Mains electricity: Students should be able to explain:

The idea

This statement is the lead-in to the mains electricity explanations that follow. In practice, an explanation links a named part or quantity to a consequence. For example, the live wire alternates between a positive and negative potential relative to earth, so a person touching it can provide a path to earth and receive a current. A fuse melts when the current exceeds its rating because the heating effect of the current raises the temperature of the fuse wire. Earthing provides a low-resistance path so a fault current is large enough to melt the fuse. Answers should name the component or quantity, state what happens to it, and say why that matters for safety.

What earns the marks

• An explanation must link a named part of the circuit or a named quantity to a stated consequence, not merely name the part.
• The live wire carries the alternating potential of the mains supply, so contact with it can drive a current through a person to earth.
• The neutral wire completes the circuit and is near earth potential in normal operation, so it is not the wire that supplies the dangerous driving potential.
• The earth wire and fuse work together: a fault connects the live wire to the metal case, a large current flows to earth, and the fuse melts to break the live connection.
• A fuse rating must be just above the normal operating current so it does not melt in normal use but does melt under fault current.
• The heating effect of a current is the mechanism by which a fuse melts; a larger current transfers more energy per second to the fuse wire.
• Explanations should use the correct terms live, neutral, earth, fuse and potential difference rather than informal words such as positive wire.

Common mistakes and how to put them right

• Saying the earth wire carries the normal operating current.

  Put it right: The earth wire carries current only when a fault occurs, providing a low-resistance path to earth.

• Thinking a fuse protects by reducing the voltage to zero instantly.

  Put it right: A fuse melts and breaks the circuit when the current exceeds its rating, which then removes the potential difference from the appliance.

• Confusing the live and neutral wires.

  Put it right: The live wire alternates in potential relative to earth and is the dangerous one; the neutral wire is near earth potential in normal operation.

Exam tips

• Use the word because to force a causal link between each component and its safety effect.
• When asked to explain, write at least one sentence per component or quantity mentioned, stating what happens and why it matters.
• Check that every wire you name is matched to its correct function before moving on to the next point.