When we plug our mobile phone into the mains to recharge its internal battery a current passes through the connecting wires. As all wires have some resistance heat is produced in them and their temperature will increase.
We can keep the heat production and so temperature increase very low by keeping the resistance of the wires very low, and that means making the wires very thick. But how low is very low, and how thick is very thick?
The heat production and temperature increase depend upon the current through the wire, and if we only need a current of a few milliamperes to recharge our phone battery it would be quite safe to use a wire of such a thickess that when it carries a current of 1 A it only gets very slightly warm.
We rate wires according to their safe current carrying capacity: a wire rated at 1 A will only get very slightly warm when carrying a current of 1 A. A problem arises however if a fault such as a short circuit develops inside your mobile phone. Then a current much bigger than 1 A will flow through the wires, they will get very hot and might even set fire to the house. Many house fires start in exactly this way, where a wire overheats because it is carrying a much bigger current than it is rated to carry. This is where fuses come in.
A fuse is a very short length of a very thin wire made of a special low melting point metal. Although the same current passes through the fuse as the other wires, because the fuse is made of such thin wire it gets much hotter than them. If too big a current passes the fuse melts and breaks the circuit.There are two main types of fuse.
In Britain ordinary 13 A plugs are fitted with cartridge fuses. The fuse wire is inside a small glass tube with a metal cap at each end. These fuses are usually rated at 1, 5 or 13 A - a 5 A fuse is designed to melt if the current through it exceeds 5 A.
The other sort of fuse consists of a short length of fusewire inserted into a fuse holder. These are usually located in a fusebox. In many houses the fusebox is in the cellar or cupboard under the stairs next to the electricity meter and main switch. The best way to find out what this sort of fuse looks like is to look at your own fusebox, and it is always a good idea for you to know where your fusebox, meter and main electricity switch are, but do not touch anything without adult supervision. The fuses in the fuse box are usually rated at 30 A for power circuits and 5 A for lighting circuits. Finally there is the main house fuse, usually next to the meter - this is usually rated at 100 A.
It is important to understand that the fuse in the plug of your mobile phone recharger is designed to protect not your mobile phone but the wire connecting the plug to the mains. Then the fuse in the fusebox is designed to protect the cable leading from the wall socket to the fusebox. Finally the main house fuse is designed to protect the cable bringing electricity into the house: this fuse belongs to the electricity company and it is a criminal offence to tamper with it.
In many new houses fuses have been replaced by circuit breakers. These work in a similar way: if too big a current passes the circuit breaker trips and breaks the circuit.An extension cable rated at 13 A will safely carry a current of 13 A. It will get slightly warm when carrying its maximum current. However if we coil it up very tightly in a confined space so that the heat produced cannot escape it may get much hotter, perhaps even dangerously hot. This is why if we have a long extension cable on a reel we must always unreel the whole cable even if it would be long enough if we unreeled just one or two metres. The same applies to vacuum cleaners, lawnmowers etc fitted with cable (American cord) storage.
We often think that electrical fires can only be started by mains (or higher) voltages and big currents but this is not true. Think about it: the only reason why a battery-operated electric torch works is because an electric current is being used to heat a small piece of wire until it is white hot! The noise you can hear while you are reading this is the fan keeping your Pentium (or Athlon or whatever) processor inside your computer cool: if the fan stopped working your processor would get so hot that it would be destroyed within a few seconds. A wire designed to carry 0.01 A but actually carrying 0.1 A is just as likely to catch fire and burn the house down as one designed to carry 13 A but actually carrying 30 A.
Most mains adapters for low voltage equipment such as personal stereos have overload protection of some sort, but rechargeable batteries in particular can give very high currents - this is why they are used in radio controlled cars and video cameras, and of course real cars. Feel how hot the battery in a camcorder has got after you have been recording for fifteen minutes. A 12 or 24 V car battery can easily deliver more than 100 A - if you accidently drop a spanner onto a car battery in such a way that it shorts out the terminals, by the time you realise you have done so it will have become too hot to remove with your bare hands. Even a single D size fully charged rechargeable battery in good condition in a plastic battery holder of the sort commonly used in schools can, if shorted out, deliver so big a current that it gets so hot that the plastic holder catches fire.
© Barry Gray May 2004