SITEMAP   School-college Physics Notes: Electricity 1.5 Dangers, live wire, earthing & fuses

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Electricity in the home: 1.5 More on the dangers of the live wire, fuses and earthing appliances for extra safety - fuse calculation

Doc Brown's Physics exam study revision notes: Explaining the dangers of live wire, how to calculate the correct fuse rating to use, why earthing appliances for extra safety is important, how do circuit breakers work?

1.5 More on the dangers of the live wire, fuses and earthing appliances for extra safety

With respect to an external electricity supply, as with the earth-ground itself, your body is at a p.d. of 0 V.

Unfortunately, this means if you touch the live wire or anything connected to it e.g. faulty metal casing touching the live wire, a large p.d. of 240 V occurs across your body i.e. between you and the 'earth'.

Therefore you are in grave danger of an electric shock because a current will flow through your body to 'earth' - unfortunately, the fluids in your body contain enough ions for quite efficient conduction with a p.d. of 230 V.

The electrical current flow will give you an electric shock, which can be sufficient to kill you.

Even if the appliance is 'switched off' there is still a danger of an electric shock because the live wire is still at high pd (eg 240 V).

DANGER - if a heating element or metal casing of an appliance is faulty and they come into contact

.Note

(i) Wearing electrically insulating rubber boots may offer some protection, but is that what you normally wear!?

(ii) Water is a poor conductor, but with a high potential difference it can conduct. You should also remember from your chemistry (electrolysis) that ions from salts increase the electrical conductivity of water and you have salt ions in your blood, cells and nervous system etc.!

(iii) It is the function of fuse to protect you and the appliance from current surges (next section).

When things go wrong!

The function of a fuse and how to calculate the fuse rating for an appliance

If a heating element or metal casing of an appliance is faulty and they come into contact, if you are touching the appliance when it is switched on, then you can be electrocuted as the current will flow through to earth (the ground).

BUT, you should be saved by an earth connection from the case to the ground and a fuse fitted in the plug or a circuit breaker (next section).

How do the earth wire and fuse work in the circuit of an appliance

In any household or industrial circuit, you can get sudden surge (increase) in current.

The current surge maybe due to a fault, but sometimes even switching appliances on and off can trigger a sensitive circuit breaker, but shouldn't blow a fuse.

A current surge due to a fault can lead to overheating, damaging the appliance or even causing a fire.

KEY: Live wire (brown), neutral wire (blue) and the earth wire (yellow/green) and switch of the plug and socket.

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.

Fuses 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:

(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.

INDEX of ELECTRICITY Notes 1. Electricity in the home

Keywords, phrases and learning objectives for ?

Be aware and able to describe and explain the dangers of live wire if a fault develops in an appliance.

Know how to calculate the correct fuse rating to use in a plug.

Know, explain and describe why earthing appliances for extra safety is important.

Know what a circuit breaker is and how it works.

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INDEX of ELECTRICITY Notes 1. Electricity in the home

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