Hazards of Mains Supply

Common hazards include:

• 1. Damaged Insulation: Can cause high currents to flow in unintended places, leading to electric shocks or fires.
• 2. Overheating Cables: Excessive current can cause cables to overheat, risking fires or damage to appliances and insulation.
• 3. Damp Conditions: Water reduces resistance to earth, causing short circuits and electric shocks. Devices used in damp areas must have high damp-proofing standards.
• 4. Overloading Plugs or Sockets: Plugging too many appliances into one outlet can cause excessive current, leading to overheating and fire risks.

House Circuits

A mains circuit consists of three wires:

• Live Wire: Carries the alternating current (a.c.) supply, with voltage varying by country (110 V to 240 V).
• Neutral Wire: Completes the circuit and is at 0 V relative to earth.
• Earth Wire: Provides a safety path to earth, also at 0 V.

All home circuits are connected in parallel across the live and neutral wires, allowing individual switching. A switch must be placed in the live wire to safely disconnect the appliance from the mains supply when turned off.

Fuses and Trip Switches

Fuses: A thin wire that melts and breaks when excessive current flows, protecting against shocks, fires, or damage. Fuses are placed in the live wire.

Trip Switches: Contain electromagnets that separate contacts and break the circuit when current is too high. They operate quickly and can be reset easily.

Choose the correct fuse rating or trip switch setting based on the maximum expected current. For example, a 13 A fuse or 10 A trip switch is suitable for a 9 A maximum current.

Earthing

Appliances with metal casings must be earthed using the earth wire to prevent electric shocks. If the live wire touches the metal casing, the earth wire provides a path for the current to flow to earth, causing the fuse to melt and disconnect the appliance.

Double Insulation

Appliances with plastic outer casings are double-insulated. Since plastic is an insulator, there is no risk of electric shock, and these appliances do not require an earth connection. However, they still have a fuse to protect against current surges.