4.4Electricity and Magnetism
Electrical Safety — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
5 min read 10 parts Lesson 42 of 65 1 quiz
Lesson summary
4.4 Electrical Safety — IGCSE PHYSICS notes: State the hazards of mains electricity, describe the live, neutral and earth wires and explain how fuses, circuit breakers, earthing and double insulation protect users.Danger level — voltages as low as 50 V can pose a serious risk of …
- Damaged insulation
- Overheating cables
- Damp conditions
- Overloading
- Other countries
- Construction
Part 1 of 8Electrical hazards
Danger level
- Danger level voltages as low as 50 V can pose a serious risk of electrocution.
Damaged insulation
- Damaged insulation exposed metal can give a lethal shock or let current flow into other metal objects.
Overheating cables
- Overheating cables excess current can melt the insulation or cause a fire.
Damp conditions
- Damp conditions water conducts, so moisture on a live wire can cause a short circuit or let current pass through a person to earth.
Overloading
- Overloading too many plugs, extension leads or sockets on the mains draw an excessive current and heat the cables.
Part 2 of 8The mains circuit
| Wire | Colour | Purpose |
|---|---|---|
| Live | Brown | Carries the alternating p.d. from the mains to the circuit; the most dangerous wire |
| Neutral | Blue | Completes the circuit; at a lower voltage so much less dangerous |
| Earth | Green and yellow stripes | Safety wire that stops the appliance from becoming live |
Position
- Position switches and fuses must be in the live wire, so that when they open the appliance is disconnected from the high p.d.
Other countries
- Other countries two-pin plugs still have an earth connection, just more hidden.
Part 3 of 8Fuses
Construction
- Construction a glass cylinder containing a thin metal wire, fitted in the plug.
Action
- Action if the current is too large the wire heats up and melts, breaking the circuit.
Rating
- Rating the maximum current the fuse wire can carry; fuses come as 3 A, 5 A and 13 A.
Symbol
- Symbol a rectangle with a wire running through it.
Must be replaced
- Must be replaced a blown fuse cannot be reset.
Part 4 of 8Choosing a fuse (Extended)
1
Find the power rating of the appliance and the mains p.d.
2
Calculate the normal current with I = P / V.
3
List the available fuse ratings.
4
Choose the smallest rating greater than the normal current.
5
Fit the fuse in the live wire.
Too low
- Too low it blows as soon as the appliance is switched on.
Too high
- Too high it lets too much current through before blowing.
Example
- Example an appliance using 3.1 A needs a 5 A fuse.
Part 5 of 8Trip switches (circuit breakers)
Action
- Action a trip switch automatically switches off when the current exceeds a set value.
Consumer unit
- Consumer unit the current enters the house here, and trip switches control the current to each circuit.
Advantages
- Advantages they protect against surges and faults, and they can be reset when the problem is fixed.
Part 6 of 8Earthing
Purpose
- Purpose the earth wire connects a metal case to earth.
Fault
- Fault if the live wire touches the case, the case becomes live.
Action
- Action a large current flows through the low-resistance earth wire, which melts the fuse.
- (or trips the breaker)
Result
- Result the supply is cut off, which stops electric shocks.
Part 7 of 8Double insulation
Two layers
- Two layers insulation around the wires and a non-metallic case.
No earth wire needed
- No earth wire needed the case cannot become live because there is no exposed metal.
Fuse
- Fuse it still protects the circuit and the cabling.
Part 8 of 8More from the specification
More from the specification
Circuit breaker electrical safety
- Circuit breaker electrical safety installing residual current devices and miniature circuit breakers provides a protective means to interrupt dangerous current surges automatically.
Fuse
- Fuse a thin wire that melts when current exceeds its rating, breaking the circuit and protecting the appliance and wiring
Earth wire
- Earth wire connects the metal case to the ground so a fault current flows safely away instead of shocking the user
Double insulation
- Double insulation plastic casings prevent contact with live parts so no earth wire is needed
Series and parallel
- Series and parallel in series the current is the same everywhere and voltages add;
- in parallel the voltage across each branch is equal and branch currents add at a junction
Fuse
- Fuse a thin wire that melts when current exceeds its rating, breaking the circuit and protecting the appliance and wiring
Earth wire
- Earth wire connects the metal case to the ground so a fault current flows safely away instead of shocking the user
Double insulation
- Double insulation plastic casings prevent contact with live parts so no earth wire is needed
Series and parallel
- Series and parallel in series the current is the same everywhere and voltages add;
- in parallel the voltage across each branch is equal and branch currents add at a junction
Potential difference
- Work done by a unit charge passing through a component.
Combined resistance
- Single resistance that could replace several resistors.
Potential divider
- Series resistors that share the supply p.d.
Circuit diagram
- Drawing of a circuit using standard symbols.
Components
- Perpendicular parts of a vector that add to give the vector.
Series
- Components in one loop, sharing the e.m.f. between them.
Parallel
- Components on separate branches, each with the full e.m.f.
Source
- Reservoir at TH providing heat QH
Branch
- Path connecting two junctions.
Junction
- Place where a pipe splits or joins; flow rates add.
Key terms
| Term | Meaning |
|---|---|
| Live wire | Brown wire that carries the alternating p.d. from the mains. |
| Neutral wire | Blue wire that completes the circuit. |
| Earth wire | Green and yellow safety wire. |
| Fuse | Thin wire that melts when the current is too large. |
| Circuit breaker | Trip switch that cuts off the current and can be reset. |
| Double insulation | Two layers of insulation so that no earth wire is needed. |
Avoid these mistakes
1
Putting the fuse in the neutral wire — it must be in the live wire.
2
Choosing a fuse far above the working current — it does not protect the flex.
3
Saying a fuse can be reset — a fuse must be replaced, a circuit breaker can be reset.
4
Believing damp conditions are safe because the voltage is low — water conducts and can cause shocks and short circuits.
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