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Surge protectors and fuses

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by Chris Woodford. Last updated: December 18, 2016.

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, which is probably the best-known brand; other popular makes include APC, Ativa, and Hubbell.

What are surges?

Closeup of a Belkin surge protector showing the indicator lights.

Photo: Closeup of a typical Belkin surge protector. Note the indicator lights on the top, both of which need to be lit up to confirm that the protector is working. The one on the left glows green to show that the appliance is protected. The one on the right (marked either 'Earthed' or 'Power') confirms that the power is switched on.

If you've read our long article on electricity, you'll know that an electric current is a flow of electrons (tiny particles inside atoms) carrying energy through a metal or another substance in a loop called a circuit. You'll also know that electricity can be extremely dangerous: it's not something to mess with if you value your life. The electricity that comes to our homes from power plants travels at incredibly high voltages because that helps to save energy. Transformers in substations near to buildings turn the high voltage power into lower voltages that the appliances in our homes can safely use. Different appliances 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 fluctuates because of sudden changes in the way power is supplied from the grid. Or it can happen if someone in a nearby factory switches on or off a huge appliance with a powerful electric motor inside it, which might 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 probably 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.

How a surgeprotector works

Artwork: Left artwork: 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 artwork: 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.

Fuses

UK wired three pin electricity plug and fuse

When a fuse goes, you can often hear it blowing out with a sharp CRACK! that plunges your home into sudden darkness. It's a real nuisance when this happens late at night—but the alternative is much worse. If we didn't have fuses, electrical faults could start fires in our homes and burn them to the ground. Thank heavens, then, for these tiny electric protectors that keep us safe. Let's find out what they are and how they work!

Photo: The fuse inside an electrical plug (wired for the UK electricity system). The fuse is the brown vertical cylinder on the right. This one is rated at 13 amps, which is the largest possible current any appliance like this should take. It's more common for small appliances to use 3 amp or 5 amp fuses.

Why do we need fuses?

For a whole variety of unpredictable reasons, the cables running into electrical appliances can suddenly find themselves carrying much more current than they should. If we didn't have fuses, those high currents could damage our televisions, radios, computers, and light bulbs, possibly causing fires, and maybe even putting lives at risk. Fuse protect electrical appliances by blocking currents that are bigger than they should be.

How fuses work

You probably know that wires get hot when electricity travels through them. That's how ordinary, incandescent lamps work. Electricity flows through a very thin wire called a filament making it so hot that it gives off light. The same idea is at work in an electric toaster. Here, electricity flows through a series of thin metal ribbons, making them so hot that they produce enough heat to cook bread. A fuse is exactly the same. It's a thin piece of wire designed to carry a limited electrical current. If you try to pass a higher current through the wire, it'll heat up so much that it burns or melts. When it melts, it breaks the circuit it's fitted to and stops the current flowing.

We fit fuses in different places in our homes. In some countries, such as the UK, fuses are fitted into plugs on every appliance that connects to an electrical outlet. Different appliances draw different amounts of current, so an electric toaster will need a higher fuse (typically 13 amp) than an electric light (usually just 3 amp).

Old fashioned fusebox using fuse wire

Types of fuseboxes

Photo: An old-fashioned fusebox. This one has four fuses inside four brown Bakelite fuse holders, each fuse protecting a separate circuit inside a house. If one fuse blows, the other three should remain unaffected. The entire supply can be switched on and off with the little red switch on the right. That switches all four circuits on or off at the same time.

There are also fuses fitted at the junction where the main electricity supply flows into your home. This is called the junction box or fusebox. It divides the incoming electricity into a number of separate circuits and feeds them to different parts of your home. A high-power circuit feeds large items like electric cookers, while lower-rated circuits feed lights and other appliances. Having different parts of your home on separate circuits means that a failure in one circuit doesn't stop the others from working.

Typically each circuit in your home is fitted with its own fuse. In older fuseboxes, the fuse is just a bare piece of wire connected between two terminals. More recent fuseboxes have replaceable cartridge fuses with the fuse wire built into a glass cylinder that you can easily snap in and out. The latest fuseboxes do away with fuses altogether and have trip switches instead. If a fault occurs, the fusebox detects the problem instantly and the trip switch automatically switches off whichever circuits are affected. Once you've identified and solved the problem, you can simply flip the switch back to get the power working again.

A modern fusebox with RCD trip switches A modern fusebox with RCD trip switches (close-up)
Photo: A modern fusebox like this one, made by Wylex, uses trip-switches instead of fuse wire or cartridges. The left photo shows the entire fusebox; the right photo shows a close-up of the trip switches. If there's too much current flowing in one of the circuits, the switch for that circuit flips over and cuts off the electricity. You can restore the power by flipping the switch back again (after correcting whatever caused the problem). Half the circuits in this fusebox are fitted with automatic RCD (residual-current device) protection, which greatly reduce the risk of electric shocks when you accidentally cut through power cables.

Which fuse should you use?

30 amp cartridge fuses from a fusebox

If you have to replace a fuse, it's generally fine to replace the one you take out with another one of the same rating (13 amp with 13 amp, 3 amp with 3 amp, or 5 amp with 5 amp). But it's always a good idea to check: most appliances (or their instruction books) will tell you what fuse you need. You can sometimes work by instinct: large appliances that heat things, such as electric kettles or electric fires, will draw high currents and need large fuses; small appliances that use smaller currents, such as table lamps or cellphone chargers, will only need small fuses. If you put a small fuse in an appliance that draws a large current the fuse will blow quite quickly and stop your appliance working; if you put a large fuse in an appliance that draws a small current, you're stopping the fuse from working and putting yourself at risk.

Photo: Two glass cylinder fuses, rated at 30 amps, from a household fusebox. You never need fuses this big in single, household appliances.

You can also calculate the fuse you need from the power rating of your appliance and the voltage of your supply, because power, voltage, and current are related by a simple equation: power (watts) = voltage (volts) × current (amps). So to find the fuse rating (which must be higher than the current the appliance draws), simply divide the power rating of your appliance by the voltage. For example, if you live in the UK and you have a 2500 watt electric kettle and a 240 volt supply, you can see that your kettle will use a current of 2500 divided by 240 or approximately 10.5 amps, so you need a 13 amp fuse. If you have a table lamp with an old-fashioned 60 watt light bulb, it will use 60/240 = 0.25 amps, so a 3 amp fuse is what you need. Here's a summary of how it works for 240 volt supplies:

Fuse rating Power rating (for 240 volt supply)
3 amp Up to 720 watts.
5 amp 720–1200 watts
13 amp Over 1200 watts

If in doubt, always use the smallest fuse; the worst that will happen is that the fuse will blow if the current is too high. If you use a fuse that's too big, it won't protect your appliance from excessive currents and you may put yourself, your home, and your life at risk.

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.

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. Indeed, if you look round the back of a typical surge protector, you'll very likely find.... a replaceable fuse!

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Woodford, Chris. (2007/2012) Surge protectors and fuses. Retrieved from http://www.explainthatstuff.com/surgeprotectors.html. [Accessed (Insert date here)]

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