Earthing is a safety feature for appliances with a metal case. Questions supply a diagram and ask what happens if the live wire touches the case, and why that is made safe. The skill is explaining a sequence of events, in order, using the parts shown.
This lesson builds on comparing a fuse with a switch, because the fuse is part of the answer.
What is connected to what on the diagram?
A typical supplied diagram shows a three-core cable to a metal-cased appliance.
- The live wire (brown) carries the supply voltage to the appliance, through the fuse and switch.
- The neutral wire (blue) completes the circuit back to the supply.
- The earth wire (green-and-yellow) connects the metal case to earth. Its resistance is very low. It carries no current when everything is working.
How do you explain a fault using the diagram?
Use a fixed sequence: fault, consequence, response, result. This keeps the explanation from stopping early.
Worked example
An invented diagram shows a metal-cased toaster. The live wire has come loose inside and is touching the metal case.
The plug fuse is 13 A. Explain why the user is protected.
Fault: the live wire touches the case, so the case is now connected to the live supply.
Consequence without earthing: the case would stay at a dangerous potential, and a person touching it could be given a current through their body.
Response with earthing: the earth wire joins the case to earth. It gives the current a very low-resistance path from the live wire, through the case and earth wire, to earth.
Result: a large current flows. It is larger than the 13 A rating, so the fuse in the live wire melts. The appliance is disconnected from the live supply, and the case is no longer at a dangerous potential.
Notice that the fuse and earth wire work together. The earth wire makes the current big enough; the fuse then breaks the circuit.
The mistake to watch for
A common slip is to say that the earth wire “takes the electricity away so it is safe”.
Mistaken answer: “The earth wire carries the electricity to the ground, so the case is safe.”
This leaves out the large current and the fuse. It does not explain why the case becomes safe.
The correction is to say that the earth wire provides a low-resistance path, which causes a large current, which melts the fuse and cuts the appliance off from the live supply. Another slip is to place the fuse in the earth wire. On the diagram, check that the fuse sits in the live wire.
Check yourself
1. Why is a metal case connected to the earth wire?
Show answer
So that if the live wire touches the case, a large current flows to earth through the low-resistance earth wire, the fuse melts, and the case is no longer live.
2. The earth wire carries no current in normal use. Explain why it is still needed.
Show answer
It is a safety connection for faults only. When a fault makes the case live, it provides the low-resistance path for the large current that melts the fuse.
3. A hairdryer with a plastic case is marked with the double-insulation symbol and has only two wires. Why is this acceptable?
Show answer
The case is an insulator and the live parts are separated from the user by double insulation, so no metal part can become live. An earth connection is therefore not needed.
Where this leads next
Next, apply the same fault-explaining habit to simple circuits in explaining a circuit fault without mains-work instructions. The module overview is electrical energy and safe interpretation.
If you understand each part but lose marks on the order of an explanation, a teacher giving online one-to-one Physics tuition can practise that sequence with new diagrams.