Most people who use PCs very regularly have accidentally tripped over a cable, yanking it out.

More likely, especially if you live in a rural area, you may have experienced a power cut during a storm, or a ‘brownout’, when the mains voltage fluctuates beyond its normal voltage range. Any of these power variations can mean lost work and lost hair, as you tear it out.

A lot of power problems with computers come not from a mains power cut, but from brownouts. We think of our mains electricity as being 240V, but in fact it’s been harmonised with the European Union, and is centred around 230V, with an allowance of -5 per cent or +6 per cent.

This means the mains voltage can lie anywhere between 218.5V and 243.8V. All domestic electrical equipment is designed to work over this range, so for most purposes you don’t need to worry about it. It’s when it drops further than that range that things start to go wrong.

If the voltage drops below the normal voltage range, but not to zero, it’s called a brownout, and if your house lights have ever dimmed or flickered for a moment then you’ve seen one in action.

In some ways, a brownout, particularly if it’s short-lived, can be more troublesome to a PC than a blackout. At least with a power interruption, you know it’s happened. Everything else electrical in your house is likely to have died too, leading to an hour of resetting the clocks and timers on everything.

With a brownout, however, you may just find that settings go haywire or that changes to files have been lost. Fortunately, there’s a piece of computer equipment that can take care of both power cuts and brownouts. It’s called an Uninterruptible Power Supply, or UPS.

The principle is quite simple: in the UPS box is a low-voltage battery, a charger circuit, an inverter to get the voltage up and something to convert the battery’s DC power to AC – suitable to feed to your PC. There’s also a voltage sensor, so the UPS knows when the power has dropped off.