Earthing : Any electrical appliance with a metal casing or with metal parts, likely to be touched by an operator is potentially dangerous. The danger is that an internal or external fault could cause the metal casing to become live and thus electrocute the operator.
Some of the electrical equipment used in your home, laboratory or workshop does not require earthing as it is fully insulated, i.e. all the parts which carry electric current are endosed in a casing of plastic or other insulating material.
When such appliances are used, normally it is impossible for your hand or other parts of your body to come into contact with the current. The appliance is thus completely safe, unless the insulator is cracked or damaged. However, larger pieces of electrical equipment are frequently encased in a metal container as part of their structure or in order to mechanically protect fragile components. All metal clad appliances must be earthed so that if any fault develops, the chasis or metal casing does not become live.
All electrical equipment that is not fully insulated should be properly earthed for safety. The correct connection of the green earth leads from the appliance to the centre terminal of the three-pin plug is generally all that is required. The end of the green cable should be attached to a terminal on the in site of the metal casing, preferably in the switch handle. When the plug is then placed into the earthed socket of the power supply there will be a direct path of earth.
A screw as well as solder should always be used when earthing metal-clad equipment, because simple soldered joints are not sufficient for a reliable earth connection.
Occasionally you may have to make an earth connection other than by using the earth lead of a flex, e.g. laboratory batteries. In this or any other case you must remember never to earth onto water pipes - there may be plastic pipes which isolate the pipe system from earth.