Making the loops in earthing circuits explicit
What the activity is for
Modelling the earthing circuit is much easier if you can show it in action. A large scale board-mounted model is not hard to make. This then provides tangible resources against which you can explain the workings of the separate loops in the circuit.
What to prepare
- 12 V power supply
- demonstration earthing board, as shown
What happens during this activity
Introduce the appliance, and the connections made to it: live, neutral and earth. Having a real plug to hand, as shown here, will help. A metal-walled container, such as the sawn kettle shown in the diagram, is ideal. Show the appliance working normally at first, without the white flying wire linking the live or neutral connections and the earth, which should usefully be as obvious as the model shows (a large copper strip is good).
Bridge the fuse so that it's replaced by a standard laboratory wire.
On making the link between the live connection and the lamp, the lamp representing the heart will glow if the voltage and lamp are well matched. This is not good for the person. Trace out the complete loops involved (remembering not to start at the power supply!), showing the extra loop that the man provides. This draws more current from the source. Break the fault link between the element and the casing by unplugging the white flying wire.
Introduce the fuse as a one–time current–limiting switch and remove the bridging link. Now adding the fault link will add in the extra loop and so cause more current in the fuse. The fuse melts and this sacrificial action prevents the man's heart from glowing.
The Physics narrative provides a much fuller explanation of the action of the earth circuit and fuse. Here we only sketch a possible series of actions and words to explain this to students.