Electrical Safety, Fuses and Earthing
Mains electricity is dangerous, and the syllabus expects you to know which wire does what, how a fuse protects a circuit, and why earthing works.
1. The three wires
| Wire | Colour | Function |
|---|---|---|
| Live (L) | brown | carries the high voltage from the supply — the dangerous one |
| Neutral (N) | blue | completes the circuit, at approximately 0 V |
| Earth (E) | green and yellow stripes | safety wire — carries current away if a fault occurs |
A useful memory aid: look at the initials. Tutors suggested exactly this — L for Live, N for Neutral, E for Earth, and match them to the colours.
The LIVE wire is the dangerous one — it is at high voltage relative to earth. A recorded error identified a metal pipe as the electrocution risk rather than the live wire; the pipe is only dangerous if a live wire touches it.
In the UK, mains voltage is about 230 V a.c. at 50 Hz — check the values used in your own region.
2. The fuse
A fuse is a thin wire that melts and breaks the circuit if the current exceeds its rating.
How it works:
- A fault causes a larger than normal current
- The fuse wire heats up (P = I²R)
- It melts, breaking the circuit
- Current stops, preventing fire and further damage
The fuse must be in the LIVE wire. This is essential and frequently examined: if the fuse were in the neutral wire, it would still break the circuit, but the appliance would remain connected to the live supply and therefore still dangerous to touch. Uncertainty about which wire the fuse connects to was recorded.
Choosing a fuse rating:
- Calculate the normal operating current: I = P / V
- Choose the next standard fuse value ABOVE it (commonly 3 A, 5 A, 13 A)
Example: a 920 W appliance on 230 V.
- I = 920 ÷ 230 = 4 A
- Choose a 5 A fuse
The fuse must be rated slightly ABOVE the normal current — too low and it blows in normal use; too high and it won’t blow when it should.
A smaller current does not by itself make an appliance safe. A recorded error suggested this; safety comes from the fuse rating being appropriate and from earthing, not simply from low current.
3. Earthing
The earth wire connects the appliance’s metal case to the ground.
How it protects you:
- If the live wire comes loose and touches the metal case, the case becomes live
- The earth wire provides a path of very low resistance to the ground
- A large current flows through it
- This large current blows the fuse, disconnecting the supply
- The case is now safe
Earthing and the fuse work TOGETHER. The earth wire creates the large current; the fuse then breaks the circuit. An answer describing only one of the two is incomplete — say that the earth causes a surge which blows the fuse.
Earthing is needed for appliances with METAL cases. Without it, a fault would leave the case live and anyone touching it would provide the path to earth.
4. Circuit breakers
A circuit breaker is an automatic switch that trips when the current is too high.
Advantages over a fuse:
- Can simply be reset rather than replaced
- Operates faster
- More accurate at its trip current
An RCCB / RCD compares the current in the live and neutral wires. If they differ — meaning current is leaking to earth, possibly through a person — it cuts off the supply very quickly. It works even when the current is too small to blow a fuse.
5. Double insulation
A double-insulated appliance has a plastic (insulating) case, so no external metal part can ever become live.
Such appliances need no earth wire and are marked with a double-square symbol.
6. Other hazards and precautions
Hazards:
- Damaged insulation — exposed live wires
- Overheating cables — too much current, or a coiled cable
- Damp conditions — water conducts, providing a path to earth
- Overloaded sockets / long extension leads
Precautions:
- Switch off and unplug before repairs
- Never handle plugs with wet hands
- Use correctly rated fuses
- Replace frayed cables
- Keep water away from sockets
Why is water dangerous? It conducts electricity, so it can complete a circuit from a live conductor through you to earth.
7. Mistakes that cost marks
Putting the fuse in the neutral wire.
Not knowing the wire colours.
Saying the neutral or earth wire is the dangerous one.
Explaining earthing without mentioning the fuse blowing.
Choosing a fuse rating below the operating current.
Saying a low current alone makes an appliance safe.
Forgetting that double-insulated appliances need no earth.
Vague hazard answers without saying why they’re dangerous.
Frequently asked questions
What are the three wires and their colours? Live (brown), neutral (blue) and earth (green and yellow).
Which wire is dangerous? The live wire — it is at high voltage.
How does a fuse work? A large current heats and melts the thin wire, breaking the circuit.
Which wire must the fuse be in? The live wire — otherwise the appliance stays connected to the live supply.
How do I choose a fuse rating? Calculate I = P/V, then take the next standard value above it.
How does earthing protect you? It gives a low-resistance path for a fault current, causing a large current that blows the fuse.
What is a circuit breaker? An automatic switch that trips on excess current and can be reset.
What is double insulation? A plastic case with no exposed metal, so no earth wire is needed.
Why is water dangerous with electricity? It conducts, completing a circuit to earth through you.
What does an RCD do? Detects a difference between live and neutral currents and cuts off the supply quickly.
Quick revision checklist
- I know the three wires, their colours and their functions
- I know the live wire is the dangerous one
- I can explain how a fuse works
- I know the fuse goes in the live wire, and why
- I can choose a fuse rating from I = P/V
- I can explain earthing as a full sequence ending in the fuse blowing
- I know earthing is for metal-cased appliances
- I know the advantages of circuit breakers
- I know what an RCD does
- I can explain double insulation
- I can list hazards and why each is dangerous
These notes cover electrical safety, fuses and earthing in the Cambridge IGCSE Physics (0625) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Physics lessons, with particular attention to the errors students make most often and the wording examiners reward. Mains voltage and plug wiring conventions vary by country — check the values used in your own syllabus and region.
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