BS 7671 Guide · Section 11
Quick reference
The numbers and checklists you reach for most, on one page. Every table is a Tradefox summary: use it to jog your memory, then confirm the detail in the current edition before you sign anything.
Key dates for Amendment 4
| Date | What changed | What it means on site |
|---|---|---|
| 15 April 2026 | BS 7671:2018+A4:2026 published | You could design to A4 from this day |
| 15 October 2026 | BS 7671:2018+A2:2022+A3:2024 withdrawn | New designs must follow A4 |
| Designed before 15 October 2026 | Transitional allowance | Work designed to A2+A3 may be finished and verified to that version |
| From 15 October 2026 | Certificates and reports | New designs state BS 7671:2018+A4:2026 on the paperwork |
| Any time | Older installations | Not unsafe just because the standard moved on; no automatic upgrade needed |
Regulation references: BS 7671:2018+A4:2026 introduction (dates and transitional note). Full guide: What changed in Amendment 4
Maximum disconnection times (230 V)
| Circuit | TN system | TT system |
|---|---|---|
| Final circuit up to 63 A with socket-outlets, or up to 32 A feeding only fixed equipment | 0.4 s | 0.2 s |
| Distribution circuit, or any other final circuit | 5 s | 1 s |
RCD test on site: one AC test at the rated residual current (IΔn), whatever the RCD type. A general non-delay device must trip within 300 ms; a time-delayed S-type between 130 ms and 500 ms. The old 5 × IΔn test is no longer required.
Regulation references: 411.3.2.2, 411.3.2.3, 411.3.2.4, Table 41.1, 415.1.1, 643.8. Full guide: Protection against electric shock
Where a 30 mA RCD is needed
| Where | Requirement | Can it be left off? |
|---|---|---|
| Socket-outlets rated up to 32 A | 30 mA additional protection | Only outside dwellings, for sockets ordinary persons and children aren’t likely to use, and only with a documented risk assessment |
| Mobile equipment up to 32 A used outdoors | 30 mA additional protection | No |
| AC final circuits feeding luminaires in dwellings | 30 mA additional protection | No |
| Cables in walls or partitions less than 50 mm deep, or in partitions with metal parts | Under 50 mm: safe zone plus 30 mA RCD. Metal-part walls: 30 mA RCD at any depth (and a safe zone if shallow). Or a 522.6.204 method instead | Yes, if the cable has an earthed metal covering, is in earthed metal conduit, or is SELV/PELV. Check Table 52.1 |
| Rooms with a bath or shower | Every circuit serving the room | No |
| Outdoor lighting the public can reach (street furniture and similar) | 30 mA additional protection | Only if supplied by SELV |
Regulation references: 411.3.3, 411.3.4, 522.6 and Table 52.1, 701.411.3.3, 714.411.3.4. Full guide: Protection against electric shock
Choosing the RCD type
| Type | What it can sense | Typical loads |
|---|---|---|
| AC | Plain AC residual current only | Fixed equipment known to produce no DC, such as simple resistive heaters |
| A | AC plus pulsating DC | General domestic circuits: LED lighting, IT equipment, most appliances |
| F | Everything Type A senses, plus mixed-frequency current | Single-phase variable-speed kit: heat pumps, air conditioning, modern washing machines |
| B | Everything Type F senses, plus smooth DC | Three-phase drives and inverters; some PV, battery and EV set-ups |
When RCDs are in series, the upstream device must not be blinded by DC that a downstream device lets through:
- No Type AC upstream of Type A, F or B.
- No Type A upstream of Type F or B.
- No Type F upstream of Type B.
The one exception: a Type AC, A or F RCD may sit upstream of a Type B where the RCD manufacturer specifically declares the arrangement suitable. This comes from an informative note to Regulation 531.3.3, so check it and Figure A53.2 in your copy.
Regulation references: 531.3.3, Figure A53.2, 722.531.3.101. Full guide: Protection against electric shock and EV, solar, batteries and heat pumps
Where AFDDs are required
The rule applies to single-phase AC final circuits supplying socket-outlets rated up to 32 A. Use AFDDs to BS EN 62606.
| Premises | AFDD on those socket circuits |
|---|---|
| High-rise residential: a domestic building with any storey more than 18 m above ground | Required |
| Houses in multiple occupation (HMOs) | Required |
| Purpose-built student accommodation | Required |
| Care homes | Required |
| All other premises | Recommended |
| EV charge-point circuits to BS EN 61851 with BS EN IEC 62196-2 connectors | Not required |
Regulation references: 421.1.7, 722.421.1.7.201. Full guide: Fire, arc fault and surge protection
Insulation resistance
| Circuit | Test voltage (DC) | Minimum reading |
|---|---|---|
| SELV and PELV | 250 V | 0.5 MΩ |
| Up to 500 V, including FELV (normal 230/400 V work) | 500 V | 1 MΩ |
| Above 500 V | 1000 V | 1 MΩ |
Regulation references: 643.3, Table 64. Full guide: Inspection and testing
Voltage drop limits
| Installation fed from the public LV network | Limit | At 230 V |
|---|---|---|
| Lighting | 3% | 6.9 V |
| Other uses (power, heating) | 5% | 11.5 V |
Limits are measured from the origin of the installation to the furthest point. Installations on a private supply are allowed more; check Table 4Ab for the figures.
Regulation references: Section 525, Appendix 4 (Table 4Ab). Full guide: Cable selection and sizing
Main protective bonding: minimum copper sizes
| Supply arrangement | Minimum main bonding conductor |
|---|---|
| Not PME (TN-S, TT) | Half the earthing conductor size, never below 6 mm²; need not exceed 25 mm² |
| PME, supply neutral 35 mm² or less | 10 mm² |
| PME, supply neutral 50 mm² | 16 mm² |
| PME, supply neutral 70 mm² | 25 mm² |
| PME, larger supplies | Check Table 54.8 |
Regulation references: 544.1.1, Table 54.8. Full guide: Earthing and bonding
EICR observation codes
| Code | Meaning | What happens next |
|---|---|---|
| C1 | Danger present, risk of injury now | Act straight away; make safe before you leave if you can |
| C2 | Potentially dangerous | Urgent remedial work |
| C3 | Improvement recommended | Not dangerous; the report can still be satisfactory |
| FI | Further investigation advised | Advisory under A4: on its own, an FI does not make the report unsatisfactory. Use it rarely, and never for things you can code, such as low insulation resistance or an unidentified circuit. |
In privately and socially rented homes in England, landlords must get remedial or investigative work done within 28 days, or sooner if the report says so. GOV.UK guidance still treats FI as needing investigation, so tell landlords to follow every FI up.
Regulation references: 653.1, 653.2, Appendix 6. Full guide: Certificates and EICR codes
Pre-energisation checklist
Do these with the supply off, in this order, before anything goes live:
- Inspection complete: terminations tight, barriers fitted, cables supported, labels on.
- Continuity of protective conductors, including main and supplementary bonding.
- Ring final circuits: end-to-end and cross-connected readings.
- Insulation resistance at the right test voltage, sensitive kit disconnected.
- Polarity: switches and single-pole devices in the line conductor only.
- Earth electrode resistance on TT installations.
- Only then energise, and move on to Ze, Zs, fault current, RCD and functional tests.
Regulation references: Chapter 64, Section 643. Full guide: Inspection and testing
Safe isolation checklist
- Identify the right circuit and the right isolation point.
- Switch off and isolate.
- Lock off, keep the key on you, and fit a warning label.
- Prove your voltage indicator works on a known live source or proving unit.
- Test dead between every combination of line, neutral and earth.
- Prove the indicator again, straight after the dead test.
- Only now start work.
Regulation references: Section 537 (isolation and switching); Electricity at Work Regulations 1989, regulations 12 and 13. Full guide: Inspection and testing
Important. This guide is written by Tradefox to help qualified electricians understand BS 7671:2018+A4:2026. It is a plain-English summary for learning and reference only. It is not the standard, it does not reproduce it, and it is not professional advice. Always work to the current edition of BS 7671, the IET guidance notes, manufacturers’ instructions, Building Regulations and your scheme provider’s requirements. Electrical work must be carried out by a competent person.
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