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By this point, most electrical professionals will be familiar with the term surge protection device (SPD). However, there is still confusion around when SPDs are required and which type should be installed. Kirsty Johnson MIET, Technical Director, Surge Protection Devices Ltd, provides a practical overview of SPD selection and clarifies the requirements under BS 7671, including specific considerations for EV charging and solar PV installations.
There are three types of SPD, selected based on what part of the installation requires protection. Type 1 and Type 2 SPDs are typically installed at distribution boards or consumer units, while Type 3 SPDs are used to protect specific items of equipment, such as a socket outlet or fused spur.
A Type 1 SPD is only required where a building has an external lightning protection system or where the installation is supplied via an overhead line. In all other cases, a Type 2 SPD will generally provide adequate protection for distribution boards. Where protection is required at the point of use for sensitive or critical equipment, a Type 3 SPD may be installed downstream.
When are SPDs required?
Regulation 443.4.1 of BS 7671 states that protection against transient overvoltages shall be provided where the consequences could:
For all other installations, SPDs are still required unless the person ordering the work formally declines surge protection and accepts the associated risk to wiring and equipment as tolerable.
The requirement relating to the protection of life is the most onerous. Any distribution board supplying equipment intended to protect life or prevent injury will require surge protection. As a result, the practical position of Section 443 is now that SPDs should be installed in the majority of installations.
In reality, most buildings contain circuits that fall within Regulation 443.4.1, meaning that most distribution boards will require an SPD. Only in limited cases, where none of the criteria apply, should a discussion take place between the electrical designer and client to assess whether the omission of surge protection is acceptable.
Where a building has an external lightning protection system, BS EN 62305 applies. In these cases, a Type 1 SPD is required at the origin of the installation, and BS 7671 requirements are secondary. Type 1 SPDs will also be needed on any distribution boards supplying external circuits.
This has direct implications for two of the fastest-growing areas of the industry: EV charging and solar PV.
EV chargers and SPDs
EV chargers are both sensitive to overvoltage and commonly installed in exposed locations. Many manufacturers now either specify the installation of an SPD or reference surge protection within their warranty conditions.
Damage to EV chargers can occur due to overvoltages originating within the installation, such as switching surges or disturbances on the supply network. These repetitive overvoltages can degrade the sensitive electronic components used in modern chargers.
Equally important is the impact EV chargers can have on the wider installation. Any cable entering or leaving a building presents a potential route for overvoltages. Because EV charging circuits often run externally, they are particularly vulnerable to the effects of nearby lightning strikes.
Where a building has an external lightning protection system, the distribution board supplying the EV charger will require a Type 1 SPD, as it is feeding an external circuit. This requirement does not apply to buildings without an external lightning protection system.
Solar PV and SPDs
The rise in solar PV installations continues as consumers seek to reduce energy costs and adopt a more prosumer-based approach to energy use. One area frequently overlooked in PV system design is surge protection.
Section 443.4 of BS 7671 applies to both AC and DC installations, meaning that surge protection requirements extend to the DC side of solar PV systems – a detail often missed during design and installation.
Some inverters claim to include overvoltage protection. For an integrated SPD to satisfy the Wiring Regulations, the manufacturer must clearly state that protection is provided on the DC side of the inverter. Many products marketed as having surge protection only provide protection on the AC side. Others include individual varistors, which do not constitute a complete SPD.
Where there is any uncertainty, installers should request a certificate of conformity from the manufacturer. This document confirms the standards the equipment complies with and provides assurance of its suitability.
It is also important to note that inverters typically only incorporate Type 2 SPDs. If a building is fitted with an external lightning protection system, additional external Type 1 SPDs will always be required to comply with BS EN 62305, as PV cabling will cross lightning protection zones. The same applies to any distribution board supplying the PV system, as it is feeding an external circuit. Again, this only applies where an external lightning protection system is present.
Although SPDs are still classified as Type 1 and Type 2 devices, DC SPDs for PV systems are specifically designed for DC applications and differ significantly from their AC counterparts. Only SPDs designed and rated for DC use should be installed on the DC side of a solar PV system.
Selecting a PV SPD is straightforward once the required type has been identified. The remaining consideration is the inverter operating voltage, most commonly 600V or 1000V, although specialist systems may require alternative ratings.
This article appeared in Electrotechnical News May/June 2026 edition – read it here
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