MCBs
The MCBs (miniature circuit breakers) in modern consumer units allow you to switch off the power to a specific circuit for repairs or alterations, but they also act as automatic protection in the event of overloading - if you plug in more appliances or use more powerful lights than the circuit is designed to supply. When overloading or a short circuit occurs, the MCB automatically switches itself off, and it cannot be reset to restore power until the fault is corrected.
Wiring systems older than about 25 years and not modernised since installation will have cartridge or rewirable circuit fuses instead of MCBs. These contain a length of special fuse wire that will melt if the circuit is overloaded or short-circuited, cutting off the supply to the circuit. See Replacing a Circuit Fuse.
RCDs
Modern consumer units also contain a more general safety feature called a residual current device (RCD) that monitors the flow of electric current through the property. If the RCD detects that current has gone to earth - usually because of a leak caused by broken insulation, or because someone has touched a live wire and received a shock - it switches off the current in a fraction of a second. This is quick enough to prevent a shock from causing heart failure and death.
Because the area outside a house is not protected by the property's earthing system, RCDs are now fitted in new power points intended for appliances being used out of doors, for instance in garages. If you don't have these, you should buy an RCD adaptor to use when you plug an outdoor appliance into an indoor socket.
The Importance of Earthing
Earthing is one of the most important safety features of your wiring system. It can prevent an electric shock, and makes it much less likely that an electrical fault will develop into a fire.
Earthing provides a safe path for any current that accidentally strays from the circuit wiring because of faulty insulation. The escaped electricity is attracted to earth (the ground) because this is an even better conductor than circuit wiring. Given the chance, electricity will flow to earth by the shortest possible route. If there is no earthing system, this could be through your body.
Every circuit is connected to earth via an earth continuity conductor, which is a wire inside the circuit cable, connected to every lighting point, every socket, and every direct appliance connection point (for instance an immersion heater or electric cooker). All the circuit earth conductors are connected to an earth terminal block in the consumer unit or fuse box. This terminal is connected by a cable with green or green-and-yellow insulation, either to a clamp on the sheath of the main supply cable, or to an earth rod driven into the ground beneath or beside the home.
You should also have similar cables connecting metal gas and water pipes, and other exposed metalwork such as sinks and baths, to each other and to the main earth terminal in the consumer unit. These cross-bonding cables ensure that the metalwork is earthed, so that if metalwork comes into contact with a live wire it will not itself become live.
