What Is an SPD and What Does It Do?
An SPD (Surge Protection Device) is an electrical safety device designed to protect electrical installations and connected equipment from short-duration voltage surges, also known as transient overvoltages.
SPDs are commonly installed inside or alongside a consumer unit or distribution board and provide protection for equipment throughout the installation.
What Does an SPD Protect Against?
Electrical equipment is designed to operate within a particular voltage range. A sudden voltage surge can exceed these limits and potentially damage sensitive components.
Surges can originate from several sources, including:
- Switching operations within the electrical supply
- Large electrical loads being switched on or off
- Supply network disturbances
- Indirect effects of lightning
- Other transient overvoltages entering the installation
Modern properties contain increasing amounts of sensitive electronic equipment, including televisions, computers, appliances, heating controls, smart-home equipment and EV charging equipment.
An SPD helps limit the voltage reaching this equipment.
How Does an SPD Work?
Under normal conditions, an SPD remains inactive and does not interfere with the electrical supply.
When a transient overvoltage occurs, the SPD responds extremely quickly and diverts the surge current away from the protected installation, limiting the voltage to a safer level.
Once the transient event has passed, the SPD returns to its normal state.
Where Is an SPD Installed?
In domestic installations, SPDs are commonly fitted within the consumer unit, usually close to the incoming supply.
They are also widely used in commercial and industrial distribution boards.
Depending on the installation, you may find:
- A separate SPD module
- A multi-pole SPD assembly
- An SPD supplied as part of a consumer unit
- Replaceable SPD cartridges
- Combined Type 1 + Type 2 protection
The exact arrangement depends on the manufacturer and the requirements of the installation.
What Does the Green or Red Indicator Mean?
Many SPDs have a small status indicator on the front.
Green generally indicates that the SPD is operational.
Red generally indicates that the protection module has reached the end of its service life or is no longer providing the intended surge protection.
The exact indication should always be checked against the manufacturer's instructions for the particular model.
If an SPD is showing a failed status, the complete SPD or its replaceable cartridge may need to be replaced.
Are All SPDs the Same?
No. SPDs are available in different types, specifications and configurations.
The three main classifications you will encounter are:
Type 1 – designed to handle partial lightning currents and typically used where protection against direct lightning effects is required.
Type 2 – commonly used for protection against transient overvoltages in electrical installations and frequently found in domestic consumer units.
Type 3 – provides additional protection closer to sensitive equipment and is normally used in conjunction with upstream surge protection.
There are also combined Type 1 + Type 2 SPDs.
Choosing a replacement based purely on appearance is not recommended. The SPD type, electrical specifications, manufacturer and compatibility with the installation should all be checked.
Need a Replacement SPD?
ReCircuit Resale stocks a range of new, surplus and discontinued electrical components, including SPDs and replacement modules from major consumer unit and distribution board manufacturers.
When looking for a replacement, the manufacturer and model or part number printed on the existing SPD are usually the best place to start.
If you're unsure which SPD you have, clear photographs of the existing unit and its markings can help identify the correct replacement.
Next Guide: SPD Types Explained – Type 1, Type 2 & Type 3