What is the difference between a standby generator and a prime power generator?

A standby generator is designed to provide temporary power during mains failures. It is rated for standby power (kVA/kW standby rating) which assumes the generator runs for a maximum of 200 hours per year, with typical average loads of 70–80% of standby rating. A prime power generator is designed for continuous operation as the primary source of power (e.g., off-grid sites, events, construction). It is rated for prime power (a lower kVA/kW rating than the same physical generator in standby mode) and can run continuously at 70–80% of prime rating, with brief peaks to the prime rating. Never operate a standby-rated generator continuously at high load — it will overheat and fail prematurely.

How long does an automatic transfer switch take to restore power?

The total changeover time depends on three components: the time for the ATS controller to detect the mains failure (typically 2–10 seconds, configurable), the engine start and warm-up time (typically 10–30 seconds for a diesel engine to reach stable voltage and frequency), and the transfer time (typically less than 1 second for the changeover contactors to operate). Total switchover time from mains failure to generator supply is therefore typically 15–45 seconds. For critical loads (data centres, hospitals, operating theatres), a UPS (Uninterruptible Power Supply) bridges this gap. On return of the mains supply, the ATS retransfers the load back to mains (typically after a 30–300 second stability timer confirms the mains has stabilised).

When does Engineering Recommendation G99 apply to a generator installation?

ER G99 applies to any generator connected to the distribution network that can export electricity to the network. The trigger threshold is above 16 A per phase (approximately 3.68 kW single phase, 11 kW three phase). Even a generator that is only intended for standby use (not for export) may need G99 compliance if it is connected to the mains in a way that could inadvertently export. Generators operating only in island mode (completely disconnected from the mains during generator operation, confirmed by a break-before-make changeover switch or interlocked ATS) do not require G99. However, generators connected via a static transfer switch that can operate in parallel with the mains, even briefly, require G99 notification and compliance.

How should a standby generator be earthed?

A standby generator operating in island mode (disconnected from the mains) should be earthed using a TN-S arrangement: the generator neutral is connected to a local earth electrode at the generator, and a protective earth conductor is run from this electrode to the changeover panel and connected to the installation PE conductors. The generator earth must not be bonded to the mains earth (via the MET) while the generator is operating in island mode — bonding the two earth systems together when the mains neutral is not present can create hazardous voltage between the two earth systems. The ATS wiring should include switching or interlock to manage the earthing. In practice, modern four-pole ATS (which switches neutral as well as the three phases) provides a clean solution for generator earthing.

What are the regulations for diesel fuel storage with a generator?

Diesel fuel storage for generators in England is regulated by the Control of Pollution (Oil Storage) (England) Regulations 2001 (for above-ground storage above 200 litres at non-domestic premises). Requirements include: a secondary containment bund with a capacity of at least 110% of the largest tank, or 25% of the total volume of all tanks (whichever is greater); bund must be impermeable, resistant to oil, and strong enough to retain oil in a leak; the bund must not have a drain valve that can be opened accidentally; tanks must be structurally sound and clearly labelled. Above 1,500 litres, Planning Permission is typically required. Above 3,000 litres in an SSSI or near a watercourse, additional Environment Agency guidance applies. Scotland has similar but separate regulations.

What is wet stacking in a diesel generator and how is it prevented?

Wet stacking occurs when a diesel engine is run for extended periods at very low load (typically below 30% of rated output). At low load, the engine does not reach full operating temperature, combustion is incomplete, and unburnt fuel and carbon deposits accumulate in the exhaust system, turbocharger, and cylinder walls. This reduces engine efficiency, increases fuel consumption, and can cause engine damage over time. Wet stacking is particularly common in standby generators that are only test-run at no-load or light load once a week. Prevention: run the generator at a minimum of 75% rated load during monthly or quarterly test runs, using a load bank if the connected site load is insufficient. Annual load bank testing at 100% rated load for 2 hours burns off wet stacking deposits.

What is the recommended maintenance schedule for a diesel standby generator?

A typical maintenance schedule for a diesel standby generator: weekly — automated self-test run (no-load, 15 minutes, log any alarms); monthly — manual test run under load if possible, check coolant level, battery electrolyte, fuel level, oil level, visual inspection; quarterly — oil and filter change (or per manufacturer hours — typically every 250 hours or 12 months whichever first), air filter inspection, coolant system inspection, fuel filter inspection, battery load test, exerciser clock setting; annually — full service per manufacturer schedule, load bank test at rated load for 2 hours, cooling system flush and inhibitor change, transfer switch operation test, ATEX inspection if in hazardous area. All maintenance should be recorded in a generator log.

INDUSTRIAL GUIDE

Generator Installation Guide UK: Standby Generator Electrical

Everything UK electricians need to know about standby generator installation — standby vs prime power ratings, automatic transfer switches, manual changeover, G99 DNO requirements, generator earthing in TN-S systems, fuel storage regulations, testing schedules, and load bank testing.

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16 min readUpdated 2026-06-10Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

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The short answer

What are the BS 7671 requirements for a standby generator?

A standby generator is a switched alternative supply under BS 7671 Section 551. Reg 551.6.1 requires precautions complying with Chapter 46 and Section 537 so the generator cannot operate in parallel with the public supply — an interlock between the changeover devices, a system of locks with a single transferable key, a three-position break-before-make changeover switch, an automatic changeover switching device with a suitable interlock, or equivalent. Reg 551.4.3.2.1 requires a dedicated means of earthing: automatic disconnection must not rely on the earthed point of the public supply when the generator runs as a switched alternative to a TN system. Where the set may also run in parallel, Reg 551.7.1(b) requires any RCD providing additional protection on the generator circuit to disconnect all live conductors including the neutral, and a grid-connected (G99) install needs DNO approval.

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Key takeaways

01Standby generators must never be connected in parallel with the mains supply without an approved isolation/changeover system — simultaneous connection causes phase opposition and can injure DNO personnel working on the network.
02An Automatic Transfer Switch (ATS) monitors the mains supply and automatically starts the generator and transfers the load within seconds of a mains failure — the switchover time depends on the engine start time (typically 10–30 seconds).
03Engineering Recommendation G99 applies to any generator connected to the distribution network that exports power (above 16 A per phase) — G99 mandates loss of mains (LoM) protection, anti-islanding, and DNO approval before connection.
04Generator earthing in a TN-S installation uses a separate earth electrode at the generator, with the generator neutral bonded to the electrode — the generator is not bonded to the site mains earth when operating in island mode.
05Diesel fuel storage above 1,500 litres requires Planning Permission and must comply with the Control of Pollution (Oil Storage) (England) Regulations 2001, including a secondary containment bund sized for 110% of the largest tank volume.
06Load bank testing at 100% rated load for a minimum of 2 hours annually verifies generator performance and burns off wet stacking deposits in diesel engines running on light load during routine weekly tests.
07Where a generating set may run in parallel with another source, BS 7671:2018+A4:2026 Reg 551.7.1(d) prohibits connecting the source to the load side of any RCD providing additional protection (Reg 415.1) that is shared with other circuits, unless that RCD disconnects all live conductors including the neutral — a common wiring mistake in closed-transition ATS panels.
08A portable generator that is isolated from earth may only supply: (a) one or more Class II items; or (b) one Class I item; or (c) one or more Class II items together with one Class I item — never more than one Class I item simultaneously (OSG Reg 2.4.3).
09Where a rotary generating set is used as a safety source (emergency lighting, fire systems, hospitals), it must conform to BS ISO 8528-12 — a mandatory requirement under BS 7671:2018+A4:2026 Reg 560.6.13.

01 · Industrial Guide

Standby Power vs Prime Power: Choosing the Right Generator

The first step in any generator installation project is correctly specifying whether a standby or prime power machine is required. Specifying a standby-rated generator for an application that requires continuous running will result in premature failure and will almost certainly void the manufacturer's warranty. Standby generators are rated for limited annual hours at 100% load; prime power generators are rated for continuous operation.

Standby power rating (ESP) — the output available for the duration of a power outage, typically assumed as no more than 200 hours per year. Average load should not exceed 70% of standby rating. No overload rating is available. Suitable for emergency power backup in commercial and industrial premises.
Prime power rating (PRP) — the maximum output available for continuous operation with a variable load. Prime rating is typically 80–90% of standby rating from the same physical generator. Suitable for off-grid sites, temporary power, events, and peak shaving installations.
Continuous power rating (COP) — the maximum output for unlimited hours at 100% load with no variable load capability. The lowest rating for a given engine. Suitable for base load generation and industrial power plant where the generator runs continuously at a constant output.
Load assessment — before specifying generator kVA, calculate the maximum coincident demand including motor starting (which can draw 6–8× running current). The generator must maintain stable voltage and frequency during the largest motor start. Generator sets are typically selected to be at least 1.5× the largest motor starting kVA to maintain voltage above 80% during starting.

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02 · Industrial Guide

Automatic Transfer Switch (ATS)

An Automatic Transfer Switch (ATS) continuously monitors the mains supply voltage and frequency. On detection of a mains failure, the ATS sends a start signal to the generator, waits for the generator to reach stable voltage and frequency, then transfers the load from mains to generator. On mains restoration, the ATS retransfers the load back to mains after a stability timer has confirmed the mains supply has recovered.

Open transition ATS — the most common type. Load is disconnected from the mains before being connected to the generator (break-before-make). There is a brief interruption (typically less than 100 ms for the transfer contactor operation) plus the generator start time. Suitable for most commercial and industrial applications.
Closed transition ATS — the generator is synchronised to the mains supply (matched voltage, frequency, and phase angle) before the transfer, allowing a make-before-break transfer with zero interruption. Used for critical loads that cannot tolerate any interruption. Requires G99 compliance as the generator is momentarily paralleled with the mains.
Four-pole vs three-pole ATS — a four-pole ATS switches neutral as well as the three phases. This is required where the generator has a separate earth (TN-S) to prevent the mains neutral and generator neutral being connected together during the transfer, which would create a neutral-to-neutral current path. A three-pole ATS leaves the neutral connected to the mains throughout, which is only safe if the generator neutral is bonded to the same earth as the mains.
Mains monitoring — the ATS controller monitors voltage on all three phases (configurable thresholds, typically trip below 85% and above 110% of nominal voltage), frequency (typically trip below 47.5 Hz or above 52 Hz), and optionally phase rotation. The mains failure detection time is configurable — a delay of 2–5 seconds prevents nuisance starts from brief transients.

A4:2026 — Reg 551.7.1(d): Connecting a Source to the Load Side of a Shared RCD

Amendment 4 redrafted Reg 551.7.1 and added indents (c) and (d). Section 551.7 applies where a generating set may operate in parallel with another source, so it catches closed-transition (make-before-break) ATS arrangements. Indent (d) states that, except where the RCD disconnects all live conductors including the neutral conductor, a source of supply shall not be connected to the load side of any RCD providing additional protection in accordance with Reg 415.1 that is shared with other circuits. Indent (b) separately requires that where an RCD provides additional protection for the circuit connecting the generating set to the installation, that RCD shall disconnect all live conductors including the neutral. Together they rule out a common panel error — terminating the generator output on the RCD-protected side of a board where that RCD also feeds other circuits. Verify the connection point on both the mains and generator sides of any ATS panel before completing the Electrical Installation Certificate.

03 · Industrial Guide

Manual Changeover Switch

Where automatic starting is not required or practical, a manual changeover switch (also called a generator changeover switch or transfer switch) provides a safe means of connecting a generator to the installation while ensuring that the mains and generator supplies can never be connected simultaneously. Manual changeover is common for smaller installations, domestic standby, and rental generator connections.

Break-before-make operation — the changeover switch must have a centre-off position (Mains — Off — Generator) or a mechanical interlock that positively prevents both mains and generator being connected simultaneously. Never use two separate isolators as a changeover arrangement — there is no guarantee both cannot be closed at the same time.
Rating — the changeover switch must be rated for the full prospective load current on both the mains and generator sides. Use a purpose- made generator changeover switch (e.g., from Socomec, ABB, Lewden), not a general-purpose isolator or double-pole switch. The switch must be AC-23 rated (motor load switching) if motor loads are connected.
Temporary generator connections — temporary connections to rental generators (via a Ceeform inlet or MK7 inlet connector) must use a purpose-made changeover panel, not a trailing lead plugged into an existing socket outlet. The inlet connector must be lockable and protected from accidental contact when not in use.
Notification and labelling — the changeover panel must be clearly labelled to indicate the mains and generator positions and the changeover procedure. The generator connection point and the changeover switch must be included in the EICR inspection scope. The presence of a generator changeover must be noted in the Electrical Installation Certificate.

04 · Industrial Guide

Portable Generator Rules: Class I and Class II Equipment (OSG Reg 2.4.3)

A portable generator that is isolated from earth — the windings isolated from the mass of Earth and from the protective conductor, with no connection between the chassis or the earth connection of the socket-outlets and either live conductor of the winding — presents a specific shock risk. The arrangement is a form of electrical separation (BS 7671 Section 413): basic protection comes from basic insulation, barriers or enclosures, and fault protection from simple separation of the circuit from other circuits and from Earth. There is no earth fault reference, so a fault to an earthed Class I enclosure will not necessarily cause a protective device to operate. The On-Site Guide (OSG) Reg 2.4.3 addresses this by restricting the combinations of equipment that may be connected to such a generator.

Permitted permutation (a) — Class II only: a portable generator isolated from earth may supply one or more items of Class II equipment. Because Class II equipment relies on double or reinforced insulation rather than a protective earth conductor, the absence of an earth reference on the generator does not create a shock risk. This is the safest and most common configuration for site power tools.
Permitted permutation (b) — one Class I item only: a portable generator isolated from earth may supply a single item of Class I equipment (which has a protective earth conductor). Only one Class I item is permitted because on a separated system a first fault goes undetected — it does not cause a protective device to operate. If a second Class I item then develops a fault, a person touching both can become the path for current flowing between the exposed-conductive-parts of the two faulty items.
Permitted permutation (c) — Class II items plus one Class I item: a portable generator isolated from earth may supply one or more Class II items together with one Class I item simultaneously. Multiple Class II items are safe; the single Class I item is the controlled variable — never connect more than one Class I item to an isolated-from-earth generator.
Verify before connecting: check the equipment marking before connecting to an isolated portable generator. Class II equipment is identified by the square-within-a-square symbol. Class I equipment has a protective earth terminal. If in doubt about the class of any item, treat it as Class I. Where the site requires multiple Class I items (e.g., portable lighting with metal fittings, power tools without double-insulation), the generator neutral must be bonded to a local earth electrode to provide an earth reference.

05 · Industrial Guide

DNO Requirements: Engineering Recommendation G99

Engineering Recommendation G99 (Requirements for the Connection of Generation Equipment in Parallel with Public Distribution Networks Operated by Distribution Network Operators) governs the connection of all generators above 16 A per phase (approximately 11 kW three-phase) that are capable of operating in parallel with the mains supply. Compliance with G99 is a legal requirement under the Electricity Safety, Quality and Continuity Regulations 2002 (ESQCR) and must be achieved before connection.

Loss of mains (LoM) protection — BS 7671 Reg 551.7.4 already requires means of automatic switching to disconnect the generating set from the public distribution system on loss of that supply, or on deviation of the voltage or frequency at the supply terminals from declared values; Reg 551.7.5 requires means to prevent connection under the same conditions. For a set with an output exceeding 16 A, Reg 551.7.4 states the type of protection and its sensitivity and operating times shall be agreed with the distributor — which in Great Britain is done through the G99 application. The generator must not continue to supply the network when mains power has been lost, as this creates a hazard for DNO staff working on the network under the belief that it is de-energised.
Frequency and voltage protection — G99 specifies minimum operating frequency and voltage ranges (Stage 1: 47.5–52 Hz, +10%/−15% voltage; Stage 2: 47.0–52 Hz, +10%/−20% voltage for Type A generators). Generators must automatically disconnect if operating conditions fall outside these ranges. Protection relay settings must be agreed with the DNO as part of the G99 application.
Application process — submit a G99 connection application to the DNO before installation. The DNO assesses the impact on the network and provides connection conditions (protection relay settings, power factor requirements, monitoring requirements). The connection cannot be energised until the DNO has issued a connection agreement. Commissioning of the protection system must be witnessed or verified by the DNO or their representative.
Island mode generators — G99 applicability — a generator that only operates in island mode (no electrical connection to the mains at any time during operation, achieved by an interlocked break-before-make changeover switch) does not export to the network and does not require G99 compliance. The changeover interlock must be robust — a simultaneous connection, even briefly, brings G99 into scope.

06 · Industrial Guide

Earthing the Generator (TN-S Separate Earth)

Generator earthing requires careful design to ensure safe operation both when the generator is running in island mode (mains disconnected) and during changeover. BS 7671 Reg 551.4.3.2.1 is the governing requirement: protection by automatic disconnection of supply shall not rely upon the connection to the earthed point of the public supply when the generator is operating as a switched alternative to a TN system, and a suitable means of earthing shall be provided. The earthing approach then depends on whether the ATS is three-pole or four-pole, and on the site mains earthing system (TN-S, TN-C-S, or TT).

Generator earth electrode — the generator neutral is connected to a local earth electrode sited near the generator (typically a driven copper-clad rod or a ring electrode). There is no single fixed Ω limit applicable to all LV generators; the electrode resistance must be assessed against the ability of the protective devices to operate under earth fault conditions (BS 7430 method, using a proprietary earth electrode resistance tester — do not use an earth fault loop impedance tester for this measurement). The generator earth must be tested and recorded at commissioning.
Four-pole ATS earthing — with a four-pole ATS, the neutral is switched between mains and generator. When on mains, the installation neutral is connected to the mains PEN (TN-C-S) or mains neutral (TN-S). When on generator, the installation neutral is connected to the generator neutral. The generator neutral is bonded to the generator earth electrode via a removable link that is only closed when the generator is selected.
Three-pole ATS earthing — with a three-pole ATS, the neutral remains permanently connected to the mains PEN terminal. The generator neutral is also connected to this same neutral bar. This is acceptable where both the mains and generator use the same earth (common on sites with private HV/LV substation where the generator shares the transformer earth). Not suitable where the generator has a separate independent earth.
PME and generators — connecting a generator to a site with a TN-C-S (PME) supply requires special consideration. The generator neutral must not be bonded to the PME earth when operating in island mode, as the generator neutral voltage under unbalanced load would cause current to flow through the PME earth network. Consult the DNO earthing requirements and follow IET Guidance Note 7 (Special Locations) Section 3.

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07 · Industrial Guide

Fuel Storage Regulations

Diesel fuel storage for generators is regulated in England primarily by the Control of Pollution (Oil Storage) (England) Regulations 2001 (for storage above 200 litres at non-domestic premises). Additional requirements apply for large storage volumes and storage near watercourses. Non-compliance can result in Environment Agency prosecution and significant clean-up costs if a fuel spill reaches a watercourse.

Secondary containment bund — must contain at least 110% of the capacity of the largest tank, or 25% of the total capacity of all tanks, whichever is greater. Bund must be impermeable to oil and water, resistant to attack by the stored oil, and strong enough to retain oil under all foreseeable conditions. The bund must not have a drain valve that is left open.
Pipework and valves — all fill points must be within the bunded area or have a drip tray. Vent pipes must discharge safely. Sight gauges must be fitted with automatic shut-off valves. Fuel transfer pipes must be secure and where possible run above ground for visual inspection. Underground pipework must be double-skinned with leak detection.
Planning permission threshold — diesel storage above 1,500 litres typically requires Planning Permission as permitted development rights do not extend to oil storage above this volume in most commercial and industrial situations. Check with the local planning authority before installing a large base tank.
Fire safety — diesel (Class C2 liquid, flash point above 55°C) has a relatively high flash point and is less volatile than petrol, but storage areas must still have adequate ventilation, no ignition sources, appropriate signage, and fire extinguishers. The generator room should have fire detection and, for large installations, automatic suppression.

08 · Industrial Guide

Testing and Maintenance Schedule

Standby generators that are never tested cannot be relied upon when needed. A structured testing and maintenance regime is required under the Electricity at Work Regulations 1989 (which require all electrical equipment to be maintained to prevent danger), and is typically a condition of the building insurance policy and business continuity planning for critical facilities.

Weekly automated test — modern ATS controllers include a programmable exerciser clock that starts and runs the generator for 15–30 minutes once a week (typically at a low-impact time such as Saturday morning). This confirms the engine starts, oil pressure builds, and coolant temperature is normal. Results are logged. Alarms must be reviewed promptly.
Monthly mains failure simulation test — physically simulate a mains failure (by opening the mains incomer) and verify the ATS operates correctly, the generator starts and reaches stable output, and the transfer occurs within the specified time. Then restore mains and verify the retransfer to mains. Record switchover times and voltage/frequency at transfer.
Quarterly service — check engine oil level, coolant level and inhibitor concentration, battery state of charge and electrolyte level (flooded lead-acid), fuel level, fuel contamination (diesel bug), belt tension, and all fluid hoses. Run the generator under available building load for at least 30 minutes. Change the oil filter and fuel filter per the engine manufacturer's hours-based schedule.
Annual service and load bank test — full engine service per manufacturer schedule, alternator insulation resistance test, ATS protection relay secondary injection test (confirm trip settings), fuel system inspection, load bank test at 100% rated load for minimum 2 hours. All results recorded in the generator maintenance log and compared to previous results for trending.

Safety Sources: Reg 560.6.13 and BS ISO 8528-12 (Hospitals, Fire Systems, Emergency Lighting)

Where a rotary generating set is used as a safety source — supplying emergency lighting, fire detection, evacuation systems, or other life-safety services — BS 7671:2018+A4:2026 Reg 560.6.13 imposes a mandatory requirement: the generating set shall conform to BS ISO 8528-12. This conformity requirement is triggered by the safety-source role, not by generator size or site type. The commissioning pack for any such installation must include documentation evidencing BS ISO 8528-12 compliance. Inspectors completing an EICR for hospitals, care homes, or premises with emergency lighting should verify this documentation is present.

09 · Industrial Guide

Load Bank Testing

Load bank testing applies a known resistive or resistive-inductive load to the generator to verify its rated output under controlled conditions. A dedicated portable load bank (resistive load units connected to the generator output) is used where the site load is insufficient or unavailable during the test. Load bank testing is the definitive verification of generator performance and is essential for burning off wet stacking deposits.

Step loading protocol — apply load in steps: 25%, 50%, 75%, 100% of rated kW. Allow voltage and frequency to stabilise at each step (typically 5 minutes) before increasing. Record voltage, frequency, current (all three phases), coolant temperature, oil pressure, exhaust temperature, and fuel consumption at each step. This detects derating issues (overloaded cooling system, fuel starvation) that would not be apparent at light load.
100% load duration — maintain 100% rated load for a minimum of 2 hours. This verifies the cooling system can sustain full-load operation and allows wet stacking deposits to burn off from the exhaust system. Coolant temperature should reach and maintain its normal operating temperature (typically 82–90°C for modern diesel engines).
Transient response test — apply a step load change from 0% to at least 60% of rated load in a single step and record voltage and frequency dip and recovery time. Compare with the generator manufacturer's specification (typically voltage recovery within 10% dip in less than 3 seconds, frequency recovery within 2% in less than 5 seconds). Poor transient response indicates governor or AVR problems.
Load bank connection — connect the load bank at the generator output terminals (or at the ATS generator bus when the ATS is in generator position and the building load is disconnected). Ensure the load bank is correctly rated and that the connection cables are sized for the full test current. Use temporary cable protection (rubber ramp covers or cable bridges) where cables cross access routes.

10 · Industrial Guide

For Electricians: Generator Installation Certification

Generator installation work — including ATS panels, changeover switches, cable installation, and earthing — requires an Electrical Installation Certificate under BS 7671. Where a G99 connection is made, the commissioning of the G99 protection relay must be documented and a copy provided to the DNO. The EIC must note the presence of a generator and confirm that the changeover arrangement prevents simultaneous connection of mains and generator.

A4:2026 Certification Requirements — Reg 133.1.3 and the Schedule of Inspections

BS 7671:2018+A4:2026 Reg 133.1.3 requires that where equipment is not in accordance with Reg 133.1.1, or is used outside the scope of its standard, the designer or other person specifying the installation confirms it provides at least the same degree of safety, and that such use is recorded as a departure on the appropriate electrical certification specified in Part 6. Separately, the Part 6 Schedule of Inspections carries dedicated line items for generators: adequate arrangements where a generating set operates as a switched alternative to the public supply (551.6), adequate arrangements where it operates in parallel with the public supply (551.7), and a dedicated earthing arrangement independent of that of the public supply (551.4.3.2.1). These must be signed off on the certification for a generator installation. The Elec-Mate EIC app captures these fields so the certification record is complete before you leave site.

A4:2026 Reg 421.1.7 — AFDDs Are Now Required in Four Premises Types

Amendment 4 redrafted Reg 421.1.7. It is no longer purely a recommendation: AFDDs conforming to BS EN 62606 shall be provided for single-phase AC final circuits supplying socket-outlets with a rated current not exceeding 32 A in (a) high rise residential buildings (HRRBs), (b) houses in multiple occupation (HMOs), (c) purpose-built student accommodation and (d) care homes. Note 1 to the regulation treats an HRRB as a residential building over 18 m in height or in excess of six storeys, whichever is met first. For all other premises the use of AFDDs is recommended for the same single-phase socket-outlet circuits not exceeding 32 A. Where used, AFDDs shall be placed at the origin of the circuit to be protected. This applies to generator-fed fixed installations in the same way as mains-fed ones — so where a generator supplies a building's final circuits, first check whether the premises fall into one of the four mandatory categories, and record the decision either way in the design rationale. Using AFDDs does not obviate the need to apply the other protective measures in BS 7671.

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