Electrical Safety – Topic 4.4 (Cambridge IGCSE / A‑Level)
1. Mains‑supply basics
1.1 Conductors and colour‑coding
| System | Live (phase) | Neutral | Earth (protective conductor) |
|---|
| UK (BS 7671) | Brown (old: red) | Blue (old: black) | Green‑yellow striped |
| EU (IEC) | Brown (or black for 3‑phase) | Blue | Green‑yellow striped |
| US (NEC) | Black (or red for second phase) | White | Green or bare copper |
The switch in a plug‑in appliance must always break the live conductor so that the appliance is de‑energised when the switch is off.
1.2 Class I vs. Class II appliances
| Class | Construction | Safety principle | Typical examples | Plug marking |
|---|
| Class I (earthed) | Single insulation + protective earth conductor | Live parts are earthed; fault current is diverted to earth | Kettle, washing‑machine, toaster | Earth pin present; “E” or earth symbol |
| Class II (double‑insulated) | Double or reinforced insulation, no earth connection | Insulation alone prevents access to live parts | Hair‑dryer, cordless drill, phone charger | No earth pin; “II” or double‑square symbol |
Important exam point: Class II appliances have no earth pin. A fuse in a circuit that powers a double‑insulated appliance protects the cable and the circuit, not the appliance itself.
2. Hazards associated with mains electricity
2.1 Damaged insulation
- Exposes live conductors → risk of electric shock.
- Provides a low‑resistance path to earth or to other conductive parts → short‑circuit.
- Short‑circuit can produce sparks, fire, or damage to equipment.
- Fault current may bypass protective devices, reducing their effectiveness.
2.2 Over‑heating cables
- Excess current raises cable temperature; if it exceeds the insulation’s thermal rating the sheath melts.
- Molten insulation exposes conductors → shock hazard.
- Heat can ignite surrounding combustible material → fire.
- Thermal degradation increases resistance, causing further heating – a self‑reinforcing cycle.
- Visible signs: discoloration, burnt smell, warm‑to‑touch, softened or cracked sheath.
2.3 Damp or wet conditions
- Water is a good conductor; moisture on equipment, hands or surfaces reduces resistance.
- Increases the likelihood of electric shock when touching live parts or a grounded metal.
- Moisture accelerates corrosion of contacts → poor connections and localized heating.
- Residual‑current devices (RCDs) give extra protection in wet locations but do not replace basic precautions.
2.4 Excess current from over‑loading (plugs, extension leads, single‑ and multiple‑socket units)
- Connecting more devices than the rating allows forces a current > rated value.
- Resulting heating can melt insulation, deform sockets, or ignite nearby material.
- If the protective device (fuse or circuit‑breaker) is correctly rated it will trip; an undersized device may trip unnecessarily, while an oversized device may fail to protect.
- Repeated thermal stress shortens the service life of both the supply unit and the connected equipment.
- Over‑heated components can damage insulation, creating a new shock hazard.
What the exam asks you to state
• List the three main hazards of a damaged mains cable.
• Explain two consequences of an overheated extension lead.
• State why damp conditions increase the risk of electric shock.
• Describe one effect of over‑loading a socket strip and one preventive measure.
3. Protective devices – operation and selection
3.1 Fuses
- Contain a thin metal strip that melts when current exceeds its rating (e.g., 13 A).
- Melting opens the circuit, stopping the flow of current.
- Rating must match the maximum safe current for the cable and the socket circuit.
- Using a higher‑rated fuse defeats protection; a lower‑rated fuse may trip during normal use.
3.2 Circuit‑breakers (trip switches)
- Thermal or magnetic devices that open the circuit when current exceeds a preset value.
- Can be reset after the fault is cleared, unlike a fuse which must be replaced.
- Same rating principles as fuses – choose a breaker whose rated current ≤ the rating of the wiring it protects.
3.3 Residual‑Current Devices (RCDs) / Earth‑Leakage Circuit Breakers (ELCBs)
- Detect an imbalance between live and neutral currents (typically 30 mA for personal protection).
- Trip within ≈30 ms, cutting off supply and greatly reducing the risk of fatal shock.
- Essential in wet locations (kitchens, bathrooms) and for portable equipment.
4. Practical safety checklist (visual inspection & basic testing)
- Plug pins – no deformation, cracks, or missing pins.
- Cable sheath – no cuts, abrasions, kinks, discoloration or signs of overheating.
- Earth pin present on Class I appliances; absent on Class II (as required).
- Feel the cable (carefully) after use – it should be only slightly warm.
- Ensure sockets are dry; never operate equipment with wet hands or in damp environments.
- Check total load on a socket or extension lead does not exceed its rating (e.g., 13 A for a standard UK socket).
- Test continuity of earth conductors with a low‑voltage tester where required.
- Use appropriate PPE – insulated gloves, safety glasses – when working on live circuits.
5. Summary of hazards, consequences and preventive measures
| Hazard source | Potential consequences | Key preventive measures |
|---|
| Damaged insulation | Electric shock, short‑circuit, fire | Regular visual inspection; replace damaged cables; use protective sleeving; keep live parts inaccessible. |
| Over‑heating cables | Melting, fire, equipment damage | Do not exceed rated current; avoid tight coils; provide ventilation; replace cables showing discoloration, burnt smell or deformation. |
| Damp conditions | Increased shock risk, corrosion, localized heating | Keep equipment dry; use RCDs in wet areas; wear insulated gloves if moisture unavoidable; dry hands before handling live parts. |
| Excess current (over‑loading) | Heating, fuse/breaker failure, fire | Observe load ratings; distribute load over several sockets; use correctly rated fuses or circuit‑breakers; avoid daisy‑chaining extension leads. |
| Improper protective devices | Inadequate fault protection, possible fire or shock | Select fuses/breakers matching cable rating; replace blown fuses with the same rating; test RCDs monthly. |
6. Suggested classroom diagram
Illustrate the following in a single figure (labelled):
- Plug with a cut in the insulation (show exposed conductor).
- Overheated extension lead – hot spot, melted sheath.
- Wet socket – droplet symbol on the socket face.
- Current paths: live → appliance → earth (for Class I) and live → neutral (for Class II).
- Location of the protective earth, fuse, and RCD in the circuit.