FAQs about SPDs
A Surge Protection Device (SPD) is designed to protect electrical installations and equipment from transient overvoltages, commonly known as surges. These surges can be caused by nearby lightning strikes, switching operations within the electrical network, or faults on the power supply. By safely diverting excess voltage to earth, an SPD helps prevent damage to sensitive electrical and electronic equipment, reducing the risk of costly downtime, repairs and replacements.
A surge is a very short, sharp increase in the voltage in the electrical system. The voltage is essentially the pressure of the electrical system, so when this increases, it leads to electrical components being damaged within the installation. You won’t be aware of the surges when they occur, but over time they lead to the degradation of electrical components, which will shorten the lifespan of the electrical equipment in your installation, whether that’s a domestic dwelling, or commercial / industrial premesis.
Modern homes are filled with valuable electronic equipment, most of which contain sensitive components that can be damaged in an instant by a surge. From smart TV’s, computers, gaming consoles, boiler controls, heat pumps, washing machines and LED lightning, today’s homes rely on technology more than ever before.
Surges can lead to costly repairs, equipment replacement and, in some cases, the loss of irreplaceable memories such as family photos, videos and important documents stored on computers and connected devices. While many homeowners insure their possessions against theft and accidental damage, few consider the risk posed by electrical surges.
Surges occur far more frequently than most people realise and installing an SPD acts as the first line of defence, helping to protect your home and the valuable equipment within it from failure. Considering the value of the electronics in a typical modern home, installing an SPD is often one of the simplest and most cost-effective forms of protection available.
Lightning strikes can generate powerful surges that travel through the electrical system, even when the strike occurs several miles away. For most installations, a Type 2+3 SPD provides effective protection against the surges commonly caused by nearby lightning strikes, switching events or power disturbances.
However, when a building is fitted with an external lightning protection system (LPS) or is supplied by overhead power lines, a Type 1+2+3 SPD is required. These devices are specifically designed to withstand and safely discharge a high-energy surge and lightning impulse event, associated with a direct lightning strike to the building.
The correct SPD selection is therefore paramount to ensuring compliance with installation requirements and providing the appropriate level of protection for the installation.
A Type 1 Surge Protection Device is designed to protect electrical installations against lightning strikes.
When and where is it Required?
Under BS 7671 and BS EN 62305, a Type 1 SPD is strictly required at the origin of an installation under the following conditions:
External Lightning Protection System (LPS):
If the building is equipped with an external lightning protection system, BS EN 62305 takes precedence, making a Type 1 SPD mandatory at the origin and distribution boards that supply sub-circuits or equipment located outside the main structure—such as external EV charging stations or solar PV arrays that cross lightning protection zones.
Overhead Lines:
If the electrical installation is supplied via an overhead supply line, it faces an elevated risk of direct lightning flashes, requiring a Type 1 SPD.
A Lightning Risk Assessment is a formal calculation performed under BS EN 62305-2 to evaluate whether a structure requires an external Lightning Protection System.
When BS EN 62305 Precedes BS 7671
While BS 7671 governs general electrical installations, BS EN 62305 takes precedence whenever a building has an external LPS or high lightning risk. In these cases, standard BS 7671 SPD rules become secondary.
Key Factors Calculated in a Risk Assessment
The assessment mathematically balances multiple risk factors, including:
- Structure Exposure: Building dimensions, height, location, and surrounding terrain (e.g., isolated high ground vs. sheltered urban area).
- Incoming Lines: Whether incoming electrical and telecommunications lines are buried underground or supplied via exposed overhead lines.
- Consequence of Damage: Evaluates risk to human life, risk of loss of essential public services, loss of cultural heritage, and economic/data loss.
- Internal Systems: Presence of external rooftop equipment (such as Solar PV arrays or HVAC units) that cross lightning protection zones.
Assessment Outcomes
- High Risk / LPS Required: Mandates a structural LPS alongside Type 1 SPDs at the main electrical intake and on all boards feeding external circuits.
- Calculated Separation Distances: Determines the required spatial separation between lightning conductors and internal metalwork/PV arrays to prevent dangerous flashover arcing. If separation cannot be achieved, direct equipotential bonding and Type 1 SPDs are required.
A Type 2 Surge Protection Device is the standard form of protection used in electrical installations to protect against transient overvoltages caused by indirect lightning strikes and everyday switching events.
When and where is it Required?
Type 2 SPDs are installed at main distribution boards or sub-distribution boards to safeguard the entire wiring infrastructure and downstream loads. A Type 2 SPD provides all the protection required at the board level where there is no external lightning protection system and no overhead supply
Type 1:
A Type 1 SPD is designed to protect electrical installations against direct lightning activity. This is only required where there is an external lightning protection system or an overhead supply. For buildings with an external lightning protection system, the Type 1 SPD is required on the main incomer or on distribution boards supplying external circuits. For installations that are overhead supplied, a Type 1 device is only required on the main incomer.
Browse our range of Type 1 SPDs
Type 2:
A Type 2 SPD is designed to protect electrical installations against indirect lightning activity and overvoltages, such as switching surges. If it is a distribution board that requires protection and it does not fall into the criteria above to require a Type 1 SPD, a Type 2 SPD will always be required. A Type 2 device can be installed on the main incomer or sub-distribution boards.
Browse our range of Type 2 SPDs
Type 3:
A Type 3 SPD is designed to protect individual electrical equipment against overvoltages. Unlike Type 1 or Type 2 SPDs, Type 3 SPDs are designed to be installed locally to the equipment you wish to protect.
To correctly select an SPD, you can follow the following steps:
1) We need to select the type of SPD
- Main Incomer of the installation and it has an overhead supply – Type 1
- Main Incomer of the installation and it has an external LPS – Type 1
- Sub board and the installation has an overhead supply – Type 2
- Sub board (internal circuits only) and the installation has an LPS – Type 2
- Sub board (includes external circuits) and the installation has an LPS – Type 1
- Sub board and the installation has no LPS and underground supply – Type 2
2) What is the earthing system
- TN/TNC/TNS/PME – CT1 or CT2
- TT – CT2 Only (if the installation could be TT in the event of an emergency make sure your SPDs are suitable for a TT)
3) Single phase or Three Phase
By following this process, it will help you select the correct SPD! If you require further help you can use our product selector tool.
To achieve full protection, SPDs must be installed at strategic points across an installation’s electrical infrastructure.
- Main Distribution Board
Every installation requires primary protection installed as close as possible to the origin of the supply (e.g., consumer unit or main switchboard):
- Type 1 SPD: Required if there is an external LPS or overhead supply.
- Type 2 SPD: Required for standard installations without external lightning protection.
- Sub-Distribution Boards
If an external LPS is installed on-site, and the DB supplied external equipment, these sub-boards require a Type 1 SPD. For all other cases a Type 2 SPD can be used
- Sensitive Equipment
Install Type 3 SPDs directly at or adjacent to sensitive, critical, or high-value equipment—such as data racks, fused spurs, or Cat 6 network runs—especially where the equipment is located a significant distance downstream from the distribution board.
- On DC Solar PV Strings
On solar installations, SPDs must be installed on the DC side between the solar array and the inverter, mounted close to the inverter or in a dedicated string box.
Yes. Under Section 443 of BS 7671 Wiring Regulations, surge protection is mandatory for the vast majority of electrical installations.
Regulation 443.4.1 Requirements
Protection against transient overvoltages shall be provided where the consequences of a surge could:
- Result in serious injury to, or loss of, human life.
- Result in significant financial or data loss.
The Practical Position
Because almost all modern domestic, commercial, and industrial distribution boards supply equipment that falls into these two categories—such as fire alarms, life-safety detection, or sensitive electronic and computing assets—the practical requirement under Section 443 is that SPDs must be installed in nearly all installations.
Can SPDs Be Omitted?
For all other installations not meeting the strict criteria above, an SPD is still required unless the person ordering the work formally declines the installation in writing and explicitly accepts the long-term risk to the wiring and equipment as tolerable.
(Note: Where a building has an external Lightning Protection System, BS EN 62305 takes priority over BS 7671, making a Type 1 SPD mandatory.
An SPD does not have a fixed calendar expiration date; instead, its lifespan depends on the frequency and intensity of the overvoltages it diverts to earth.
Each time a transient overvoltage or switching surge occurs, these components divert energy to earth, undergoing a small amount of internal degradation in the process. Over time, repeated small surges or a single massive direct spike will cause the module to reach the end of its operational life.
An SPD would only need replacing when the indication changes over on the module.
No. You should not electrically test an SPD.
An SPD acts as a voltage-controlled variable resistor. Under normal operating voltages, it maintains high electrical resistance. When exposed to high voltage, its resistance drops instantaneously to establish a low-impedance path to earth.
An electrical testing instrument (such as an insulation resistance tester or high-voltage flash tester) injects high DC voltages into the circuit. The SPD cannot distinguish between test voltage and a dangerous surge event. Consequently:
- The tester will yield false readings (e.g., failing an insulation resistance test).
- High voltage injected by test equipment forces unnecessary internal degradation on the SPD components, directly shortening its operating lifespan.
Correct Maintenance Procedure
During routine inspections or Electrical Installation Condition Reports (EICRs), the proper procedure is purely visual inspection:
- Check the visual status flag or LED indicator on the front of the unit (Green = Active, Red = failure).
- If performing an insulation resistance test on the installation wiring, either disconnect/unplug the SPD module or use the tester’s test-isolation switch to avoid damaging the device.
Yes. Solar photovoltaic (PV) systems require surge protection on both the AC and DC sides under BS 7671 Section 443.4, which applies equally to AC and DC electrical circuits.
Common Misconceptions to Avoid
- Integrated Inverter Protection: Installers often assume an inverter with “built-in” surge protection removes the need for external SPDs. Many inverters only provide protection on their AC side or use basic varistors that do not satisfy the full product standards (BS EN 61643-31). Installers should request an official Certificate of Conformity from the manufacturer to confirm DC-side compliance.
- Using AC SPDs on DC Lines: Standard AC SPDs must never be used on the DC side of a solar system. Because DC arcs do not cross a zero-voltage point like AC, DC SPDs are specifically engineered for continuous DC voltage loading and must bear the official PV symbol.
Selecting the Correct PV SPD
- Standard Installations: A dedicated Type 2 DC SPD rated for the inverter’s DC operating voltage (typically 600V or 1000V) must be installed on the DC side.
- Buildings with an External LPS: If the property has an external Lightning Protection System, the PV cabling crosses lightning protection zones. In these cases, external Type 1 DC SPDs are mandatory under BS EN 62305.
Find out more in our solar blog.
Electric Vehicle (EV) charging infrastructure is particularly vulnerable to transient overvoltages due to its physical placement and delicate internal electronics.
Why EV Chargers are Vulnerable
- Exposed Locations: EV chargers are typically mounted externally on walls or pedestals. Outdoor cabling acts as an antenna for induced voltage spikes caused by nearby secondary lightning strikes.
- Switching Surges: Repetitive grid disturbances and internal switching surges degrade the charger’s control electronics over time.
Critical Life-Safety Implications (Open PEN Protection)
Beyond protecting the charger hardware, surge protection on EV circuits serves a critical life-safety function. Many modern EV charge points utilize internal Open PEN fault detection electronics to safely isolate the EV if a protective earth neutral (PEN) supply conductor breaks.
If a PEN conductor breaks without detection, the metallic chassis of the connected vehicle can rise to a lethal voltage relative to true earth. If repetitive switching surges or transient overvoltages damage these sensitive internal electronic sensors, the disconnection mechanism can fail silently, exposing users to severe or fatal electric shock upon touching the vehicle.
Installation Requirements
- Standard EV Installations: The board supplying an external EV charger requires a Type 2 SPD as standard.
- Sites with External LPS: If the building features an external Lightning Protection System, the distribution board supplying the EV charger requires a Type 1 SPD because it feeds an external circuit.
Read our EV blog to find out more.
While power SPDs protect main electrical distribution boards, transient overvoltages can easily bypass these units by traveling along signal, communication, and data cables. Protecting data networks requires localized Type 3 SPDs engineered specifically for low-voltage signal lines.
How Data Networks Experience Surges
Data cables (such as Cat 6 Ethernet runs or Power-over-Ethernet lines) running between buildings, across outdoor spaces, or in close proximity to power cabling can pick up induced overvoltages from nearby lightning strikes or electromagnetic interference. These surges travel directly into network switches, servers, and connected IP hardware.
Selecting a Data SPD
Data SPDs differ significantly from power distribution SPDs:
- Operating Voltages: Data devices operate at much lower voltages, starting from as low as 6V.
- Bandwidth & Connectivity: Data SPDs must be selected to match network bandwidth requirements (e.g., Cat 6 or PoE) to ensure transient protection does not attenuate signal quality or restrict data transfer rates.
- Form Factors: Available as inline RJ45 modules, DIN-rail terminal blocks, or multi-port rack-mounted units installed directly at the equipment rack or cable entry point.
