What Is an SPD in Electrical Installations? A Guide to Surge Protection and Wiring

What is an SPD in electrical installations?

An SPD, or surge protective device, is a device designed to limit transient overvoltages and divert surge current away from an electrical installation and its connected equipment. BS 7671 defines it as a device intended to limit transient overvoltages and divert surge currents, containing at least one non-linear component.

Transient overvoltages are very brief increases in voltage. They can be caused by lightning activity, switching operations on the electricity network, large motors or transformers being switched, power being restored following an interruption, electrical equipment operating within the property, or nearby industrial or commercial installations. Although a surge may last only a fraction of a second, it can damage sensitive electronic equipment, degrade insulation and shorten the service life of connected devices.

Modern properties commonly contain numerous electronic products worth protecting, televisions, computers, routers, smart-home equipment, heating controls, LED drivers, kitchen appliances, alarm systems, EV chargers, solar PV inverters, battery storage systems and heat pumps. An SPD fitted within the consumer unit helps protect these by directing transient surge energy towards earth while limiting the voltage reaching the installation. The IET notes that surges can cause either immediate equipment failure or damage that only becomes apparent over a longer period, which is part of why SPDs are now standard provision in most new consumer units rather than an optional extra.

On this page

• What does SPD stand for, and how does it work?

• Do you need to fit an SPD?

• Types of SPD: 1, 2, 3, and connection type

• How to wire an SPD in a consumer unit

• Wylex and Crabtree SPD compatibility

• How to test an SPD

• Common SPD wiring mistakes

• How much does it cost to fit an SPD?

• FAQs

What does SPD stand for, and how does it work?

SPD stands for surge protective device. It's sometimes informally called a surge protector, surge arrestor, consumer-unit surge protection, whole-house surge protection, or lightning surge protection. An SPD installed in a consumer unit should not be confused with a plug-in extension lead marketed as a "surge-protected" socket strip, a consumer-unit SPD forms part of the fixed electrical installation and must be selected according to the supply, earthing arrangement, prospective fault current, required protection level and manufacturer's instructions.

Under normal supply conditions, an SPD has a high impedance and has little effect on the installation. When the voltage rises above its designed threshold, it rapidly becomes conductive, creating a low-impedance route through which surge current can be diverted, normally towards the earthing system. Once the transient event has passed, the device returns to its normal high-impedance state.

An SPD does not disconnect the supply in the way an MCB, RCBO or RCD does. Its job is to limit the transient voltage reaching equipment, divert surge current safely, and reduce the risk of insulation breakdown and equipment damage, it cannot guarantee that every connected appliance survives every lightning or switching event, and its effectiveness depends entirely on correct selection, installation, coordination and conductor arrangement.

Does an SPD protect against electric shock?

No. An SPD is not primarily an electric-shock protection device, and it does not replace earthing, protective bonding, RCD protection, RCBO protection, overcurrent protection, correct cable sizing, automatic disconnection of supply, or lightning-protection measures where required.

These devices perform different functions and are commonly all required within the same installation, none of them is a substitute for the others.

Do you need to fit an SPD?

Under Regulation 443.4 of BS 7671, protection against transient overvoltages must be provided in three cases: where it could result in serious injury to, or loss of, human life; where it could cause failure of a safety service (as defined in Part 2); or where it could result in significant financial or data loss. This applies across domestic, commercial and industrial premises, the previous single-dwelling exemption (which turned on whether the value of the installation justified the cost of protection, not on whether the supply was overhead or underground) has been removed along with the calculated-risk-level method it was tied to. For any other case, protection is still required unless the owner of the installation formally declines it under Regulation 443.4.1, recording that any loss or damage is tolerable and that they accept the risk, an owner sign-off, not a designer's documented risk assessment (the separate risk-assessment method previously set out in Regulation 443.5 was deleted by Amendment 2, along with the Calculated Risk Level factor and the associated map of the UK).

In practice: fit an SPD, unless a documented risk assessment justifies leaving it out. Since Amendment 2 to the 18th Edition, that's the default position most electricians work from, and SPDs are inexpensive relative to the cost of the installation and equipment they protect, genuinely one of the easier upgrades to justify to a client. The final decision should be recorded as part of the electrical design, not omitted simply because the supply is underground or the property hasn't previously shown visible surge damage.

Types of SPD: 1, 2, 3, and connection type

SPDs are classified by where they sit in the installation and what kind of surge they're built to handle.

A Type 1 SPD should not be selected automatically simply because the incoming supply happens to be overhead, the complete lightning and surge risk needs assessing. Where several SPDs are used together, they must be correctly coordinated rather than simply stacked.

Connection type: CT1 and CT2

BS 7671 also specifies connection type, which matters for how the SPD is wired relative to the earthing system:

CT1 (4+0 connection): all live conductors (L1, L2, L3 and N) connect to the common of the protective earth. Suitable for TN-S and TN-C-S systems.

CT2 (3+1 connection): the live conductors connect to a common neutral point, with a separate component from neutral to earth. This is the connection type generally used on TT systems, though Section 534 also permits CT1 on a TT system where the SPD is fitted downstream of the RCD, a CT1/4+0 device connected ahead of the RCD on a TT system will instead conduct continuously between neutral and earth, degrading the device and risking nuisance RCD operation. Confirm the SPD's position relative to the RCD before assuming CT2 is the only valid choice on a TT installation.

Getting the connection type wrong for the earthing system isn't a minor spec mismatch, it's the kind of error that quietly shortens the SPD's working life and can trip RCDs for no obvious reason months after installation.

For more on classification tests and where each type belongs in an installation, see Voltimum's guide to sizing surge protection devices.

How to wire an SPD in a consumer unit

The precise arrangement depends on the SPD manufacturer, consumer unit design, supply arrangement, earthing system, number of phases, position of the main switch, required SPD type, short-circuit protection and the assembly manufacturer's instructions. There is no universal SPD wiring diagram suitable for every consumer unit, what follows is the professional process, not a single diagram to copy.

1. Assess whether surge protection is required

Establish the type of premises, the consequences of an overvoltage, whether an external lightning-protection system is present, whether safety services are present, the supply arrangement, the building's lightning exposure, the value and sensitivity of connected equipment, whether additional downstream SPDs are needed, and whether the client has specifically requested surge protection. Treat this as a genuine design decision for the installation as a whole, not a box to tick on a consumer-unit spec sheet.

2. Identify the supply and earthing arrangement

Confirm whether the installation is TN-S, TN-C-S, TT, IT or another permitted configuration, and select an SPD built for that connection type (CT1 or CT2), not simply the cheapest or most available module. Manufacturer documentation should confirm compatible earthing arrangements, maximum continuous operating voltage, nominal and maximum discharge current, voltage protection level, short-circuit withstand capability and required backup protection. Electrium, for example, lists its Wylex NMT2SPD3W/1 Type 2 device as suitable for TN-C-S, TN-S and TT systems, with a nominal discharge current of 20kA and a maximum continuous AC voltage of 275V.

3. Confirm consumer unit compatibility

An SPD should form part of a verified consumer-unit assembly, not simply a module that happens to fit. Before installation, confirm the SPD is approved for the selected consumer unit range, physically fits the enclosure with adequate module space, the busbar or supply connection is compatible, the consumer unit maintains its declared ratings, manufacturer-provided connection kits are used, and the enclosure can be closed correctly with acceptable clearances.

Use only the devices approved by that manufacturer for the selected consumer unit, mixing brands, even where a device fits mechanically, can affect busbar contact, short-circuit performance and the assembly's certification.

4. Select the correct SPD type

A typical dwelling without an external lightning-protection system may use a Type 2 SPD at the origin; a Type 1 or combined Type 1+2 device may be required where lightning-current protection is necessary. Selection should consider Type 1/2/combined protection, single-phase or three-phase supply, system voltage, maximum continuous operating voltage (Uc), impulse current (Iimp), nominal and maximum discharge current (In, Imax), voltage protection level (Up), temporary overvoltage performance, short-circuit current rating, and coordination with any downstream devices. The voltage protection level should be low enough to protect the equipment, while the SPD must withstand the expected operating conditions.

5. Determine the backup overcurrent protection needed

Some SPDs require a dedicated fuse, MCB or other backup protective device; others may be protected by the upstream supply fuse, subject to the conditions stated by the manufacturer and the verified assembly. This is genuinely debated: some manufacturers market single-module SPDs as "fully rated, no backup device needed" based on their own coordination testing, while other guidance points to BS 7671 clauses within the 534.4.1.x and 534.4.4.x groups as requiring overcurrent protection (confirm the exact sub-clause against the current edition before citing it). Don't assume either way: check the specific manufacturer's instructions and the verified assembly's conditions, and if in doubt, treat a dedicated backup device as the safer default.

Wylex's instructions for its NMT2SPD3W/1, for example, state it's fully rated at 100A with no backup device needed, but exclude certain dual-RCD arrangements, directing installers to a different product for those boards. A useful illustration of why this has to be checked per device and per assembly, not assumed as a blanket rule.

6. Position the SPD correctly

Both "before" and "after" the main switch exist as valid arrangements depending on the consumer unit design and manufacturer's instructions. Installing an SPD after the main switch allows it to be isolated using that switch, though incoming terminals may still remain live; an SPD connected before the main switch may remain active while the consumer unit is switched off and may need separate isolation arrangements. Don't alter the manufacturer's intended configuration to suit a preference.

Similarly, in many domestic consumer units the SPD sits upstream of individual final-circuit RCBOs or downstream RCCBs, but the correct arrangement depends on the consumer unit configuration, RCD type, SPD connection mode, TT/TN earthing arrangement and manufacturer instructions. An SPD should not simply be added to the load side of an existing RCCB without checking the arrangement is approved and technically suitable, some manufacturers specifically exclude certain SPD kits from particular dual-RCD consumer unit arrangements, as above.

7. Keep the connecting leads as short as possible, the 0.5m rule

This is the detail most responsible for a poorly performing SPD installation, even when the device itself is correctly specified. The total length of the SPD's connecting leads, live, neutral and earth combined, from the busbar/OCPD to the SPD terminals and back to the earth bar, should not exceed 0.5m, and should never exceed 1m even in a difficult layout.

The reason is inductance: during a fast-rising surge current, every extra metre of connecting lead adds inductance that generates additional voltage across that lead, roughly in the order of 1kV per metre for a typical 8/20µs surge. That voltage adds directly to the protection level the SPD is supposed to deliver, so a technically excellent device wired with long, looped leads can still let through a damaging voltage spike to the equipment downstream. Route leads by the shortest practical path, straight rather than coiled or looped, and locate the SPD as physically close to the busbar as the board layout allows, a neat-looking installation is not the same thing as an effective one if the leads take a long or indirect route.

Where 0.5m genuinely can't be achieved for the phase and neutral conductors, the “V” connection method lets the earth conductor be extended to 0.5m instead, by routing the SPD electrically in a “V” between the incoming supply and the outgoing busbar rather than as a simple branch connection, keeping the more critical live/neutral path short even when the physical earth run is longer.

8. Connect the line, neutral and earth conductors correctly

The connection arrangement varies by SPD design, depending on the product, connections may include line to SPD, neutral to SPD, SPD to protective earth, combined line-and-neutral modules, a neutral-to-earth protection module, or integrated busbar/terminal connections. Follow the terminal markings and manufacturer's wiring diagram; conductors should never be connected based solely on colour or the terminal layout of a different manufacturer's product. Incorrect connections can leave the installation unprotected, damage the SPD, cause a short circuit, create a shock risk, compromise the consumer unit assembly, or produce misleading status indication, the earth connection in particular must terminate at the correct main earthing or protective-conductor terminal specified for that assembly.

9. Torque, segregation and enclosure closure

Tighten all terminals to the manufacturer's specified torque using calibrated equipment, too loose risks overheating or arcing, too tight can damage the terminal or conductor. Keep incoming and outgoing conductors separated, with short earth routes and no large loops, using the manufacturer's formed connection kit where supplied, so surge energy can't couple back into the protected side. Before closing up, confirm barriers and blanking plates are correctly fitted, the enclosure still meets its protection rating, no copper is exposed, conductors aren't trapped by the cover, labels are visible, and no swarf or debris remains inside.

10. Label the installation

Depending on the installation and manufacturer requirements, labelling should identify the presence of surge protection, the SPD type, the need to check the status indicator, replacement-cartridge information, multiple SPDs within the installation, alternative supply sources, and any testing limitations. The circuit schedule and installation records should also identify the SPD.

Typical arrangement: the SPD sits in parallel, not in series

A simplified conceptual arrangement: incoming supply → main switch or supply connection → SPD connection point → final-circuit protective devices. The SPD is connected in parallel with the supply rather than in series with the normal load current, so the full installation load doesn't normally pass through it, instead it provides a diversion path only when a transient overvoltage occurs. The precise sequence of main switch, SPD and overcurrent device must still follow the verified consumer-unit design and manufacturer's instructions rather than this simplified description.

Wylex and Crabtree SPD compatibility

Both Wylex and Crabtree offer consumer units with factory-fitted Type 2 surge protection, Wylex's NM1806LS, for example, ships with an integrated miniature SPD as part of a manufacturer-configured system, and Crabtree's Starbreaker range offers compatible miniature SPDs for its own assemblies. A consumer unit is a tested assembly, not an empty enclosure any modular device can be dropped into: even where two devices share the same module width, they may not be electrically or mechanically interchangeable. The safest approach is a factory-fitted SPD consumer unit, a manufacturer-approved retrofit kit, or a separate approved enclosure, using the manufacturer's own conductors and accessories throughout.

How to test an SPD

An SPD should be inspected as part of initial verification and periodic inspection. Checks include confirming the correct product type and consumer-unit compatibility, correct line/neutral/earth connections, tightness of accessible terminations, conductor size and routing, backup protection, status indication, signs of overheating or mechanical damage, correct labels, coordination with other SPDs, and compliance with manufacturer instructions.

Standard installation testers don't fully reproduce the high-energy transient conditions an SPD is designed for, so testing is largely based on visual inspection, status indication, manufacturer-specific test equipment where available, verification of connections, and confirmation of protective-device coordination. Insulation-resistance testing needs particular care, since the test voltage could affect connected SPDs or produce misleading results, follow BS 7671 testing procedures alongside both the SPD and test-instrument manufacturers' instructions, and only disconnect or isolate the SPD for a specific test using the approved procedure.

Status indicator and replacement

Most SPDs show status via a visual window, commonly green for operational, red for replacement required, though colours and meanings vary by manufacturer. A green light doesn't guarantee the whole installation is correctly designed, just that the replaceable protective element still works (BS 7671 Amendment 4 model forms specifically check this). Many SPDs can divert several smaller surges over their service life, but one large enough surge can still take a cartridge to end-of-life in a single event, which is why checking the indicator matters more than following a fixed replacement schedule. Replace where the indicator shows failure, a cartridge has operated, there's visible damage, overheating, a cracked enclosure or water ingress, or testing identifies an unsatisfactory condition; otherwise it doesn't need replacing on a schedule, just checking at every periodic inspection.

What SPDs don't do

Can an SPD stop a consumer unit tripping? No, it isn't intended to prevent overload, short-circuit or residual-current tripping. Repeated tripping is more likely a faulty appliance, insulation breakdown, an overloaded circuit or a loose connection, surge protection isn't a fix for unexplained MCB, RCD or RCBO operation.

Will an SPD protect against a direct lightning strike? A Type 2 SPD alone can't manage the full current of a direct strike, that needs an external lightning-protection system, a Type 1 or combined device, and protection across other incoming services (telecoms, broadband, data). A consumer-unit SPD doesn't protect equipment through those other routes automatically.

What's the “10-metre rule”? Sensitive equipment more than roughly 10m of cable from the upstream SPD may need additional protection, since longer runs allow oscillations or increased voltage to appear downstream, a second Type 2 SPD at a sub-board, a Type 3 SPD near the equipment, or both. One SPD at the main board isn't automatically enough for every downstream board in a larger installation.

Can an SPD be retrofitted to an existing consumer unit?

Sometimes, but not every consumer unit is suitable. Before retrofitting, assess available spare ways, manufacturer-approved SPD options, the age and condition of the board, busbar compatibility, main-switch configuration, short-circuit rating, earthing arrangement, available cable space, the location of earth and neutral terminals, the ability to maintain short connection lengths, and whether the enclosure can be modified without compromising its rating.

A separate manufacturer-approved SPD enclosure can sometimes be more appropriate than cramming a device into an overcrowded consumer unit. If the existing board is obsolete or its manufacturer doesn't approve a compatible SPD, replacing the board may be the safer and more practical route than forcing a retrofit.

Common SPD wiring mistakes

Exceeding the 0.5m lead length rule. The most common fault, a neat-looking but indirect cable route quietly defeats the SPD's rated protection, even with a healthy status indicator.

Wrong connection type for the earthing system. A CT1 device wired ahead of the RCD on a TT system conducts continuously between neutral and earth, degrading it and risking nuisance tripping.

Assuming every SPD needs, or doesn't need, a separate MCB. Check the manufacturer's instructions rather than assuming either way.

Mixing manufacturers. A device that physically fits may still invalidate the consumer unit's verified assembly.

Installing in an overcrowded board. Restricted space leads to poor routing and trapped cables, a separate approved enclosure is sometimes the better answer.

Failing to coordinate downstream SPDs. A Type 3 device needs suitable upstream protection already in place to be effective.

Incorrect insulation-resistance testing. Testing without considering the connected SPD can damage the device or distort the results.

Failing to check the status indicator. A failed SPD can stay installed, invisibly unprotective, until someone checks it.

Treating an SPD as complete lightning protection. A Type 2 SPD alone doesn't replace full lightning protection, and doesn't cover other incoming services like telecoms or data.

How much does it cost to fit an SPD?

Costs vary depending on whether the SPD is fitted during a new consumer-unit installation, added using a manufacturer-approved kit, installed in a separate enclosure, combined with a full consumer-unit replacement, Type 1/2/combined, and whether the system is single-phase or three-phase. As an indicative UK framework:

These are broad indicative estimates rather than fixed market prices, real trade quotes for adding a Type 2 SPD module alongside a consumer unit job commonly sit around £80–£120, with the bare device itself (excluding labour) typically £15–£40 for a basic Type 2 module. The final price depends on SPD type, consumer-unit compatibility, available space, required overcurrent protection, access, earthing arrangement, testing, certification, local labour rates and whether remedial work is required.

Is fitting an SPD notifiable under Part P?

Installing an SPD as an alteration isn't automatically the same as replacing the whole board or adding a new circuit, but all electrical work must still comply with Building Regulations. Notification may be needed if the work also involves a consumer-unit replacement, a new circuit, a special location, or another notifiable alteration, Approved Document P covers England, with different rules across Wales, Scotland and Northern Ireland. Assess certification and notification for the complete scope of work, not the SPD alone.

A retrofit SPD with no new circuit or full replacement may suit a Minor Electrical Installation Works Certificate; a full replacement or larger installation is likely to need an Electrical Installation Certificate. Either way, record the SPD type, manufacturer reference, location, earthing arrangement, backup device, test results, status indication, any limitations, and the BS 7671 edition used.

FAQs

What is an SPD in electrical installations?

A surge protective device, it limits transient overvoltages and diverts surge current to reduce the risk of damage to wiring and electrical equipment, working alongside (not instead of) RCDs and MCBs.

What does SPD stand for?

Surge protective device.

How does an SPD work?

It stays high-impedance under normal conditions, then rapidly becomes conductive when voltage rises above its threshold, diverting surge current to earth before returning to its normal state once the transient has passed.

Is an SPD the same as an RCD?

No, an RCD protects against electric shock by detecting residual current to earth, while an SPD protects equipment and wiring from transient overvoltage damage; most modern consumer units include both, doing different jobs.

Does every consumer unit need an SPD?

In most cases, yes, under BS 7671 Regulation 443.4, the alternative is for the owner to formally decline it under Regulation 443.4.1 and accept the risk, not a designer's own risk assessment.

What's the difference between Type 1, Type 2 and Type 3 SPDs?

Type 1 handles direct lightning-related surges at the origin; Type 2 handles more common induced/switching surges and is what most domestic consumer units use; Type 3 is a final stage of fine protection close to sensitive equipment, used alongside, not instead of, Type 1/2 protection.

How do you wire an SPD in a consumer unit?

Confirm the earthing system and correct connection type (CT1/CT2), fit appropriate backup overcurrent protection, keep connecting leads to a maximum 0.5m total (never more than 1m), connect strictly per the manufacturer's diagram, torque terminals correctly, then test, label and certify the completed work.

How long can the SPD connecting leads be?

The combined live, neutral and earth lead length should not exceed 0.5m, and should never exceed 1m, longer leads add inductance that increases the voltage let through to equipment downstream.

Does an SPD need a backup MCB or fuse?

Not always, some manufacturers rate certain SPDs for use without a separate backup device under specific, tested assembly conditions, while others require dedicated overcurrent protection, so check the manufacturer's instructions rather than assuming either way.

How do you test an SPD?

Mainly through visual inspection, status-indicator checks, verification of connections and protective-device coordination, standard multifunction testers don't reproduce the transient conditions an SPD is designed for, and insulation-resistance testing needs care around connected SPDs.

How do you know when an SPD needs replacing?

Check the status indicator, a red or failed reading normally means the cartridge needs replacing, and watch for visible damage, overheating, a cracked enclosure or water ingress.

Can an SPD be retrofitted to an existing consumer unit?

Sometimes, it depends on spare ways, busbar compatibility, board condition and whether the connection leads can be kept short enough; a separate approved enclosure or full replacement may be safer where it can't.

Will an SPD protect against a direct lightning strike?

A standard Type 2 SPD alone is not designed to manage a direct lightning strike, that needs an external lightning-protection system and a Type 1 or combined Type 1+2 device, plus coordinated protection across other incoming services.

How much does it cost to install an SPD?

Roughly £75–£175 added to a new consumer-unit installation, £150–£350 to retrofit one to an existing suitable board, or £600–£1,200+ where an incompatible consumer unit needs replacing to accommodate one.

Can I fit an SPD myself?

No, this is specialist work inside a consumer unit involving potentially lethal voltages, competent design, safe isolation, specific test procedures and certification.

The bottom line

Understanding what an SPD is in electrical installations matters more every year as homes and businesses fill up with sensitive electronic equipment. A correctly selected SPD reduces the risk of damage from switching surges and indirect lightning effects, but performance depends on far more than simply fitting a device into an available consumer-unit way.

The electrician needs to consider SPD type, earthing arrangement, consumer-unit compatibility, backup overcurrent protection, short and direct conductor routes, manufacturer instructions, coordination with any downstream devices, and proper inspection and certification. For new installations, a factory-configured Wylex or Crabtree consumer unit with approved surge protection is often a simpler and more reliable route than retrofitting an unrelated device, and where an SPD is added to an existing installation, the complete assembly, available space and short-circuit performance all need assessing before work begins.

If you're wiring the SPD into a garage or outbuilding consumer unit specifically, our guide to wiring a garage consumer unit covers the earthing and submain decisions that come with a detached supply.