Section 1 of 6Why mains is dangerous
Large potential difference
- Large potential difference The live wire is at about 230 V compared with the ground, so touching it can drive a harmful current through the body.
Complete circuit through you
- Complete circuit through you A person standing on the ground who touches a live wire makes a path from live, through the body, to earth.
Effects
- Effects A current of a few tens of milliamps can cause muscle spasm, burns, interference with the heart and death.
Neutral is not a risk
- Neutral is not a risk The neutral wire is close to 0 V,
- but the live wire stays dangerous even when the switch is off if the switch is in the neutral side.
Section 2 of 6Everyday hazards
| Hazard | Why it is dangerous |
|---|---|
| Damaged or frayed cable | Exposed live wire can touch a person or other wires |
| Long cables left coiled | Current heats the coil and the insulation can melt |
| Water near sockets or appliances | Water conducts, so it provides a path to earth through the user |
| Overloaded sockets with adaptors | Total current exceeds the safe limit and the wires overheat |
| Pushing metal into a socket | Can contact the live terminal directly |
Section 3 of 6Safety devices
Protection from faults
Fuse
- Fuse Contains a thin wire that melts when the current exceeds its rating, breaking the circuit.
- It must be replaced after it blows.
Circuit breaker
- Circuit breaker An electromagnetic switch that trips when the current is too high.
- It acts faster than a fuse and can be reset by pressing a switch.
Earth wire
- Earth wire Connects a metal casing to the ground so a fault current is large enough to blow the fuse.
Double insulation
- Double insulation Plastic-cased appliances such as hair dryers have insulation that surrounds all live parts, so no earth wire is needed.
Section 4 of 6What happens in a fault
1
Insulation wears or a wire works loose so the live wire touches the metal casing.
2
The casing is now at the live potential and would give a shock to anyone who touches it.
3
Because the casing is earthed, a very large current flows along the earth wire to the ground.
4
The large current blows the fuse (or trips the breaker) in the live wire, cutting off the supply and making the appliance safe.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Live | The wire at high p.d. (about 230 V) that carries the supply to the appliance. |
| Earthing | Connecting a metal casing to the ground through the earth wire so that faults are made safe. |
| Circuit breaker | An automatic, resettable switch that opens when the current is too large. |
| Double insulation | A design in which all live parts are enclosed in insulating material, removing the need for earthing. |
| Overload | Drawing more current than a circuit or cable can safely carry. |
Section 6 of 6Exam technique
1
Saying birds are safe on a power line because 'electricity cannot hurt them' — there is no complete path through the bird to earth, so no large p.d. across it.
2
Thinking a switched-off appliance is safe if the switch is in the neutral wire; the live wire is still connected.
3
Describing a fuse as stopping the voltage rather than breaking the circuit when the current is too high.
4
Using a fuse of a far higher rating than needed, which will not blow in time to prevent overheating.