Section 1 of 6Why mains electricity is dangerous
Heating effect of a current
- Heating effect of a current When a current passes through a wire, free electrons collide with the metal ions and transfer energy to them,
- so the wire gets hotter.
- Useful in kettles, toasters and hobs, but unwanted in cables that carry more current than they are designed for.
Electric shock
- Electric shock A current passing through the human body can disrupt the heart and cause burns,
- and voltages as low as 50 V can be a serious risk of electrocution.
- Mains voltage is far above this level, so mains equipment must be treated with great care.
Section 2 of 6The four main hazards
| Hazard | Why it is dangerous | How to reduce the risk |
|---|---|---|
| Damaged insulation | Exposed metal wire can give a lethal shock and may touch other metal objects | Replace damaged cables and never touch exposed wire |
| Overheating cables | Too much current heats the cable, melting its insulation and possibly starting a fire | Use cables rated for the current and uncoil them fully |
| Damp conditions | Water conducts, so it can create a short circuit or a path for current through a person to earth | Keep mains devices dry and use them away from water |
| Overloaded sockets | Too many appliances draw an excessive current, which overheats the cable | Plug only a limited number of devices into each socket or extension lead |
Section 3 of 6How an overload becomes a fire
1Too many appliances
- Too many appliances Plugging many devices into one extension lead means a very large total current flows through it.
2Cable heats up
- Cable heats up The large current produces a strong heating effect in the thin wires of the lead.
3Insulation melts
- Insulation melts The plastic insulation softens and melts, exposing the live and neutral wires.
4Fire or shock
- Fire or shock Exposed wires can touch to cause a short circuit, giving sparks and a fire, or expose people to shock.
Section 4 of 6What to do in practice
1
Inspect plugs, cables and appliances before use, and do not use any with frayed or cracked insulation.
2
Never touch plugs, switches or appliances with wet hands, and keep drinks away from sockets.
3
Do not plug many high-power appliances into the same socket or extension lead.
4
Unplug an appliance before attempting to look inside it, and never push metal objects into sockets.
5
If someone is in contact with a live wire, switch off the supply at the mains before touching them, then call for help.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Heating effect | The rise in temperature of a conductor when a current passes through it. |
| Insulation | A non-conducting layer, usually plastic, that covers the wire and stops contact with the metal. |
| Short circuit | An unintended low-resistance path that lets a very large current flow. |
| Overloading | Drawing more current from a supply than the circuit or cable is designed to carry. |
| Electrocution | Injury or death caused by electric current passing through the body. |
Section 6 of 6Exam technique
1
Saying that water 'is' dangerous because it is wet. It is dangerous because it conducts electricity, allowing current to pass through a person or into the device.
2
Thinking only high voltages matter. The current through the body is what causes harm, and voltages as low as 50 V can be a risk.
3
Writing that overloading increases the voltage. It increases the current drawn from the supply.
4
Forgetting to explain the heating effect with free electrons colliding with ions in the wire.
5
Describing damaged insulation as a danger only to the appliance, when it is mainly a danger to people.