4.3.6Electricity and Magnetism
Electrical Safety — Cambridge IGCSE Co-ordinated Sciences 0654 - Physics Revision Notes & Topical Questions
5 min read 9 parts Lesson 24 of 31
Section 1 of 9Electrical hazards
Mains danger
- Mains danger voltages as low as 50 V can pose a serious risk of electrocution
Damaged insulation
- Damaged insulation the metal of the wire becomes exposed
- Touching it can give a lethal electric shock.
- Current can also flow between the exposed wire and any metal it touches.
Overheating cables
- Overheating cables an excess current in a wire makes it overheat
Damp conditions
- Damp conditions water is a conductor
- Moisture on a live wire can cause a short circuit inside the device, which can start a fire.
- It can also give a conductive path through a person to earth, causing electrocution.
Overloading
- Overloading too many plugs, extension leads or sockets on one mains supply draws an excessive current
- The heat can melt the insulation or cause a fire.
Section 2 of 9Mains circuits
- Three wires
- live, neutral and earth
| Wire | Colour | Purpose |
|---|---|---|
| Live | Brown | Carries the alternating p.d. from the supply; the most dangerous wire |
| Neutral | Blue | Forms the opposite end of the circuit to complete it; at a lower p.d. so much less dangerous |
| Earth | Green and yellow stripes | Safety wire that stops the appliance becoming live |
Section 3 of 9Double insulation
1Wire insulation
Wire insulation the metal conductor is covered with an insulator such as rubber so it cannot be touched
2Double-insulated appliance
- Double-insulated appliance has two layers of insulation
- The first layer is the insulation around the wires.
- The second layer is a non-metallic case, so there is no risk of the case becoming live.
- It does not need an earth wire, or the earth wire is designed so that it cannot touch the metal casing.
Section 4 of 9Earthing
Earthed metal case
- Earthed metal case many appliances have a metal case that could become live
- If a live wire touches the case, anyone touching it risks electrocution.
Earth wire action
- Earth wire action gives a low-resistance path from the case to earth
- A large surge of current flows in the earth wire and in the live wire.
- The high current melts the fuse and breaks the circuit.
Section 5 of 9Fuses
Fuse
- Fuse a thin metal wire, usually in a glass cylinder, fitted in the plug to protect an individual appliance
- If the current becomes too large the wire heats up and melts, breaking the circuit.
- The fuse is fitted in the live wire so that the appliance is disconnected from the high p.d. when it blows.
- A fuse protects the circuit and cabling of a double-insulated appliance that has no earth wire.
Fuse rating
Fuse rating the maximum current that can flow through the fuse wire without melting
- Fuses come in values of 3 A, 5 A and 13 A.
- The correct fuse is the value just above the current needed by the appliance.
Wrong rating
- A fuse with a rating below the working current blows as soon as the appliance is switched on.
- A fuse with a rating far too high lets a large current flow before it blows, so it gives little protection.
Section 6 of 9Trip switches
Consumer unit
- Consumer unit where the current enters the house, and it contains a series of trip switches
- (circuit breakers)
- Each trip switch controls the current supplied to one circuit in the house.
Trip switch action
- Trip switch action when the current is too high the switch trips and flicks to off, stopping the current in that circuit
Advantages of trip switches
- They protect against current surges and faults.
- They can be reset when the problem is fixed.
Section 7 of 9More from the specification
More from the specification
Schematic circuit representation
- Schematic circuit representation using standard circuit symbols provides an unambiguous graphic means to describe electrical component configurations.
Series and parallel circuits
- Series and parallel circuits in series the current is the same everywhere, in parallel the current splits at a junction between branches
Potential difference
- Potential difference in parallel, the voltage across each branch is equal to that of the source;
- in series the voltage is shared across components
Circuit components
- Circuit components ammeters are in series, voltmeters are connected across a component in parallel
Power and safety
- Power and safety power equals current times voltage, and fuses and earth wires protect from excess current from the source
Series and parallel circuits
- Series and parallel circuits in series the current is the same everywhere, in parallel the current splits at a junction between branches
Potential difference
- Potential difference in parallel, the voltage across each branch is equal to that of the source;
- in series the voltage is shared across components
Circuit components
- Circuit components ammeters are in series, voltmeters are connected across a component in parallel
Power and safety
- Power and safety power equals current times voltage, and fuses and earth wires protect from excess current from the source
Potential difference
- The work done by a unit charge passing through a component.
Potential divider
- Two resistors in series across a supply that share the supply p.d. in the ratio of their resistances.
Circuit diagram
- A diagram using symbols to show a circuit.
Component
- A part of a circuit such as a lamp or buzzer.
Source
- A part of the plant where substances are made or released.
Branch
- One of the paths in a parallel circuit.
Junction
- A point where branches meet; the current entering equals the current leaving.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Live wire | The brown wire that carries the alternating p.d. from the supply; the most dangerous wire. |
| Neutral wire | The blue wire that completes the circuit and is at a much lower p.d. than the live wire. |
| Earth wire | The green and yellow wire that provides a low-resistance path to earth if a metal case becomes live. |
| Fuse | A thin wire in the live wire that melts and breaks the circuit if the current exceeds its rating. |
| Fuse rating | The maximum current a fuse can carry without melting. |
| Trip switch (circuit breaker) | An automatic switch that opens when the current is too high and can be reset. |
| Double insulation | Two layers of insulation, including a non-metal case, so no earth wire is needed. |
| Short circuit | A very low-resistance connection between points at different p.d. that causes a very large current. |
Section 9 of 9Exam technique
1
Putting the fuse in the neutral wire — it must be in the live wire so that a blown fuse disconnects the appliance from the high p.d.
2
Choosing a fuse rating below the working current — it would blow in normal use, so the rating is the next standard value above the current
3
Thinking the earth wire carries current in normal use — it carries current only when a fault makes the case live
4
Thinking double-insulated appliances need an earth wire — their non-metal case cannot become live
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