SAS Urban Survival Handbook by John Wiseman Page A

battery-operated doorbell to carry a full-strength mains current. The wire will heat up, melting the insulating plastic coating and then melting itself at the weakest point—possibly starting a fire in the process!
    Trying to pass a heavy current in this way is causing the wire to act like a fuse (see Fuses ). It is also the principle of the light bulb—but the filament does not burn out because air, which is needed for combustion, is excluded from the bulb.

High voltages
     
    Very high voltages carried in overhead cables are insulated by the air. Ceramic insulators are used to protect the pylons and the points at which the current comes down to earth. The currents carried may be as high as 400,000 volts, and are reduced at substations to be used domestically.

WARNING
     
    Although air is a good insulator, very high voltages can ‘arc’ across quite a gap (up to 18 m/60 ft). Never fool around near overhead cables with a kite or a carbon-fibre fishing rod. You could quite easily bring the power down into your body and kill yourself.
     

Earthing/grounding
     
    Electricity always wants to return to earth. This process of ‘earthing’ is what happens when you receive an electric shock from an appliance, or from a live wire. The electricity is trying to return to earth through you. The human body is not a very good conductor, but so strong is the tendency for electricity to earth itself, it will take any route offered to it. Effectively, you form a circuit.
    Wearing rubber boots, which cut off the route to earth and therefore break the circuit, should protect you. Standing on wet ground makes you a more efficient circuit and is therefore deadly.
    For this reason, mains circuits and most appliances carry a third wire (apart from the live and the neutral)—the earth wire. In appliances, particularly those with metal casings, the earth wire will be fixed to the inside of the casing. It is intended to offer a swifter route to earth for the electricity than you do—thereby protecting you. In reality, it does more to protect the appliance—the fuse does more to protect you (see Fuses ).

REMEMBER
     
    The earthing of your domestic wiring is designed to save your life and prevent fires. Do NOT tamper with it or disconnect it. It is advisable to have your earth checked every few years or after work has been done on your wiring.
     
    The electricity should pass down the earth wire to the socket. The socket is connected to the mains supply, which is also earthed. All the earth wires on the domestic circuit should run to one heavy earth cable at the meter. It used to be quite common for the mains supply’s earth to be connected to the water pipes to guarantee a quick and easy journey for the electricity, but the introduction of plastic plumbing has rendered this method unreliable. You will still find that water and gas pipes are earthed to protect anyone touching part of these systems should any part of the electrical wiring come into contact with either of them.

WARNING
     
    Plastic plumbing is becoming more common. This is not effective as a means of earthing your domestic electricity. If all or even part of your plumbing has been replaced with plastic, your mains supply may no longer be earthed. CHECK!
     

ELECTROCUTION
     
    It is VITAL to understand electrocution, and what to do if it occurs. Through touching an appliance which has become ‘live’ or by touching a live wire, the current is carried through the body. Since many materials, including water and even urine, are effective conductors, it is possible to receive a shock without realizing that you are in any immediate danger.
    It is quite safe to touch properly-insulated wiring in dry conditions, but use electrical screwdrivers with properly-insulated handles and stand on non-conductive surfaces—a rubber mat will do, or thick rubber boots. Do NOT touch or lean on pipes or scaffolding as there is a possibility that these could give the current an earth again.
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