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

Know that a mains circuit consists of a live (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.

Components of a Mains Supply

A typical single‑phase domestic mains supply in the UK (and many other countries) has three conductors:

• Live (Line) wire – carries the alternating voltage from the supply. It is the “hot” conductor.
• Neutral wire – provides the return path for current and is connected to earth at the transformer, keeping it at (or near) zero volts relative to ground.
• Earth (Ground) wire – a safety conductor that provides a low‑resistance path to ground for any fault current, helping to protect users from electric shock.

Standard Colour Coding (UK)

Why the Switch Must Be on the Live Wire

When a switch is opened, the circuit is broken and current can no longer flow. If the switch were placed on the neutral conductor, the live conductor would remain permanently connected to the appliance or wiring downstream of the switch. This creates several hazards:

1. Risk of electric shock – The exposed parts of the appliance or wiring would still be at the full mains voltage relative to earth. Anyone touching them could receive a dangerous shock.
2. Fault protection failure – Protective devices such as fuses or circuit breakers rely on the fault current returning via the live conductor. If the live is always present, a fault may not be detected correctly.
3. Unintentional energising of equipment – Maintenance or cleaning work performed on a “switched‑off” circuit could still be hazardous because the equipment is still live.

Placing the switch on the live wire ensures that when the switch is in the OFF position, the entire downstream circuit, including the appliance and its exposed parts, is at (or very close to) earth potential. The neutral conductor, being at earth potential, does not present a shock hazard.

Illustrative Diagram

Key Points to Remember

• The live wire carries the full mains voltage; the neutral is near earth potential.
• The earth wire is solely for safety and does not normally carry current.
• All switches and circuit‑breaking devices must be connected to the live conductor to guarantee a safe OFF condition.
• Never assume a circuit is safe simply because a switch is OFF; always verify that the switch is on the live side.

Example Calculation (Optional)

If a faulty appliance draws a leakage current of \$I_{\text{leak}} = 0.5\ \text{A}\$ to earth, the earth wire must be able to carry this safely. The resistance of a typical earth conductor is low; using Ohm’s law, the voltage rise on the earth conductor is:

\$ V{\text{earth}} = I{\text{leak}} \times R_{\text{earth}} \$

Assuming \$R_{\text{earth}} = 0.1\ \Omega\$,

\$ V_{\text{earth}} = 0.5\ \text{A} \times 0.1\ \Omega = 0.05\ \text{V} \$

This small voltage confirms that the earth path effectively limits the shock hazard.