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    Life Safety Generator selection guide

    On a recent London office refurbishment, with a steady running load of 353 kVA, our load profile showed a life safety generator of 800 kVA with every load starting together at 15 s and 715 kVA with staggered starting: the size depended on the starting step, not the running load. This guide covers the decisions a designer makes for a standby set to BS 8519:2020, from the load list to fuel, sizing, changeover, earthing, emissions and testing.

    Checklist

    NoDecisionWhat it depends onReference
    1Loads on the secondary supplyThe fire strategy. Fire alarm and emergency lighting are outside the scope of BS 8519[BS 8519:2020, 1; BS 9999:2017, 37.2.3.3]
    2Start timeWithin 15 s of the failure of the primary supply, for the critical life safety and fire-fighting load[BS 8519:2020, 6.3; BS 7671:2018+A4:2026, 560.4.1]
    3Motor data and start sequenceLocked rotor current, starting current and hot burn-out time from the pump/motor manufacturer[BS 8519:2020, 20.1; BS 7671:2018+A4:2026, 560.7.11]
    4Rating and sizePrime or standby rating; for standby, variable average power output not over 70 %; worst-case transient load; class G2 or better[BS 8519:2020, 6.3; BS 7671:2018+A4:2026, 560.6.13]
    5Regenerative liftsRegenerated power declared by the lift supplier; initial load acceptance of the set[BS 8519:2020, 20.4; BS EN 81-72:2020, Annex C]
    6Fuel storageStart signal (fire only, or any mains failure) and fault indication to a permanently manned control room: 4 h, 8 h or 72 h at full load; service tank for at least 120 min at rated power[BS 8519:2020, 6.3 and Table 2]
    7EnclosurePosition of the set: 120 min EI enclosure with 120 min ESa doors, unless the set is outside the building at ground floor or roof level; roof slab under a roof-mounted set 120 min REI[BS 8519:2020, 6.3]
    8Changeover and earthingAutomatic transfer switch (ATS) to BS EN 60947-6-1, switch technology (PC classification), four-pole; generator neutral point connected to the main earthing terminal[BS 8519:2020, 9; BS 7671:2018+A4:2026, 463.1.4 and 551.4.3.2.1; BS 7430:2011+A1:2015, 7.2.5]
    9Generator breaker and discriminationGenerator fault current with and without excitation boost; short time delay setting; time-current curves[BS 8519:2020, 6.3; Schneider Electric Electrical Installation Guide 2018, chapter N (manufacturer guidance)]
    10Indication at the main fire alarm panelPanel in the fire control room; mains healthy, generator running and generator fault, with a voltmeter and ammeter for the total load[BS 8519:2020, 6.3]
    11Emissions and planningRated thermal input of each unit (medium combustion plant (MCP) permit from 1 MWth to less than 50 MWth); standby use; planning conditions[gov.uk, Medium combustion plant: when you need a permit; GLA London Plan Guidance, Air Quality Neutral (February 2023), 3.3.1]
    12TestingManual control of on-load and off-load test conditions; no test interval in BS 8519 or BG 70. On a recent London office refurbishment: factory acceptance test (FAT), site load bank tests and integrated systems test (IST), under one protocol signed off by all parties[BS 8519:2020; BSRIA BG 70/2017, 3.1]
    Independent Primary & Secondary Power Supplies:
    • The secondary supply should be completely independent of the primary supply, preferably an automatically started standby generator. [BS 8519:2020]
    Fire-Resistant Enclosures:
    • Generators should be housed in a 120 min fire-resisting enclosure rated EI, with doors rated ESa, unless installed outside the building at ground floor or roof level. [BS 8519:2020, 6.3; BS EN 13501-2]

    Key Steps in Life Safety Generator Selection

    Identify Life Safety Loads

    List every load the fire strategy requires on the secondary supply. The list of equipment on the safety supply, with power, voltage, current, starting current and duration, shall be available. [BS 7671:2018+A4:2026, 560.7.11]

    LoadStandardNote
    Sprinkler and fire pumpsBS EN 12845Supply within 15 s [BS 8519:2020, 6.3]
    Smoke control: smoke extract fans, corridor and staircase pressurisation fans, automatic opening vents (AOVs)BS 9999Supply within 15 s [BS 9999:2017, 37.2.3.3]
    Fire and rescue service communicationBS 9999Supply within 15 s [BS 9999:2017, 37.2.3.3]
    Firefighters liftsBS EN 81-72Rated to run the lift at rated load and speed for 120 min [BS 8519:2020, 20.4]
    Evacuation liftsBS 9999:2017, Annex G; BS 9991Excluded from the 15 s supply items [BS 9999:2017, 37.2.3.3 a)]
    Fire alarm and emergency lightingBS 5839-1; BS 5266-1Outside the scope of BS 8519 [BS 8519:2020, 1]

    Fire Alarm and Emergency Lighting: “This British Standard does not give recommendations for those installations covered in BS 5839-1, BS 5839-8, BS 5839-9 and BS 5266-1, but makes reference to these standards.” [BS 8519:2020, 1]

    15 s Supply: It applies to smoke control, fire and rescue service communication and “any other fire protection or fire-fighting equipment, except automatic fire detection and fire alarm systems and evacuation lifts”. [BS 9999:2017, 37.2.3.3 a) and b)]

    Evacuation Lifts: Connect them to both supplies where a secondary supply is provided for other life safety reasons: “it is preferred that the evacuation lifts be connected to both these supplies”. [BS 8519:2020, 20.4, NOTE 3]

    Determine Starting Inrush & Locked-Rotor Current

    Each system requires power, but not all loads are calculated in the same way.

    To size both the primary mains supply and the secondary generator, “the pump/motor manufacturer should provide the following motor parameters”: [BS 8519:2020, 20.1]

    • pump rating, kW;
    • method of starting, star/delta, soft start;
    • voltage, V;
    • voltage (min), V;
    • full load current, A;
    • starting current, A;
    • duration of star phase, s;
    • starting current (delta), A;
    • duration of delta phase, s;
    • starting power factor;
    • locked rotor current star, A;
    • hot burn-out time, s; and
    • starter fuse selection.

    Requirements for Sprinkler Pumps

    Hot Burn-out Time

    Worked example

    Duty pump stalled at LRA 199 A, standby at full starting current 66 A (TB210 8.2.1(a)6: one pump in a stalled rotor condition and any other(s) at full start current).

    Combined current = 199 + 66 = 265 A
    Apparent power = 1.732 x 400 V x 265 A = 183.6 kVA, held while the stall persists.

    How long it persists: the fuse must not clear the stalled current before 75% of the motor hot burn-out time (BS 8519 20.1; TB210 8.2.1(e)) and clears by the burn-out limit. BG 70 models clearance at approximately 75%. On a 13 second burn-out that is a 10 to 13 second window at 183.6 kVA.

    That transient does not need a 200 kVA generator: an alternator can supply kVA above its rating for seconds, provided it holds the G2 limits. It needs a set whose manufacturer run proves this step, and a start strategy that keeps every deferrable load (fans, lifts, boosters) out of the stall window. Staggering them past it is how the generator gets smaller. You can run this case in the free calculator; the worst case is the default.

    Burn-out time is motor-specific: BG 70 Table 6 lists typical values from about 6 to 50 seconds by rating and efficiency class, with high-efficiency IE3 and IE4 motors surviving longest. BG 70 then makes it a design parameter: the hot burn-out time “should be limited to 15 seconds and the motor protection selected accordingly”, or the stability of the distribution suffers. Field practice for smaller pumps sits at 9 to 13 seconds; the supplier datasheet applies. Convention worth adopting: enter the LRA on the duty pump line only, the standby line’s role in the worst case is starting at full current, not stalling.

    Motor Interlocking

    The secondary supply should be sized to account for all connected loads. If the secondary supply also powers load other than the firefighter’s lift(s), a sequenced start-up may be used to help manage the total power demand. [BS EN 81-72:2020, Annex C] 

    Motor Interlocking:

    • Sequential Load Start: Where possible, design your control system to stagger the load start time. For example, it may be possible to delay the start of other loads while the sprinkler pump is in the LRA

    • Staggered Fan Start: Where possible start smoke fans after pumps reach full speed.

    • Firefighters Lift: “When the power supply is re-established the lift shall become available for service within 1 min.” [BS EN 81-72:2020, 5.10]

    • When lifts other than the designated firefighters lift(s) are connected to the secondary power supply (for example to return them to the fire service access level), sequential activation and speed reduction may be used to limit the total power demand. [BS EN 81-72:2020 Annex C]

    Two TB210 points that get missed. The sequenced start-up concession is for wet riser pumps, not sprinklers: the first wet riser starts in a stalled rotor condition 15 seconds later, further pumps at 15 second steps, and it is “subject to the agreement of the authorities” (TB210 8.2.1(a)7). Sprinkler pumps start the moment power is available. And the generator arrangement itself “shall be agreed by the fire insurer at the planning stage” (TB210 8.2.1(a)2), so get that agreement in writing before the design freezes.

    Generator rating: prime or standby, to ISO 8528-12

    BS 8519:2020, 6.3: “The rating of the generator should be selected in accordance with BS 7698-12, ISO 8528-12 as either prime or standby rated; for the standby set the variable average power output should not exceed 70%.”

    The standard permits either rating, however conformity with ISO 8528-12 is not optional: BS 7671:2018+A4:2026 Reg 560.6.13 states a rotary generating set used as a safety source shall conform to BS ISO 8528-12. A set quoted without it fails the wiring regulations, not just the design guide.

    • Prime (PRP): unlimited running hours on a varying load, with a limited average load and a short-period overload allowance under the ISO 8528-1 conditions. Suits sets that also serve non-emergency duty.
    • Standby (ESP): emergency-only duty. Higher rating from the same engine, but no overload capability at all and limited expected hours per year (manufacturer literature; ISO 8528-1 defines the rating conditions). The usual life safety choice, because the set runs only on mains failure.

    The 70% rule, read correctly: it limits the variable average power, the average of the fluctuating load over a 24 hour running period, not any instant of it. It protects the engine thermal life. The seconds-long starting transient is judged separately, against the alternator short-time capability and class G2 (below).

    Note: “Mission Critical Standby” and similar tiers are manufacturer commercial ratings, not ISO 8528 classifications. Specify prime or standby to ISO 8528-12 and let the manufacturer map their range onto it.

    Generator Sizing Considerations

    On a life safety set the transient sets the frame size, so a low steady-state load factor is normal, often 20 to 30%. Two limits then apply: the standby variable average should not exceed 70% (BS 8519 6.3), and sustained light running risks wet stacking, so provide an on-load periodic test regime. BSRIA BG 70 puts the other bound plainly: “it is equally important that the generator set is not oversized.”

    Performance class G2

    BS 8519:2020, 6.3 recommends that the electrical output is “classified as G2 or better”. The class G2 operating limits for the transient study are below. The values are from the 2013 edition of BS ISO 8528-5 (Table 4); the current edition was published in 2025, and the values have not been checked against it:

    • Transient voltage deviation on sudden power increase: within -20%, recovering within 6 seconds.
    • Transient frequency deviation: within -(10 + droop)%, recovering within 5 seconds.
    • Steady-state voltage deviation: within plus or minus 2.5%.

    The evidence rule. Require the manufacturer sizing run (GenSize, SpecSizer, GenSelect or equivalent) to show your worst load step, stalled pump plus assist start, with the resulting dip and recovery against these limits, at the installed ambient and enclosure conditions. A verbal confirmation that “the software allows for it” is not design evidence.

    Worked Example: Start Sequence and Generator Size

    On a recent London office refurbishment we sized a standalone life safety standby set (no synchronising to the mains). The smoke fans are star-delta started, and the load profile includes their locked rotor and starting characteristics.

    OptionStarting arrangementGenerator size
    AEvery load starting together at 15 s800 kVA
    BStaggered starting715 kVA

    Note: A third option, with the smoke fan ratings as currently selected (smaller), needs no staggering.

    The steady running load is 353 kVA: 44 % of 800 kVA and 49 % of 715 kVA. The size is set by the starting step, not the running load.

    What the Generator Supplier Checks: The generator supplier will check our sizing independently for:

    • G2 compliance [BS 8519:2020, 6.3];
    • minimum load acceptance;
    • light loading: the supplier’s target is normal running at not less than 30 % of rating.

    At 44 % and 49 %, the steady running load on both options is above the supplier’s 30 % target and below the 70 % limit on the variable average power output of a standby set. [BS 8519:2020, 6.3]

    Rating Hours: On that project the supplier advised that standby and prime ratings are both based on a 70 % average load, and that the standby rating limits planned running hours per year. In a real mains failure the set runs as long as needed; the hour limits relate to the warranty, not to safety. These are the supplier’s statements, not requirements of a standard.

    Best Practice: Agree the start sequence before the set is selected; the generator size depends on it.

    Regenerative Drives in Lifts

    “The designer should establish from the lift supplier whether the proposed equipment incorporates regenerative braking, by which the lift installation is capable of exporting energy back into the electrical distribution system.” [BS 8519:2020, 20.4]

    “Life safety generators are typically limited in their ability to absorb regenerative energy.” [BS 8519:2020, 20.4, NOTE 1]

    Annex C of BS EN 81-72:2020 (informative) states:

    “Care needs to be taken when selecting a generator as the initial load acceptance on change-over may be much less (e.g. in the region of 60 %) than the full-load rating. If the lift(s) regenerates energy onto its supply, this should be declared to the building designer since generators are typically limited in their ability to absorb such energy. The generator should be sized either to accept the regenerated power or alternative measures agreed to absorb this energy.” [BS EN 81-72:2020, Annex C]

    CheckWhat to obtainAction
    Initial load acceptanceThe first load step after changeover, from the load profileCheck the step against the initial load acceptance of the set offered, not against its full-load rating
    Regenerated powerThe regenerated power, declared in writing by the lift supplierSize the generator to accept it, or agree an alternative measure to absorb it

    Note: Annex C gives no percentage limit for regenerated power. The 60 % is an example of initial load acceptance on changeover, not a limit. Ask the generator supplier for the initial load acceptance of the set offered.

    Timing

    The secondary supply must be capable of providing power within 15 seconds of the failure of the primary supply (BS 9999:2017, 37.2.3.3 b); BS 8519:2020, 6.3, citing BS 9999 and BS EN 12845). In BS 7671:2018+A4:2026 terms this is an automatic supply classified “medium break” (Reg 560.4.1, medium break). The 15 seconds is consumed by the start-up sequence itself: ATS detects, generator starts and accelerates, holds stable voltage and frequency, ATS closes. Loads can only start after it.

    In the event of a mains or sub-mains power failure, the ATS should detect the loss of supply, initiate the start-up of the life safety generator, and automatically switch to the secondary power source. “When power is re-established the firefighters lift should become available for service within one minute.” [BS 8519:2020, 20.4]

    Short-Circuit Protection: The standby generator should “be able to support the worst-case transient load and fault conditions”. [BS 8519:2020, 6.3] Generator fault current depends on the excitation system:

    ExcitationFault current
    With field forcing or compound excitation (excitation boost)Rises “for 10 seconds, normally to 2 to 3 times the full-load current of the generator”
    Without excitation boostFinal current “normally around 0.5 In”

    [Schneider Electric Electrical Installation Guide 2018, chapter N (manufacturer guidance)]

    Obtain the short-circuit characteristic of the offered set from the generator supplier before the protection is set.

    Generator Breaker: “Setting of the STD must allow for the short-circuit characteristic of the generator set” (STD: short time delay). [Schneider Electric Electrical Installation Guide 2018, chapter N (manufacturer guidance)]

    Best Practice: Specify an electronic trip unit with an adjustable short time delay, so the delay can be set against the short-circuit characteristic of the set.

    Note: On the London office refurbishment the 1,250 A generator breaker is a moulded case circuit breaker (MCCB) with an electronic trip.

    Discrimination: Prove discrimination between the generator breaker and the downstream devices with time-current curves, with and without excitation boost. Energy-based discrimination relies on the manufacturers’ cross-tables. Where the generator breaker and the downstream switchgear are made by different manufacturers, there may be no cross-tables between them. On the London office refurbishment there were none, so energy-based discrimination could not be proven, and time-current discrimination could.

    Locked Rotor Current Takes Precedence Over Harmonics

    While non-linear loads introduce harmonics, the primary concern in generator sizing is ensuring adequate capacity for Locked Rotor Current (LRA) of motors. As the generator is already oversized to handle LRA, additional oversizing for harmonics is typically unnecessary. However, in cases of excessive harmonic distortion, Harmonic Filters, Line Reactors, or K-Factor Rated Alternators may be used for mitigation.

    Automatic Transfer Switch (ATS)

    ATS Location

    • must be located where the fire and rescue service can reach it, in the plant room or fire-fighting shaft positions set by BS 8519:2020, clause 9 (BS 9991:2024, 21.1 defers electrical design for life safety and firefighting equipment to BS 8519)
    • must be installed within a fire-protected area. [BS EN 81-72:2020 5.1.7]
    • must be accessible either directly from this vertical fire compartment or via a fire-protected route. [BS EN 81-72:2020 Annex I, I.5]
    • within plant room(s) housing the life safety, fire-fighting or other critical system equipment;
    • If The ATS is serving the firefighters’ lift, it should be located within the fire-fighting shaft outside the lift well or within a fire-protected building fabric enclosure directly adjacent to the fire-fighting shaft [BS 8519:2020, Section 9]
    • The main car park extract fan control panel should be located in the same fire-resisting building fabric enclosure as the run and standby extract fans, fed by an ATS located in the same building fabric enclosure. [BS 8519:2020, 20.3]
    • When smoke clearance system  uses impulse or jet fans, their power supply should originate from the same fire-protected building enclosure as the smoke extract fans. They may be connected through a shared ATS. [BS 8519:2020, 20.3]

    ATS Requirements

    • Any switchgear space outside the lift well and beyond a fire compartment must have a fire resistance level equivalent to that of the fire compartment(s). Similarly, any connecting elements, such as electrical cables or hydraulic pipes, that pass between fire compartments must also be adequately fire-protected. [BS EN 81-72:2020 5.7.2 ]
    • The primary and secondary supplies should be terminated in a changeover device (automatic transfer switch). It should conform to BS EN 60947-6-1 and be based on switch technology (PC classification), rather than circuit breakers or contactors. [BS 8519:2020, 9]

    • If building occupancy depends on the functionality of life safety and fire-fighting equipment, a single or dual bypass system should be implemented.

      Note: If a single bypass is used, it should be on the primary power supply. [BS 8519:2020, 9]

    Typical ATS sizes based on Mertech

    • SBP: single bypass
    • DBP: dual bypass

     

    Frame Size (Amps)Height (SBP | DBP)Width (SBP | DBP)Depth (SBP | DBP)
    45600 | 800800210
    63600 | 800800210
    100600 | 800800210
    125600 | 800800210
    160800 | 12001200300
    250800 | 12001200300
    3501000 | 16001600300
    4001000 | 16001600300

    Automatic Transfer Switch (ATS): BS 9991:2024 compliance

    • Single component with an integral controller from the same manufacturer
    • Status monitoring for: 
      • Primary and secondary supply availability
      • Switchover position (“On Primary Supply” or “On Secondary Supply”)
    • Fault indication, which must be linked to:
      • The fire alarm system
      • The firefighting lift control switch (if applicable)

    Remote indication of Life Safety Generator status

    “Visual indication should be provided at the main fire alarm panel of the fire control room, giving the operational state of the standby generator, including mains healthy, generator running and generator fault. A voltmeter and ammeter should also be provided to indicate the total load on the generator, in accordance with BS EN 12101-10.” [BS 8519:2020, 6.3]

    Best Practice: Position the remote indication (mimic panel) where the fire and rescue service can identify the generator status on arrival.

    This can be achieved by the use of a configurable addressable matrix panel, such as the DS52 series, which offers a practical and compliant method for providing clear and accessible remote indication of life safety generator and ATS status at designated fire service locations, including reception or entrance lobbies.

    The panel shall be interfaced with the building’s fire alarm system, and provide key operational states, such as generator running, fault, changeover status, and battery health.

     

    ATS Panel Integration

    Each ATS need a signal cable running back to the generator. The ATS panels send a two-wire start/stop signal to the generator and manage all monitoring and switching. Typically timers for mains restoration are built-in, which matters for life safety compliance.

    Remote Indication

    • Typically remote indication panels are supplied with LED indicators showing:

      • S1 available

      • S1 on-load

      • S1 bypass

      • S2 available

      • S2 on-load

    Fuel Level Monitoring

    • Fuel percentage is displayed directly on the generator controller.

    Cabling, Commissioning, and Communication

    • Site electricians are typically responsible for all cabling.

    • Generator commissioning and sign-off are performed by the supplier.

    • Communications use RS485 (APM303 controller as standard).

    • Fixed volt-free contacts pack includes:

      • Genset running

      • General shutdown

      • Low fuel level

    Ancillary Supplies

    • A 13A single-phase supply is needed for the generator water jacket heater and battery charger.

    • This supply can be sourced from the nearest ATS or a local fused connection unit.

    For more detailed guidance on the Remote indication of Life Safety Generator status please go to:

    Mimic Panels for Life-Safety Generators: a UK Designer’s Guide

    Car park smoke control systems

    Car park smoke control systems are part of the Life Safety System, fed from the Generator. The main car park extract fan control panel should be located in the same fire-resisting building
    fabric enclosure as the run and standby extract fans, fed by an ATS located in the same building fabric enclosure.

    Cabling , Containment and Fixings

    Both the primary and the secondary supplies should be protected against fire and water damage and be separated from each other throughout the installation, by adopting diverse cable routes.

    Cables for life safety, fire-fighting, and other critical systems must be installed on a dedicated cable support system, separate from other cable installations. [BS 8519:2020, Section 7.3]

    If both the primary and secondary power supplies are located on the ground floor or roof level, they must be adequately protected to minimise the risk of a fire affecting both sources. This can be achieved by enclosing each supply within a 120-minute fire-resistant enclosure (REI, EI, or ESa) in accordance with BS EN 13501-2:2016, or by providing sufficient separation, supported by a fire safety engineering analysis conducted by a qualified fire safety engineer to demonstrate that fire or smoke will not spread between them.

    Where the generator is at ground floor or roof level, 120 min fire compartmentation (EI or ESa as appropriate) should also be provided to separate it from any adjacent fire risk. [BS 8519:2020, 6.3]

    Fire-Resistant Cable Routing & Protection

    • Category 1 (30 minutes): means of escape applications
    • Category 2 (60 minutes): means of escape: evacuation lift supplies, means-of-escape smoke control
    • Category 3 (120 minutes): fire-fighting: firefighters lift supplies, sprinkler and wet riser pumps, fire-fighting smoke control (including any firefighter override)

     

    Fire alarm circuits follow BS 5839-1 rather than this category list. The application-by-application assignment is BS 8519:2020 Table 1.

    • Dedicated Cable Support Systems: Cables for life safety systems must have dedicated supports rated for the same fire survival time.
    • Fire-fighting applications (firefighters lift supplies, sprinkler and wet riser pumps, fire-fighting smoke control) are Category 3, 120 minutes. [BS 8519:2020, Table 1]
    • Category 3 fire-resistant control cables must be installed between each ATS and the generator to transmit the start signal and status indication in case of a supply failure. In areas prone to mechanical damage, unprotected fire-resistant control cables should be additionally protected by using armoured cables or enclosing them within a cable tray with a lid.

     

    • Cables in protected escape routes: only cables meeting Regulation 422.2.1 may be installed there (limited smoke production with 60% light transmittance to BS EN 61034-2, halogen acid gas not exceeding 0.5% to BS EN 60754-1, and flame propagation to the BS EN 60332-3 series or BS EN 60332-1-2 as applicable); cable management systems must be classified non-flame propagating to their product standard (BS EN 61386 conduit, BS EN 50085 trunking and ducting, BS EN 61537 tray and ladder). Cables in a fire-resisting enclosure rated not less than the compartment, or as specified by the fire engineer, are deemed outside the route. [BS 7671:2018+A4:2026, 422.2.1 and 422.2.202]
    • Fire-Stopping: Any cable penetrations in fire-rated barriers must be sealed with fire-resistant materials per BS 8519.
    • Cable support systems for fire-resistant and non-fire-resistant cables should be kept separate. When running along the same route, the fire-resistant cable support system should, where feasible, be installed above the non-fire-resistant system. [BS 8519:2020. 16]
    • For further guidance, refer to Wiring in Protected Escape Routes.

    Fuel Storage & Containment

    Minimum Fuel Storage Requirements: “The fuel storage capacity should be not less than the minimum shown in Table 2.” [BS 8519:2020, 6.3]

    Start signal and fault indicationMinimum at full load
    Dedicated to life safety and fire-fighting, starts only on a fire signal, fault indication to a permanently manned control room4 h
    Starts whenever the primary power source fails, fault indication to a permanently manned control room8 h
    Any other arrangement (“Otherwise”)72 h

    [BS 8519:2020, Table 2]

    Note: “For applications other than life safety and fire-fighting, the time period is as determined by the fire engineered strategy.” [BS 8519:2020, Table 2, note A]

    PD Position on the 8 h Row: Remote supervision is accepted as immediate notification [BS EN 12101-10, Note 1 to 6.2.2], and an alarm receiving centre is “continuously staffed premises” [BS 5839-1:2025, 3.3]. A set that starts on any mains failure, with running, fault, low fuel and ATS position signals sent through the fire alarm to the alarm receiving centre, meets the manned control room condition of the 8 h row. Confirm it with the fire engineer.

    Service Tank: Provide a bunded local fuel service tank within the generator enclosure, “large enough for at least 120 min of operation at rated power”. [BS 8519:2020, 6.3]

    Bulk Storage: Where the service tank holds less fuel than the set needs to run for the Table 2 minimum at full load, provide the balance in separate bulk storage. [BS 8519:2020, 6.3 and Table 2]

    Firefighters Lift: The secondary supply should be “rated to run the firefighters lift at rated load and rated speed for a period of 120 min”. [BS 8519:2020, 20.4] That is a rating, not a fuel quantity.

    Fuel Polishing: Provide it “where recommended by the generator specialist”. [BS 8519:2020, 6.3]

    • Pipe-in-Pipe Systems: Use Brugg Pipe-in-Pipe or equivalent double-containment system.
    • Vacuum Leak Detection: SGB System recommended.
    • Thermal Shut-off Valve: Stops fuel flow in case of fire.
    • Fuel Dump System: Emergency drainage mechanism.
    • Alternative Systems: Durapipe PLX, KPS OPW.

    Location & Fire Compartmentation

    To protect against fire and structural failures, BS 8519:2020 recommends:

    • Generators should be housed in a dedicated plant room with 120-minute fire separation. 
    • Switchrooms and distribution boards must be fire-protected and separated from other services.
    • If both primary and secondary power sources are on the ground or roof level, they must be enclosed separately to prevent fire spread. (Requirement for 120-minute fire separation of roof slab)

    Emissions and Planning

    In England and Wales, a new standby set running less than 500 hours a year (3 year rolling average) is exempt from the medium combustion plant (MCP) emission limit values, not from the permit. [gov.uk, Medium combustion plant (MCP): comply with emission limit values; gov.uk, Medium combustion plant: when you need a permit]

    RegimeRequirementSource
    MCP permitRated thermal input of each unit from 1 MWth to less than 50 MWth[gov.uk, Medium combustion plant: when you need a permit]
    Specified generatorExcluded from the specified generator controls when used only for emergency power, with testing limited to 50 hours a year; still “a MCP and requires a permit by the appropriate deadline”[gov.uk, Specified generator: when you need a permit]

    London Planning Conditions: Emergency and life safety backup plant “may be excluded from the calculation of predicted building emissions”. Use other than emergency and operational testing (less than 50 hours per year) “would be prevented by planning conditions”, and “local authorities could seek a more restrictive condition”. [GLA London Plan Guidance, Air Quality Neutral (February 2023), 3.3.1] Check the site’s planning conditions.

    SCR: On the London office refurbishment a selective catalytic reduction (SCR) system is fitted to meet an emissions planning condition. It is effective only once the exhaust reaches 200 to 300 °C, about 5 to 10 minutes after start (supplier’s figure).

    AdBlue Access: On that project the 275 litre AdBlue tank is mounted on the generator canopy. The supplier recommends maintenance headroom above the SCR, ideally 2 m. Where steelwork prevents it, provide lift-up floor sections with steps down, or a ground-level hand pump for filling.

    Plant Coordination

    ItemData on the London office refurbishmentAction
    Hot air discharge75 to 80 °C at full load (supplier’s figure)Keep PV panels clear of the discharge
    Anti-vibration mounts95 % isolation or better; 97 to 98 % in practice (supplier’s figure)Send the mount data to the structural engineer for the structural design
    BaseSCR support legs straddle the enclosureProvide one flat, level slab, not separate plinths, slightly larger than the enclosure so the legs can land

    Hot Air Discharge: On that project, PV panels were shown above the generator enclosure. The generator position is fixed, so the PV panels are to be relocated. On another project, the same conflict was resolved with SCR insulation and an extended exhaust discharge. Check the discharge path against the PV layout before the roof layout is fixed.

    Anti-Vibration Mounts: If the structural design does not include the mount data, structure-borne noise and vibration are difficult to resolve later.

    Generator Earthing

    Generator as a Switched Alternative to a TN System: “Protection by automatic disconnection of supply shall not rely upon the connection to the earthed point of the system for distribution of electricity to the public when the generator is operating as a switched alternative to a TN system. A suitable means of earthing shall be provided.” [BS 7671:2018+A4:2026, 551.4.3.2.1]

    Four-Pole ATS: “Functional switching devices ensuring the changeover of supply from alternative sources shall switch off all live conductors and shall not be capable of putting the sources in parallel, unless the installation is specifically designed for this condition.” [BS 7671:2018+A4:2026, 463.1.4] A live part includes the neutral conductor. [BS 7671:2018+A4:2026, Part 2] The changeover device switches the neutral, so specify a four-pole ATS.

    Generator Neutral to Earth: With a four-pole ATS, the generator neutral is disconnected from the supply neutral when the set is on load. Each source that can operate independently should remain earthed when the other sources are not connected. [BS 7430:2011+A1:2015, 7.2.1] “Where there is only one generating set, the neutral point of its windings, the generator frame, all exposed-conductive-parts and extraneous-conductive-parts should be connected to the main earthing terminal.” [BS 7430:2011+A1:2015, 7.2.5] Connect the generator neutral point to the main earthing terminal, so the installation stays earthed on either supply.

    No Parallel Operation: Take precautions so the generator cannot operate in parallel with the public supply, for example an interlock, a single transferable key, a break-before-make switch or an automatic changeover with interlock. [BS 7671:2018+A4:2026, 551.6.1]

    Fault and Overload Protection

    Disconnection Times: Check that each protective device disconnects within the time required on the generator fault current, not only the mains. In a TN system, Table 41.1 applies to final circuits up to 63 A with socket-outlets and 32 A supplying only fixed equipment; 5 s is permitted for distribution circuits and other circuits. [BS 7671:2018+A4:2026, 551.4.1, 411.3.2.2 and 411.3.2.3]

    Where the Times Cannot Be Met: Apply Section 419. Provide supplementary protective equipotential bonding to Regulation 415.2, and limit the voltage between simultaneously accessible exposed-conductive-parts and/or extraneous-conductive-parts to 50 V AC. [BS 7671:2018+A4:2026, 411.3.2.5, 419.1 and 419.3]

    RCDs: Connect the generating set so that RCD protection in accordance with Chapter 41 remains effective for every intended combination of sources. In a TN-S system where the neutral is not isolated, position any RCD to avoid incorrect operation due to a parallel neutral-earth path. [BS 7671:2018+A4:2026, 551.4.2 and 551.6.2]

    Safety Circuits: Select and erect overcurrent devices so that an overcurrent in one circuit does not impair the correct operation of safety circuits. Where loss of supply may cause a greater hazard, overload protection may be omitted; monitor the circuit for overload instead. [BS 7671:2018+A4:2026, 560.7.3 and 560.7.4]

    Final Design Validation

    The standby generator should be capable of providing the supply to the critical life safety and fire-fighting load within 15 s of the failure of the primary supply in accordance with BS 9999 and BS EN 12845.

    Sample Cause-and-Effect Matrix

    Base the project cause-and-effect matrix on this sequence.

    StepEventActionReference
    1Primary supply failsThe ATS detects the loss of supply and signals the generator to start.[BS 8519:2020, 20.4]
    2Load bank test in progressThe mains failure start signal also trips the load bank breaker (shunt trip, reset by hand).Project practice, recent London office refurbishment
    3Generator at stable voltage and frequencyThe ATS changes over to the secondary supply. The set should be capable of supplying the critical life safety and fire-fighting load within 15 s of the failure.[BS 8519:2020, 6.3 and 20.4; BS 9999:2017, 37.2.3.3 b)]
    4Secondary supply on loadSprinkler pumps start. The first wet riser pump starts 15 s later in a stalled rotor condition, and further pumps start in 15 s steps, “subject to the agreement of the authorities”.[LPC TB210:2022, 8.2.1(a)7]
    5Pumps at full speedWhere possible, start the smoke fans.Best Practice
    6Lift on secondary supplyThe firefighters lift becomes available for service within 1 min of power being re-established.[BS 8519:2020, 20.4; BS EN 81-72:2020, 5.10]
    Any timeFire signal during a load bank testAn electrical interlock disconnects the load bank and the set transfers to the building load.Project practice, recent London office refurbishment
    Any timeATS faultFault indication is provided to the fire alarm system and duplicated to the firefighting lift control switch where the ATS serves a firefighters lift within a firefighting shaft.[BS 9991:2024, 21.1] (residential buildings)

    Best Practice: Start the loads after the ATS closes.

    The record BS 7671 requires you to produce

    Reg 560.7.11: a list of all the current-using equipment permanently connected to the safety power supply, indicating the nominal electrical power, rated nominal voltage, current and starting current, together with its duration.

    Generator Load Calculator

    The Generator Load Calculator runs everything on this page: pick your loads from a typical library or enter your own, and it builds the time-step profile with the TB210 worst case as the default, applies the BS 9999 15 second timebase, states the G2 acceptance criteria and recommends the standby set. Free to run, no signup, every figure traceable to the clause it came from.

    The Engineering Toolkit (79 GBP a year) turns it into a report, with a Reg 560.7.11 equipment record and secondary-supply schematic in editable Word and Excel, the sequential start advisor that helps to reduce the Gen size, the fuel and endurance check, and reports saved to your projects. The same subscription covers the cable tray and duct sizing calculators, so the generator you size here can have a compliant tray and duct route.

    Generator Testing

    BS 8519:2020 and BSRIA BG 70/2017 state no test interval. “The generator control system should provide manual control of the periodic test conditions for both on-load and off-load testing.” [BSRIA BG 70/2017, 3.1]

    TestWhereLoad
    Factory acceptance test (FAT)Off site; optional, usually providedSupplier’s normal FAT: 3 h at 100 %, 1 h at 110 % where the set has an overload rating
    Site testsInstalled switchgearLoad banks
    Integrated systems test (IST), “black building”Whole building, all systems runningBuilding load; load banks not normally needed

    Note: Parties use the terms site acceptance test (SAT) and IST for different tests.

    Agree one test protocol, signed off by all parties.

    Load Bank Type: A resistive load bank test proves the engine only. An inductive load bank test (0.8 power factor) proves the engine and alternator together and is closer to real motor load.

    Load Bank Connection: On the London office refurbishment the mains failure signal that starts the generator also trips the load bank breaker (shunt trip, reset by hand), so a mains failure during a test does not overload the set. On a fire signal, an electrical interlock disconnects the load bank and the set transfers to the building load.

    ATS Transfer Test: Do not inhibit the generator. The ATS cannot transfer without the secondary supply.

    No-Load Running: On that project the supplier recommended no more than 2 to 3 minutes of no-load running per exercise.

    Start Attempts: BS 8519 states no number. On that project the specification required six; the supplier’s standard is three.

    Note: Health Technical Memorandum (HTM) 06-01 is healthcare guidance, not a requirement for other buildings. It states: “(Purely resistive load banks may damage the cylinder or cylinder liners due to the high carbon build-up.)” [HTM 06-01 (2017), 17.84]

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