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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What are the BS 7671 requirements for a standby generator?

A standby generator is a switched alternative supply under BS 7671 Section 551. The installation must prevent the generator and the public supply being connected in parallel unless designed for it — normally via a changeover or automatic transfer switch that switches all live conductors. Where an RCD provides additional protection on the generator circuit it must disconnect all live conductors including the neutral (Reg 551.6.2), and the earthing/neutral arrangement must suit the system. A grid-connected (G99) install also needs DNO approval.

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Key Takeaways

  • 1Standby 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.
  • 2An 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).
  • 3Engineering 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.
  • 4Generator 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.
  • 5Diesel 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.
  • 6Load 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.
  • 7BS 7671:2018+A4:2026 Reg 551.7.1(d) prohibits connecting a generator (or other source) to the load side of an RCD under the conditions specified in that regulation — a common wiring mistake in ATS panels that must be avoided.
  • 8A 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).
  • 9Where 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): Generator Must Not Connect to RCD Load Side

BS 7671:2018+A4:2026 Reg 551.7.1(d) introduces an explicit prohibition: a generator (or other source such as an inverter) must not be connected to the load side of an RCD under the conditions specified in that regulation. This is a common wiring mistake in ATS panels where the generator output is inadvertently terminated on the RCD-protected side of the distribution board. Verify the connection point on both the mains and generator sides of any ATS panel and confirm compliance 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 (i.e., has no connection between its neutral and a local earth electrode) presents a specific shock risk: 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 (double-insulated) equipment. Because Class II equipment relies on supplementary 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 the absence of an earth reference means a concurrent fault on a second Class I item would not be detected.
  • 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 — G99 requires that generators above the threshold are fitted with loss of mains detection to disconnect from the network if the mains supply fails (anti-islanding protection). 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. The earthing approach 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 411.9.3 Checklist

BS 7671:2018+A4:2026 Reg 133.1.3 now requires that certain equipment usage is explicitly recorded on the appropriate Part 6 electrical certification. For generator installations, the Reg 411.9.3 commissioning checklist includes a specific item confirming that adequate arrangements for a generating set operating as a switched alternative to the public supply are provided in accordance with Reg 551.6 — this confirmation must appear on the EIC. The Elec-Mate EIC app captures this field so the certification record is complete before you leave site.

A4:2026 Reg 421.1.7 — AFDD Recommendation for Generator-Fed Fixed Installations

BS 7671:2018+A4:2026 Reg 421.1.7 recommends the installation of arc fault detection devices (AFDDs) in AC final circuits of fixed installations to mitigate the risk of fire from arc fault currents. This recommendation applies to generator-fed fixed installations in the same way as mains-fed installations — where a generator supplies a building's final circuits, consider AFDD protection as part of the installation design, particularly in higher-risk premises (sleeping risk, older wiring, high fire consequence). Note that the regulation uses recommendatory wording rather than a mandatory 'shall', but the recommendation should be recorded in the design rationale and discussed with the client.

Complete the EIC on Site

Use the Elec-Mate EIC app to record generator installation test results — earth electrode resistance, insulation resistance, loop impedance, ATS transfer time — and complete the Electrical Installation Certificate on your phone. Export a professional PDF certificate before you leave site, including notes on the G99 connection if applicable.

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