Get Listed
Surge protection helps protect an electrical installation from sudden, short-duration increases in voltage known as transient overvoltages. These spikes can happen in a fraction of a second and are most commonly caused by lightning activity (direct strikes or nearby induced surges) or by switching events on the electricity network and within larger electrical systems.
Although they last only microseconds, these voltage spikes can reach thousands of volts and cause immediate or cumulative damage to consumer units, boilers, EV chargers, LED lighting, IT equipment and other sensitive electronics. Surge Protection Devices (SPDs) work by safely diverting excess voltage to earth before it can reach connected equipment.
Why Types 1, 2 and 3 are different
Each SPD type is designed and tested for different levels of surge energy and different locations within an installation.
Type 1 devices are built to handle very high currents associated with lightning entering the installation. Type 2 devices protect against indirect lightning effects and switching surges on the supply network. Type 3 devices provide a final stage of protection for sensitive electronics at the point of use.
Using the correct type in the right place allows surges to be managed in stages throughout the installation.
Types of Surge Protection Devices
Type 1 SPD: protection at the origin
Type 1 SPDs are installed as close as possible to the origin of the installation (service intake or main distribution point), in line with the requirements of BS 7671. They are typically specified where lightning current could be introduced into the installation, such as buildings with external lightning protection systems, overhead supply lines, or locations with higher exposure.
Type 1 protection is commonly used as part of a coordinated arrangement with a downstream Type 2 SPD to further reduce residual overvoltage. In some cases, combined devices provide Type 1, Type 2 and Type 3 protection within a single unit.
Type 2 SPD: main installation protection
Type 2 SPDs are the most common choice for domestic and light commercial installations. They are typically installed within the consumer unit or distribution board to protect the installation from transient overvoltages caused by indirect lightning and supply switching events.
For many UK properties without an external lightning protection system and with lower exposure, a Type 2 SPD at the consumer unit forms the primary layer of protection. Many modern consumer units now come with factory-fitted surge protection or allow an SPD module to connect directly to the busbar.
Type 3 SPD: point-of-use protection
Type 3 SPDs are installed close to sensitive equipment, often as surge-protected socket outlets or extension leads. They provide a final stage of limitation at the equipment terminals but have a lower discharge capacity. Type 3 devices are intended to be used downstream of Type 2 protection as part of a coordinated scheme.
For best performance, connection lengths should be kept short and manufacturer guidance followed closely.
Quick reference: which SPD type do you need?
For most homes and small businesses, a Type 2 SPD fitted at the consumer unit is the standard starting point.
Add a Type 1 SPD where lightning current influence is a realistic concern, such as where an external lightning protection system is installed or where exposure is higher.
Add Type 3 protection where you want extra protection for sensitive or high-value electronics, installed downstream of Type 2 and correctly coordinated.
SPD selection and installation is covered in BS 7671 Section 534, including guidance on device choice, voltage protection levels and correct connection methods.
Type 2 only vs Type 1 + Type 2 + Type 3: what’s the practical difference?
A Type 2 SPD is designed to handle the majority of surges seen in typical installations. A coordinated Type 1 and Type 2 arrangement expands protection to higher-energy lightning-related events, and Type 3 adds further limitation at the point of use where required.
Type 3 devices can reduce the overvoltage seen by sensitive electronics, but only when installed and coordinated correctly with upstream protection.
Surge waveforms and testing standards
Each SPD type is tested using standardised surge waveforms that represent the disturbance it is designed to withstand.
Type 1 devices are tested using a 10/350 µs waveform, representing high-energy current from direct lightning. Type 2 devices use an 8/20 µs waveform, representing indirect lightning and switching surges. Type 3 devices are typically tested using a combination waveform to simulate residual surges reaching equipment.
These waveforms help ensure SPDs are suitable for the energy levels expected at their installation point.
Single-phase and three-phase installations
The principles are the same, but three-phase installations require SPDs designed to protect all live conductors and neutral where applicable. In higher-risk environments, Type 1 protection at the origin should be considered for three-phase systems as well, coordinated with downstream protection.
Meter supply isolator with surge protection
Where space is limited — or where protection is preferred upstream of the consumer unit — a meter supply isolator with integrated surge protection can be a neat solution. Positioned between the meter and the consumer unit, it limits transient overvoltages before they reach the distribution board, helping protect the entire installation.
This approach can be particularly useful for consumer unit upgrades, retrofits where board space is tight, and installations where a clean, pre-board protection solution is preferred.
As with all SPDs, correct installation location, earthing arrangement and compliance with BS 7671 are essential.
Conclusion
For many UK installations, a Type 2 SPD at the consumer unit provides the core level of protection. Add Type 1 where lightning current influence is a realistic concern, and use Type 3 near sensitive equipment where additional limitation is required. Whatever the approach, devices must be correctly selected, coordinated and installed in line with current standards and manufacturer instructions.
FAQs: Surge Protection Devices
Do SPDs stop lightning striking a building?
No. SPDs do not prevent lightning strikes. They limit the overvoltage that can enter the installation during lightning activity or switching disturbances by diverting excess energy safely to earth.
Are SPDs required under BS 7671?
Under the 18th Edition (as amended), surge protection must be considered for all installations. In many cases, an SPD is required unless the installation owner accepts the risk and protection is considered unnecessary.
Do SPDs replace MCBs or RCDs/RCBOs?
No. They do different jobs. MCBs protect against overcurrent, RCDs/RCBOs protect against earth fault shock risk, and SPDs protect against transient overvoltages.
What happens if an SPD operates?
SPDs are connected in parallel, so they do not usually interrupt power to circuits when diverting a surge. If the device reaches end of life, indicators typically show it needs replacing, but the installation will usually remain energised.
Is a Type 2 SPD enough for most homes?
In many typical UK homes without an external lightning protection system, a Type 2 SPD at the consumer unit provides suitable protection against most surges. Where lightning current could enter the installation, Type 1 protection should also be considered.
Do underground supplies mean surge protection isn’t needed?
Not necessarily. Surges can still occur due to network switching events or distant lightning activity, even when supplies are underground.
Do SPDs protect against all power problems?
No. SPDs protect against short-duration voltage spikes. They do not correct sustained overvoltage, undervoltage, power cuts or frequency issues.
Does an SPD need its own MCB?
Traditionally, many Type 2 SPDs used a dedicated MCB for backup protection and isolation. Some newer designs are engineered to connect directly to the busbar and do not require a separate MCB, simplifying installation when used correctly.
For more information on Surge Protection Quick Reference: Type 1, Type 2 and Type 3 SPDs (UK) talk to Expert Electrical Supplies Ltd