Fuse: melts above rated current, in live wire
Fuse: a thin wire that melts and breaks the circuit when the current exceeds the rated value — both elements are needed for the mark. It is always fitted in the live wire. Rate it slightly above the normal operating current: too low blows in normal use; too high fails to blow in a fault, letting cables overheat. Get the current from — a 1150 W appliance on 230 V draws A, so choose a 13 A fuse (next rating above 5 A).
Earth wire and double insulation
Earth wire: a low-resistance path to earth. If the live wire touches the metal case, the case reaches live potential → a large fault current flows through the earth wire → this melts the fuse and cuts off the supply. "Prevents shock" alone scores zero — give the mechanism. Double insulation: two insulation layers and no exposed metal, so no earth wire is needed; symbol = two concentric squares (□ inside □).
Three electrical hazards
Cambridge names three hazards, each worth 2 marks (name + mechanism):
Drawn from real examiner reports.
Bigger fuse is not safer
A higher-rated fuse is NOT safer. Too low (1 A for a 5 A appliance) blows in normal use — annoying, not dangerous. Too high (13 A for a 1 A appliance) will NOT blow even if a fault drives 10 A, so the cable overheats → fire or shock. The rule: rate the fuse slightly above the normal operating current so it blows in a fault but not in normal operation.
November 2023 Paper 11 (fuse-rating MCQ) — examiner report confirms the correct answer logic: "a 4 A fuse would melt (operating current is 5 A); the 10 A fuse is the correct choice." The "biggest fuse available" misconception is the primary distractor.
Earth wire: state the mechanism
"The earth wire prevents electric shock" scores zero. Two elements are required: (1) it provides a low-resistance path to earth for the fault current; (2) the resulting large current melts the fuse (or trips the breaker) and cuts off the supply. Answers like "protects the user" or "makes it safe" state the outcome, not the mechanism, so they earn no marks.
Fuse belongs in the live wire
The fuse must sit in the live wire, never the neutral. If it were in the neutral and blew, the appliance's live terminal would stay connected to the mains — the case can rest at live potential and give a shock even when "off". Placed in the live wire, a blown fuse isolates the whole appliance from the supply. June 2024 examiner report: few could explain this.
June 2024 Paper 31/32/33 Q9(b)(ii) — examiner report: "Only a few were able to give a correct explanation for positioning the fuse in the live wire" — confirms fuse-position reasoning is systemically weak across candidates.
Wires are not positive/negative
Few candidates can name all three mains wires. They are live (brown), neutral (blue) and earth (green-and-yellow stripes) — never "positive", "negative" or a "list of metals". Live is a.c., so the d.c. terms positive/negative are wrong. Cambridge accepts "line" for live and "ground" for earth. Swapping the live and neutral colours loses both colour marks.
June 2024 Paper 31/32/33 Q (mains cable wire names) — examiner report: "Few candidates could recall all three wire names; common errors were positive, negative, or lists of metals." Confirmed by November 2024 Paper 31 mark scheme Q10(b): "live OR line | earth OR ground | neutral" [B2].
Double insulation ≠ two earth wires
Double insulation does NOT mean two earth wires. A double-insulated appliance has no earth wire — live parts are separated from the user by two independent layers of insulation and no exposed metal can be touched (symbol: two concentric squares). A metal-cased appliance is the opposite: it DOES need an earth wire and is not double insulated.
A fuse is replaced; an MCB resets
A blown fuse must be replaced — its wire has melted. A circuit breaker (MCB) trips a switch, so it can be reset with a button, and it operates faster than a fuse. For "two advantages of an MCB over a fuse" give BOTH: (1) it can be reset / needs no replacement; (2) it operates faster. Giving only one is the usual half-mark loss.
Earth-wire chain: one mark per step
For "explain how the earth wire protects the user" (4 marks) write one sentence per link: live touches case → case at live potential → fault current flows through the earth wire → large current melts the fuse → supply cut off. Each arrow is a mark; give at least four.
Fuse rating: current first, then round up
Fuse-rating questions need first. Find the operating current, then pick the next standard fuse above it (3 A, 5 A or 13 A). A 1150 W appliance on 230 V draws 5 A → a 13 A fuse. Never choose a rating below the operating current — it blows in normal use.
Two marks per hazard: name + mechanism
Hazard questions give two marks each: (1) name/identify the hazard; (2) state the mechanism and its consequence — shock OR fire. "The wire is damaged" scores zero: it names the fault but neither the hazard nor the mechanism. Finish with "so… electric shock" or "so… fire".
Give BOTH advantages of an MCB
For advantages of an MCB over a fuse, give TWO: it can be reset (no replacement) and it operates faster. One point caps you at half marks. Mechanism: current exceeds the rated value → the electromagnet pulls the switch open → the circuit is broken.
| Formula | Meaning | Use in §4.4 |
|---|---|---|
| Current = Power ÷ Potential difference | Calculate operating current to select fuse rating | |
| Power dissipated = Current² × Resistance | Explains why overloaded cables overheat | |
| Charge = Current × Time | Background formula (charge flow context) |
Symbol key: = current (A), = power (W), = potential difference (V), = resistance (Ω), = charge (C), = time (s).
Cambridge notation note: always write "potential difference" — never "voltage" — in definitions and explanations. Cambridge mark schemes reject "voltage" in definition answers.
State two advantages of a circuit breaker over a fuse.
State two electrical hazards in the home and, for each hazard, explain why it is dangerous. [4]
