains swells.
- Up (voltage protection level): residual voltage across the SPD during discharge. Standard equipment withstands 2.5 kV, so aim for Up ≤ 1.5 kV; lower is better.
- In: nominal 8/20 µs current the SPD can discharge repeatedly; at least 5 kA for Type 2, 20 kA in practice.
- Imax: highest single 8/20 µs current it survives (e.g. 40 kA).
- Iimp: 10/350 µs current for Type 1; at least 12.5 kA per pole where a lightning protection system exists.
3. Connection scheme by system type
| System | Three-phase | Single-phase | Note |
|---|---|---|---|
| TN-S | 4+0: four varistors from L1, L2, L3, N to PE | 2+0 | Most common board type |
| TT | 3+1: three varistors L1, L2, L3 to N plus a spark gap N to PE | 1+1 | May be installed upstream of the RCD; no leakage to earth as varistors age |
| TN-C | 3+0: L1, L2, L3 to PEN | 1+0 | No neutral pole |
A 4+0 scheme is not used in TT systems because an end-of-life varistor could put a dangerous voltage on PE. To identify the system type see Earth Rod Selection and TT / TN Systems.
4. Lead length and conductor sizes
The 0.5 m rule: the total length of the conductors from the line busbar to the SPD and from the SPD to the earth bar must not exceed 0.5 m. Under surge current each metre of conductor adds roughly 1 kV; one extra metre lifts a 1.5 kV Up above 2.5 kV at the equipment. If the length cannot be shortened, use a V connection where incoming and outgoing conductors meet directly on the SPD terminal.
| Conductor (IEC 60364-5-53, clause 534) | Type 1 | Type 2 |
|---|---|---|
| Line and neutral connecting conductor | Min. 6 mm² copper | Min. 2.5 mm² copper |
| Earth (PE) connecting conductor | Min. 16 mm² copper | Min. 6 mm² copper |
- Run conductors straight and short without loops; do not lay protected and unprotected cables side by side in one duct.
- Backup fuse: each model has a maximum backup fuse (e.g. 125 A gG). If the upstream fuse is larger, add a dedicated fuse or miniature circuit breaker in the SPD branch.
- Coordination: if the cable between a Type 2 SPD and the equipment exceeds 10 m, add a Type 3 or another Type 2 near the equipment.
5. Status indicator and replacement
In varistor SPDs a thermal disconnector isolates the element at end of life and the window turns from green to red. Pluggable types need only a new module. Models with a remote contact report the fault to a pilot lamp or automation system. Remove SPD modules before insulation resistance testing; otherwise the test voltage makes the varistor conduct and the reading is wrong.
Frequently asked questions
What do class B, C and D arresters mean?
They are the former German classes: B = Type 1 (lightning current, 10/350 µs), C = Type 2 (8/20 µs, distribution board), D = Type 3 (fine protection near equipment). B+C means a combined Type 1 + 2 device.
Which SPD is used in a home?
A dwelling without a lightning protection system and fed by underground cable needs a Type 2 (In 20 kA, Imax 40 kA) in the board after the meter. With a lightning protection system or overhead service, fit Type 1 + 2 in the main board.
Is the SPD installed before or after the RCD?
Preferably before. Surge current through an RCD can trip or damage it. In TT systems an upstream SPD must be a 3+1 (1+1 for single-phase) design.
What size should the SPD earth conductor be?
At least 16 mm² copper for Type 1 and 6 mm² for Type 2, taking the shortest, straightest route to the earth bar.
What if the SPD indicator turns red?
The protective element is disconnected; the installation keeps running but without surge protection. Replace the module or device with the same Uc and class.