C&G 2391-52 cheat sheet

Health and Safety at Work etc. Act 1974 imposes general duties on employers and self-employed persons towards everyone affected by their work.

Electricity at Work Regulations 1989 are the principal statutory instrument for electrical systems in Great Britain; Reg 4 requires conductors to be insulated or protected.

EAWR Reg 13 requires adequate precautions for work on or near live conductors; Reg 14 permits live work only where it is unreasonable in all circumstances to work dead.

BS 7671 is a non-statutory British Standard; compliance is generally accepted as evidence of reasonable practicability under HSWA 1974 and EAWR 1989.

Approved Document P to the Building Regulations (England and Wales) covers fixed installations in dwellings; notifiable work is certified by a registered competent person or building control.

ESQCR 2002 set distributor duties, declared supply voltage of 230 V +10%/-6%, and supply earthing arrangements.

Non-statutory guidance includes HSE GS38, IET Guidance Notes, the IET On-Site Guide, IET Codes of Practice and the EAWR Approved Code of Practice.

Other legislation commonly engaged: CDM 2015, PUWER 1998, COSHH 2002, RIDDOR 2013 and the Work at Height Regulations 2005.

Part 6 covers inspection and testing: Chapter 61 initial verification, Chapter 62 periodic inspection and testing, Chapter 63 certification and reporting.

BS 7671 specifies minimum insulation resistance of 1 MΩ at 500 V DC for circuits up to 500 V, and 0.5 MΩ at 250 V DC for SELV and PELV.

Maximum earth fault loop impedances for protective devices are given in Tables 41.1 to 41.5 and assume conductors at their operating temperature.

Appendix 6 contains the model certificates, the schedule of inspections and the schedule of circuit details and test results.

IET Guidance Note 3: Inspection and Testing expands Part 6 with test methods, expected values, sampling tables and worked examples.

Guidance Notes 1 (Selection and Erection) and 8 (Earthing and Bonding) plus the On-Site Guide are the usual companion publications.

Chapter 41 covers protection against electric shock; Reg 411.3.3 requires 30 mA RCD additional protection for general-use socket-outlets rated up to 32 A.

Part 7 adds requirements for special locations, e.g. Section 701 locations containing a bath or shower, 702 swimming pools, 704 construction sites, 705 agriculture.

Identify every source of supply before isolation: mains, generators, PV, UPS, EV charging, batteries and stored energy.

Sequence: identify, notify affected persons, isolate, secure the isolation, prove the tester, prove dead at the point of work, prove the tester again.

Proving dead means testing all live conductors at the point of work - line to neutral, line to earth, neutral to earth, and line to line on polyphase systems.

Voltage detectors must meet HSE GS38: two-pole, fused leads, finger barriers, no more than 4 mm exposed probe tip, correct overvoltage category.

Secure the isolation with a personal padlock and a caution notice; the key remains with the person carrying out the work until completion.

Multi-point lock-off devices are used where several supplies or isolators can energise the point of work.

Never rely on a neon screwdriver, a single-pole detector or the position of a switch as proof of isolation.

After isolation, check for back-feeds from parallel circuits, borrowed neutrals, interconnected services and control systems.

Gather previous certificates, EICRs, drawings, schedules and design data, and agree the extent and scope of the work with the client.

Establish supply characteristics: earthing system, nominal voltage, external loop impedance or electrode resistance, and prospective fault current at the origin.

Confirm instruments hold valid calibration (typically within 12 months) and are proved against a known source before and after use.

Check the installation is safe to test: effective isolation, safe access, adequate lighting, and a clear working area.

For periodic inspection, agree sample sizes - typically a minimum of 10% of circuits, increased where the installation is in poor condition or records are missing.

Agree and record limitations (inaccessible areas, occupied premises, sealed equipment) before inspection begins.

Arrange PPE, safe access equipment, warning notices and notification of building users in advance.

Select the correct instrument function and range for each test and inspect leads and probes for damage before use.

A risk assessment and method statement should be produced before work; a permit to work is used where the site requires it.

Work dead unless EAWR Regulation 14 applies; live working needs justification, a safe system of work and a recorded risk assessment.

Use GS38-compliant test leads and instruments rated for the overvoltage category of the point of test.

Use insulated tools, temporary barriers and shrouds to prevent inadvertent contact with adjacent live parts.

Beware of parallel paths, borrowed neutrals, back-feeds from generators or PV, and stored energy in capacitors and drives.

Do not work alone on high-risk or live tasks; ensure first aid arrangements and emergency contact details are in place.

Use suitable access equipment and maintain three points of contact where practicable; never work from wet or unstable surfaces.

Discharge capacitors and prove dead before touching conductors; confirm the instrument is on the correct function and range.

Initial verification confirms compliance by inspection and testing before the installation is put into service; periodic inspection assesses the condition of an existing installation.

Inspect for damage, corrosion, overheating, mechanical protection, cable support and routing, and correct IP rating of enclosures.

Check the type, rating and setting of protective devices against the design, and the cross-sectional areas of line, neutral and protective conductors.

Verify main protective bonding is connected to the main earthing terminal and to every metallic service at its point of entry, correctly sized and identified.

Confirm live parts are not accessible (IP2X or barriers), unused openings are closed, and enclosures and covers are secure.

Check that labels, warning notices, RCD notices, diagrams and schedules are present and correct.

In special locations, verify supplementary bonding, RCD protection and suitability of equipment for the zone.

Record findings on the schedule of inspections; every departure from BS 7671 must be recorded with reasons.

Use a low-resistance ohmmeter and either null the leads or record and subtract their resistance from every reading.

R1+R2 method: link line and circuit protective conductor at the distribution board and measure at each accessory; compare with the calculated value for the circuit.

For radial circuits the highest reading is at the furthest point and approximates r1 + r2 for the whole circuit.

Ring final circuit: measure end-to-end r1, rn and r2, then cross-connect line of one leg to the CPC of the other; socket readings should be about (r1+r2)/4.

Verify continuity of main protective bonding conductors between the main earthing terminal and each bonded service; the value should be negligible.

Continuity of protective conductors must be confirmed before insulation resistance testing, otherwise the insulation test may be invalid.

Expected values can be derived from conductor resistance tables (mV/A/m multiplied by length) or from Guidance Note 3 tables.

Also confirm continuity of exposed conductive parts, protective screens, trunking, earthing terminals and circuit protective conductors at every accessory.

Apply 500 V DC for circuits up to 500 V; the minimum acceptable result is 1 MΩ.

For SELV and PELV circuits apply 250 V DC with a minimum acceptable result of 0.5 MΩ.

Test between live conductors (line to neutral and line to line) and between each live conductor and earth.

Disconnect or remove sensitive equipment - dimmers, electronic ballasts, IT equipment, surge protective devices - before testing.

Isolate the supply, close all switches, remove lamps or link them out, and ensure all circuit conductors are included in the test.

Test each circuit separately where practicable and record results on the schedule; a whole-installation test is acceptable if readings are satisfactory.

Readings below 1 MΩ require investigation; subdivide the circuit to locate moisture, damaged insulation or a connected load.

Insulation resistance is a dead test and must be completed before polarity, earth fault loop impedance and RCD live tests.

Zs = Ze + (R1+R2); compare measured Zs with 80% of the tabulated maximum Zs because tabulated values assume conductors at 70 °C.

Alternatively confirm Zs does not exceed Uo divided by Ia, where Ia is the current causing disconnection within the required time.

Typical maximum external loop impedance values are 0.8 Ω for TN-S and 0.35 Ω for TN-C-S; TT systems rely on RCD protection.

For TT systems the electrode resistance must not exceed 50 V divided by the RCD residual operating current: 1667 Ω at 30 mA, 500 Ω at 100 mA, 167 Ω at 300 mA, 100 Ω at 500 mA.

Earth electrode resistance is measured by the fall-of-potential (62%) method, the three-pole method, or a clamp meter where a suitable parallel path exists.

Space the test stakes adequately and repeat the measurement with the potential stake moved to confirm the reading is stable.

Disconnect the main earthing conductor from the main earthing terminal when measuring electrode resistance to avoid parallel paths through bonding.

Prospective fault current is measured at the origin and compared with the breaking capacity of the protective devices; check both short-circuit and earth fault values.

Half rated residual current (½ IΔn): the RCD must not operate.

Rated residual current (1× IΔn): a general RCD must operate within 300 ms.

Five times rated residual current (5× IΔn): a general RCD must operate within 40 ms where IΔn is 30 mA or more.

Type S time-delayed RCDs: 130 ms to 500 ms at 1× IΔn, and 50 ms to 150 ms at 5× IΔn.

Test at both 0° and 180° and record the longer operating time on the schedule of test results.

Select the correct test waveform for the device type: alternating current, pulsating direct current (Type A), Type F, or smooth direct current (Type B).

Operating the integral test button confirms mechanical and electrical function only and is recorded separately from instrument tests.

Confirm the RCD is correctly connected (supply to line terminals), and that its sensitivity and rating match the design.

Typical maximum inspection intervals: domestic 10 years or change of occupancy; rented domestic 5 years; commercial and industrial 5 years; many industrial premises 3 years.

Sampling typically covers at least 10% of circuits including at least one from every distribution board, and is increased where defects are found or records are absent.

Observation codes: C1 danger present and immediate action required; C2 potentially dangerous and urgent action required; C3 improvement recommended; FI further investigation required.

The report is satisfactory only where no C1, C2 or FI observations are recorded; any one of these makes it unsatisfactory.

Record the extent and limitations of the inspection and state whether the conclusions are affected by them.

Assess the installation against the current edition of BS 7671, but do not require upgrading of compliant older work unless it is unsafe.

The report includes the schedule of inspections, circuit details and test results, and a summary of the overall condition.

The next inspection date is recommended by the inspector based on the type of installation, its use, its environment and its condition.

An Electrical Installation Certificate covers new installations, alterations and additions, and carries declarations from designer, constructor and inspector/tester.

A Minor Electrical Installation Works Certificate covers work that does not extend or alter a circuit and does not require a schedule of inspections.

An Electrical Installation Condition Report is used for periodic inspection and includes schedules of inspections and of circuit details and test results.

Certificates record client and installation details, extent of work, supply characteristics, departures, test results and signatures.

Every departure from BS 7671 must be recorded on the certificate together with the reasons for it.

Give the original certificate to the person ordering the work and retain a duplicate for the period until the next inspection or as required.

The schedule of test results records R1+R2, insulation resistance, Zs, RCD operating times, polarity, prospective fault current and electrode resistance where applicable.

The recommended date of the next inspection must be entered on the certificate or report.

Compare each measured value with its design or calculated value: R1+R2 against the expected value, insulation resistance against 1 MΩ, Zs against 80% of the tabulated maximum, RCD times against the limits.

A high Zs suggests a loose termination, an overlong run, an undersized protective conductor or a high external loop impedance; investigate before concluding.

A low insulation resistance indicates moisture, damaged insulation or connected equipment; subdivide the circuit to locate the cause.

C1 observations require immediate action - isolation or other measures to remove the danger - and the client must be informed without delay.

Distinguish C2 (potentially dangerous, urgent remedial work) from C3 (improvement recommended) in plain language and give indicative timescales.

State clearly how recorded limitations affect the conclusion; never claim full compliance for parts that were not inspected.

Advise clients that the RCD test button is a functional check only and does not verify operating times or sensitivity.

Provide a written, prioritised summary of recommended remedial actions alongside the formal report.