Electrical safety
Section: Electricity & Magnetism | Syllabus: Cambridge AS Level Physics 9702
Electrical Hazards The following conditions create electrical hazards: Hazard Risk Damaged insulation Exposed live wires can cause electric shock or short circuits Overheating cables Cables carrying excessive current heat up, risking fire Damp conditions Water conducts electricity, increasing shock risk Overloading Too many appliances on one socket causes overheating and fire risk Mains Wiring A mains circuit consists of three wires : Wire Colour Function Live (L) Brown Carries current to appliance at high voltage (~230 V) Neutral (N) Blue Completes circuit, returns current to supply (~0 V) Earth (E) Green/Yellow stripes Safety wire connected to ground (0 V) Why the Switch Must Be on the Live Wire When the switch is open , no current flows and the appliance is disconnected from the high voltage live wire.
If the switch were on the neutral wire, the appliance would still be connected to 230 V even when "off" – dangerous for maintenance. Fuses A fuse is a safety device containing a thin wire that melts when too much current flows, breaking the circuit.
How Fuses Work Normal operation: current flows through fuse wire If current exceeds rating: fuse wire heats up and melts Circuit broken → appliance and wiring protected Choosing the Correct Fuse Rating The fuse rating should be just above the normal operating current.
Example: Selecting a Fuse Question: A 920 W heater operates at 230 V. Which fuse should be used: 3 A, 5 A, or 13 A? Answer I = P/V = 920/230 = 4 A The 5 A fuse is suitable (just above 4 A, protects against overcurrent).
Circuit Breakers (Trip Switches) A circuit breaker (trip switch) is an automatic switch that opens when current exceeds a set value. Fuse Circuit Breaker Must be replaced after blowing Can be reset (switched back on) Cheap More expensive but reusable Slower response Faster response Earthing The earth wire is connected to the metal casing of an appliance and to the ground.
How Earthing Protects If a fault causes the live wire to touch the metal casing Current flows through earth wire to ground (path of least resistance) Large current flows → fuse blows → circuit broken User protected from electric shock FIG 4.4.3: Earth wire protection Diagram showing appliance with metal case.
Live wire accidentally touches case. Current flows through case → earth wire → ground. Large current causes fuse to blow, disconnecting supply. Double Insulation Some appliances have double insulation (plastic outer casing) and do not need an earth wire.
Double Insulation Symbol Square within a square □□ on appliance rating plate. Outer casing is plastic (non-conducting) Internal components are insulated from casing No metal parts can become live No earth wire needed – fuse still required to protect circuit and cabling
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