Know that a mains circuit consists of a live wire (line wire), a neutral wire and an earth wire and explain why a switch must be connected to the live wire for the circuit to be switched off safely

4.4 Electrical Safety

Objective

Students will be able to:

  • Define the three conductors of a single‑phase mains supply – live (line) conductor, neutral conductor and earth (ground) conductor – using the exact terminology from the Cambridge IGCSE 0625 syllabus.

  • Identify the four main hazards associated with a mains supply.

  • Explain why any switch or circuit‑breaking device must be placed in the live conductor for the circuit to be switched off safely.

  • Describe the operation of protective devices (fuse, MCB, RCD) and select an appropriate fuse rating for a given load.

  • Distinguish between double‑insulated and earthed appliances and explain why a fuse alone protects a double‑insulated appliance.

  • Apply this knowledge in a short practical activity (identifying conductors and safety breaches on a wiring diagram).

1. Components of a Single‑Phase Mains Supply

A typical domestic single‑phase supply (≈230 V r.m.s. in the UK and many other countries) consists of three conductors:

Conductor (syllabus term)Common nameFunctionTypical colour (UK)
Live (line) conductorLive wireCarries the alternating voltage from the supply; always at full mains potential.Red (pre‑2021) / Brown (post‑2021)
Neutral conductorNeutral wireProvides the return path for current; bonded to earth at the supply transformer (the “point of earthing”), so its potential is essentially earth potential throughout the installation.Black (pre‑2021) / Blue (post‑2021)
Earth (ground) conductorEarth wireSafety conductor that carries fault current to earth; it does not normally carry load current.Green‑yellow striped (both periods)

Note: Colour‑coding varies internationally (e.g. IEC 60446 uses brown‑live, blue‑neutral, green‑yellow earth). The three‑wire principle – live, neutral, earth – is universal.

2. Main Hazards of a Mains Circuit (AO 1)

HazardWhat can happen?
Damaged insulationLive conductor becomes exposed → risk of electric shock or short‑circuit.
Over‑heating of cablesExcess current raises conductor temperature → insulation melts, fire risk.
Damp or wet conditionsWater lowers skin resistance and can create unintended conductive paths → increased shock hazard.
Excess current (over‑load)Too many appliances or a fault draw more current than the cable/fuse rating → protective device must act.

3. Why the Switch (or Any Circuit‑Breaking Device) Must Be on the Live Conductor

  1. Eliminates voltage from downstream parts – When the switch opens, the live conductor is disconnected; all exposed parts downstream are at (or very close to) earth potential.

  2. Prevents electric shock – If the switch were on the neutral side, the live conductor would remain permanently connected to the appliance and its metal parts, leaving them at full mains voltage.

  3. Ensures correct operation of protective devices – Fuses, MCBs and RCDs rely on the fault current flowing through the live conductor. A live‑side switch guarantees that a fault will be cleared when the device is opened.

  4. Safe maintenance – Technicians can be confident that a circuit marked “OFF” is truly de‑energised.

All single‑pole switches, isolators, fuses, miniature circuit breakers (MCBs) and residual‑current devices (RCDs) are therefore installed in the live conductor.

3.1 Protective Devices and Their Placement

  • Fuse – A metal strip that melts when the current exceeds its rating, permanently opening the live circuit.

  • Miniature Circuit Breaker (MCB) – A resettable device that trips magnetically (high‑instantaneous current) or thermally (over‑load) on the live side.

  • Residual‑Current Device (RCD) – Continuously monitors the difference between live and neutral currents. If the imbalance exceeds ≈30 mA, it trips within 30 ms, protecting against earth‑leakage shock.

3.2 Linking Hazards to Protective Devices (AO 2)

HazardProtective device that mitigates it
Damaged insulation → possible short‑circuitFuse / MCB (opens live circuit)
Over‑heating (over‑load)Fuse / MCB (trips before temperature reaches dangerous levels)
Damp/wet conditions → earth‑leakageRCD (detects imbalance and trips quickly)
Excess current from faultFuse / MCB (opens live) + RCD (if fault involves earth)

4. Worked Example – Selecting a Fuse Rating (AO 2)

Suppose a circuit supplies a single 2 A lamp from a 13 A fused spur. Choose an appropriate fuse for a new circuit that will power a 4 A heater together with the lamp.

  1. Calculate total normal load: 2 A (lamp) + 4 A (heater) = 6 A.

  2. Choose a fuse rating higher than the normal load but lower than the cable’s safe‑current rating. A common practice is to select a fuse ≈1.25 × normal load.

  3. 1.25 × 6 A = 7.5 A → round up to the next standard rating → 10 A fuse.

  4. Because the circuit is protected by a 13 A MCB upstream, the 10 A fuse will open first if the load exceeds 10 A, preventing the MCB from tripping unnecessarily.

5. Appliance Design: Double‑Insulated vs. Earthed

  • Double‑insulated appliances – Have two independent layers of insulation; the outer casing is non‑conductive. Consequently, there is no need for an earth connection. A fuse in the live conductor protects the appliance because any internal fault that creates a short will cause the fuse to open, removing the live voltage.

  • Earthed appliances – Contain exposed conductive parts (metal casings). An earth wire is attached to the casing so that, if the live conductor faults to the case, a large fault current flows to earth, causing the protective device (fuse/MCB/RCD) to trip almost instantly.

6. Practical Activity – Identifying Conductors and Safety Breaches (AO 3)

Materials: printed wiring diagram (or a simple schematic on a sheet), coloured markers, a worksheet.

  1. Label the three conductors (live, neutral, earth) on the diagram using the correct colour conventions.

  2. Locate the switch, fuse/MCB and any RCD. Verify that each is drawn in the live conductor.

  3. Identify any safety breach (e.g., switch drawn on neutral, missing earth connection to a metal‑cased appliance, or an unprotected overload).

  4. For each breach, write a short explanation of the risk and how it should be corrected.

Extension*: Using a low‑voltage (≤12 V) DC supply, connect a lamp through a switch placed on the neutral side and another switch on the live side. Observe that the lamp remains “live” (measurable voltage) when the neutral‑side switch is open, reinforcing the theoretical explanation.

7. Putting It All Together – Safe Circuit Design

  1. The live conductor is switched or protected on the supply side.

  2. The neutral conductor is bonded to earth at the transformer (the “point of earthing”) and therefore stays at earth potential throughout the installation.

  3. The earth conductor provides a low‑impedance path for fault currents only; it does not normally carry load current.

  4. All protective devices (fuse, MCB, RCD, isolator) are installed in the live conductor.

  5. Appliances are either double‑insulated (no earth required) or have a reliable earth connection.

  6. Each of the four hazards is mitigated by the appropriate protective device (see Table 3.2).

8. Illustrative Diagram

Simple UK mains circuit – live (brown), neutral (blue), earth (green‑yellow). Shows a single‑pole switch, a 13 A fuse/MCB, an RCD, and either a double‑insulated lamp or an earthed metal‑cased kettle.

9. Key Points to Remember

  • Live conductor carries full mains voltage; it must be interrupted to make a circuit safe.

  • Neutral conductor is bonded to earth at the transformer and is effectively at earth potential.

  • Earth conductor is a safety path only; it carries current only when a fault occurs.

  • All switches and circuit‑breaking devices are placed in the live conductor so that the OFF position removes voltage from downstream parts.

  • Four main hazards: damaged insulation, overheating, damp conditions, excess current – each is mitigated by a specific protective device.

  • Double‑insulated appliances are protected by a fuse on the live side; earthed appliances need the earth conductor to ensure a fault trips the protective device quickly.

  • RCDs detect a live‑neutral current imbalance (≥30 mA) and trip within 30 ms, providing vital protection against earth‑leakage shock.