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Household electrical safety features

The electrical circuit that supplies electric power to homes is the mains power supply.

In the UK, Europe and Singapore, the mains supply is $$230\text{ V}$$ A.C. current with a frequency of $$50\text{ Hz}$$.

Electrical appliances are connected to the mains power supply in parallel. This means that the same voltage is supplied to each appliance.

The mains power supply is made up of two wires that connect the home and the substation:

  • The high-voltage ($$230\text{ V}$$) live wire carries current into the home.
  • The zero-voltage ($$0\text{ V}$$) neutral wire carries current out of the home.

When electrical appliances are plugged in, they are connected to a live wire and a neutral wire, creating a complete circuit. This allows current to flow through the appliance from the live wire to the neutral wire.

The mains power supply is made up of the live wire (brown) and the neutral wire (blue).
The mains power supply is made up of the live wire (brown) and the neutral wire (blue).

A fuse is a component that protects circuits from excessive currents.

High currents cause wires to overheat and can lead to fires.

A fuse contains a thin fuse wire that melts (blows) if too much current flows through it. This breaks the circuit and prevents current from flowing.

A blown fuse must be replaced for the circuit to function again.

The rating of a fuse is the current at which the fuse blows. The rating of the fuse should be slightly higher than the intended maximum current in the circuit.

If the intended maximum current in an appliance is $$4\text{ A}$$, a $$3\text{ A}$$ fuse would be useless because it would blow immediately. A fuse with a rating of $$5\text{ A}$$ should be used.

Fuses come in three standard ratings: $$3\text{ A}$$, $$5\text{ A}$$ and $$13\text{ A}$$.

Fuses are always connected to the live wire . This is because fuses are used to prevent excessive current flow into the home or appliance.

A common fuse.
A common fuse.

A circuit breaker is a switch that automatically turns off when too much current flows through it, breaking the circuit. A circuit breaker that has been activated is said to have been tripped.

Like a fuse, a circuit breaker is connected to the live wire to prevent excessive current flowing into the home.

Unlike a fuse, (which has to be replaced) a tripped circuit breaker only has to be reset (the switch has to be turned on again) for the circuit to function normally again. The circuit breaker can be reset when the current has fallen to safe levels again.

A simple circuit breaker with two switches (the blue levers).
A simple circuit breaker with two switches (the blue levers).

Earthing refers to the connection of the external metal parts of an electrical appliance to the earth.

This is done by connecting an earth wire from the appliance to a metal item that is buried within the ground. Any build-up of charge in the body of an earthed appliance will be distributed over a vast volume of soil and become harmless.

The earth wire is often connected to water pipes!

In the UK, Europe and Singapore, the earth wire is coloured yellow and green. The connection is made via the plug socket on the wall where the appliance is plugged in.

A person touching an earthed appliance will not get an electric shock if there is a current leakage as the current will pass harmlessly through the earth wire.

Earthing is used in many electrical appliances as a failsafe in case the fuse and circuit breaker do not work.

This yellow-green earth wire is connected to a conducting rod driven into the ground. Any build-up of charges would dissipate into the wet soil.
This yellow-green earth wire is connected to a conducting rod driven into the ground. Any build-up of charges would dissipate into the wet soil.

A double insulated electrical appliance has two layers of insulation between the live wire and any accessible parts of the appliance.

Hairdryers and electric drills are often double insulated.

The wires are first surrounded by a layer of inner insulation. The whole appliance is then surrounded by a second layer of outer insulation.

If anything goes wrong with the appliance, no live conductor can touch the outer casing because of the two layers of insulating plastic.

This prevents you from getting an electric shock when you touch the appliance, even if it has been damaged slightly.

Double insulated appliances are not required to have an earth connection.

Left: A double insulated appliance. Right: Double insulated appliances must display the double box symbol.
Left: A double insulated appliance. Right: Double insulated appliances must display the double box symbol.

A mains plug is the connector between an electrical appliance and the mains power supply.

Plugs used in the UK and Singapore have three pins:

  • The top pin is connected to the earth wire which is coloured yellow and green.
  • The left pin is connected to the neutral wire which is coloured blue.
  • The right pin is connected to the live wire which is coloured brown.
How to wire a three-pin plug.
How to wire a three-pin plug.

When wiring a plug:

  • Do not leave any bare wires as they could cause short circuits.
  • Connect all the wires correctly.
  • Connect the fuse to the live wire.
  • Tighten the cable grip so that the wires stay firmly in place.

If an appliance is double insulated, it is quite common for the earth pin of the plug to just be plastic! In some countries, two pin plugs can be used for double insulated appliances.