Fuse:
The fuse is a devise which protects equipment from an excessive flow of current. When a fuse becomes hot and the current flowing through it is greater than it’s rated value, it melts. For safety precautions, fuses should have a rating which is slightly more than the current drawn by the appliance. Fuses must be connected to the live wire so that after the fuse has melted the appliance does not become live. The mains must be switched off when replacing a use. The current rating of the fuse should be just above the value of the current that flows when operating properly. The fuse rating is the maximum current that the fuse can carry without melting. The fuse rating should be slightly higher than the normal working current of an appliance.
Switches:
A switch breaks off or completes an electric circuit. The switch must be connected to the live wire, so that it disconnects the high voltage from an appliance. When connected to the neutral wire a shock may occur.
Plugs and sockets:
The life and neutral pin holes are blinded for protection and are opened by the earth pin. Within a fused plug a fuse is fitted and when it blows, it isolates the appliance from others plugged in the mains socket.
Earthing:
The earth wire provides an alternative path for the current to flow should the metal casing accidently become life. It protects a user form an electric shock. The current that flows from the loose live wire through the metal casing and the earth wire will blow the circuit fuse and cut off the supply to the appliance.
Circuit breakers:
Mostly two circuit breakers are found in the consumer unit circuit breaker:
the miniature circuit breaker (MCB);
the earth leakage circuit breaker (ELCB).
The MCB will break the circuit when a short circuit occurs or when a too large current flows through the circuit. This type of breaker has a limiting current which when exceeded will trip the circuit and cut off the power supply. The MCB can be switched on again but after the faulty wire is rectified. The device can be reused by resetting the spring-loaded switch. The ELCB detects leakages from the live wire to the earth wire. When a leakage is detected in an earth wire the ELCB will switch off the electricity supply.
Double insulation:
This is used as a substitution for a earth wire. It provides two types of insulation. Firstly, the electric cable is insulated from the internal components of the appliance; and secondly, the internal metal parts which in case of a fault may become life are insulated from the external casing. Appliances with this feature usually have non-metallic casings.
Need for Earth Wire and Metal Case
Earth wire is connected to the metal casing of an electrical appliance. In case of a leakage of current, the user may get an electric shock if the live wire is connected to the metal casing. The earth wire allows the current to flow through it to the ground and protects the user from a fatal electric shock.
Double insulation protects the user of the appliance from an electrical shock by preventing any possibility of the external casing becoming live, thus eliminating the need for an earth connection. Firstly, the electric cable is insulated from the internal components of the appliance. Secondly, the internal metal parts, which could become live if a fault developed is also insulated from the external casing.
Wires
Live wire carries electric current to the appliances.
Neutral wire completes the circuit by forming a path for the current back to the mains. It is usually at zero volts.
Earth wire has very low resistance and is usually connected to the metal casing of the appliances. It forms a safety outlet for current in case the appliance becomes live.
Switches, fuses and circuit breakers must be fitted onto the live wire so that when the switch is off or when the fuse/circuit breaker is broken, they disconnect the high voltage from the appliances.
Mains Plug
Wire/ pin
