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SPD Installation in a Consumer Unit: Placement, Wiring and Checks
Updated October 2026 · Meteor Electrical guides · 9 min read
Quick answer: For SPD installation in a consumer unit, fit a Type 2 SPD as close as possible to the main switch, upstream of the RCDs and RCBOs. Keep the line, neutral and earth leads short and straight, ideally 0.5m in total and never over 1m. Make sure it has the right backup protection, label it, disconnect it for insulation resistance testing and check the status indicator before energising.
A surge protection device (SPD) limits transient overvoltages from lightning and switching before they can damage boilers, alarms, routers, EV chargers and other electronics. Even a good SPD will underperform if it is poorly placed or connected with long leads. This guide covers SPD types, what BS 7671 requires, placement, connections, backup protection, labelling, testing and replacement.
In this guide
What are the types of SPD?
SPDs are classified by the surges they are tested to handle. Most domestic and light commercial consumer units use a Type 2 device.
| SPD type | What it protects against | Where it is fitted |
|---|---|---|
| Type 1 | Partial direct lightning currents (10/350 µs test) | At the origin, where there is a lightning protection system or high exposure |
| Type 2 | Indirect lightning and switching surges (8/20 µs test) | In the consumer unit or distribution board, close to the main switch |
| Type 1+2 | Both of the above in one device | At the origin, where one device must do both jobs |
| Type 3 | Residual surges at the equipment | Near sensitive equipment, always with a Type 2 upstream |
Type 3 SPDs, such as some protected sockets and plug-in units, are not a substitute for a Type 2 SPD at the board. They only work properly when coordinated with one upstream.
Do you need an SPD in a consumer unit?
Under BS 7671, protection against transient overvoltage must be provided where the consequences of an overvoltage could:
- cause serious injury to, or loss of, human life;
- interrupt public services or damage cultural heritage;
- interrupt commercial or industrial activity; or
- affect large numbers of people in one place.
In all other cases, an SPD is required unless a risk assessment allows it to be left out. The current amendment words this as a decision by the installation owner to accept the risk. In practice, SPDs are now generally expected in new and replacement domestic consumer units. The exact wording changes between editions and amendments, so check the current version of BS 7671, or I.S. 10101 in Ireland.
Planning a full board replacement? Our consumer unit upgrade guide covers boards, protective devices and surge protection together.
Where should an SPD be installed in a consumer unit?
Fit the SPD as close as possible to the main switch. In most consumer units it sits next to the main switch, on its load side, and upstream of the RCDs and RCBOs. That keeps the leads short, and it means surge current diverted to earth does not pass through an RCD and trip it.
- Plan the SPD position when you choose the board, so a way next to the main switch is free.
- Choose a board with a dedicated SPD position or kit where possible.
- Keep the SPD where its status indicator can be seen easily.
- Leave room to remove a plug-in cartridge without disturbing other devices.
Common SPD placement mistakes
- Fitting the SPD at the far end of the board, away from the main switch.
- Feeding it through an RCD, so surges cause nuisance tripping.
- Using long leads coiled up to look tidy.
- Hiding the status indicator behind other devices or labels.
How should an SPD be connected in a consumer unit?
The SPD connects line and neutral (or each phase and neutral on a three-phase board) to earth. The connecting leads add impedance, and during a surge the voltage across them adds to the protection level the equipment actually sees. That is why lead length matters so much.
- Safely isolate the supply and prove the board dead. On a new board, work before the supply is connected.
- Fit the SPD and its backup device, if one is needed, in the way next to the main switch.
- Connect line and neutral from the load side of the main switch or the busbar, as the manufacturer specifies.
- Run the earth lead directly to the main earthing terminal or the board earth bar.
- Keep the total length of the line, neutral and earth leads to 0.5m or less where possible. BS 7671 sets 1m as the maximum.
- Route leads straight, with no loops or sharp bends, and keep them away from outgoing circuit cables.
- Use at least the conductor size required by BS 7671 and the manufacturer: typically 4mm² copper for a Type 2 earth connection and 16mm² for Type 1.
- Tighten all terminals to the manufacturer’s torque setting.
Pro tip: Many SPDs are supplied with pre-cut leads sized for the board. Use them as supplied rather than swapping in longer leads.
Does an SPD in a consumer unit need its own MCB or fuse?
Every SPD needs backup overcurrent protection in case it fails short-circuit at the end of its life. The SPD manufacturer states the maximum rating of the upstream protective device.
- If the device upstream, such as the supplier’s main fuse, is within that maximum, the SPD may be connected without a separate device.
- If it is higher, fit a dedicated MCB or fuse of the stated rating in series with the SPD.
- A dedicated MCB also lets you isolate the SPD for testing without opening up the board.
Choose the backup device so a fault in the SPD does not trip the whole installation, and follow the board manufacturer’s instructions. Our guide to the types of circuit breakers explains MCB ratings and curves.
What labels and status indicators does an SPD need?
Most SPDs have a status indicator: a window that shows green, or another normal colour, while the device is healthy and changes when it needs replacing. Some also have a remote signalling contact.
- Fit a label at the board stating that surge protection is installed and that the status indicator should be checked regularly.
- Mark the SPD and its backup device on the circuit chart.
- Record the SPD type and details on the Electrical Installation Certificate.
- Make sure the indicator is still visible once the cover and labels are in place.
How do you test an installation with an SPD fitted?
An SPD can affect an insulation resistance test and may be damaged by the test voltage. BS 7671 says it should be disconnected before testing.
- Disconnect the SPD, or switch off its dedicated MCB if that fully isolates it, before insulation resistance testing.
- If disconnection is not practicable, test that circuit at 250V DC; the minimum acceptable reading is 1 megohm.
- Carry out the other tests as normal: continuity, polarity, earth fault loop impedance and RCD tests.
- Reconnect the SPD and confirm the status indicator shows normal before energising.
Safety: Working inside a consumer unit is dangerous: the supply side of the main switch stays live. Fitting an SPD is work for a qualified electrician, and new work must be tested and certified. In dwellings in England and Wales, a consumer unit replacement is notifiable under Part P.
For the full board process, see our step-by-step guide to how to install a consumer unit.
When and how should an SPD be replaced?
An SPD wears each time it diverts a surge, so it will not last for ever. Replace it when:
- the status indicator shows the end of its life;
- it has been through a known lightning strike or major fault;
- it is damaged or discoloured; or
- a periodic inspection finds it is unsuitable or not working.
Many SPDs have plug-in cartridges that can be swapped without rewiring. Isolate first and replace like for like, with the same type, voltage rating (Uc) and current rating. Then check the indicator and record the change.
Video: Electrician 247 London on YouTube, showing a surge protection device being fitted.
Which SPDs suit consumer units?
Always pick an SPD that the board manufacturer approves for that consumer unit, or choose one of our consumer unit boards with space planned for it. Our surge protection range includes:
Live Electrical Type 2 surge protection device
A Type 2 SPD with a 20kA/40kA rating in a two-module format, for standard domestic and light commercial boards. Shop the Live Electrical Type 2 SPD. A single-module 1P+N Type 2 SPD is also available.
GARO 2-pole Class II single module SPD
A single-module Type 2 (Class II) SPD supplied with earth, neutral and live leads, useful where space in the board is tight. Shop the GARO single module SPD, or see the GARO 2-pole Class II surge protector and the wider GARO range of boards and devices.
Three-phase boards
For TP&N boards, a 4-pole Type 2 SPD protects all three phases and the neutral. Our three-phase boards come with one already fitted.
Key takeaways
- Most consumer units need a Type 2 SPD; Type 3 devices are only for extra local protection.
- BS 7671 requires SPDs where overvoltage could have serious consequences, and in most other cases too.
- Fit the SPD next to the main switch, upstream of the RCDs and RCBOs.
- Keep the total lead length to 0.5m where possible, and never more than 1m.
- Check the backup protection rating and fit a dedicated MCB or fuse if needed.
- Label the SPD, disconnect it for insulation resistance testing and check the status indicator.
Getting SPD installation in a consumer unit right
Good SPD installation in a consumer unit comes down to the right type, a position next to the main switch, short and straight leads, correct backup protection and clear labelling. Disconnect the SPD for insulation resistance testing, check the indicator before energising and replace it when it reaches the end of its life. If you are not qualified, use a registered electrician.
Shop surge protection and consumer units
Type 2 SPDs, consumer units and boards from GARO, Live Electrical and more. Trade prices, next working day delivery across NI & ROI on in-stock orders before 5pm.
Shop surge protection Shop consumer unitsNeed help choosing? Call our team on 028 8675 1515 (048 from ROI), Monday to Friday, 8am–5pm.
FAQs
Do I need an SPD in my consumer unit?
In most cases, yes. BS 7671 requires surge protection where an overvoltage could cause serious injury, affect public services, commercial activity or many people. Elsewhere it is required unless a risk assessment, or the installation owner under current wording, justifies leaving it out. SPDs are now generally expected in new and replacement domestic consumer units.
Where should an SPD be installed in a consumer unit?
Fit the SPD as close as possible to the main switch, normally on the load side of the main switch and ahead of the RCDs and RCBOs. This keeps the connecting leads short. BS 7671 recommends a total lead length of no more than 0.5m, and it must not exceed 1m.
Does an SPD need its own MCB?
It depends on the SPD and the board. Every SPD needs backup overcurrent protection. If the device upstream, such as the main fuse, is rated higher than the SPD manufacturer allows, a dedicated MCB or fuse must be fitted. Many consumer unit SPDs are designed to be fed through their own MCB.
Should you disconnect an SPD for insulation resistance testing?
Yes. An SPD can affect an insulation resistance test and may be damaged by it, so disconnect it before testing. If disconnecting it is not practicable, BS 7671 allows the test voltage to be reduced to 250V DC for that circuit, with a minimum reading of 1 megohm. Reconnect it afterwards.
How do I know if an SPD has failed?
Most SPDs have a status indicator window. Green, or the manufacturer’s normal colour, means the device is working. Red, or a changed colour, means the protective element has reached the end of its life and the SPD or its cartridge needs replacing. Check the indicator at every inspection and after storms.
What is the difference between Type 1, Type 2 and Type 3 SPDs?
Type 1 SPDs handle direct lightning currents and are fitted at the origin where there is a lightning protection system or overhead supply risk. Type 2 SPDs are the usual choice in consumer units and distribution boards. Type 3 SPDs give local protection near sensitive equipment and must be used with a Type 2.