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Introduction

Getting your kettle to boil involves the delivery of electrical energy to your kettle from a power station that could be in a neighbouring country. Fortunately, the systems needed to do this were worked out over one hundred years ago. This activity looks at one of the safety devices incorporated in the design of domestic wiring.

Task 1: The earth wire - wired correctly


Model 1

  1. Open the Yenka Model 1.
  2. The circuit diagram shows the wiring arrangement of a metal cased device such as an electric kettle or security lamp. The bulb represents the working parts, like the element of the kettle or the bulb in the security lamp. The metal case is labeled C. What is the case connected to?
    Answer
    The earth wire (and then to the plug and earth connection).
  3. Turn on switch B. The bulb switches on and operates normally. Is the metal casing connected to the supply at all?
    Answer
    No.
  4. Internal wiring can become loose due to poor wiring, vibration, or if the appliance is dropped by accident. If it touches the metal casing, it can make the casing live. Close Switch A. This simulates a piece of internal wiring touching the casing. Describe what happens to the fuse. (In real life the mains a.c. supply would not be damaged if the fuse operated correctly.)
    Answer
    It blows.
  5. What would have happened if the fuse had been replaced by one designed to blow at a much higher current, or if it was bypassed with a wire, or another piece of metal?
    Answer
    Fuse wouldn't blow. Circuit remains closed. Low resistance path for charges to go to earth. High current. Wires heat up. Fire can start.
  6. Move the model human and let it touch the metal casing. If the light on his chest turns on, then it means charges can find a path to earth, and that would be the same as a human touching an electric kettle and receiving a possibly fatal electric shock. Does the model human receive a shock? Why?
    Answer
    No, because fuse has blown and isolated the casing.

Summary

The earth wire provides a low resistance path to earth for charge if the casing becomes live. As the path has low resistance, there is an immediate high current that blows the fuse in the plug, breaking the circuit. The appliance is isolated, and shock risk is avoided. This also prevents excessive current from heating wire insulation that could start a fire.

Teacher Summary

  • The supply is destroyed in the simulation. Explain that a properly chosen fuse would blow before the house wiring had a chance to heat up.
  • The supply has been chosen to be d.c. because it shows the role of the neutral connection more clearly than if a signal generator is used. This becomes necessary in Earth Wire Faults as it will show why it is important not to switch the live and neutral wires over.