Surge Protective Device (SPD) Selection: Type 1, Type 2 and Type 3
Surge protective device selection: Getting surge protection right inside the panel: SPD types, coordination and installation rules.
ContentsSurge protective device selection is one of the decisions that most directly determines the damage lightning and switching transients cause to an installation. If the right type is not installed in the right position with the right wiring, the device cannot do its job and may even become a new risk inside the assembly.
Where do surges come from?
There are two main sources. Lightning transients reach the supply through a direct strike or by induction nearby, rising to kilovolt levels within microseconds. Switching transients arise when motors, transformers or capacitor stages are energised and de-energised; they are smaller but repeat far more often.
SPD types and their roles
The IEC 61643 series classifies surge arresters into three test classes. They are not alternatives but complementary stages.
| Type | Test class | Test wave | Location | Role |
|---|---|---|---|---|
| Type 1 | Class I | 10/350 µs (Iimp) | Building entrance / main panel | Discharge direct lightning current |
| Type 2 | Class II | 8/20 µs (In, Imax) | Main and sub-distribution boards | Limit induced transients |
| Type 3 | Class III | Combination wave | Close to sensitive equipment | Reduce the remaining residual voltage |
Which type does a given site need?
If the building has a lightning protection system or is fed by an overhead line, a Type 1 device is mandatory at the service entrance. Where the supply is by underground cable and there is no air termination system, Type 2 in the main board is usually sufficient. In data centres, medical installations and automation panels a Type 3 device is added close to the equipment.
Protection level Up and lead length
The Up value in a datasheet is the voltage remaining across the device during a surge, and it must sit below the withstand category of the equipment being protected. In the field, however, the connecting leads cause most of the trouble: every metre of lead adds hundreds of volts during a surge current. The total length of the phase and earth connections should therefore not exceed 0.5 m, and a V-connection should be used where possible.
Back-up fuse and end of life
An SPD can fail to a short circuit at the end of its life, so the back-up fuse or breaker specified by the manufacturer must always be fitted. Modern SPDs include a thermal disconnector and a visual status indicator, with a volt-free contact option for remote monitoring. Models with replaceable cartridges shorten maintenance time.
Points to watch in panel design
The most common site errors are avoided by observing the following:
- Connect the SPD earthing conductor directly to the main earth bar
- Keep phase and earth leads short, straight and close together
- Connection schemes (3+1 or 4+0) differ between TN-S, TN-C-S and TT systems
- The SPD compartment must remain accessible for maintenance
- Calculate Iimp from the lightning protection level rather than guessing
For sizing the earth bar, see neutral and earth bar sizing.
Key takeaways: surge protective device selection
- Where do surges come from — There are two main sources.
- SPD types and their roles — The IEC 61643 series classifies surge arresters into three test classes.
- Which type does a given site need — If the building has a lightning protection system or is fed by an overhead line, a Type 1 device is mandatory at the service entrance.
- Protection level Up and lead length — The Up value in a datasheet is the voltage remaining across the device during a surge, and it must sit below the withstand category of the equipment being protected.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
Is a Type 2 still needed when a Type 1 is fitted?
Usually yes. A Type 1 discharges the large energy but its residual voltage is still high for sensitive equipment. Combined Type 1+2 devices are also widely used.
How long does an SPD last?
There is no fixed life; it depends on the number and energy of the surges it absorbs. The indicator window should therefore be checked at every periodic inspection and the module replaced when the colour has changed.
Does an SPD affect residual current devices?
It can. If the SPD is placed downstream of an RCD, surge current may cause nuisance tripping. It is preferably arranged upstream of the RCD, or combined with a time-delayed (type S) device.
Related articles
- Residual Current Device Types: The Difference Between AC, A, F and B
- Choosing the Incoming Device: MCCB or ACB?
- What is the IEC 61439 Standard? Type Testing and Conformity in Low Voltage Switchgear.
- All technical articles
Devpan configures surge protection according to the site’s lightning risk assessment and earthing arrangement. Tell us how your project is supplied and we will define the right SPD stages with you.
