IEC 61643-11 classifies surge protection devices into three types by installation level and surge current magnitude. Installing the wrong type in the wrong location leaves equipment unprotected.
This guide explains what each type does, how to read their test parameters, and a step-by-step selection method for any installation.
A single lightning strike injects 50 to 200 kA into the mains network. Even a residual surge of 1 to 5 kA at the wall socket is enough to destroy unprotected equipment.
SPDs at each level of the installation intercept the surge at the right point.
Type 1, Type 2 and Type 3 SPDs: What Each One Does
Installed at the origin of the installation, at the service entrance or main distribution board. Handles the highest surge currents from a direct or nearby lightning strike on the supply network.
Test current: Iimp (impulse current), 10/350 µs waveform. Typical Iimp values: 12.5 kA to 100 kA per pole.
Device type: spark gap or combined spark gap and varistor. Built to absorb the high energy of a 10/350 µs impulse.
Installed at sub-distribution boards and branch panels downstream of the main distribution board. Handles residual surges that pass through the Type 1 SPD and switching-induced surges.
Test current: In (nominal discharge current) and Imax (maximum discharge current), 8/20 µs waveform. Typical In values: 5 kA to 20 kA per pole.
Device type: varistor (MOV) or transient voltage suppressor. Lower energy handling than Type 1, lower residual voltage Up.
Installed close to the protected equipment at the socket outlet or equipment terminal. Handles residual surges that pass through Type 1 and Type 2 SPDs.
Test current: Uoc (open-circuit voltage) and Isc (short-circuit current), combination wave. Typical Uoc: 1 kV to 10 kV.
Device type: transient voltage suppressor, suppressor diode, or filter. Very low Up voltage protection level, typically below 1.5 kV.
Where Each SPD Type Is Installed in the Building
This is why a device rated only for 8/20 µs testing (a Type 2 SPD) must never be used at the service entrance without a Type 1 SPD in front of it. The 10/350 µs energy of a real lightning event would destroy a Type 2 device instantly.
Key SPD Parameters Compared
SPD Selection Tool
Coordination Between SPD Types
SPDs at different levels must be coordinated so that each one clamps only the surge energy it is designed for, and passes the residual energy to the next level downstream.
| Coordination Rule | Requirement | Why It Matters |
|---|---|---|
| Minimum cable distance between Type 1 and Type 2 | At least 10 m of cable between the Type 1 SPD and the Type 2 SPD, or a decoupling inductance of at least 1.5 µH | Without separation, the surge voltage from the Type 1 device can couple directly into the Type 2 device before the Type 1 has finished clamping. This overstresses the Type 2. |
| Minimum cable distance between Type 2 and Type 3 | At least 5 m of cable between the Type 2 SPD and the Type 3 SPD, or a decoupling element | Same coupling risk at a lower energy level. The Type 3 device is small and will fail if exposed to the full discharge current of the Type 2 before the Type 2 has clamped. |
| Up of upstream SPD must exceed Up of downstream SPD | Up(Type 1) greater than Up(Type 2) greater than Up(Type 3). Typical: 3 kV, 1.5 kV, 1.0 kV. | The upstream device must clamp at a higher voltage so that it conducts first and absorbs most of the surge energy before the downstream device sees it. |
| Maximum cable length from Type 3 to equipment | Less than 10 m total cable from Type 3 SPD to the protected equipment | A long cable between the Type 3 and the equipment can re-develop a significant surge voltage from the residual current. Keep the Type 3 physically close to the equipment it protects. |
Uc is the maximum AC voltage the SPD can withstand continuously without conducting. For a 230 V system, IEC 61643 requires Uc of at least 1.1 x 230 = 253 V, but practical designs use Uc of 275 V or 320 V to allow for supply tolerance. An SPD with a Uc below the local supply voltage will conduct continuously, overheat, and fail. Always check the Uc rating on the datasheet, not just the kA discharge rating.
SPD connection varies between TN-S, TN-C, TT, and IT systems. See the TN, TT and IT earthing systems guide and the earthing resistance calculation guide for earth electrode requirements.
Watch: SPD Selection and IEC 61643 Explained
SPD Selection Questions Engineers Ask
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External References
What We Learn Today
- IEC 61643-11 defines three SPD types by installation level and test waveform. Type 1 (10/350 µs, Iimp) goes at the service entrance. Type 2 (8/20 µs, In and Imax) goes at sub-distribution boards. Type 3 (combination wave, Uoc) goes at equipment level, always downstream of Type 1 or Type 2.
- The voltage protection level Up decreases from Type 1 (2 to 4 kV) to Type 2 (0.9 to 2.5 kV) to Type 3 (below 1.5 kV). Each stage reduces the residual surge voltage seen by the next level of equipment.
- SPDs must be coordinated: at least 10 m of cable or 1.5 µH decoupling inductance between Type 1 and Type 2, and at least 5 m between Type 2 and Type 3. Without coordination, the downstream SPD is destroyed by the upstream clamping transient before the upstream device has finished conducting.