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GCSE Science/Safety in Mains circuits

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GCSE Science/Electricity

Mains electricity is potentially dangerous. There are, however, safety features included in plugs. This module looks at how to wire a plug correctly.

The three pin plug

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Nowadays most appliances are sold with moulded plugs already fitted. Nevertheless, it is still important to understand the correct wiring of a plug because enough of the old plugs still exist. It is also the case when you bring in equipments overseas. British Standard compliant adaptors are not always available for such non-UK plugs. You are very likely to need to change a plug at some time in your life. In the UK mains electricity is 230 V. (In Hong Kong, it is 220 V.) If you were to touch a live wire a current would flow through your body to the ground. This current may be enough to kill you.

The cable from the appliance usually consist of three wires, an earth and two other wires, live and neutral, which carry the current to and from the power station (live is from the power station and neutral is back to the power station). The wires are made of copper surrounded by an insulation casing. The casing is made of plastic and is coloured:

  • The live wire is brown
  • The neutral wire is blue
  • The earth wire is green and yellow

The three wires are covered by an outer sheath made of plastic.

Q1) Use your knowledge of insulators and conductors to explain

Why the wires are insulated.
Why the sheaths are made of plastic.

 

The plug has the following features:

  • A cable grip, to grip the outer sheath of cable and prevent it being pulled out of the plug
  • Three pins made of brass, one of which is the earth pin.
  • A fuse.
  • A case made of plastic.

Q2) Why are the pins made of brass and why is the case plastic?

The purpose of the parts of a plug

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The live and neutral wires

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The live and neutral wires carry the current around the circuit. Mains current is A.C. (alternating current); this means that it is going backward and forwards in cycles (clockwise and anticlockwise around the circuit). The frequency of the cycle is 50 hertz (50 times per second). This cycling of current is achieved by varying the voltage on the line wire from about +325V to – you to the earth. This is where the earth wire is included, for your safety. The earth wire connects the case of the appliance out down the flex to the earth pin on its plug. This connection goes into the socket, then inside the wiring of your house down to the earth through the earthing system (not necessarily plumbing).

If the live wire were to touch the case a huge current would flow through the earth wire. This would probably blow the fuse and break the circuit (see next section). However even if the fuse doesn't blow the current would still prefer to flow through a wire with low resistance than a human body with relatively higher resistance.

Thus the earth wire helps protect you if you touch the case of an appliance that is "live".

The earth pin on a plug is longer than the live and neutral pins. This ensures that the earth pin always connects with the socket first. All sockets have shutters which prevent access to the live contacts when there is no plug in the socket. On some sockets these shutters are operated by the earth pin pushing the shutter mechanism down to uncover the line and neutral socket contacts, on other sockets there is a mechanism which opens the shutters when the two live pins are inserted simultaneously, and on other sockets when all three pins are inserted simultaneously.

The fuse

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A fuse is simply a very thin wire. The wire has quite a low melting point. As current flows through the wire it heats up. If too large a current flows, it melts, breaking the circuit. Fuses are used to protect the flexible lead between the plug and the appliance. If too large a current flows through a lead it may overheat or catch fire. Fuses are unlikely to act quickly enough to prevent human electrocution – their main purpose is to prevent fires due to large currents.

Fuses are rated according to how much current they can carry before melting. In plugs fuses are usually 3A (red), 5A (black), or 13A (brown). The correct fuse is the one that matches the current rating of the lead.

All plug fuses must comply to British Standard BS1362. The rating and "BS1362" should be explicitly marked on such fuses.

Q4) A table lamp usually carries a current of 0.5A. What fuse should be put in the plug: 3A, 5A, or 13A?

Q5) An iron usually carries a current of 5.2A. What fuse should be put in the plug: 3A, 5A, or 13A?

Q6) A kettle is protected by an earth wire and a 13A fuse. The line wire comes loose and touches the side of the kettle. The fuse blows. Explain why.

Q7) Explain why the fuse is always located on the line wire and not the neutral wire?

Q8) Describe and Explain what happens in the following scenarios:

  • The earth and live wire switch terminals on the plug
  • The live and neutral wire switch terminals on the plug
  • The neutral and earth wire switch terminals on the plug
  • When the earth wire is removed
Summary of this topic
The line wire is brown, and has A.C. voltage of 230 V

The neutral wire is blue and has A.C. voltage of approximately zero.
The earth wire is green and yellow and is a safety device designed to protect people from shocks.
The fuse is a device that melts and breaks the circuit when the current gets too high.
The case is made of plastic (an insulator) for safety.

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