Ten 2391-52 questions that catch out electricians who know the test sequence well, each with the correct answer and why it trips people up, checked against BS 7671:2018+A4:2026, GN3 and the On-Site Guide.
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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.
The short answer
The hardest 2391 questions test what readings mean rather than how to take them: why insulation resistance rises during a test, how temperature affects it, parallel paths, and why a cold measured Zs differs from the design value. The rest are second-tier figures such as the 30% leakage limit behind an RCD (Reg 531.3.2) and GN3 inspection intervals for special premises.
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01 · Exam Guide
The City & Guilds Level 3 Award in Initial and Periodic Inspection and Testing of Electrical Installations (2391-52) is, in City & Guilds’ words, assessed “through a multiple-choice on-line test and a practical task”. Your centre will confirm the current paper format and what you can take in with you.
The multiple-choice test is where understanding gets tested. The ten questions below are the kind that catch out people who know the test sequence well: each comes with the answer and why it trips people up, tied to BS 7671:2018+A4:2026, IET Guidance Note 3 or the On-Site Guide. They are all in our free 2391 mock exam.
02 · Exam Guide
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 cable charges like a capacitor and the insulation draws a polarisation current. As that current falls away, the displayed resistance climbs. A slowly rising reading is normal on sound insulation, especially on long runs; the wrong options suggest a fault or an instrument problem.
A Zs reading is taken on a cold circuit. How is it compared with the BS 7671 maximum?
Answer: Against 0.8 × the tabulated maximum, or by correcting R1 + R2 to operating temperature
Why people miss it: The Table 41.2 to 41.4 maximums apply with conductors at operating temperature. BS 7671 Appendix 3 allows a cold reading to be compared with 0.8 × the tabulated value. By calculation, the conductor part (R1 + R2) is multiplied up to operating temperature (the On-Site Guide multipliers are 1.20 for 70 °C thermoplastic and 1.28 for 90 °C thermosetting) before Ze is added. Ze itself is not corrected, which is the trap.
What maximum Ze do distributors typically quote for a TT supply?
Answer: 21 Ω, excluding the installation’s own earth electrode
Why people miss it: Everyone knows 0.8 Ω for TN-S and 0.35 Ω for TN-C-S; the TT figure in the On-Site Guide is 21 Ω, and it excludes the consumer’s earth electrode. The electrode is a separate check: BS 7671 notes that an electrode resistance above 200 Ω may not be stable (Table 41.5, Note 2), and fault protection on TT normally relies on an RCD.
What is the minimum CSA for supplementary bonding conductors?
Answer: 4 mm² where not mechanically protected (2.5 mm² with mechanical protection)
Why people miss it: Supplementary bonding has two tiers in Section 544: 2.5 mm² copper is acceptable with mechanical protection, 4 mm² without. Candidates who learn a single number get caught whichever way the question is framed.
How much standing protective conductor current can be downstream of a 30 mA RCD?
Answer: No more than 30% of IΔn: 9 mA
Why people miss it: Regulation 531.3.2 limits the accumulated protective conductor and earth leakage current downstream of an RCD to 30% of its rated residual operating current, to avoid unwanted tripping. On a 30 mA device that is 9 mA. Many training notes still say one third (10 mA), which is the answer the wrong option relies on.
What effect does temperature have on insulation resistance readings?
Answer: Higher temperatures give lower readings
Why people miss it: Insulation resistance falls as temperature rises, quite steeply in many insulating materials. It is the opposite of conductor resistance, which rises with temperature, and that reversal is exactly what the wrong options exploit.
What maximum period between inspections does GN3 recommend for cinemas?
Answer: 1 to 3 years, with a yearly routine check
Why people miss it: GN3 Table 3.2 lists cinemas at a 1 to 3 year maximum period between inspections, with a routine check every year, and notes that local authority licence conditions can set the period. Candidates who only remember 10 years domestic and 5 years commercial lose this mark; the special-premises rows are 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, so the whole-installation reading is 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 minimum measurement category for test equipment used on a fixed LV installation?
Answer: CAT III (CAT IV at the origin)
Why people miss it: Measurement categories describe where an instrument can safely be connected. Work on the fixed installation calls for CAT III as a minimum, with CAT IV at the origin and supply side. CAT II belongs on plug-connected equipment, not distribution boards.
Why does a measured Zs usually differ from the calculated value?
Answer: The conductors are cooler when tested than the design assumes, and parallel paths can lower it further
Why people miss it: The calculated value (Ze + R1 + R2 corrected to operating temperature) will normally exceed a live reading on a cold circuit, and parallel earth paths through bonding and other services can pull the measurement lower still. Knowing which way the difference runs, and why, is what the question tests.
03 · Exam Guide
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.
In the app
AI-generated mocks, instant marking, and explanations on every question — targeted at your weakest topics. From £6.99/mo.
04 · Exam Guide
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