Electrical safety

Work out the normal operating current with $I = P/V$, then pick the next standard rating above it (3 A, 5 A, 13 A). A rating below the operating current melts during normal use. A rating far above protects nothing: a fault current can overheat the cable and melt its insulation without ever growing large enough to blow the fuse quickly.

A fuse does not supply, control or hold constant the current, and it does not reduce it. It is only a thin wire that heats up and melts when the current exceeds its rating, breaking the circuit. Describe it as melts and breaks the circuit at excess current, never as keeps the current constant or reduces the current.

When several appliances share one socket or extension lead, the total current is the sum of their currents. Do not average or subtract them, and do not quote just one appliance's current. You can instead add the powers first and then use $I = P/V$. It is this large total that overheats the wiring, so it is the figure the fuse rating has to cover.

Inverting the formula is a standing error: current is power divided by potential difference. Convert the power to watts before substituting (a 2.3 kW kettle is 2300 W), and always write the unit — a bare number is routinely penalised. Ohm's-law arithmetic runs the same way round: $I = V/R$, not $R/V$.

Heating power in a cable is $P = I^2R$, not $IR$. The squaring matters: doubling the current gives four times the heating, which is why a modest overload makes a cable dangerously hot. Substitute $I^2$ first — for 6.0 A through 0.80 Ω, $I^2 = 36$, so $P = 28.8$ W — and give the answer in watts.

Explaining earthing as the earth wire carries the current away stops at the mechanism and drops the marks. The full chain: a live wire touches the casing, so the earth wire gives a low-resistance path and a large current flows; that current blows the fuse (or trips the switch); the circuit breaks and the supply is disconnected.

These are alternatives, not partners. Earthing needs a metal casing, an earth wire and a fuse — the blown fuse is what finally makes it safe. Double insulation needs a non-conducting casing with no exposed metal, so it has no earth wire at all. Drawing an earth wire onto a plastic double-insulated appliance is a common slip.