Mains Electricity: a.c., Earthing and Fuses
Mains supply is alternating, changing direction many times a second, unlike the direct current from a cell. The live, neutral and earth wires each have a job, and a fuse or breaker cuts the circuit when the current climbs too high.
What a learner can do afterwards
- States the difference between alternating and direct current and names the mains voltage and frequency
- Describes what each of the three wires in a plug does and why the live one is dangerous even when a switch is off
- Chooses a suitable fuse rating for an appliance from its power and explains how earthing plus a fuse work together
1 · Read
A cell pushes current one way only, which is direct current. Mains supply is alternating current: it swaps direction 50 times a second in the UK, at about 230 V. That swapping lets transformers step voltage up and down.
A plug holds three wires with three jobs. Live brings the 230 V in and is the dangerous one. Neutral completes the circuit back to the supply, and earth is a safety path that normally carries nothing. A wrongly wired switch may break neutral instead, leaving the appliance live even when it looks off.
A 2300 W kettle at 230 V draws 10 A, so it needs a 13 A fuse. If a fault joins live to the metal case, the earth wire carries the surge away and the surge melts the fuse, cutting the circuit. Test a suspect fuse with a multimeter: a good one reads near zero, a blown one reads open.
Pick the next standard rating above the normal current: 3 A for small appliances, 13 A for heavy ones like kettles. Never fit a bigger fuse to stop blowouts, because oversizing lets dangerous currents flow until wires overheat. A correct fuse that keeps blowing means the appliance has a fault.
Live brings power, neutral returns it, earth guards you, and the right fuse cuts the danger.
More to read
20.5 Alternating Current versus Direct Current
A cell pushes current one way only, which is direct current. Mains supply is alternating current: it swaps direction many times a second, 50 times a second in the UK at about 230 V. That rapid swapping lets transformers step the voltage up and down, which is why the grid uses AC.
Because AC keeps reversing, we quote an effective voltage that delivers the same heating as the matching DC value. Appliances built for mains expect that effective 230 V, so running them from the wrong supply or the wrong frequency can damage them or make them unsafe.
10.6 Household Wiring and Electrical Safety
A plug holds three wires with three jobs. Live brings the 230 V in and is the dangerous one, neutral completes the circuit back to the supply, and earth is a safety path that normally carries nothing. If a fault connects live to a metal case, the earth wire carries the surge away instead of through you.
That surge is what blows the fuse or trips the breaker. Fuses hold a thin wire that melts when the current passes its rating, cutting the circuit. Pick a fuse just above the normal current: a 3 A fuse for small appliances, 13 A for heavy ones like kettles.
How to Test a Fuse With a Multimeter: 7 Steps
A fuse looks fine from outside even when its inner wire has melted, so test it instead of guessing. With the appliance unplugged, set a multimeter to continuity or low resistance and touch the probes to the two fuse ends. A good fuse reads near zero resistance or beeps, while a blown one reads open circuit.
Never swap in a higher rated fuse to stop blowouts. The rating matches the flex and the appliance, and oversizing it lets dangerous currents flow until wires overheat. If a correct fuse keeps blowing, the appliance has a fault that needs fixing, not a bigger fuse.
2 · Watch
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Where it sits
Optional connections
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Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.