Syllabus coverage
- State the hazards of damaged insulation, overheating cables, damp conditions and excess current from the 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
- Explain the safety reasoning behind the use of fuses, trip switches, earthing and double insulation
Part of the IGCSE Physics syllabus 0625.
Why mains electricity is dangerous: the four hazards
Mains electricity is supplied at about 230 V and alternates 50 times a second. The danger is not the voltage by itself but the current it can drive through you. Dry skin has a resistance of tens of thousands of ohm, but damp skin has a resistance of only a few thousand ohm, so from I = V / R a wet hand touching 230 V can carry a serious current: 230 V across a resistance of 5000 ohm gives I = 230 / 5000 = 0.046 A, which is 46 mA. A current of about 50 mA through the chest is enough to stop the heart, which is why every rule in this topic exists.
Damaged insulation. The plastic or rubber covering of a cable is the only thing keeping the live conductor away from your hand. If it is cut, cracked, worn or chewed, the bare live wire is exposed at 230 V. Touching it lets a current flow through your body to earth, which can kill, and if two bare conductors touch each other they can spark and start a fire.
Overheating cables. Every cable is rated for a maximum current. The current warms the wire, and the power wasted as heat is I x I x R, so doubling the current makes the heating four times as great. If the current is above the rating, the copper gets hot, the insulation becomes brittle, and it can melt, smoke or burn. That is both a fire risk and a shock risk, because burnt insulation exposes the live wire. A cable coiled up or buried under a carpet cannot lose its heat, so it reaches a dangerous temperature at a lower current.
Damp conditions. Pure water hardly conducts at all, but tap water and sea water contain dissolved salts, so they conduct well. Water on your skin lowers its resistance, and water around a plug or socket provides a conducting path from the live parts to earth. The contact that would give a slight tingle with dry hands can be fatal with wet hands, because the current is much larger.
Overloading plugs, extension leads and sockets. A socket, an adaptor and an extension lead each have a maximum current, typically 13 A for a domestic socket but often only 5 A or 10 A for a lead. Plug a heater, a kettle and a toaster into one socket through a multi-way adaptor and the total current can exceed the rating of the socket and the lead. The cable then overheats and the insulation can melt and start a fire. The fuse in each plug protects that one appliance; it cannot protect the shared socket or lead when several appliances together draw too much.
- Damaged insulation: the live wire is exposed at 230 V, so touching it gives a shock, and bare wires touching can spark and start a fire.
- Overheating cables: too much current heats the wire, melts the insulation and can start a fire.
- Damp conditions: water lowers the resistance of your skin and of the path to earth, so the current through you is larger.
- Overloading: the total current through a plug, socket or lead exceeds its rating, so the cable overheats.
The three wires of a mains circuit
A mains cable that supplies an appliance contains three wires, and each one has a job. The live wire (also called the line wire) carries the current from the supply to the appliance and is at about 230 V. The neutral wire carries the current back to the supply and is at about 0 V, close to earth potential. The earth wire is connected to the metal case of the appliance and to the ground, and it carries no current at all while everything is working properly.
The colours are examinable, so learn them exactly. The live wire is brown, the neutral wire is blue, and the earth wire is green and yellow striped. Older installations used red for live and black for neutral, so if a question shows an old cable, check which set of colours it is using. A three-pin plug also contains a fuse in the live connection, and its earth pin is longer than the other two so that the earth connects first and disconnects last.
The current flows round a complete circuit: out along the live wire, through the appliance, and back along the neutral wire. The earth wire carries no current in normal use. It is a safety route that is used only if something goes wrong, and the whole reason for earthing is that the earth wire is joined to the metal case, so a fault inside the appliance cannot leave the case sitting at 230 V.
| Wire | Colour | Potential | Job |
|---|---|---|---|
| Live (line) | Brown | About 230 V | Carries the current to the appliance at the full mains potential, so it is the dangerous wire |
| Neutral | Blue | About 0 V | Completes the circuit and carries the current back to the supply |
| Earth | Green and yellow | 0 V | Carries no current in normal use; it is the safety route from the metal case to the ground |
Why the switch must be in the live wire
A switch connected in the live wire disconnects the appliance from the 230 V supply when it is open. Everything beyond the switch, including the cable and the metal case, is then at 0 V, the same potential as you and the earth. No potential difference means no current, so you can safely touch the appliance or change a lamp, even though the appliance is still joined to the supply at the other end.
If the switch were in the neutral wire instead, opening it would still stop the appliance working, because the circuit would be broken. But the appliance would remain joined to the live wire, so it would stay at 230 V while it looked switched off. A person touching it would then complete the path from the live wire, through the appliance, through the person, to earth, and would receive a shock. This is the trap in the question: off does not mean safe unless the switch is in the live wire.
The same reasoning explains why the fuse goes in the live wire too. If a fuse were fitted in the neutral wire, melting it would stop the appliance working but would leave the appliance connected to the live wire, so it would still be dangerous to touch. With the fuse in the live wire, blowing the fuse isolates the appliance from the high potential.
Remember that a switch does not stop the supply reaching the appliance; it only breaks the path at the point where it is fitted. For the circuit to be switched off safely, the break must be in the wire that carries the high potential, and that is the live wire.
Fuses and trip switches: choosing the right rating
A fuse is a short length of thin wire that melts when the current through it is too large. It is fitted in the live wire, usually inside the plug. When a fault makes the current exceed the fuse rating, the fuse wire heats up, melts and breaks the circuit, so the current stops. The fuse must then be replaced with one of the same rating.
A trip switch, or circuit breaker, does the same job without being destroyed. Inside it, the current through a coil produces a magnetic effect, or heats a bimetallic strip, and when the current exceeds the setting the switch is pulled open automatically. Because nothing melts, it can simply be reset by hand once the fault is put right. Trip switches work faster than fuses, can be set to any chosen current, and the same switch protects a whole circuit rather than a single appliance.
A residual current circuit breaker (RCCB) protects against shock rather than overload. It compares the current flowing out in the live wire with the current returning in the neutral wire. In a healthy circuit the two are equal; if some current leaks to earth, perhaps through a faulty appliance or through a person, they are not equal and the RCCB trips. A typical setting is 30 mA, small enough to disconnect the supply before a shock becomes fatal.
To choose a fuse rating, first find the normal current of the appliance from P = V x I, that is I = P / V, then choose the next standard rating above that current. A 2000 W heater on a 230 V supply takes I = P / V = 2000 / 230 = 8.7 A, so it needs a 13 A fuse. A 60 W lamp takes 60 / 230 = 0.26 A, so a 3 A fuse is right, and an 800 W microwave takes 800 / 230 = 3.5 A, so a 5 A fuse is right. Never fit a fuse with a higher rating than the cable can carry, because a large current could then flow for a long time and overheat the cable without the fuse ever melting.
| Appliance | Power | Current = P / V | Fuse rating |
|---|---|---|---|
| Table lamp | 60 W | 60 / 230 = 0.26 A | 3 A |
| Microwave oven | 800 W | 800 / 230 = 3.5 A | 5 A |
| Electric heater | 2000 W | 2000 / 230 = 8.7 A | 13 A |
Earthing and double insulation: keeping the case safe
A metal case can become live if a wire comes loose inside the appliance and touches it, or if a heating element fails and makes contact with it. If nothing is done, the case sits at 230 V, and anyone who touches it provides a path to earth and receives a shock. Earthing prevents this by connecting the metal case to the earth wire.
If the live wire touches an earthed case, a very large current flows from the live wire through the case and the earth wire to the ground. The resistance of that path is well under 1 ohm, so the fault current is hundreds of amperes, far above the fuse rating. The fuse melts at once, or the trip switch opens, and the live wire is disconnected before anyone can touch the case. The case is then at 0 V, so it is safe.
A metal-cased appliance therefore needs a three-core cable and a three-pin plug with the earth wire properly connected. The earth wire is a safety device only: it does no work in a healthy appliance, and if it is left off the appliance still works perfectly, but the protection has gone. That is why an earth connection must never be omitted.
Double insulation is the other way to make an appliance safe. Instead of earthing the case, the appliance is built with a strong non-conducting outer casing, usually plastic, with all the live parts sealed inside it, so there is no metal surface that can become live. The user is protected by the insulation itself, so no earth wire is needed. Double-insulated appliances carry a two-core cable, are marked with the symbol of a square inside a square, and are often fitted with a two-pin plug, or a three-pin plug whose earth pin is not connected.
What protects a double-insulated appliance, then? The fuse, which is still fitted in the live wire. A double-insulated appliance has no earth wire, so there is no path for a fault current to earth and the fuse cannot be blown by a live-to-case fault. The fuse protects the circuit and the cabling of the appliance: if the current becomes too large for any reason, the fuse melts and breaks the circuit, so the cable cannot overheat and cause a fire. Earthing protects the person; the fuse protects the cable.
Key terms you must be able to define
- Live wire (line wire)
- The brown wire of a mains cable, which carries the current to the appliance at about 230 V; the switch and the fuse must both be connected in it.
- Neutral wire
- The blue wire of a mains cable, at about 0 V, which completes the circuit and carries the current back to the supply.
- Earth wire
- The green and yellow wire of a mains cable, at 0 V, connected to the metal case of an appliance; it carries no current in normal use and exists to carry a fault current to the ground.
- Fuse
- A short thin wire in the live connection that melts and breaks the circuit when the current exceeds its rating; domestic ratings include 1 A, 3 A, 5 A and 13 A.
- Trip switch (circuit breaker)
- An automatic switch that opens when the current exceeds its setting and can be reset by hand, so it does not have to be replaced like a fuse.
- Residual current circuit breaker (RCCB)
- A trip switch that compares the current in the live wire with the current in the neutral wire and trips when current leaks to earth, typically at a setting of 30 mA.
- Double insulation
- A method of making an appliance safe by using a non-conducting outer casing with no exposed metal parts that could become live, so that no earth wire is needed; marked with a square inside a square.
- Overloading
- Drawing more current through a plug, socket, adaptor or extension lead than it is rated for, so that the cable overheats and may start a fire.
Applied to the real world
Cambridge sets physics in unfamiliar contexts - a car braking, a power station, a hospital scanner, a satellite. Use these as models for how to apply the physics, then write your own.
A 2000 W electric heater is fitted with a 13 A fuse. The current it takes is I = P / V = 2000 / 230 = 8.7 A, which is below 13 A, so the fuse does not blow in normal use. A 5 A fuse would melt as soon as the heater was switched on, while a fuse of 20 A would let a faulty current of 15 A flow without melting, so the cable could overheat.
A kettle, a toaster and a microwave are plugged into one multi-way adaptor in a single 13 A socket. A 3000 W kettle alone takes about 13 A, so adding the toaster takes the total current well above the 13 A rating of the socket and the adaptor. The adaptor cable overheats even though every appliance has its own correct fuse, because no single fuse protects the shared lead. The remedy is to spread the appliances over separate sockets.
Ordinary sockets are not allowed in a bathroom, apart from a shaver socket supplied through an isolating transformer, because damp skin has a much lower resistance than dry skin. A person with wet hands touching a live part would carry a far larger current at the same 230 V, so a shock that would be minor elsewhere could be fatal. Water and electricity are kept apart for exactly this reason.
Portable tools on a building site are run from a 110 V supply rather than 230 V, provided by a yellow site transformer. The supply is centre-tapped to earth, so the highest potential any wire can have above earth is 55 V. From I = V / R, halving the voltage halves the current through a person who touches a live part, which makes a serious shock much less likely in wet and muddy conditions.
A desktop computer has a plastic case and is double insulated, so it needs no earth wire and is supplied with a two-core lead. The metal chassis inside is separated from the user by the insulating case, and a fuse in the plug still protects the lead and the internal wiring from overheating if a fault draws too much current. The square-in-square symbol on the label confirms that the appliance is double insulated.
Flashcards
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Exam-style questions
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Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Exam technique for this topic
- Learn the wire colours exactly: brown live, blue neutral, green and yellow earth. Red and black are the old colours, so check which set a question is using before you answer.
- For the switch, the mark is for the reason: with the switch in the live wire, the appliance is at 0 V when the switch is open, so no current can flow through a person who touches it. 'So that it can be switched off' scores nothing.
- In fuse-rating questions, calculate I = P / V first and then choose the next standard rating above that value. Write the calculation and the chosen rating; the rating on its own will not score full marks.
- Keep the two protective jobs apart: the fuse (or trip switch) protects the cable and the circuit from overheating, while the earth wire protects the person by making a large fault current flow. Mixing them up is the commonest error in this topic.
- For a hazard question, name the hazard and then say what it causes, either a shock or a fire. A bare list of hazards with no consequence will not earn the explanation marks.
- Remember that a double-insulated appliance still has a fuse. It needs no earth wire, but the fuse still protects its circuit and its cabling.
Why students lose marks here
Where this fits in IGCSE Physics
This page is one topic of Cambridge IGCSE Physics 0625. IGCSE Physics is the course home: every area and every topic in Cambridge's own order, the papers you sit and what each one tests, and the exam-practice tools. The full topic list, with each topic linked to its revision page, is on the IGCSE Physics syllabus page.