Knowledge Organiser: Mains Electricity and Safety
Key Terms
- AC: Alternating current — direction reverses
- DC: Direct current — one direction only
- Fuse: Melts to break circuit if I too high
- Earth wire: Safety path for fault current
- Double insulation: Plastic case, no earth needed
Wire Colours
- Live = brown (dangerous)
- Neutral = blue (~0 V)
- Earth = green and yellow
- Fuse on the LIVE wire only
UK Mains Values
- 230 V (UK mains)
- 50 Hz
- AC (not DC)
Exam Tips
- Fuse rating: I = P/V, then choose just above
- Earth wire causes large current → blows fuse
- Circuit breaker = resettable fuse
- Double insulated = no earth needed
Key Equations
- I = P ÷ V (to calculate correct fuse rating)
- UK mains: about 230 V, 50 Hz
- P = I × V (power in electrical circuit)
Common Mistakes
- Choosing a fuse rating that is too low: The fuse must be rated just above the normal operating current — too low and it blows during normal use; too high and it fails to protect
- Confusing earth and neutral wires: The neutral wire completes the circuit; the earth wire is a safety wire that only carries current during a fault
- Saying AC and DC are interchangeable: Mains electricity is AC (alternating current); batteries supply DC (direct current) — they are not the same
- Thinking double-insulated devices are safer in all situations: Double insulation means no earth wire is needed — it does not mean no fuse is needed
Memory Aid
- Wire colours — "Brown Lives, Blue Neutral, Green-Yellow Earth":
- Brown = Live (Be careful — it's Lethal!)
- Blue = Neutral (think: bland/safe at 0 V)
- Green/Yellow = Earth (think: grass and sun = the earth and ground)
- Fuse selection rule — "Just Above, Never Below":
- Calculate the normal current using I = P/V. Choose the fuse that is just above this value from the available options (3 A, 5 A, 13 A). Never choose one that's below — it would blow immediately. Never choose one that's much higher — it won't protect.
- Earth wire purpose — "Earth Escapes, Fuse Finishes":
- The Earth wire provides an escape route for fault current. The large current then causes the Fuse to Finish (melt), cutting the circuit.