Mains: 230 V AC, 50 Hz; cells give DC
Mains electricity (UK) is alternating current (AC) — current reverses 50 times a second (50 Hz) at a nominal ≈ 230 V. A cell or battery gives direct current (DC) — one direction, steady voltage. A cathode-ray oscilloscope (CRO) tells them apart: AC gives a sinusoidal trace, DC a flat line. Plug wires: live (brown) carries the high AC voltage, neutral (blue) completes the circuit at ≈ 0 V, earth (green/yellow) is the safety conductor to the casing.
Fuse, breaker, RCCB, earth, insulation
Four protective ideas: a fuse melts if current exceeds its rating, breaking the live wire. A circuit breaker trips on excess current and can be reset. An RCCB/RCCD cuts off within milliseconds when the live and neutral currents differ (earth leakage). Earthing links the metal casing to ground so a fault current melts the fuse. Double insulation (two-squares symbol) uses two insulation layers, no earth wire. Full comparison is in the note.
Fuse choice via I = P/V
rearranges to . To pick a fuse: (1) find the operating current with the 230 V mains; (2) choose the standard fuse just above it (3 A, 5 A or 13 A). Example: a 920 W toaster draws A → a 5 A fuse (3 A would blow in normal use; 13 A would not protect against a 7 A fault). The rating is the current at which the fuse melts: above the working current, but as low as possible for protection.
Drawn from real examiner reports.
Earth wire carries no current normally
Normally the earth wire carries NO current; it acts only if the live wire touches the metal casing. Then a large current flows down it to ground, blowing the fuse or tripping the breaker. Saying the earth wire "completes the circuit" confuses it with the neutral wire and scores zero. State: low-resistance path → large fault current → fuse melts.
June 2024 Paper 2P — candidates who gave vague or incorrect descriptions of safety wire functions scored zero; the examiner noted that imprecise safety explanations (e.g. "it stops you getting a shock" without mechanism) were not creditworthy.
Fuse rating too low or too high
Two errors: a rating BELOW the operating current (e.g. 3 A for a 4 A appliance) blows in normal use; a rating far ABOVE it (e.g. 13 A for a 4 A appliance) gives no protection — a 6 A fault would not melt it. Correct method: find , then take the next standard rating (3, 5 or 13 A) just above it. Also state the unit, amperes (A).
November 2024 Paper 2P — the examiner report noted that candidates lose marks by omitting units on calculations, and the summary explicitly stated that candidates should recall the units given in the specification and use them appropriately.
Old wire colours (red/black)
Many quote pre-2004 UK colours (red = live, black = neutral). The current colours the 4PH1 spec tests are: live = brown, neutral = blue, earth = green and yellow stripes. A related trap: calling neutral the "safe" wire is loose — live is at high AC voltage and neutral near 0 V, but current flows through BOTH in use; only the earth wire normally carries none.
June 2024 Paper 2P — candidates who used pre-2004 wire colour designations (red/black) rather than the current brown/blue/green-yellow lost marks on wire identification questions.
Double insulation vs earthing
A double-insulated appliance (two-squares symbol) has no exposed metal and no earth wire — two insulation layers keep live parts away from the user. An earthed appliance has a metal casing, an earth wire and a fuse. You cannot earth a double-insulated appliance: there is no exposed metal for the earth wire to connect to.
Fuse placed in the neutral wire
The fuse (and the switch) must be in the live wire. If the fuse is in the neutral, then after it blows the appliance is still connected to the live wire at high voltage — a shock hazard remains even though no current flows. Diagrams that put the fuse in the neutral lose the fuse marks. Always draw the fuse in the live wire.
Mains is AC, not DC
UK mains is alternating current (AC) at ≈ 230 V, 50 Hz — not DC. On a CRO, AC is a repeating sine wave and DC a flat horizontal line. Read the frequency from the trace with , where is the period (one full cycle) from the time-base. Do not describe mains as a steady, one-direction voltage.
Safety: give the mechanism, not the result
Safety questions want the MECHANISM, not just the outcome. "How a fuse protects" → current exceeds the rating → the fuse wire heats and melts → the circuit breaks. "It stops the current" or "prevents a shock" alone scores zero. Do the same for earth-wire and breaker answers.
One physics point per mark
Count the marks and give that many distinct points. A safety explanation worth 3 marks needs three separate physics statements — fault current → earth path → large current → fuse melts is four linked points, one per mark. Write one sentence per step so each can be ticked.
Fuse answers: show I = P/V and the unit
On fuse-selection questions, show the working with the 230 V mains, then justify the chosen rating (the next standard value above it). State the unit, amperes (A), on the current — the report flags dropped units. Do not just name a fuse without the calculation.
Name wires by colour AND function
When identifying a plug wire, give BOTH the colour and the function — e.g. "live, brown, carries the high AC voltage". Colour alone or function alone can lose a mark. Use the current colours (brown/blue/green-yellow), and put the fuse and switch in the live wire on any diagram.
| Quantity | Formula | Symbols | Unit |
|---|---|---|---|
| Power | = power (W), = current (A), = voltage (V) | watt, W | |
| Current from power | Use to select a fuse rating | ampere, A | |
| Energy transferred | = time (s) | joule, J |
UK mains supply values (memorise these):
State the colour and function of the live wire in a UK mains plug.
A toaster is rated at 920 W and is connected to the 230 V mains supply.
(a) Calculate the current drawn by the toaster during normal operation. Give the unit.
(b) Select the most appropriate fuse from: 3 A, 5 A, 13 A. Justify your answer.
