Electrical safety

Key points

Hazards of mains electricity

• The mains supply is the electricity supply to homes, schools and offices. Its p.d. is large enough to kill. So mains electricity has hazards.
• Damaged insulation: if the plastic insulation on a cable is cracked or worn, the metal wire inside is exposed. A person who touches the live wire gets an electric shock. Bare wires can also touch each other. This makes a very large current, which can start a fire.
• Overheating cables: a current heats a cable. A cable that carries too much current gets very hot. The insulation can melt and the cable can start a fire.
• Damp conditions: water can conduct electricity. Water on a switch, socket or appliance can make a path for current from the live wire to a person. This gives an electric shock. So never touch a switch or a plug with wet hands.
• Overloading: too many appliances in one plug, extension lead or socket (single or multiple) draw a total current that is too large. The cables and sockets overheat, and this can cause a fire.

The mains circuit

• A mains circuit has three wires:
  • the live wire (also called the line wire): it carries the alternating p.d. from the supply
  • the neutral wire: it completes the circuit. It stays at a p.d. close to 0 V (close to the p.d. of the earth)
  • the earth wire: a safety wire. It connects the metal case of an appliance to the ground (earth). Normally no current flows in it.
• The switch must be in the live wire. Then, when the switch is off, the working parts of the appliance (the circuit after the switch) are cut off from the high p.d. of the live wire.
• The wire on the supply side of the switch is still live. So switching off is not enough to make it safe to open an appliance. Unplug it first.
• If the switch were in the neutral wire, the appliance would stop working when switched off. But its working parts would still be connected to the live wire, at a high p.d. Touching them could still give a shock.
• The fuse is also in the live wire, for the same reason.

Fuses and trip switches

• A fuse is a short, thin piece of wire inside a holder. It is connected in the live wire.
• If the current becomes larger than the fuse rating, the fuse wire gets hot and melts. This breaks the circuit and stops the current.
• This protects the cables and the appliance from overheating, so it reduces the risk of fire.
• Once a fuse has melted, it must be replaced with a new one of the correct rating. First find and fix the fault.
• A trip switch (a circuit breaker) does the same job. It is a switch that opens automatically when the current is larger than its setting.
• Inside a trip switch, the current passes through an electromagnet. When the current is too large, the electromagnet becomes strong enough to pull the switch contacts apart. This breaks the circuit.
• A trip switch can be reset (switched back on) after the fault is fixed. It does not need to be replaced.
• Choosing a fuse rating: find the normal current the appliance takes, using I = P / V. Choose the fuse with the smallest rating that is larger than this current.
• If the rating is too low, the fuse melts even when the appliance works normally.
• If the rating is too high, the fuse does not melt when there is a fault. The cable could overheat.
• Plug fuses are commonly made with ratings such as 3 A, 5 A and 13 A.
• Choosing a trip switch setting: add up the currents of all the appliances that can be used at the same time on that circuit. Choose the setting just above this total.
• The setting must also not be higher than the safe current for the cables and sockets of that circuit. If the appliances need more current than this, use fewer appliances on that circuit at the same time.

Earthing and double insulation

• Many appliances have a metal casing. If there is a fault, the live wire can touch the casing. The casing becomes live (at the high p.d. of the live wire).
• A person who touches a live casing gives the current a path through their body to the ground. They get an electric shock.
• So the outer casing must be either earthed or non-conducting (double-insulated).
• Earthed casing: the earth wire connects the metal casing to the ground.
  1. If the live wire touches the casing, a current flows from the live wire, through the casing and the earth wire, to the ground.
  2. The earth wire has a very low resistance. So this current is very large.
  3. The large current melts the fuse (or opens the trip switch).
  4. The appliance is now cut off from the live wire. The casing is no longer live, so it is safe to touch.
• Double-insulated appliance: the outer casing is made of an insulator, such as plastic. The live parts inside are also insulated from the casing. So the casing cannot become live, even if there is a fault. A hairdryer is often double-insulated.
• A double-insulated appliance needs no earth wire. It still has a fuse.
• This fuse, without an earth wire, protects the circuit and the cabling. If too much current flows, the fuse melts, so the cable does not overheat.
• The fuse in a double-insulated appliance does not protect the user from a shock. The double insulation does that.

Common mistakes

• Students write that a fuse protects the person from electric shock. / The mark scheme wants: the fuse protects the cables and the appliance from overheating. The earth wire and the fuse together protect the user when the casing becomes live.
• Students write that the switch can go in either wire. / The mark scheme wants: the switch in the live wire, so the working parts of the appliance are disconnected from the live wire when it is switched off.
• Students choose a fuse rating equal to the normal current or far above it. / The mark scheme wants the next rating above the normal current.
• Students write that the earth wire “takes the electricity away” and stop there. / The mark scheme wants the full chain: low-resistance path → large current → fuse melts → live wire disconnected.
• Students write that the earth wire carries current all the time. / The mark scheme wants: current flows in the earth wire only when there is a fault.
• Students write that a double-insulated appliance needs no fuse. / The mark scheme wants: it needs no earth wire, but it still has a fuse to protect the circuit and cabling.

Exam tips

• State a hazard and its result together. Example: “Damaged insulation can expose the live wire, so a person could get an electric shock.”
• Explain earthing as a numbered chain of steps. Each step is usually worth a mark.
• Choose a fuse: calculate I = P / V, show the answer, then name the fuse and say why the smaller one would not do.
• Explain why the switch goes in the live wire: when it is off, the working parts of the appliance are disconnected from the high p.d. of the live wire.
• Command word Suggest: use the information in the question. If an appliance has a plastic case, the answer is double insulation, not earthing.
• A typical 1-mark answer: “A trip switch can be reset; a fuse must be replaced.”
• A typical 2-mark answer to “How does a fuse protect a circuit?”: “If the current is larger than the fuse rating, the fuse wire melts (1). This breaks the circuit, so the current stops and the cable does not overheat (1).”
• A typical 3-mark answer to “Explain how the earth wire protects the user”: “If the live wire touches the metal case, a large current flows through the low-resistance earth wire (1). This large current melts the fuse (1). The appliance is disconnected from the live supply, so the case is safe to touch (1).”