Earthing

The Earth wire is a low-resistance wire. It is usually connected to the metal casing of the appliance and is used to protect the user from electric shock.

Scenario 1: No earth wire

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When a fault occurs and the live wire touches the metal casing, the metal casing becomes “live”, i.e. at high potential.

Since circuit is not completed, there is no current flow and the fuse does not blow. 

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If a user accidentally touches the metal casing, the user completes the circuit and a large current flow through him. The user gets electrocuted.

Our body resistance is very high (dry skin about 100 kΩ, wet skin about 100Ω), current less than fuse rating, fuse will not blow. 

Scenario 2: With earth wire

Courtesy of Marshall Cavendish

When a fault occurs and the live wire touches the metal casing, a surge of current flows from live wire, through metal casing, to the ground via the earth wire creates a short circuit. Current exceeds the fuse rating, so the fuse melts and opens the circuit. When a user accidentally touches the metal casing, he is protected as the “live” has already been cut off.

Steps in explaining the function of Earth wire

Step 1: State the fault
When a fault occurs and the live wire touches metal casing
Step 2: Describe how the current flows
Current flows from live wire, through the metal casing, to earth via Earth wire which creates a short circuit.
Step 3: State what happens to the fuse
Current exceeds the fuse rating, so the fuse melts and open the circuit.
Step 4: State what happens to user touching the appliance accidentally.
Electricity supply to appliance is cut off and user is protected from electric shock when he touches the casing (“live” already cut off).

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