Electricity is an effective method for transferring energy in many domestic situations. Often, electric circuits are used to transfer energy to movement (using an electric motor) or to heating. Electrical heating can be used in cooking and in heating a building (for example. through fan heaters or radiators).
Electricity can cause hazards in domestic situations. For example:
- Electric shock and burns from contact with live parts.
- Injury from exposure to arcing.
- Fire from faulty electrical equipment or installations.
- Water conducts electric current and so you should never touch an electric appliance with wet hands and operate electric equipment in wet conditions
If there is a fault in an electrical appliance, it could take too much electrical current. This might make the appliance itself dangerous, or it could cause the flex between the appliance and the wall to become too hot and start a fire.
Electrical appliances can be damaged if the current flowing through them is too high. The electric current usually has to pass through a fuse or circuit breaker before it reaches the appliance. If there is a sudden surge in the current, the wire in the fuse will heat up and melt – it ‘blows: This breaks the circuit and stops any further current flowing. If a circuit breaker is used, then the circuit breaker springs open (trips) a switch if there is an increase in current in the circuit. This can be reset easily after the fault in the circuit has been corrected.
In many houses there will be a distribution box that takes all of the electricity for the house and sends it to the different rooms. In old houses this box may still use fuses, but in modern installations, the box will use miniature circuit breakers, often known as MCBs.
Where a fuse is fitted to the plug, it must have a value above the normal current that the appliance needs but should be as small as possible. The most common fuses for plugs are rated at 3 A, 5 A and 13 A.
Metal-cased appliances must have an earth wire as well as a fuse. If the live wire worked loose and came into contact with the metal casing, the casing would become live and the user could be electrocuted. The earth wire provides a very tow resistance route to the 0 V earth – usually water pipes buried deep underground. This low resistance means that a large current passes from the live wife to earth, causing the fuse to melt and break the circuit.
Appliances that are made with plastic casing do not need an earth wire. The plastic is an insulator and so can never become live. Appliances like this are said to be double insulated.
In situations that may expose people to electricity unexpectedly, for example using an electric drill, especially drilling into a wall with hidden power cables, or using power too ls out of doors, perhaps in wet conditions, a residual current circuit breaker (RCCB) must be used in the power socket on the wall. If any of the charges starts to leak out, the RCCB will turn off the power in 30 ms or less. This may or may not be quick enough to save the user’s life.
Safety features of the 3 pin plug and the two pin plug:
- If the live wire comes loose and touches the metal casing of the appliance, the earth wire carries the current safely to the ground and the fuse melts
- In the 2 pin plug, the earth wire is carried in grooves in the plastic case.
- If the current in an appliance becomes too large, the thin fire inside the fuse melts and breaks the circuit.
- The plastic coating over the cable insulates the conducting wires.
At Home, circuit breakers protect us from electric fires.
- If the current flowing through the wire becomes too large, the iron core becomes strongly magnetized.
- It also will attract enough force to pull it down .
- This opens the contacts and breaks the circuits so that the current can no longer flow .
- Circuit breaker can be reset by flicking a switch, this will push down the springy piece of metal and pulls the rocker back up to close the contacts
A blown fuse has to be replaced but a circuit breaker can easily be reset by lifting the iron rocker back into position.
- Joule meter: Instead of using an ammeter and a voltmeter to measure the electrical energy transferred by an appliance, a Joule meter can be used to obtain it directly in joules .
- Fuse: Short length of wire with a low melting point, breaks the circuit when a current exceeds a certain value.