City & Guilds 2391-51 common mistakes
City & Guilds 2391-51 · Understand the traps and how to avoid them.
Stating that BS 7671 is a statutory document or that the IET Guidance Notes have legal force, instead of identifying HASAWA, EAWR and ESQCR as the statutory framework.
Candidates memorise document titles but not their legal status, so they treat all publications as equally binding.
What to do instead
Build a two-column table: statutory (HASAWA, EAWR, ESQCR, CDM) and non-statutory (BS 7671, IET Guidance Note 3, HSE HSG107). For each, state its purpose and whether breach is an offence.
Describing safe isolation as 'switch off and test' without proving the tester on a known source before and after proving dead, without locking off, and without proving all conductors at the point of work.
The procedure is learned as a slogan rather than a sequence, and exam pressure causes steps to be omitted.
What to do instead
Write out the full sequence every time: identify circuit, isolate, lock off and retain key, prove tester on known source, prove dead on all conductors at the point of work, re-prove tester, prove dead again.
Beginning a periodic inspection without a written agreed extent and limitations, and without reviewing existing certificates, schedules and previous reports.
Candidates focus on test results and treat the report as the starting point rather than agreeing scope first.
What to do instead
State that the extent, limitations and sampling approach must be agreed with the client in writing before the inspection, and that existing documentation must be reviewed to inform sampling and identify changes.
Carrying out visual inspection only at the consumer unit and missing damaged cable sheaths, unsealed enclosure entries, missing grommets, absent circuit labels, inadequate IP protection and disconnected main protective bonding.
Testing is seen as the main task and visual inspection is treated as a quick look rather than a systematic examination.
What to do instead
Use the BS 7671 inspection schedule as a checklist and inspect by location: origin, distribution boards, accessible cable routes, accessories, equipment and special locations. Record what was and was not inspected.
Describing periodic inspection as requiring 100% inspection and testing, or as including design verification and the issue of an initial verification certificate.
The candidate has learned one procedure for initial verification and applies it unchanged to periodic inspection.
What to do instead
Compare the two in a table: initial verification confirms a new installation complies with BS 7671 and includes design verification; periodic inspection assesses condition, is sampled and limited by agreement, and results in an EICR with classifications and recommendations.
Testing a ring final circuit only by measuring end-to-end r1, rn and r2 at the board and never cross-connecting conductors to verify ring continuity and identify spurs or breaks.
The end-to-end readings alone appear to confirm continuity, so the three-stage method is not completed.
What to do instead
Perform the full sequence: stage 1 end-to-end r1, rn and r2; stage 2 cross-connect opposite legs; stage 3 confirm readings at each socket and compare with expected values. Investigate any socket that does not follow the expected pattern.
Carrying out insulation resistance testing with equipment, SPDs or electronics still connected, or testing only between line and earth rather than all conductor combinations.
Isolation and disconnection are rushed, and the test is remembered as a single L-E measurement.
What to do instead
Isolate and prove dead, disconnect sensitive equipment and electronics, then test L-N, L-E and N-E at 500 V DC (or 250 V DC where required for SELV/PELV), and record the lowest valid reading.
Measuring earth fault loop impedance only at the origin, or recording prospective fault current for only one fault type without stating whether it is line-neutral or line-earth.
Ze and Zs are confused, and PFC is treated as a single quantity.
What to do instead
Measure Ze at the origin and Zs at the relevant points of the circuit, including the furthest point. Record PFC for both line-neutral and line-earth, and state the point of measurement and supply characteristics used.
Testing an RCD only by pressing its integral test button and recording 'RCD tested', without instrument tests at half, one and five times rated residual operating current and without checking both polarities.
The test button is mistaken for a full verification of RCD performance.
What to do instead
State that the integral button is a mechanical/functional check only. Use an RCD tester at ½IΔn (no trip), IΔn and 5IΔn, on both positive and negative half-cycles, record operating times and compare with BS 7671 limits.
Assigning the wrong classification code, such as recording a missing RCD on a socket circuit as C3, using C3 for a dangerous exposed live part, or using FI without a specific reason why further investigation is needed.
The definitions of C1, C2, C3 and FI are memorised loosely and applied by impression rather than by risk.
What to do instead
Apply the definitions literally: C1 = danger present, C2 = potentially dangerous, C3 = improvement recommended, FI = further investigation required without delay. For each observation, state the risk and justify the code in the report.