When the 18th Edition went out for public comment, the Part 6 changes were mostly about layout. The technical content changed less than people expected. But the numbering changed completely, and in an open-book exam the numbering decides whether you find a regulation in thirty seconds or spend three minutes looking for it.
This post covers the restructure, the wording changes that matter on site, and the reasons behind them. That structure carried through into the published 18th Edition and its amendments, and it is the Part 6 you will be working from for the 2391-52.
Old chapters, new chapters
The old Part 6 had three chapters. One covered initial verification, one covered periodic inspection, and certification and reporting had a chapter of its own. The 18th Edition dropped that third chapter and moved its content into the two chapters that use it.
| 17th Edition | 18th Edition | What it covers |
|---|---|---|
| Chapter 61 | Chapter 64 | Initial verification: inspection and testing of new work |
| Chapter 62 | Chapter 65 | Periodic inspection and testing |
| Chapter 63 | Split into 644 and 653 | Certification moved into initial verification, reporting moved into periodic |
| Table 61 | Table 64 | Minimum insulation resistance values |
This helps you under exam pressure. If the question is about new work, look in Chapter 64. If it is about an existing installation, look in Chapter 65. The paperwork for each now sits in the same chapter as the work it records.
641.3: precautions against danger
The general requirement is that precautions are taken to avoid danger to persons and livestock, and to avoid damage to property and installed equipment, during inspection and testing. The video points out that the wording has tightened. The duty is to prevent danger, not just to show that you took precautions.
The periodic chapter goes further. The old wording said precautions should be taken so that periodic inspection and testing does not cause danger. The new wording simply says it shall not cause danger.
If you cause danger, that’s a non-compliance.
You can no longer argue that you took precautions, just the wrong ones. Safe isolation, GS38-compliant probes and leads, and proving your voltage indicator before and after use are how you meet that requirement. The safe isolation and GS38 guide covers the procedure.
642.3: new inspection items
The inspection checklist kept its existing items and added these at the end:
- selection and erection of wiring systems (previously described as erection methods only)
- selection and installation of surge protective devices (SPDs), where required
- measures against electromagnetic disturbances
The 18th Edition introduced the SPD and electromagnetic disturbance requirements, so inspection has to check them. If you are not confident about SPD types and where they have to be fitted, brush up on that before you sign off an inspection schedule that includes them.
643.2: continuity of conductors
Two old regulations were merged into one. The requirement is now to verify continuity by measuring resistance on:
- protective conductors, including protective bonding conductors
- live conductors, for ring final circuits
The wording about using a test supply with a no-load voltage of 4 V to 24 V was removed from the regulation. That does not mean any instrument will do. Your continuity tester must still meet BS EN 61557-4, which sets the output requirements, so the method you use on site is unchanged.
The presenter’s main complaint applies to every tester: there is still no explicit requirement to verify neutral continuity on radial circuits. Good practice does not wait for a regulation. Confirming the neutral end to end, and confirming polarity, on a radial circuit takes very little extra time and shows up the faults that a protective conductor reading alone will miss.
643.3: insulation resistance and Table 64
The minimum values are now in Table 64. The bigger change is that a statement which used to be a note is now part of the regulation itself:
The values are usually much higher than those of Table 64. When such values show evident differences between circuits, further investigation to identify the reasons is required.
On a new installation, a circuit that only just passes the minimum, or reads far lower than similar circuits, is not finished just because it passed. Investigate the low reading. It could be a damaged cable, a trapped conductor, or equipment that was left connected.
Test order: continuity comes before insulation resistance. Then polarity. Then the live tests: earth fault loop impedance and RCD operation. You prove the protective conductor is continuous before you rely on it, and you prove the installation is sound while it is dead before you put voltage on it. The test sequence post explains each step.
643.7: verifying automatic disconnection
This section got the most new wording, and most of it helps.
Earth fault loop impedance. Compliance with the Chapter 41 disconnection requirements is verified by measuring earth fault loop impedance. Where measurement is not possible, Regulation 643.7 accepts verifying the continuity of the protective conductors, provided calculations of earth fault loop impedance or protective conductor resistance are available.
In practice that is the familiar relationship:
Zs = Ze + (R1 + R2)
Take the R1 + R2 value you measured during the dead tests, add Ze, and compare the result with the maximum Zs for the protective device. This is only allowed where measuring Zs is not possible. It does not replace a Zs test you could have done. The earth fault loop path post works through it.
Continuity first. Regulation 643.7.3 says a continuity test under 643.2 has to be done before you measure earth fault loop impedance. That is the test sequence written into the regulation. Never put a live loop test on a protective conductor whose continuity you have not proved.
RCDs. RCDs are verified with suitable test equipment to BS EN 61557-6, taking account of the device’s operating characteristics. The protective measure works if the RCD disconnects at a fault current equal to or less than its rated residual operating current. Disconnection times are recommended to be verified, and they must be verified for additions and alterations to an existing installation where existing RCDs are relied on for disconnection.
Downstream of a verified point. Where the protective measure has been confirmed at a point downstream of an RCD, the installation beyond that point can be verified by confirming continuity of the protective conductors.
Supplementary bonding. Where ADS cannot be achieved and supplementary bonding is used as additional protection, its effectiveness has to be checked against Chapter 41. That means measuring the resistance between simultaneously accessible parts and checking it against Regulation 415.2.
644.1: the consumer unit clarification
A short addition to the initial verification certification wording makes clear that replacing a distribution board or consumer unit is work that needs an Electrical Installation Certificate. Some electricians had been certifying board changes as minor works. The EIC, Minor Works or EICR post sets out the full decision test.
Chapter 65: periodic inspection additions
Periodic inspection is still carried out without dismantling, or with partial dismantling, to check the safety of persons and livestock and the protection of property. Two checks were added:
- confirm the rating and setting of protective devices required by Chapter 41, including RCDs with adjustable settings
- confirm the rating and setting of monitoring devices
A note recognises that existing installations may have been designed and installed to earlier editions. That does not make them unsafe. It does mean you judge what is in front of you on its merits when deciding on a classification.
Another note says that where a circuit is permanently monitored by an RCM or IMD, insulation resistance measurement is not needed if functional checks confirm the device works. The device has been monitoring insulation continuously. Record on the report that you relied on it.
Reporting (653). The report must include the parts inspected, any limitations, damage, deterioration, defects, dangerous conditions and non-compliances, together with the schedule of inspections and schedule of test results.
Why this matters in the 2391-52
The exam is open book: 60 questions in 120 minutes, with a 75% pass mark. That is two minutes per question. Knowing that new work is in 64, periodic work is in 65, test methods are in 643 and reporting is in 653 means you can find the right section of BS 7671 straight away. Tab those sections before the exam.
How 2391 Prep Fits Into This
2391 Inspection & Testing Pro is built around how Part 6 is examined. It has 556 practice questions across 7 subject areas covering initial verification, periodic inspection and the documentation each one needs. The 4 working calculators let you practise the Ze + (R1 + R2) calculation that Regulation 643.7 allows. The 15 quick reference cards put the chapter structure and test sequence in one place, and the 6 field tools support the same checks on site.
