BS 7671:2018 Amendment 3 — What Changed for Industrial Installations

Amendment 3 to BS 7671:2018 came into effect on 28 September 2022. If you work on industrial installations in the UK or any jurisdiction that adopts the IET Wiring Regulations, this is not optional reading. Several changes directly affect cable sizing, fault protection design, and protective device selection for industrial environments.

I have been through every clause change in Amendment 3. Most summaries you will find online focus on domestic installations — socket outlets, AFDDs in houses, that sort of thing. This article covers what matters for industrial and commercial work: substations, motor circuits, distribution boards, cable tray runs in plant rooms, and the equipment that keeps a mine or a process facility running.

Fault Protection — Regulation 411.3.3

The most consequential change for industrial installations is the revision to Regulation 411.3.3, which governs additional protection by RCDs.

BS 7671, Regulation 411.3.3 — Additional protection

Under Amendment 2, RCD protection (30 mA) was required for socket outlets up to 32 A in AC systems. Amendment 3 expands this requirement. The revised Regulation 411.3.3 now requires 30 mA RCD protection for:

• All socket outlet circuits rated up to and including 32 A
• Mobile equipment rated up to and including 32 A for use outdoors
• Circuits supplying luminaires in certain conditions

For industrial installations, the critical question is: does "all socket outlets up to 32 A" include industrial socket outlets on a factory floor? The answer is yes. A 32 A CEE-form industrial socket on a production line now requires 30 mA RCD protection unless a specific exception applies.

The practical impact on existing industrial installations is significant. Many factory distribution boards have 32 A industrial sockets protected by MCBs alone. Retrofitting RCD protection — either as RCBOs replacing MCBs or as upstream RCDs — requires reviewing discrimination with existing protection schemes and confirming that earth fault loop impedance values are still valid under the modified arrangement.

Cable Selection — Section 521

Section 521 deals with the selection and erection of cables, and Amendment 3 introduced several refinements that affect industrial cable installations.

BS 7671, Section 521 — Selection and erection of wiring systems

Regulation 521.10 — Cables in Thermally Insulated Walls

The revised Regulation 521.10.1 clarifies requirements for cables passing through thermally insulated spaces. For industrial buildings undergoing energy efficiency upgrades — adding insulation to walls and roofs — existing cable runs that were previously in free air may now be enclosed in thermal insulation. The current-carrying capacity must be reassessed using the appropriate derating factor from Table 4C5.

This matters for retrofit projects. A cable that was correctly sized for Method C (clipped direct) at installation may be inadequate after insulation is added around it. Amendment 3 makes explicit what was previously implied: the cable rating must be reassessed whenever the installation conditions change.

Regulation 521.11 — Cables in Accessible Floor Voids

For data centres, control rooms, and commercial buildings with raised access floors, Regulation 521.11 has been clarified. Cables in accessible floor voids must comply with Method B requirements unless they are laid on perforated cable trays (Method E) with adequate ventilation.

Protective Conductors — Regulation 543.1

Amendment 3 revised the requirements for sizing protective conductors (CPCs), which directly affects the cost and complexity of industrial cable installations.

BS 7671, Regulation 543.1 — Cross-sectional areas of protective conductors

The adiabatic equation method for sizing CPCs remains in Regulation 543.1.3:

The Adiabatic Equation

The minimum CPC cross-sectional area is calculated as:

S = (I squared t) / k

Where:

• S is the cross-sectional area in mm squared
• I is the fault current in amperes (RMS)
• t is the disconnection time of the protective device in seconds
• k is a factor dependent on conductor material, insulation, initial and final temperature (from Table 54.2 to 54.6)

Amendment 3 clarified that when using the adiabatic equation, the fault current I must be the prospective earth fault current at the most remote point of the circuit — not the prospective short circuit current at the distribution board. This distinction matters for long industrial cable runs where the impedance of the cable significantly reduces the fault current at the far end.

The practical consequence: for long circuits (which are common in industrial installations — I have sized 300-metre runs in processing plants), the reduced fault current at the remote end may allow a smaller CPC than the short circuit current at the origin would suggest. But you must also verify that the reduced fault current is still sufficient for the protective device to operate within the required disconnection time per Regulation 411.3.2.

AFDD Requirements — Regulation 421.1.7

Arc Fault Detection Devices (AFDDs) are the most debated addition in Amendment 3. The new Regulation 421.1.7 recommends AFDDs for:

• Higher risk residential premises (HMOs, purpose-built flats)
• Premises with sleeping accommodation
• Locations with combustible constructional materials (timber-framed buildings)
• Locations with combustible stored materials

BS 7671, Regulation 421.1.7 — Protection against the effects of arc faults

For industrial installations, the key phrase is "locations with combustible stored materials." A warehouse storing paper, textiles, timber, chemicals, or grain falls squarely within this recommendation. So does a paint shop, a solvent store, or a wood-working facility.

The practical challenge for industrial installations is nuisance tripping. AFDDs detect series and parallel arc faults by analysing the current waveform for characteristic arc signatures. In industrial environments, variable frequency drives (VFDs), welding equipment, soft starters, and other equipment produce current waveforms that can trigger AFDD false trips.

AFDD manufacturers have improved their algorithms significantly, but in my experience, installing AFDDs on circuits feeding VFDs still requires careful coordination. The AFDD must be compatible with the harmonic content of the VFD output. Some AFDD manufacturers provide compatibility lists for specific VFD brands and models — consult these before specifying.

Cable Sizing — Table 4D1B Adjustments

The current-carrying capacity tables in Appendix 4 received corrections in Amendment 3. Table 4D1B (multicore thermosetting insulated cables, copper conductors) had several values adjusted.

BS 7671, Table 4D1B — Current-carrying capacity for multicore cables with thermosetting insulation

The adjustments are small — typically 1-3 A on individual entries — but they compound when derating factors are applied. For borderline cable selections where the derated current is within 5% of the design current, the Amendment 3 values must be used.

These reductions mean that some cable selections made under Amendment 2 are now marginal under Amendment 3. A 25 mm² cable that was carrying 112 A with zero margin under Amendment 2 tables is now at its exact table value under Amendment 3 — meaning any derating factor application will push it over.

Overload Protection — Regulation 433.1

The coordination conditions between the protective device and the cable have been clarified in Amendment 3. The fundamental requirements remain:

BS 7671, Regulation 433.1 — Coordination between conductors and overload protective devices

Condition 1: I_b is less than or equal to I_n, which is less than or equal to I_z

Condition 2: I_2 is less than or equal to 1.45 times I_z

Where I_b is the design current, I_n is the nominal current of the protective device, I_z is the continuous current-carrying capacity of the cable, and I_2 is the conventional operating current of the protective device.

Amendment 3 added a note clarifying that I_z must be the derated value — the current-carrying capacity after ALL applicable correction factors (ambient temperature, grouping, thermal insulation, soil thermal resistivity) have been applied. This was always the correct interpretation, but the explicit note removes any ambiguity.

For industrial installations with multiple derating factors (high ambient temperature in a plant room, grouped cables on a tray, possible thermal insulation contact), the fully derated I_z may be significantly lower than the table value. The protective device must coordinate with this derated value, not the base table value.

What Existing Industrial Designs Need Review

If you have industrial installations designed under BS 7671:2018 Amendment 2 (or earlier), the following warrant review against Amendment 3:

Must Review

1. Socket outlet circuits up to 32 A — check RCD protection compliance with revised Regulation 411.3.3
2. Cables in areas where insulation has been added — verify current-carrying capacity under new thermal conditions
3. Borderline cable selections — check against adjusted Table 4D1B values
4. Protective conductor sizing on long circuits — verify fault current calculations use the correct point

Should Review

5. Facilities with combustible storage — consider AFDD recommendation under Regulation 421.1.7
6. Protection coordination — confirm I_z values used are fully derated per the Amendment 3 clarification
7. Earth fault loop impedance — verify Zs values if protection arrangements have changed due to RCD additions

What New Designs Must Incorporate

For new industrial installations designed after 28 September 2022:

1. RCD protection on all socket outlets up to 32 A — including industrial CEE-form sockets, unless a documented risk assessment justifies an exception
2. Use Amendment 3 current-carrying capacity tables — not the Amendment 2 values
3. Document AFDD decisions — even if you decide not to install AFDDs, document the risk assessment that led to that decision, particularly for locations with combustible materials
4. Use the clarified protective conductor sizing method — fault current at the most remote point, not the origin

Amendment 3 is not a wholesale rewrite of BS 7671. The fundamental principles of electrical installation design remain unchanged. But the specific changes — particularly around RCD requirements and cable rating corrections — have practical consequences that can affect material costs, panel layouts, and protection coordination schemes in industrial installations. Ignoring them is not an option.

Related Resources

• BS 7671 vs IEC 60364: Same Origin, Different Paths — How BS 7671 diverges from the international standard
• Earth Fault Loop Impedance: The Number That Decides Your Protection — Critical for verifying Zs after protection changes
• Protection Discrimination: MCB vs Fuse Coordination — Relevant when adding RCDs to existing schemes