Mains electricity: the dangers of providing any connection between the live wire and earth.

The idea

In the UK, the live wire alternates between about +325 V and −325 V relative to earth, while the neutral wire stays near 0 V and the earth wire is a safety route at 0 V. If a person or a conductor creates a low-resistance path between live and earth, a large current flows. Through a person this can cause electric shock, burns, or death by disrupting heart rhythm; through a metal case it can overheat cables and start a fire. The earth wire and fuse or circuit breaker reduce this danger by giving fault current a low-resistance path so the fuse melts or the breaker trips and disconnects the live supply. Double insulation avoids the need for an earth connection. The danger is greatest when the path bypasses the normal circuit resistance, because current is then limited only by the resistance of the path.

What earns the marks

• The live wire is at a high alternating potential difference relative to earth, so touching it can drive current through a person to earth.
• A low-resistance connection between live and earth produces a very large current because current increases as resistance of the path decreases.
• A large current through the body can cause electric shock, burns, or fatal disruption of heart rhythm.
• A large current in cables or a metal case can overheat the wiring and cause a fire.
• The earth wire provides a low-resistance path for fault current so that a fuse melts or a circuit breaker trips and disconnects the live wire.
• Correct use of double insulation or an RCD can reduce the risk by preventing a person from becoming the path to earth or by quickly cutting off the current.

Common mistakes and how to put them right

• Thinking the earth wire carries current in normal operation.

  Put it right by stating that it carries current only during a fault, when it provides a low-resistance path to earth.

• Believing a shock is caused by voltage alone.

  Put it right by explaining that current through the body causes harm, and that current depends on the potential difference and the resistance of the path.

• Assuming a fuse protects a person directly.

  Put it right by explaining that a fuse protects cables and equipment by melting when the current is too large, while an RCD or double insulation gives additional protection against shock.

Exam tips

• Link the danger to the size of the current, not just to the voltage, and state that the live-to-earth path has low resistance.
• Use the correct terms live, neutral and earth, and describe the earth wire as a safety connection at 0 V.
• When explaining protection, state the sequence: fault creates low-resistance path, large current flows, fuse melts or breaker trips, live supply is disconnected.