Mains Electricity and Safety

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CognitoMains electricity, plugs and safety

Key facts & equations

AC and DC

DC (direct current): current flows in one direction only. Cells and batteries produce DC.
AC (alternating current): current reverses direction repeatedly. UK mains = AC.
UK mains supply: frequency = 50 Hz (50 cycles per second). Voltage = 230 V (rms).
Peak voltage of UK mains ≈ 325 V (higher than the rms value of 230 V).
AC can be stepped up/down by transformers — efficient for long-distance transmission.

Plug wiring (UK 3-pin plug)

Live wire: brown. Carries the alternating voltage (~230 V). Most dangerous.
Neutral wire: blue. Completes the circuit. Near 0 V.
Earth wire: green and yellow stripe. Safety wire. Usually no current flows.
Fuse: in the live wire. Melts if current too high, breaking the circuit.
The earth wire connects the metal casing of appliances to earth.

Fuses and circuit breakers

Fuse: thin wire that melts if current exceeds its rating. Breaks circuit. Must be replaced.
Circuit breaker: magnetic/electronic switch that trips. Can be reset.
Fuse rating: must be above operating current but as low as possible.
RCDs (residual current device): detect imbalance between live and neutral → trip instantly. Used in bathrooms, gardens. Faster than fuses.

Earthing and double insulation

Earthing: if live wire touches metal casing → large current through earth wire → fuse blows → circuit broken → no shock hazard.
Double insulation: appliances with plastic casings (e.g. hairdryer, some drills) — no earth wire needed as the plastic cannot become live.
NEVER touch live wire or insert objects into sockets.
Extension leads in series increase total resistance. Do not overload sockets.

Exam questions — 4 questions · 10 marks · AQA 8463 style

1 markUK mains frequencyAQA 8463 style

What is the frequency of UK mains electricity?

A 50 kHz

B 50 Hz

C 230 Hz

D 60 Hz

1 markCorrect wire colourAQA 8463 style

In a UK plug, which wire is brown?

A Neutral

B Earth

C Live

D Fuse wire

4 marksWhy fuses and earth wires protectAQA 8463 style

Explain how the earth wire and fuse work together to protect a user from electric shock if the live wire inside a metal-cased appliance comes loose and touches the casing. (4 marks)

Hint: Chain: casing becomes live → earth wire provides low-resistance path → large current → fuse blows → circuit broken → safe. Explain what would happen without each component.

+40 XP

1 markDouble insulationAQA 8463 style

Why does a plastic-cased hair dryer not need an earth wire?

A Plastic appliances use DC not AC, which is safer

B The plastic casing cannot conduct electricity, so even if the live wire touches it, the casing remains safe to touch

C Hair dryers always have RCDs which replace the earth wire

D Hair dryers operate at a lower voltage that is not dangerous

Mains Electricity and Safety

AC and DC

Plug wiring (UK 3-pin plug)

Fuses and circuit breakers

Earthing and double insulation

Module complete! 🎉