Explain why the outer casing of an electrical appliance must be either non-conducting (double-insulated) or earthed

4.4 Electrical Safety

Learning Objective

Explain why the outer casing of an electrical appliance must be either non‑conducting (double‑insulated) or protectively earthed.

1. Why the Casing Must Never Become Live

• If a live part touches the outer casing, the casing can rise to the supply voltage (≈ 230 V AC in the UK).
• A person touching the casing then provides a path to earth, so a current flows through the body.
• Shock severity depends on voltage, body resistance and fault duration.

  • ≈ 1 mA – tingling
  • ≈ 5–10 mA – painful muscular contraction
  • ≥ 30 mA – can be fatal

2. Hazards Associated with the Mains Supply (Core)

3. The Three‑Wire Mains System (Core)

• Live (L) – brown: carries the supply voltage (≈ 230 V AC).
• Neutral (N) – blue: returns current to the supply and is at earth potential.
• Earth (E) – green‑yellow: provides a low‑impedance path for fault currents.

The switch in a domestic circuit must be placed on the live conductor so that, when the switch is off, the appliance is completely de‑energised even if the neutral remains connected.

4. Over‑Current Protection (Core)

• Fuse: a thin metal strip that melts when the current exceeds its rating, opening the circuit.
• Circuit‑breaker (trip‑switch): a spring‑loaded mechanism that trips mechanically when the magnetic field from an over‑current reaches a set value.
• Rating selection – the fuse or breaker is chosen so that its rating is ≤ the maximum safe current for the appliance’s wiring (e.g., a 3 A fuse for a low‑power hand‑held tool).
• Both devices clear a fault quickly, preventing the casing from staying live for long.

5. Two Safe Design Approaches (Core)

5.1 Double‑Insulated (Non‑Conducting) Appliances

• All live parts are surrounded by two independent layers of insulation. If the first layer fails, the second still prevents the live part from reaching the outer case.
• No protective‑earth connection is required.
• Typical for handheld tools, hair dryers, electric shavers, etc.
• Identified by the double‑insulation symbol – a square within a square (□ □).

5.2 Protectively Earthed Appliances

• The outer case is conductive (usually metal) and is permanently linked to earth via the protective‑earth (PE) conductor.
• If a live part contacts the case, a large fault current flows through the PE conductor to earth.
• The high fault current causes the fuse or circuit‑breaker to operate almost instantly, disconnecting the supply.
• Because the earth path has a very low resistance (< 0.1 Ω), the voltage that can appear on the case is limited to < 50 V, a level that is not hazardous.
• Identified by the earth symbol (⏚).

6. Comparison of Double‑Insulation and Protective Earthing (Core)

7. Symbols Used on Appliances (Core)

8. How Earthing Limits the Voltage on the Casing (Core Idea)

When a fault occurs, the PE conductor provides a very low‑resistance path (typically ≤ 0.1 Ω). The resulting fault current is large, so the protective device trips almost instantly. Because the earth path is so low‑impedance, the voltage that can appear on the case is kept below the hazardous limit of 50 V.

Extension – Quantitative Example (Supplement)

9. Key Points to Remember (Core)

• The outer casing must never become a source of live voltage.
• Two ways to achieve this:

  • Double‑insulation – two independent insulating barriers; no earth required.
  • Protective earthing – a low‑impedance earth path forces the protective device to open the circuit quickly and limits the case voltage to < 50 V.

• Identify the safety method by the symbols: ⏚ for earth, □ □ for double‑insulation.
• Over‑current protective devices (fuses, circuit‑breakers) are the final safeguard that disconnects the supply when a fault occurs.