IGCSE Physics 4.4 Electrical Safety Notes | Electricity and magnetism 2025

High voltage contact can be fatal; small currents through the body can kill.
Low-voltage circuits can still overheat and cause fires if a fault allows excessive current.
Short circuit = unintended low-resistance path → very large current → rapid overheating and potential fire.

Damaged insulation on cables exposes live conductors.
Overloading sockets or extension leads increases current → overheating.
Damp/wet conditions provide unwanted conducting paths and increase shock risk.
Overheated cables can melt insulation, exposing bare wires and risking short circuits.

Operate exposed switches using an insulated pull cord/tool where damp or heat is present.
Use switches with an insulating cover and, where possible, locate them outside the damp/hot area.

Fuse action: a thin wire that melts when current exceeds its rating, opening the circuit.
Choose the rating by calculating normal current and rounding up to the next fuse size (e.g. 3 A, 5 A, 13 A).
Example: 800 W appliance on 240 V → I = P/V = 800/240 ≈ 3.3 A → use a 5 A fuse.
Placement rule: the fuse must be in the live wire so a fault disconnects the live supply and reduces shock risk.

Automatically cut off the supply when current exceeds a preset limit. Reset after the fault is cleared.
Provide fast protection against overheating and wiring damage; similar purpose to fuses.

Earthing metal cases provides a low-resistance path to ground if a live conductor touches the case, reducing shock risk.
Three-pin plug: live, neutral and earth. The earth pin is longer so the appliance is earthed first.
Double-insulated appliances have non-conducting outer casings and do not require an earth; a correctly rated fuse still protects the cable/appliance.
Switch in live: ensure switches break the live conductor so the appliance is truly isolated when off.

If a live wire touches the metal case of an earthed appliance, a large current flows to earth → the fuse or breaker trips → supply is cut.
If the fault current is below the fuse rating, the circuit may remain on → overheating risk remains; treat the appliance as unsafe until repaired.

Placement answers: Fuse and main switch go in the live conductor; not neutral alone.
Fuse choice method: compute I = P/V, then pick the next higher standard fuse rating.
Protection summary: Fuse/CB protect against overcurrent; earthing protects users from exposed metal becoming live; double insulation avoids exposed metal altogether.
Use hazard terms precisely: overheating, fire risk, electric shock, electrocution — link each to its cause (short, overload, damaged insulation, damp).

4.4 Electrical Safety