electrical safety

4.4.1 hazards of electricity

damaged insulation

• exposes live wires
• can cause electric shock
• may lead to short circuits

overheating cables

• caused by excessive current
• insulation may melt
• can result in fire hazards

damp conditions

• water conducts electricity
• increases risk of electric shock
• electrical appliances become unsafe

overloading plugs and sockets

• excess current drawn from the supply
• plugs and cables may overheat
• common with extension leads and multi-sockets

4.4.2 mains electricity and wiring

a mains circuit contains three wires.

• live wire carries the alternating potential difference
• neutral wire completes the circuit
• earth wire is a safety wire connected to metal casing

the switch must be connected to the live wire.

this ensures the appliance is safely disconnected when switched off.

4.4.3 fuses and trip switches

fuses

• contain a thin wire that melts when current is too large
• break the circuit and stop current flow
• protect appliances and wiring

the fuse rating should be slightly higher than the normal operating current.

trip switches

• automatically switch off when a fault is detected
• operate faster than fuses
• can be reset after tripping

4.4.4 earthing and insulation

earthing

• metal casing is connected to the earth wire
• fault current flows to earth
• fuse blows or trip switch operates

double insulation

• outer casing is made of non-conducting material
• no earth wire is required
• commonly used in plastic-cased appliances

4.4.5 protection in double-insulated appliances

• no earth wire is needed
• fuse protects the circuit and cabling
• non-conducting casing prevents electric shock