Two safety devices used in domestic electric circuits are the electric fuse and the earth wire. Answer the following: (i) State the specific hazard each device is designed to protect against. (ii) Explain the physical principle by which each device operates when the hazard occurs. (iii) A student argues that if a circuit has a fuse, an earth wire is unnecessary. Do you agree? Justify your answer with a specific scenario in which the fuse alone would fail to prevent an electric shock.
Generated by claude-sonnet-4-6 · 2026-06-26 01:13 · grounding rag
Model Answer
(i) Hazard each device protects against:
- Electric fuse protects against overloading and short-circuiting — situations where excess current could damage the circuit or cause fire.
- Earth wire protects against electric shock due to leakage of current to the metallic body of an appliance.
(ii) Physical principle of operation:
- Fuse: When current exceeds the rated value, Joule heating melts the fuse wire, breaking the circuit and stopping current flow.
- Earth wire: It provides a low-resistance path to the earth for any leaked current, keeping the metallic body at earth potential so the user does not receive a shock.
(iii) Disagreement with the student's argument:
No, the earth wire is not made unnecessary by a fuse. Consider a scenario where the live wire inside a refrigerator loosens and touches its metallic body — current leaks to the body, but this leakage current is too small to blow the fuse. A person touching the refrigerator would receive a severe shock. The earth wire, however, immediately provides a safe conducting path, preventing this shock. The fuse alone cannot protect against such leakage.
Source: Chapter 12, Section 12.4 — Domestic Electric Circuits
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Explanation
- Examiners expect two distinct roles clearly separated — fuse = overcurrent/fire protection; earth wire = shock protection from leakage.
- The key physics terms to use: Joule heating (fuse), low-resistance conducting path / earth potential (earth wire).
- For part (iii), the critical insight is that leakage current to a metallic body is usually too small to blow a fuse but large enough to electrocute — this is the scenario that earns full marks. Many students lose marks by only saying "they protect against different things" without giving a concrete scenario.