How do I protect electronics from a power surge?

A surge is a brief spike in voltage, from microseconds to milliseconds, that pushes far above the 230V the supply normally sits at. The dramatic ones come from lightning, which does not have to strike your house — a strike anywhere on the local network or even nearby ground can induce a spike large enough to destroy sensitive electronics. The mundane ones happen daily, when a motor switches off in the house or on the street, and they degrade electronics gradually rather than dramatically.

Safety

What you'll need

• A surge protector with a stated joule rating and a protection indicator light
• An uninterruptible power supply, for a computer or a NAS (optional)
• A note of what is on each circuit
• An electrician, for anything at the consumer unit

Step by step

1. Decide what actually matters.The television, the computer, the router, the boiler controls, and anything storing data. Protecting everything is expensive and unnecessary; protecting the six things that would be painful to lose is not.
2. Buy on joule rating, not on the number of sockets.The joule rating is how much energy the protector can absorb over its life. Under about 500 joules is close to decorative; 1,000 to 2,000 is a sensible domestic figure for a TV or computer.
3. Check it has a protection status indicator.Surge protectors wear out — each surge consumes some of their capacity — and one that has stopped protecting looks identical to one that still works. A light or an audible warning is the only way to know.
4. Protect every path into the device, not just the mains.A surge can arrive down an aerial or a network cable as readily as down the mains. A protector with coaxial and network pass-through matters for a TV with an aerial or a router with a phone line.
5. Ask an electrician about a surge protection device at the board.A Type 2 SPD in the consumer unit protects the whole installation and is now standard on new and rewired installations. It is particularly worth it with solar panels, a heat pump, a car charger, or an overhead supply in a rural area.
6. Use a UPS where a sudden shutdown matters.A small uninterruptible power supply gives a computer, a NAS or a home server long enough to shut down cleanly. It also smooths brownouts, which damage more equipment than spikes do.
7. Unplug in a thunderstorm if it is directly overhead.No surge protector is rated for a direct strike. Unplugging the mains and the aerial is the only absolute protection, and it takes a minute.
8. Do not daisy-chain protectors or extension leads.Plugging one surge protector into another does not add protection and creates a fire risk from cumulative load. One protector, plugged directly into a wall socket.
9. Replace the protector after a known large surge.If the lights flashed, something tripped and the protector's indicator has changed, it has done its job and is finished. Replace it — a spent protector still passes power, so nothing tells you otherwise.
10. Back up rather than relying on protection.The irreplaceable thing in a computer is the data, and no surge protector is a substitute for a backup that lives somewhere else.

Tips

• Damage after a power cut is often caused by the restoration rather than the cut, as everything comes back at once. Switching off sensitive equipment at the wall during a cut, and back on a few minutes after power returns, avoids most of it.
• Home insurance often covers lightning damage to appliances as a standard peril, but excludes gradual damage and sometimes excludes surges that are not caused by lightning. Read that section before deciding what to spend.
• Rural properties on overhead lines see far more surges than urban ones on underground supply. If you can see the wires coming to your house, protection at the board is worth more.
• A surge protector does nothing about a power cut, and a UPS does nothing about a lightning strike. They solve different problems and a computer usually wants both.
• Solar inverters and EV chargers are both expensive and surge-sensitive, and both are usually installed with an SPD as standard now — worth confirming with the installer.

Common mistakes

• Assuming a cheap multi-socket extension is a surge protector — Most are not, and the ones that are often have a very low joule rating. Look for a stated rating and a protection indicator, not just the words on the packaging.
• Leaving a spent protector in service for years — It degrades with every surge it absorbs and eventually protects nothing while still passing power. The indicator light is the only clue and most people never look at it.
• Protecting the mains but leaving the aerial or network cable unprotected — A surge coming down the aerial into the TV destroys it just as effectively. Protect every conductor that enters the device.
• Fitting a device at the consumer unit yourself — It is work inside a live board, notifiable in England and Wales, and dangerous. It is an electrician's job and the certificate matters for insurance.

If it doesn't work

Questions people ask