(i) Describe, with a diagram, the domestic electric circuit used in homes, identifying the live wire, neutral wire, and earth wire by their insulation colours and stating the potential difference between live and neutral wires.
(ii) Why are appliances with metallic bodies connected via a three-pin plug, but an ordinary electric bulb uses only a two-pin plug?
(iii) State two causes of overloading in a domestic circuit and explain how a fuse protects the circuit in such a situation.
Generated by claude-sonnet-4-6 · 2026-06-26 01:11 · grounding rag
Model Answer
(i) In domestic circuits, power is supplied through three wires:
- Live wire — red insulation (positive)
- Neutral wire — black insulation (negative)
- Earth wire — green insulation (connected to a metal plate buried in earth)
The potential difference between live and neutral wires is 220 V.
Diagram:
```
Live (Red) ──┬── Switch ── Appliance ──┐
│ │
Neutral(Blk)─┴─────────────────────────┘
Earth (Grn) ── Metallic body of appliance
```
Appliances are connected in parallel so each gets equal potential difference.
(ii) Appliances with metallic bodies (e.g., electric press, refrigerator) are earthed via the third pin. If current leaks to the metallic body, the earth wire provides a low-resistance path, keeping the body's potential equal to earth's, preventing electric shock. An ordinary bulb has no metallic body to touch, so earthing is unnecessary; a two-pin plug suffices.
(iii) Two causes of overloading:
- Short circuit — live and neutral wires touch directly (damaged insulation or faulty appliance), causing current to increase abruptly.
- Too many appliances connected to a single socket, or an accidental hike in supply voltage.
Fuse protection: The fuse wire has low melting point. When excess current flows, Joule heating melts the fuse wire, breaking the circuit and protecting appliances from damage.
Source: Chapter 12, Section 12.4 — Domestic Electric Circuits
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Explanation
- Part (i): Examiners expect the three wire colours, the 220 V figure, and a neat labelled diagram showing parallel connections. Mention the meter board/main fuse if space allows.
- Part (ii): The key phrase is "low-resistance conducting path" keeping the metallic body at earth potential. Contrast with a bulb — no exposed metallic part the user touches.
- Part (iii): Name both causes clearly (short circuit + excess appliances/voltage hike). For the fuse, mention Joule heating and melting — these are the two marks-earning terms examiners look for.