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6.3.2Generating and Distributing Electricity and Household Use

Electrical Safety

5 min read 5 parts Lesson 16 of 22 Unit quiz

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Section 1 of 5Wires in a Three-Pin Plug

6.3.2 Electrical Safety
Live wire
  • The brown wire at about 230 V, which carries the alternating supply current into the appliance.
Neutral wire
  • The blue wire at about 0 V, which completes the circuit back to the supply.
Earth wire
  • The green and yellow wire at 0 V, which carries current only if a fault connects the live wire to the metal case.
Switch position
  • The switch is placed in the live wire, so the appliance is isolated from the live supply when it is switched off.

Section 2 of 5Protecting the Circuit

Protecting the Circuit
Fuse
  • A thin wire that melts when the current exceeds its rated value, breaking the circuit and cutting off the supply.
Circuit breaker
  • A switch that trips open when the current is too large, and can be reset once the fault has been repaired.
Earth connection
  • The earth wire connects to the metal case, so a live fault produces a large current that blows the fuse.
Double insulation
  • Some appliances have a plastic casing and no earth wire, because the casing itself provides the protection.

Section 3 of 5Choosing a Fuse

1
Find the power rating of the appliance from its label, for example 2.5 kW.
2
Calculate the normal current using I = P ÷ V, with V = 230 V.
3
Choose the lowest standard fuse rating above the calculated current, from 3 A, 5 A or 13 A.
4
Fit the fuse in the live wire, so that the appliance is isolated from the supply when the fuse blows.

Section 4 of 5Worked Example

Question

  • A 2.5 kW electric heater operates from a 230 V supply.
  • Calculate the normal current and choose a suitable fuse rating from 3 A, 5 A and 13 A.

  1. Use P = I × V, so I = P ÷ V.
  2. P = 2500 W, so I = 2500 ÷ 230 = 10.9 A (to 3 s.f.).
  3. The lowest suitable fuse rating above 10.9 A is 13 A.

Answer The normal current is 10.9 A, so a 13 A fuse should be fitted.

Section 5 of 5Exam Checklist & Tips

6.3.2 – Key Points for the Exam

  • Live wire The wire that carries the alternating supply current at about 230 V.
  • Neutral wire The wire that completes the circuit back to the supply at about 0 V.
  • Earth wire A safety wire that carries current only during a fault.
  • Fuse A safety device containing a wire that melts when the current is too large.
  • Double insulation A safety design in which the casing is insulated, so no earth wire is needed.

Common Mistakes

  • Stating the earth wire carries normal current in every appliance; it carries current only during a fault.
  • Fitting the fuse in the neutral wire rather than the live wire.
  • Choosing a fuse rating much higher than the normal current, which fails to protect the wiring.
  • Confusing the live wire, which is brown, with the neutral wire, which is blue.
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