Safety secrets of the three-pin plug

Do you know why one pin is longer than the others?

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Professor Brian Cox Promotes Science at London School

You probably have at least ten three-pins plugs in your home, and more at work, but have you ever stopped to look at the pins? You'll notice that one of the three is longer than the others. Find out why.

The top pin is the earth pin. The other two are live (also known as line) and neutral, respectively. The fact that the earth pin is the longest means that it's the first to connect – because it goes into the socket ahead of the other two.

So the connection you make when you plug in is safer because it's earthed before live current starts flowing between the mains from the socket and the plug.

The fact that it's the longest also means that it's the last to disconnect, so it also keeps you safer as you withdraw the plug from the socket. The earthing connection is maintained after the live connection through the shorter pin has been stopped.

But what does 'earthing' actually mean? And how does it keep you safer?

Many electrical appliances, including washing machines, fridges, kettles, cookers and toasters, have metal cases. The metal is capable of conducting electricity. If a live wire becomes loose and touches the metal casing, the whole of the outside of the appliance could conduct electricity. Touching it might result in a serious electric shock.

The earth wire in your plug keeps you safe because even if the live wire touches the metal casing, the current would be diverted through the earth wire (which has very low resistance) and causes very high current in the fuse fitted to the live wire. The current surges through the earth wire, breaks the fuse, and disconnects the appliance.

So next time you have to change a fuse, wire a plug, or even just push a plug into a socket, remember how well the whole system is designed to keep you safe.