Aggregated results from hundreds of timed attempts at our free 2391 mock exam reveal exactly which questions candidates fail — from an 86%-fail insulation resistance question down. Each one below comes with the answer and, more usefully, why people miss it.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
Based on real results from hundreds of mock exam attempts, the hardest 2391 questions cluster in insulation resistance behaviour (why readings rise during testing — 86% fail rate; temperature effects; parallel paths) and earth fault loop impedance corrections (temperature factors, TT maximum Ze of 21 Ω, measured vs calculated Zs). The pattern: candidates know test procedures but stumble on why readings behave as they do.
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Most "hardest exam questions" articles are guesswork. This one is not: it is built from the aggregated, anonymous results of hundreds of real timed attempts at our free 2391 mock exam — a 300-question bank covering the full inspection and testing syllabus. Every time someone submits an attempt, each question's shown-and-answered record updates. The ten below are the questions with the highest failure rates among those shown at least fifteen times (snapshot: July 2026).
The average attempt scores just under 80% and about five in six attempts pass — which makes these outliers interesting. These are the questions that people who mostly know the material still get wrong.
What might cause an insulation resistance reading to gradually increase during testing?
Answer: Absorption of current by the insulation (polarisation)
Why people miss it: When the test voltage is first applied, the dielectric draws a charging and polarisation current. As the insulation polarises, that current falls away — so the displayed resistance climbs. Most candidates pick options suggesting a fault or instrument error, but a gradually rising reading is normal behaviour on sound insulation, especially on long cable runs.
What correction factor should be applied to measured Zs values?
Answer: Multiply by 1.2 for thermoplastic (1.04 for thermosetting)
Why people miss it: Conductors are tested cold but operate hot, and resistance rises with temperature. To compare a cold site reading against the maximum values (which assume operating temperature), the measured value is corrected upward — 1.2 for 70°C thermoplastic insulation. Candidates muddle this with the alternative "0.8 rule" applied to the tabulated limit; both express the same physics from opposite ends.
What is the typical maximum Ze for a TT system?
Answer: 21 Ω
Why people miss it: TN values (0.35 Ω and 0.8 Ω) are drilled into everyone; the TT figure is not. 21 Ω is the conventional maximum for a stable TT earth electrode — and the reason TT protection leans on RCDs rather than overcurrent devices for shock protection.
What is the minimum CSA for supplementary bonding conductors?
Answer: 4 mm² (where not mechanically protected)
Why people miss it: Supplementary bonding sizing has two tiers — 2.5 mm² is acceptable with mechanical protection, 4 mm² without. Candidates who memorise a single number get caught whichever way the question is framed. The full sizing logic is in Section 544.
What is the maximum permissible standing earth leakage for a 30 mA RCD protected circuit?
Answer: 10 mA — one third of the rated residual current
Why people miss it: The rule of thumb is that standing (protective conductor) leakage should not exceed one third of IΔn to avoid nuisance tripping — 10 mA on a 30 mA device. Candidates guess at half (15 mA) or the full rating; the one-third convention is the one guidance uses.
What effect does temperature have on insulation resistance readings?
Answer: Higher temperatures typically result in lower readings
Why people miss it: Insulation resistance falls as temperature rises — roughly halving for every 10°C in many materials. It is the mirror image of conductor resistance (which rises with temperature), and that inversion is exactly the trap the wrong options exploit.
What is the recommended inspection interval for cinemas?
Answer: 1 year
Why people miss it: Places of public entertainment carry short recommended intervals — one year for cinemas — because of occupancy risk. Candidates who only remember "10 years domestic, 5 years commercial" lose this mark; the special-premises table is worth a dedicated revision pass.
When testing a complete installation, what is the likely reading if all circuits pass individually?
Answer: Lower than the individual readings, due to parallel paths
Why people miss it: Insulation resistances in parallel combine like resistors in parallel — the whole-installation reading is always lower than the best individual circuit. A 200 MΩ circuit and a 100 MΩ circuit together read about 67 MΩ. Candidates expecting the "sum" or the lowest single value miss it.
What is the recommended minimum CAT rating for test equipment used on LV installations?
Answer: CAT III
Why people miss it: Overvoltage category ratings describe where an instrument can safely be used. Fixed LV installation work calls for CAT III as the minimum (CAT IV at the origin/supply side). CAT II instruments belong on plug-connected equipment, not distribution boards.
What causes the difference between measured Zs and calculated Zs values?
Answer: Temperature — conductors are cooler when tested than the values assume
Why people miss it: Calculated Zs (Ze + R1+R2 corrected to operating temperature) will typically exceed a live measured Zs on a cold circuit — and parallel earth paths through bonding can pull the measured figure lower still. Understanding which direction the difference runs, and why, is what the question is really testing.
Line the ten up and two patterns account for most of them:
If that first pattern describes you, the highest-value revision is not more questions — it is the underlying guides: insulation resistance testing and earth fault loop impedance calculation cover the two topics that dominate this list.
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