Surge protectors

Last updated: October 24, 2007.

When lightning strikes, it's exciting and exhilarating—but it's scary too. It's scary because it's dangerous: leaping bolts of lightning contain huge amounts of electrical energy that are released in a fraction of a second. If lightning strikes near your home, all that electricity has to go somewhere. One place it may go is through the electrical wiring system in your home, damaging or destroying any electrical items that are plugged in at the time. It's almost impossible to stop lightning from damaging your things, and it's generally best to unplug whatever you can well before a storm arrives. Another helpful thing you can do is install surge protectors. These cheap, compact cubes and power strips help to even-out sudden peaks of electricity in the supply and reduce the chances of damage to sensitive electronic equipment. Let's take a closer look at how they work.

Photo: A typical UK surge protector built into a cube. This one is made by Belkin; other brands include APC, Ativa, and Hubbell.

What are surges?

Electricity comes into your home from a wiring system that starts at a power plant some distance away. Different appliances in your home need larger or smaller amounts of electric power. Things that get hot (electric showers, toasters, and stoves) need large currents that supply a lot of power at once, whereas electronic equipment (CD players, televisions, and so on) needs much smaller currents and uses less power. All these appliances assume that the electricity coming into your home has a reasonably constant voltage.

But sometimes the voltage fluctutates because of sudden changes in the way power is supplied from the grid. Or it can happen if someone switches on an appliance with a powerful electric motor inside it (such as a drill, a vacuum cleaner, or an electric lawn mower), it can cause a sudden surge or drop in power in the whole circuit in your home. A very brief change in voltage is called a spike. A longer-lasting change is called a surge. A spike or surge won't affect other big appliances, but it could harm tiny components in sensitive electronic equipment. What we need is something that smooths out any peaks in the voltage—and that's what surge protectors do.

How surge protectors work

The appliances you use draw their power from sockets in the wall. The power from the sockets feeds straight into the appliance down a length of cable. In a surge protector, the main power line (known as the hot wire or live wire) has an extra connection (a kind of "side road") linked to it that feeds to the ground wire (sometimes also called the Earth wire; the protective wire in an electric circuit that sends any unwanted current safely into the earth). Normally, the surge connection is inactive. However, if a larger than normal voltage appears, and produces too much electric current, the excess current is diverted safely down the side road to ground. That means no more current than normal flows into your appliance, so it's better protected from harm.

How does the surge connection know when to divert the current? It is actually a device called a varistor (voltage-dependent resistor), made from a substance called a metal-oxide semiconductor, which is usually a bad conductor (carrier) of electricity. When an excessive voltage appears, the semiconductor in the varistor becomes a good conductor and starts to carry electricity normally. For as long as the surge voltage lasts, the semiconductor channels harmful current to ground. Once things return to normal, the semiconductor switches back again.

All this means your appliance is not only protected during a surge—it should keep on working normally.

Artwork: Left: Without a surge protector, the hot/live (brown) and neutral (blue) connections provide power to your appliance. The ground (green) connection is typically wired to the metal case to provide a safe way for stray currents to escape, but it's not involved in powering the appliance.

Right: With a surge protector, there is an extra connection from the hot/live wire to the ground. If a surge current flows in down the hot/live wire, any excess current is safely diverted round the surge wire (red) to the ground/earth. NB: This example features typical UK wiring.

Why surge protectors don't give you total protection?

It's important to note that surge protectors don't give you complete protection. A direct lightning strike is an absolutely massive discharge of electricity; a surge protector probably won't stop such a huge surge from damaging things in your home. Surge protectors are also of limited value when surges last some time and they don't protect against higher than expected currents from the power grid.

Photo: Surge protectors may not save you from a direct lightning strike. Picture by Dave Parsons courtesy of US DOE/NREL (Department of Energy/National Renewable Energy Laboratory).

What's the difference between a surge protector and a fuse?

A fuse is designed to stop sudden large electric currents from damaging the equipment in your house. Sounds the same as a surge protector, doesn't it? But it actually works a different way. Most fuses are very thin pieces of wire designed to allow only so much current through them. The thicker the wire, the more current can flow; so fuses rated for higher currents usually have thicker pieces of wire inside them.

Photo: A pair of 30-amp household fuses. These are designed to let very large household currents of up to 30 amps (and no larger) flow through them.

How does a fuse work? If there's too much current (for example, if you've put too many appliances together on one socket), the fuse literally burns out: the wire gets so hot that it melts and interrupts the circuit to protect you. Occasionally, fuses actually "blow": the current flowing through them is so great that they burn out instantly with a loud cracking noise. A fuse, then, is a very drastic form of protection: if anything happens, it shuts off the electricity completely. A surge protector is designed to smooth out smaller fluctuations in voltage and it doesn't normally shut down the circuit when a problem occurs. You need both fuses and surge protectors for good protection from electrical problems.