Physics · 4 Electricity and magnetism

4.4 Electrical safety

Before you start

$P = I V$ , so a current can be worked out from a power rating (4.2.5).
Conductors and insulators, and that the human body conducts (4.2.1).
That a fuse melts and breaks the circuit if the current is too large (4.3.1).

Syllabus objectives covered (5)

State the hazards of: (a) damaged insulation (b) overheating cables (c) damp conditions (d) excess current from overloading of plugs, extension leads, single and multiple sockets when using a mains supply
Know that a mains circuit consists of a live wire (line wire), a neutral wire and an earth wire and explain why a switch must be connected to the live wire for the circuit to be switched off safely
Explain the use and operation of trip switches and fuses and choose appropriate fuse ratings and trip switch settings
Explain why the outer casing of an electrical appliance must be either non-conducting (double-insulated) or earthed
State that a fuse without an earth wire protects the circuit and the cabling for a double-insulated appliance

Part 1 of 4

The four hazards

Every hazard in this subtopic comes down to one of two dangers: a person touching something live, or a wire getting hot enough to start a fire.
Damaged insulation. If the plastic covering a wire is cracked or worn, the bare conductor is exposed. Touching it gives an electric shock, and two exposed wires touching each other cause a short circuit.
Overheating cables. A cable carrying more current than it is designed for heats up, because $P = I^{2} R$ means the heating rises with the square of the current. The insulation can melt and the cable can start a fire.
Thin cables and coiled-up extension leads are both prone to this — a coiled lead cannot lose its heat to the surroundings.
Damp conditions. Water conducts electricity, so damp hands or a wet floor give the current a path through the body to earth, making a serious shock far more likely.
This is why mains sockets are not fitted in bathrooms, and why you should never touch a switch with wet hands.
Overloading. Plugging several high-power appliances into one socket or extension lead means their currents add. The total can exceed what the wiring can safely carry, causing overheating and fire.

Part 2 of 4

The three wires

A mains circuit uses three wires, and each has a distinct job.
The live wire (or line wire) carries the alternating p.d. from the supply. It is the dangerous one — it is at a high p.d. relative to earth.
The neutral wire completes the circuit, carrying the current back to the supply. It stays close to earth potential.
The earth wire is a safety wire. It carries no current in normal operation, and provides a low-resistance path to earth if a fault occurs.
The switch must be in the live wire. This is the point most often asked and most often answered loosely.
Here is why. If the switch is in the live wire, opening it disconnects the appliance from the high p.d., so everything beyond the switch is safe to touch.
If the switch were in the neutral wire, opening it would stop the current — the appliance would appear off — but the whole appliance would still be connected to the live supply, and touching it could still give a fatal shock.
So the answer is not 'so the current stops'. It is that the appliance is isolated from the high p.d., leaving nothing live to touch. The same reasoning applies to a fuse, which is also fitted in the live wire.

Part 3 of 4

Fuses and trip switches

A fuse is a thin wire in the live wire of the circuit. If the current exceeds its rating, the wire heats up, melts and breaks the circuit.
A trip switch (circuit breaker) does the same job electromagnetically. An excess current magnetises a coil strongly enough to release a catch, which snaps the switch open.
The trade-off between them is worth knowing. A trip switch operates faster and can simply be reset, whereas a blown fuse must be replaced — but fuses are cheaper.
Choosing a rating is the calculation this objective is built around. Work out the normal operating current from $P = I V$ , then choose the next fuse rating above it.
A rating too low means the fuse blows during normal use, which is a nuisance rather than a danger.
A rating too high is the real problem: a dangerously large current could flow without ever blowing the fuse, and the cable overheats exactly as if no fuse were fitted.
Common fuse ratings are 3 A, 5 A and 13 A, so an appliance drawing 4.2 A takes the 5 A fuse — the smallest rating above the normal current.

Part 4 of 4

Earthing and double insulation

The danger with a metal-cased appliance is a fault in which the live wire touches the casing. Without protection, the casing becomes live and anyone touching it completes a circuit to earth.
There are two solutions, and an appliance uses one or the other, never both.
Earthing. The metal casing is connected to the earth wire. If the live wire touches the casing, a very large current flows through the earth wire, because it is a low-resistance path straight to earth.
That surge blows the fuse almost immediately, disconnecting the appliance from the live supply. So the earth wire's job is to cause the fuse to blow, not to carry the fault current away indefinitely.
Explaining that two-step chain — large current through the earth wire, which blows the fuse and isolates the appliance — is what earns full marks.
Double insulation. The appliance is built with a non-conducting outer casing, usually plastic, and the live parts are enclosed in a second layer of insulation.
There is then no conducting path from the live wire to anything a person can touch, so no earth wire is needed. These appliances carry a double-square symbol and are fitted with two-core cable.
A double-insulated appliance still has a fuse, though. The fuse is not protecting the user from the casing — it is protecting the circuit and the cabling from an excess current that could cause overheating or fire.

Worked examples

Example 1. An electric iron is rated at $1400$ W and operates from a $230$ V mains supply. Fuses of $3$ A, $5$ A and $13$ A are available. Determine which fuse should be fitted.

Show the working

Find the normal operating current first, using $P = I V$ rearranged.
Substitute the values.
$I = \frac{P}{V} = \frac{1400}{230} = 6.1 A$
Now apply the rule: choose the smallest available rating that is above the normal current.
A 3 A or 5 A fuse would blow in normal use, since $6.1$ A exceeds both. So the 13 A fuse is the only workable choice.
Note the reasoning has to be stated. 'Choose 13 A' alone is one mark; 'because $6.1$ A exceeds 3 A and 5 A, which would blow in normal use' is what earns the rest.

Answer: The $13$ A fuse. The iron draws $6.1$ A, so the $3$ A and $5$ A fuses would blow during normal operation.

Example 2. Explain how the earth wire and the fuse together protect a user from a metal-cased appliance in which the live wire has come loose and touched the casing.

Show the working

State the fault and the danger: the live wire touching the casing would make the casing live, and a person touching it would receive a shock as current passed through them to earth.
The earth wire connects the casing directly to earth, providing a path of very low resistance.
Because the resistance of that path is very low, a very large current flows through the earth wire — much larger than the normal operating current.
That large current exceeds the fuse rating, so the fuse wire melts and breaks the live connection.
The appliance is now disconnected from the live supply, so the casing is no longer live and is safe to touch.

Answer: The earth wire gives the fault current a very low-resistance path to earth, so a very large current flows. This exceeds the fuse rating, the fuse melts, and the appliance is disconnected from the live supply, leaving the casing safe.

Example 3. Explain why a switch must be fitted in the live wire rather than the neutral wire.

Show the working

Consider what a switch in the live wire achieves: opening it disconnects the appliance from the high p.d. of the supply.
Everything beyond the switch is then at close to earth potential, so it is safe to touch.
Now consider the alternative. A switch in the neutral wire also stops the current, so the appliance appears to be off.
But the appliance would still be connected to the live wire, so it would remain at the high supply p.d.
Anyone touching the inside of the appliance would then complete a circuit to earth and receive a shock, even with the switch open.

Answer: A switch in the live wire isolates the appliance from the high p.d., so nothing beyond it is live. A switch in the neutral wire would stop the current but leave the appliance connected to the live supply, so touching it could still give a fatal shock.

Where people go wrong

Mistake: Saying a switch goes in the live wire 'so that the current stops'.

Instead: A switch in either wire stops the current. The reason is that the live switch isolates the appliance from the high p.d.

The examinable point is the danger that remains with a neutral switch — the appliance stays live even when apparently off.

Mistake: Saying the earth wire 'carries the current safely away' and stopping there.

Instead: The large current through the earth wire blows the fuse, which disconnects the appliance.

Without the fuse blowing, the fault would persist. The two components work as a pair, and the mark scheme wants the chain.

Mistake: Choosing a fuse rating far above the operating current 'to be safe'.

Instead: Choose the smallest available rating above the normal operating current.

An over-rated fuse allows a dangerously large current to flow without blowing, which defeats the point of fitting one.

Mistake: Saying a double-insulated appliance needs no fuse.

Instead: It has no earth wire, but it still has a fuse.

The fuse protects the circuit and the cabling from excess current and overheating, which double insulation does nothing about.

Mistake: Explaining overheating as 'too much electricity'.

Instead: A current larger than the cable is designed for produces heating at a rate proportional to $I^{2} R$ .

'Electricity' is not a physical quantity. Name the current, and if you can, quote the relationship.

Check yourself

State two hazards of damaged insulation.
Answer
A person touching the exposed conductor receives an electric shock, and two bare wires touching cause a short circuit, which can lead to fire.
Why is it dangerous to use mains appliances in damp conditions?
Answer
Water conducts, so it provides a path for current through the body to earth, making a severe shock much more likely.
Name the three wires in a mains circuit and state the job of each.
Answer
Live carries the supply p.d.; neutral completes the circuit and returns the current; earth is a safety wire providing a low-resistance path to earth in a fault.
Why must the fuse, as well as the switch, be in the live wire?
Answer
So that when it blows, the appliance is disconnected from the high p.d. and is no longer live.
A $920$ W appliance runs from a $230$ V supply. Which of a $3$ A, $5$ A or $13$ A fuse should be used?
Answer
$I = P / V = 920 / 230 = 4.0$ A, so the $5$ A fuse — the smallest rating above the operating current.
Give one advantage of a trip switch over a fuse.
Answer
It operates faster and can be reset rather than replaced.
State the two ways the casing of an appliance can be made safe.
Answer
Either it is made of a non-conducting material (double-insulated), or it is metal and connected to the earth wire.
What does the fuse protect in a double-insulated appliance?
Answer
The circuit and the cabling, by breaking the circuit if an excess current flows and could cause overheating.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1. A metal-cased electric heater is rated at $2300$ W and runs from a $230$ V mains supply. It is connected using a three-core cable.

(a) Calculate the normal operating current of the heater. 2 marks

Mark scheme

C1 I = P / V used
A1 10 A; unit required

(b) Fuses of $3$ A, $5$ A and $13$ A are available. Determine which should be used, and explain why the others are unsuitable. 3 marks No calculator

Mark scheme

B1 13 A; ecf from (a)
B1 3 A and 5 A are below the operating current, so would blow in normal use
B1 a rating much higher than 13 A would allow a dangerous current without blowing, owtte

(c) Explain how the earth wire and the fuse protect the user if the live wire touches the metal casing. 3 marks No calculator

Mark scheme

B1 the earth wire provides a low-resistance path to earth
B1 a very large current flows through it
B1 this blows the fuse, disconnecting the appliance from the live supply so the casing is no longer live

(d) Explain why the switch for this heater is connected in the live wire. 2 marks No calculator

Mark scheme

B1 opening it isolates the appliance from the high p.d. of the supply
B1 a switch in the neutral wire would leave the appliance connected to the live wire and still dangerous to touch, ORA

Question 2. A plastic-cased hairdryer is connected using a two-core cable and carries a double-square symbol.

(a) Explain why this appliance does not need an earth wire. 2 marks No calculator

Mark scheme

B1 the outer casing is non-conducting / it is double insulated
B1 so there is no conducting path from the live wire to anything the user can touch

(b) State what the fuse in this appliance protects. 1 mark No calculator

Mark scheme

B1 the circuit and the cabling (from excess current / overheating)

Question 3. A student plugs a kettle, a heater and a toaster into a single extension lead.

(a) Explain the hazard created by this arrangement. 3 marks No calculator

Mark scheme

B1 the currents drawn by the appliances add together
B1 the total current may exceed the rating of the extension lead's cable
B1 the cable overheats, the insulation may melt and a fire may result

(b) State one other electrical hazard in the home and how it can be avoided. 2 marks No calculator

Mark scheme

B1 any one from: damaged insulation / damp conditions / overheating cables
B1 a matching precaution, e.g. replace worn cables / keep appliances away from water / uncoil extension leads

Words to know

Live wire: The wire carrying the supply p.d. It is the dangerous one, and both the switch and the fuse are fitted in it.
Neutral wire: The wire completing the circuit and returning the current to the supply, at close to earth potential.
Earth wire: A safety wire connecting a metal casing to earth, carrying no current in normal use.
Fuse: A thin wire in the live wire that melts if the current exceeds its rating, breaking the circuit.
Trip switch: An electromagnetic switch that opens on excess current, faster than a fuse and resettable.
Double insulation: Construction with a non-conducting outer casing and a second layer of insulation, so no earth wire is needed.

All Physics lessons Next: 4.5.1 Electromagnetic induction