An appliance is fitted with an earth wire and a fuse in the live wire - and before the appliance's ON/OFF switch.

From the diagram above where the appliance could be an electric toaster or a kettle:

1. Appliance in safe condition, earth wire connected, fuse intact, no faults and switched off.

2. Appliance in safe condition, earth wire connected, fuse intact, no faults and switched on and working safely.

3. Heating element broken (maybe from corrosion) and touching the metal casing, switched off, but NOT safe.

4. The appliance is switched on and the current flows through the casing and down to the ground through the earth wire, in doing so, the heat generated in the fuse wire, melts it, breaking the circuit and making it safe.

 

So, if a fault develops and the live wire comes into contact with the metal case, then, as long as the metal case is 'earthed' (connected to the earth wire) the current surge flows harmlessly from the live wire, through the case and down the earth wire to earth.

 

The current surge should melt the fuse, as long as the correct fuse rating is used, and the current surge is over the fuse rating (in amperes).

This is why the fuse must be connected in the live wire before the appliances heating element!

Once the fuse has melted the circuit is broken and the live wire supply is cut off.

This isolates the whole appliance so you cannot get an electric shock from touching the case.

 

Fuse wires rely on an 'overheating' effect to protect an appliance from damage (e.g. so it might be repaired) and ourselves from electrocution from a high voltage current running through our body to earth.

If the temperature of a resistor becomes too high from a current surge causing overheating, the resistance increases and so does the heat transfer to an external thermal energy store around the wire.

This may interfere with the working of an appliance due to the increase in temperature of a resistor.

The temperature might rise sufficiently to melt the wire in a circuit component and the break in the circuit stops the 'device' working.

 

This is how a fuse works

If a fault develops and too much current flows, a fuse wire melts from this overheating effect, this breaks the circuit and makes it safe.

The larger the current in the appliance the thicker the wire should be to minimise the resistance and overheating. Generally speaking the fuse rating increases with increase in cable thickness.

 

Note on circuit breakers

(i) As well as appliances, the ring main circuits to the 'plug in' sockets and lighting are protected with fuses in the same way.

(ii) You can protect circuits with circuit breakers.

There are several types of circuit breakers e.g. some work off the magnetic effect of a solenoid so that a current surge produces a magnetic field strong enough to make a magnet open two contacts to break the circuit.

Circuit breakers are safer than normal household fuses.

A wire does not melt, but the circuit is broken by a fast 'switching off' action - faster than a fuse melts.

They also have the advantage of being reset, which is less trouble than fitting a replacement fuse. They are however, more expensive, but safer!

 

How to calculate the correct fuse to fit?

Fuse ratings and how you choose the safest fuse to use?

For domestic appliances in the home the most common fuse ratings in the UK are 3A, 5A, 8A, 10A and 13A.

The fuse should have a rating of close to, but, just above the maximum safe current that will run through an appliance.

If a fault develops, and the current rises a few amps above expected, the fuse must melt and break the circuit making it safe.

This means you have to work out the current flowing from the power rating of the appliance from the formula ...

power (W) = current (A) x potential difference (V)

P = IV

Example 1. A 2kW electric fire works of a 230 V ac mains supply of electricity.

Calculate the current flowing in the appliance and suggest a suitable fuse rating.

2kW = 2000 W

P = IV,  I = P/V = 2000/230 = ~8.7 A

Ideally a 10A fuse would do, but its likely that in this case the appliance would be fitted with a 13A fuse.

Obviously, you choose the nearest fuse rating from what is available.

 

Example 2. What fuse would you choose to put in the plug of a 700 W electric iron working off 230V mains electricity?

I = P/V = 700/230 = ~3.0 A

Ideally a 4A fuse would be best, but a 5A would be acceptable.

 

Double insulation

To protect you from electric shock, all appliances with metal cases should be earthed i.e. the metal case is connected to the earth wire, using three core cable as previously described.

An earthed conductor can never become live.

A metal casing is obviously an electrical conductor, but if the appliance has plastic casing (electrical insulator) with no external electrically conducting metal parts that can be touched, it is said to double insulated.

(The internal wiring is insulated apart from heating elements!)

This means the appliance doesn't need an earth wire and so is only connected with two core cable - live and neutral wires only - which are all that is required to power an appliance.