Electrical safety
Section: Electricity & Magnetism | Syllabus: Cambridge IGCSE Physics (0625)
Electrical Hazards
Several conditions create electrical hazards when using a mains supply:
| 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 the risk of shock |
| Overloading | Excess current from too many appliances on plugs, extension leads or sockets causes overheating and fire risk |
Mains Wiring
- A mains circuit consists of three wires:
| Wire | Colour | Function |
|---|---|---|
| Live (L) | Brown | Carries current to the appliance at high voltage (~230 V) |
| Neutral (N) | Blue | Completes the circuit, returning current to the supply (~0 V) |
| Earth (E) | Green and yellow stripes | Safety wire connected to the ground (0 V) |
Live, neutral and earth wires connected inside a mains plug
- The switch must be connected to the live wire
- switch on the live wire: when open, no current can flow and the appliance is fully disconnected from the high-voltage live wire
- switch on the neutral wire: the appliance would still be connected to 230 V even when "off", which is dangerous during maintenance
Fuses and Trip Switches
- A fuse is a safety device containing a thin wire that melts when too much current flows, breaking the circuit
- normal operation: current flows through the fuse wire as usual
- if the current exceeds the fuse's rating: the wire heats up and melts, breaking the circuit and protecting the appliance and wiring
- A trip switch (circuit breaker) is an automatic switch that opens when the current exceeds a set value
- unlike a fuse, it can simply be reset (switched back on) rather than replaced
| Fuse | Trip Switch (Circuit Breaker) |
|---|---|
| Must be replaced after blowing | Can be reset (switched back on) |
| Cheap | More expensive, but reusable |
| Slower response | Faster response |
Choosing the Correct Fuse Rating or Trip Switch Setting
The rating chosen should be just above the appliance's normal operating current, so it doesn't blow or trip during normal use but still protects against a fault.
Worked Example: Selecting a Fuse
Muchindu has a 920 W heater that operates at 230 V, and needs to choose between a 3 A, 5 A or 13 A fuse.
- Step 1: find the normal operating current. I = (P/V) = (920/230) = 4 A
- Step 2: choose the rating just above this value - the 5 A fuse is suitable, since it's just above the 4 A operating current
Protecting Appliances: Earthing and Double Insulation
- The outer casing of an electrical appliance must be either earthed or non-conducting (double-insulated), so that it can never become live and give the user an electric shock
Earthing
- The earth wire is connected to the metal casing of an appliance and to the ground
- if a fault causes the live wire to touch the metal casing, current flows through the earth wire to the ground (the path of least resistance) instead of through a person touching the casing
- this surge of current is large enough to blow the fuse, breaking the circuit and protecting the user from electric shock
The earth wire gives fault current a safe path to ground, blowing the fuse before anyone is harmed
Double Insulation
- Some appliances have double insulation (a plastic outer casing) and do not need an earth wire
- the outer casing is plastic (non-conducting), and internal components are insulated from it, so no metal part can ever become live
- marked with a square-within-a-square symbol on the appliance's rating plate
- Even without an earth wire, a double-insulated appliance still needs a fuse
- the fuse protects the circuit and the cabling itself from overcurrent, even though it can no longer protect the user via an earth-fault surge
Interactive revision notes, videos and practice questions load below.