What is a power surge?

Glossary · Wiring, circuits and faults

How a power surge works

A power surge is a short burst of voltage far above the normal level. The IET calls these transient overvoltages and defines them as short-duration surges of electricity that occur when energy previously stored, or induced by other means, is suddenly released.^{[1]The Institution of Engineering and Technology, Surge Protective Devices (SPD)}

There are two families of cause. Natural ones come mainly from indirect lightning: a strike on a nearby overhead power or telephone line sends the surge along the line and into the installation. Man-made ones come from the switching of motors and transformers, and some types of lighting.^{[1]The Institution of Engineering and Technology, Surge Protective Devices (SPD)}

The IET says this used to be less of an issue in homes. New technologies such as electric vehicle charging, air and ground source heat pumps and speed-controlled washing machines have made transients much more likely in domestic installations.^{[1]The Institution of Engineering and Technology, Surge Protective Devices (SPD)}

What a surge can damage

According to the IET, a surge can affect the consumer unit, wiring and accessories. It can also damage sensitive electronics such as computers, televisions and washing machines, and safety circuits such as fire detection systems and emergency lighting. The result can be instant failure, or damage that shows only over a longer period.^{[1]The Institution of Engineering and Technology, Surge Protective Devices (SPD)}

That second point explains why a device can die weeks after a storm with no obvious cause.

Surge protective devices

A surge protective device (SPD) is the protection. The IET says SPDs are usually installed within the consumer unit, though different types protect against surges arriving on other services such as telephone lines and cable TV. Protecting the installation alone and not those other services could leave another route open for transients.^{[1]The Institution of Engineering and Technology, Surge Protective Devices (SPD)}

When several devices are needed they must be coordinated, and items from different manufacturers should be checked for compatibility.^{[1]The Institution of Engineering and Technology, Surge Protective Devices (SPD)} Choosing and fitting an SPD is work for a registered electrician. It sits with surge protection device in this glossary, and with our consumer unit upgrades service.

Do you need one?

The IET says BS 7671:2018 requires protection against transient overvoltage where the consequences could, for example, risk serious injury or loss of life, or affect a large number of co-located people, unless a risk assessment is carried out. It says the older exception for some domestic dwellings, such as those with an underground supply cable, has been removed, so the requirement now applies to all types of premises. It applies to new builds and properties being rewired. The regulations are not retrospective, and for other homes the decision to buy SPDs is in the customer’s hands, with enough information to decide.^{[1]The Institution of Engineering and Technology, Surge Protective Devices (SPD)}

Extension leads with surge protection

Electrical Safety First says the surge protectors in extension leads are generally self-resetting. If a lead stops working after a surge, the protection fuse may have gone open circuit. If the device has a replaceable fuse you can try changing it, or the fuse in the plug. If that does not work, replace the lead.^{[2]Electrical Safety First, FAQ for Electrical Safety - Your Questions Answered}

Example

After a summer storm over Burley, Marcus finds his router and one television will not switch on, though the lights work. His electrician finds the consumer unit has no surge protection. Marcus decides to have a combined Type 1 and 2 device fitted when the consumer unit is next replaced, and his electrician also checks protection for the aerial and phone lines, because the IET says other services can carry a surge in.

What people get wrong

The common mistake is to assume a surge-protected extension lead protects the whole house. It protects what is plugged into it. The IET describes SPDs in the consumer unit as the protection for the installation itself, with Type 3 devices only as a supplement close to the load.

The second mistake is to wait for a visible failure. The IET says the effects of a surge can be damage that only becomes evident over a longer period.^{[1]The Institution of Engineering and Technology, Surge Protective Devices (SPD)}

The bottom line

A power surge is a transient overvoltage, from lightning on nearby lines or from switching equipment. It can break electronics outright or wear them down over time. Surge protective devices, usually in the consumer unit, are the protection. Whether to fit one in an existing home is the customer’s decision, so ask for the information you need. Fitting is a job for a registered electrician.