P2bL9  Fuses

Key Words

Coulomb - unit of charge equivalent to 1 amp flowing for one second.

Current - Flow of charge.

Earth Wire - wire connected to the ground through a spike in the ground.

Fuse - weak link in a circuit that melts when too big a current flows.

Joule - unit of energy

Miniature Circuit Breakers - electromagnetic switches which turn off when too big a current flows.

Power - rate of using energy.

Residual Current Device - Switch that turns off when it detects a "leakage" to the ground.

Watt - unit of power.  1 watt = 1 joule per second (J/s)

Test Yourself

Homework
Physics GCSE
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Grade E

Fuses are weak links in a circuit which melt when too big a current flows.  They are thinner than the other wires in the circuit and get hotter more quickly, saving costly or catastrophic damage to the appliance in case of a fault.

 

Faulty appliances can be dangerous.  They can:

  • Give a shock which is at best unpleasant, at worst fatal.

  • Electrical burns that can go deep and cause permanent damage.

  • Cause a fire.

The danger signs are:

  • Over-loaded sockets.

  • Scorched plugs and sockets.

  • A hot "fishy" smell indicating a plug getting hot.

  • Fuses blowing all the time.

  • Loose or split wires.

You must never:

  • replace a fuse that keeps on blowing all the time with one of a higher value.  If the fuse goes for no obvious reason, the chances are that there is a fault.

  • use water on an electrical fire; you could get a fatal shock.  Use a carbon dioxide extinguisher.  If you don't have one, get out, call the Fire Brigade out, stay out.

 

Fuses are available in three common sizes, 3 amp, 5 amp, and 13 amp (there are other sizes).  It is important to chose the correct fuse:

 

  1. Look at the power rating in watts, given on the appliance (on the bottom or the back).

  2. Use current = power ÷ voltage to work out the current.

  3. Chose the next size up.

Worked example:

An appliance is rated at 1 kW.  What fuse should be used?

  1. 1 kW = 1000 W.

  2. Current = 1000 W ÷ 230 V = 4.3 A

  3. Next size up is 5 A

 

The Earth wire goes to the ground through a spike in the ground.   It used to go through the water pipes, but then plastic pipes were used, which don't conduct electricity.  This rather defeats the object of the Earth wire.  If a live wire touches the metal case, the current goes to ground, and the fuse blows.

 

Grade C

Replacing a fuse in a fuse box is a pain.  It means having to scrabble for the fuse wire and, if you find it, rewire the fuse by torch light (as the battery goes flat).  Also there is the temptation to replace the wire with thicker wire (purely as a temporary measure, of course).  This can be highly dangerous.

 

In more modern houses, the fuse box is replaced by miniature circuit breakers (MCB).  When too big a current flows, the switches turn off, breaking the supply.  All you need to do, having dealt with the fault, is to turn the MCB back on again.  If it turns off again, the fault is still there.

 

 

The MCB does NOT protect against shock.  If you use an appliance outside, you should use a residual current device (RCD)

 

 

This detects leakage from any split in the cable, for example, cutting the wire with the mower, and turns off in about 1/20 s.  You can get a shock, but it won't harm you.  It will NOT protect if too big a current flows; a fuse is needed.

 

 

Some electrical equations:

 

Current is a flow of charge, the number of electrons passing every second.  It is measured in units called coulombs.  1 amp is 1 coulomb per second.  This is 6 × 1018 electrons every second.  It's like sugar.  You buy a 1 kg bag of sugar; you don't count all the crystals.

 

current (A) = charge (C) ÷ time (s)

 

We can relate energy to power by:

Energy (J) = power (W) × time (s)

 

Power and Voltage are related by:

Power (W) = voltage (V) × current (A)

 

So we can related energy and charge by:

Energy (J) = charge (C) × voltage (V)

 

 

Grade A

Earth Wire

We have seen how important the Earth wire is in the safety of appliances.  Now let us look at how this works.

 

Suppose we had a metal cased appliance that was not earthed.  It would work perfectly well.  However a fault may occur as in the picture below:

 

 

The appliance would carry on working perfectly as before, but if you touched it, you would get a massive shock.

 

The Earth wire carries the electricity to the ground.  There would be a flash and the fuse would blow, but the case would no longer be live.

 

 

The idea of electricity flowing through the ground may seem odd, as soil is not immediately obvious as a good conductor.  However there is an awful lot of it.  So the ground can carry quite a big current back to the substation. 

 

Therefore there is a large spike driven into the ground at your house.  There is a similar spike connected to the neutral wire at the substation.  This is why the neutral wire is at zero potential relative to the ground.

 

 

Mains Voltage

If we were to connect a CRO across a mains socket, we would see a pattern like this:

 

 

We would see that the 230 volt level was NOT at the peak, but about 70 % of the amplitude.  This is called the root mean square voltage (rms).  The peak voltage is Ö2 times the rms voltage.  The rms voltage give the same energy per coulomb as the same DC voltage.

 

Peak voltage = Ö2 × 230 V = 325 V

 

  The peak voltage is NOT 230 V.