2.3 understand the uses of insulation, double insulation, earthing, fuses and circuit breakers in a range of domestic appliances

Insulation
All insulation is is covering a live wire (a wire that is conducting electricity) with a material that is not an electrical conductor. This means the wire is safe to touch as the current is contained within the wiring as it can't pass through the layer of insulation.


Earthing

Many electrical appliances, for example cookers, washing machines and fridges have metal cases. The earth wire creates a safe route for the current to flow through if the live wire touches the metal casing (otherwise the current would flow through the case and it is likely you would get an electric shock).

However, the earth wire is connected to the metal casing so that the current goes through the earth wire instead of causing an electric shock. A strong current surges through the earth wire because it has a very low resistance. This breaks the fuse and disconnects the appliance.

Double insulation
Some appliances, for example vacuum cleaners and electric drills, do not have an earth wire. This is because they have plastic casings, or they have been designed so that the live wire cannot touch the casing. As a result, the casing cannot give an electric shock, even if the wires inside becomes loose and touched the casing.

Fuses

If a fault in the circuit causes too much current to flow, the fuse breaks the circuit. The fuse contains a piece of wire which melts easily. If the current going through the fuse is too great, the wire heats up until it melts and breaks the circuit.

Fuses in plugs are made in standard ratings. The most common are 3 A, 5 A and 13 A. The fuse should be rated at a slightly higher current than the device needs. Foe example, if the device works at 3 A, use a 5 A fuse, if the device works at 10 A, use a 13 A fuse.

Circuit Breakers
Circuit breakers contain an electromagnet that activates if the current goes above a certain limit (different limits for different appliances). Should the current go above the limit of an appliance, the electromagnet pulls an iron switch towards it, this opens the switch, consequently breaking the circuit.

Residual current circuit breakers (RCCBs) protect some circuits. They detect a difference in the current between the live and neutral wires. RCCBs work much faster than fuses do. (For exams, you do not need to know how they work, just what they do)