4.4 Electrical safety
- Syllabus
- 0625–2026–2027
- Topic
- 4.4
- Level
- —
Learning objectives
4.4.1Hazards of: (a) damaged insulation (b)• State the hazards of: (a) damaged insulation (b) overheating cables (c) damp conditions (d) excess current from overloading of plugs, extension leads, single and multiple sockets when using a mains supply4.4.2A mains circuit consists of a live• Know: 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 safely4.4.3Use and operation of trip switches and• Explain the use and operation of trip switches and fuses and choose appropriate fuse ratings and trip switch settings4.4.4Why the outer casing of an electrical• Explain why the outer casing of an electrical appliance must be either non-conducting (double-insulated) or earthed4.4.5A fuse without an earth wire protects• State: a fuse without an earth wire protects the circuit and the cabling for a double- insulated appliance
Recognise mains-electricity hazards
Mains electricity can cause electric shock, burns and fire. A hazard becomes dangerous when a person can contact a live conductor or when excessive current heats wiring beyond its safe temperature.
| Hazard | Why it is dangerous |
|---|---|
| damaged insulation | exposes a live conductor, allowing current through a person or a short circuit |
| overheating cable | insulation may melt and nearby material may catch fire |
| damp conditions | water containing dissolved substances conducts, making shock and short-circuit paths more likely |
| overloaded plug, extension lead or socket | parallel appliances draw a larger total current than the wiring or connector is rated to carry |
| thin or coiled extension cable carrying high current | its resistance causes heating, and a coil slows heat loss |
A correct fuse does not make damaged, wet or overloaded equipment safe to keep using. Isolate the supply and remove the hazard; never rely on insulation tape as a permanent mains repair.
Wire and switch a mains appliance safely
| Wire | Function |
|---|---|
| live (line) | carries alternating potential relative to earth and delivers energy to the appliance |
| neutral | completes the circuit and is held near earth potential at the supply |
| earth | safety conductor connected to exposed metal casing; normally carries no current |
The switch must be in the live wire. Opening it then disconnects the appliance's internal circuit from the live supply. A switch in the neutral wire could stop normal current while leaving internal parts connected to live, so touching a fault could still cause shock.
Neutral is not a substitute for earth, and earth is not part of the normal operating-current path. Treat mains conductors as unsafe even when an appliance appears switched off.
Choose and explain fuses and trip switches
| Device | Operation when current is excessive | After operation |
|---|---|---|
| fuse | its thin wire heats, melts and opens the live circuit | replace with the correct rating |
| trip switch / circuit breaker | detects an excessive current and opens contacts | remove the fault, then reset |
Choose the smallest available rating above the appliance's normal operating current but below the maximum safe current for the cable. This prevents operation during normal use while disconnecting before the cable overheats. A 10 A kettle with choices 3 A, 5 A and 13 A needs 13 A.
A small overload may take longer to operate a protective device; a much larger fault current should disconnect much faster. A fuse or trip switch disconnects the circuit—it does not reduce and hold the current at its rating.
Never replace a fuse with a higher value merely to stop it blowing. Repeated operation indicates an overload or fault that must be corrected.
Make an appliance casing safe
| Casing protection | How it prevents shock |
|---|---|
| non-conducting / double-insulated | accessible insulating material cannot become live; two insulating barriers separate live parts from the user |
| earthed metal casing | a fault from live to the case has a low-resistance path to earth, producing a large current that operates the fuse or trip switch |
The earth wire keeps the case close to earth potential while the protective device disconnects the live supply. Without earth, a live fault could leave the metal case at mains potential and a person could complete the path to earth.
An earth wire does not normally carry the appliance current and is not added to a double-insulated plastic appliance. Earthing works with a fuse or breaker; it does not simply make fault current disappear.
What a fuse protects without an earth wire
A double-insulated appliance does not need an earth wire because the user cannot touch a conducting case that could become live. It still needs overcurrent protection for its circuit and cable.
If a fault or overload makes the current too large, the fuse in the live wire melts and disconnects the supply before the appliance wiring or flexible cable overheats. The fuse therefore protects the circuit and cabling even though there is no earth wire.
In this arrangement the fuse is not replacing an earth connection to a metal case. Double insulation provides shock protection from the casing; the fuse provides overcurrent and cable protection.