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    Life Safety Generator selection guide

    If the secondary power supply is provided for firefighter’s lift, it must be capable of operating the firefighter’s lift at its full rated load and speed for a minimum duration of 120 minutes. [BS 8519:2020] However; the minimum fuel storage is 4h if the generator activates only in fire conditions. [BS 8519:2020, Table 2 (BS EN 12101-10:2005)]

    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-minute fire-resistant enclosure [REI, EI or ESa – BS EN 13501-2:2016]
    Minimum Fuel Storage Requirements :

    • 4 hours if the generator activates only in fire conditions.

    • 8 hours if it activates whenever the primary power source fails.

    • 72 hours for applications beyond life safety and fire-fighting.

    For use applications than life safety and fire-fighting, the time period is determined by the fire-engineered strategy.

    [BS 8519:2020, Table 2]

    Key Steps in Life Safety Generator Selection

    Identify Life-Safety Loads

    Life safety generators must support critical systems that require backup power, including:

    • Fire Sprinklers & Hydrant Pumps (TB210, BS EN 12845)

    • Smoke Extract Fans (BS 9999)

    • Fire-Fighting Lifts (EN 81-72)

    • Automatic Opening Vents (AOVs) (BS 9999)

    • Corridor & Staircase Pressurization Fans (BS 9999)

    • Fire Alarm Panels & Emergency Lighting (BS 9999, BS 8519)

    Regulatory References:

    • BS EN 12845 – Sprinkler system electrical supply

    • BS 9999 / BS 9991 – Fire engineering & evacuation strategy

    • BS 8519 – Fire-resistant cables for life-safety systems

    Determine Starting Inrush & Locked-Rotor Current

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

    To correctly size both the primary mains supply and secondary generator, BS 8519:2020 requires pump/motor manufacturers to provide:

    • 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

    • The generator must support one pump at Locked Rotor Amps (LRA) while another is at full Starting Current (SC).
    • Fuses must sustain at least 75% of the motor burn-out time.

    [TB210 & BS 8519:2020, Section 20.1]

    Burnout Time Considerations:

    Burnout Time Definition
    Cold Burnout Time (CBT)
    The typical max time a motor can sustain LRA when starting from ambient temperature (~13s).
    Hot Burnout Time (HBT)
    The typical max time a motor can sustain LRA after running at full temperature (~9s).

    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.

    • Firefighting Lift: Once the power supply is restored, the lift must be operational and ready for use within 60 seconds. [BS EN 81-72:2020, 5.10]

    • When lifts other than the designated firefighters’ lift(s) are connected to the secondary power supply – such as for returning them to the fire service access level – strategies like sequential activation and speed reduction may be implemented to regulate overall power demand. [BS EN 81-72:2020 Annex C]

    Select Generator Type & Rating

    Life Safety Generators Must Be ESP-Rated (BS ISO 8528-12)

    Life safety generators are only used during power failure and must comply with ESP classification.

    • Rating: ESP (Emergency Standby Power)
    • Usage: Backup-only for fire pumps, lifts, and smoke fans
    • Overload Allowed?: ❌ No overload allowed
    • Life-Safety Example: A 600 kVA ESP-rated generator must handle the worst-case emergency load, but cannot be overloaded beyond this limit.

    Why Only ESP (Emergency Standby Power) Applies:

    • Emergency Standby Power (ESP): Generator sets rated ESP should deliver rated output throughout the entire emergency outage duration. The average load factor must typically not exceed 70%, with typical operation around 50 hours per year and a maximum expected usage of 200 hours per year.
    • Mission Critical Standby Power: For critical facilities, select generators that allow higher load factors (up to 85%) with typical operation around 200 hours per year and up to 500 hours maximum.

    Why ESP Applies to Life-Safety Generators:

    • Must be sized for peak inrush loads (e.g., sprinkler pump LRA, firefighting lift SC, smoke fans startup demand).
    • No overload allowance – Must operate within rated capacity at all times.
    • Designed for emergency conditions – Must start within 15 seconds of power failure (BS EN 12845).
    • Connected via Automatic Transfer Switch (ATS) – Instantly shifts life-safety loads to generator power when mains fail.

    🚫 Why PRP (Prime Power) Is NOT Used for Life Safety Generators:

    • PRP allows overload operation (not permitted for life-safety).
    • PRP assumes a steady load, whereas life-safety generators must handle full inrush loads instantly
    • PRP Suitable for applications without utility supply or for extended outages. These generators should operate at an average load factor of around 70%, allowing a 10% overload capability for emergency use (one hour in every 12, not exceeding 25 hours per year).

    Generator Sizing Considerations

    The standby generator should start automatically and be adequately sized to maintain in operation the maximum design load and be able to support the worst-case transient load and fault conditions.

    Optimising the load factor is critical for the efficiency and longevity of generator systems. Operating generators between 75% and 85% load factors maximises fuel efficiency and reliability, reducing lifecycle costs and enhancing system performance.

    Consider load management strategies such as motor interlocking to achieve these optimal load factors.

    Regenerative Drives in Lifts

    Modern traction lifts often use regenerative drives that feed braking energy back into the power supply. While this is efficient during normal operation, it presents a challenge when the lift is running on a standby generator. BS EN 81-72:2020 Annex C warns that if the generator cannot absorb this reverse power – typically beyond 10% of its rated output it may become unstable, leading to overspeed or damage. For example, a 100 kW generator should only handle up to 10 kW of regenerative power safely; exceeding this can trip the generator or disrupt other life safety systems.

    To address this, there are two main design strategies: size the generator large enough to absorb expected regenerative energy, or provide an alternative means of dissipation. Oversizing the generator is often the most straightforward approach.

    Concurrent loads like smoke fans can help absorb regeneration, but designers should check cause & effect strategy to ensure these are being active. The safest route is to size the generator to handle regen independently.

    Alternatively, braking resistors can be fitted to the lift drive to convert excess energy into heat during generator operation.

    For larger or multi-lift systems, a permanent load bank may be installed to soak up reverse power automatically. While effective, these options add cost and complexity, so are typically used only when upsizing the generator is impractical.

    In all cases, Annex C makes clear that regenerative energy must be explicitly addressed to ensure system safety.

    Timing

    The standby generator should be capable of providing the supply to the critical life safety and fire-fighting load within 15 seconds of the failure of the primary supply [BS 9999, BS EN 12845, BS 8519:2020, 6.3].

    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. Once power is restored, the firefighter’s lift must be operational and ready for use within 60 seconds. [8519:2020, 20.4].

    Short-Circuit Protection: Generators have lower short-circuit currents than the grid. Thermal-Magnetic (TMD) circuit breakers are recommended due to their ability to operate effectively with limited fault current.

    Motor Contributions:

    Running motors can briefly contribute to fault current by feeding energy back into the system during a fault. If the total motor load exceeds 25% of transformer capacity, its contribution must be considered. The estimated fault contribution is 3.5 times the nominal motor current (Iscm = 3.5 * In per motor). This can impact the selectivity of protective devices and must be evaluated when setting short-circuit protection.

    If the lift(s) regenerates energy:

    Generators generally have limited capacity to absorb regenerated energy. The generator should either be appropriately sized to handle this energy or alternative methods should be implemented to manage the excess power. [BS EN 81-72:2020 Annex C]

    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 installed in a location easily accessible to firefighters. [BS 9991:2024, 21.2]
    • 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 power supply cables must terminate at a changeover device (ATS). [BS EN 60947-6-1]

    • 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)

    Car park smoke control systems

    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 minimize 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.

    Additionally, 120-minute fire compartmentation (EI) or (ESa – doorsets) should be implemented to physically separate the generator from any adjacent fire risks, whether located at the ground floor or roof level.

    Fire-Resistant Cable Routing & Protection

    • Dual Circuits & Separation: BS 8519 requires that primary and secondary supply cables be physically separated using diverse routes.
    • Fire Survival Times:
      • Category 1 – 30 minutes (Evacuation systems, standard alarms)
      • Category 2 – 60 minutes (Emergency lighting, smoke control)
      • Category 3 – 120 minutes (Fire-fighting lifts, sprinkler pumps, smoke clearance systems)
    • Dedicated Cable Support Systems: Cables for life safety systems must have dedicated supports rated for the same fire survival time.
    • Acording to BS 9999 the minimum required fire resistance is 120 minutes.
    • 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.
    • Containment Systems: When cables do not meet fire performance classifications (B-s3, d2), they must be installed in steel conduit or fire-rated trunking (BS EN 50085 & BS EN 61386-1).
    • 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

    A local fuel service tank should be provided within the generator building fabric enclosure large enough for at least 120 min of operation at rated power, including the appropriate bunding to protect against the risk of a fuel leak.

    However, the minimum Fuel storage capacity shall be based on BS EN 12101-10:2005. Therefore, additional fuel storage may be required if the ‘day tank’ capacity is less than shown in the table minimum fuel storage requirements for life safety generators.

    Criteria Minimum fuel storage at full load
    Generator set dedicated to the building life safety/ fire-fighting systems:

    a) only starts in case of a fire signal; and
    b) provides fault indication to a permanently manned control room
    4h
    Generator operates whenever the primary power source fails and provides fault indication to a permanently manned control room
    8h
    Otherwise
    72h (a)
    a) For applications other than life safety and fire-fighting, the time period is as determined by the fire engineered strategy.
    Source: Table 2 of BS 8519
    • 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. The same rules are introduced in (Table 4 of BS 9991:2024 based on building height) this is also stated in BS 9999 – in practice, it means a requirement for 120-minute fire separation for generator housing.
    • 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)

    Parallel Operation of Generators

    Paralleling generator systems offers substantial reliability and redundancy advantages, especially with an N+1 configuration, achieving up to 99.96% reliability. 

    Earthing Considerations

    Earthing Considerations for Life Safety Generators in TN-S Systems

    In the UK, TN-S earthing is the most common system, and integrating a life-safety generator into such a system requires careful planning to ensure fault clearance and safety.

    TN-S System Approach
    • Single Neutral-Earth Bonding: Ensure that the neutral-to-earth bond exists only at one location, typically at the main LV switchboard, not at the generator.
    • Generator Frame Earthing: The generator frame should be earthed to the Main Earth Terminal (MET) using a suitably rated conductor, while ensuring it does not create a second neutral-earth bond.
    • Avoid Neutral Bonding Conflicts: If the generator creates an additional neutral-to-earth bond while being connected to a TN-S system, it can lead to parallel paths, circulating currents, and malfunctions of protective devices.
    • Selective Fault Protection: The generator’s lower short-circuit current must be accounted for when setting earth fault protection to ensure timely disconnection of faults.
    IT Earthing for Life Safety Generators

    For life-safety generators, an IT earthing configuration is often preferred to enhance resilience. The following strategies should be considered:

    • Neutral Separation at the ATS: The generator’s neutral must be isolated from the building’s TN-S neutral to prevent unintended parallel paths. This is typically achieved by using a four-pole Automatic Transfer Switch (ATS), which switches the neutral along with the phases.
    • High-Impedance Neutral Earthing Resistor (NER): Instead of solid earthing, a high-impedance NER can be installed to limit earth fault currents, reducing damage risk and improving system stability.
    • Insulation Monitoring Devices (IMDs): Since an IT system does not provide a low-impedance path for earth faults, an IMD continuously monitors insulation resistance. If a first fault occurs, it alerts maintenance personnel without disconnecting power, ensuring continuity for life-safety loads.
    • Selective Earth Fault Protection: Protection settings must be adjusted to ensure earth fault detection is effective while avoiding miscoordination between the TN-S system and the generator’s IT earthing.
    • Integration with Fire Strategy & Compliance: Any modifications to the earthing scheme must align with BS 7671:2018+A2:2022 wiring regulations and BS 9999 fire strategy.

    Selective Fault Protection & Earth Fault Clearance

    One of the key challenges with life safety generators is ensuring that earth faults are cleared effectively despite the generator’s lower short-circuit current capability. The following measures help prevent faults from going undetected or protection devices failing to trip in time:

    • Adjusting Earth Fault Protection Sensitivity: Standard protection settings used for grid supply may not work correctly with a generator. Lower trip thresholds and time-delayed responses should be used to account for the reduced short-circuit current.
    • Ensuring Adequate Disconnection Times: BS 7671 specifies maximum allowable disconnection times for protective devices. These must be verified against the generator’s fault current output, ensuring that protective devices trip within required timescales.
    • Using Residual Current Devices (RCDs) Where Necessary: In cases where low fault current may not be enough to trip conventional overcurrent protection, RCDs with appropriate settings can be used to ensure faults are detected and cleared. (RCD protection is not recommended for life safety systems
    • Neutral-Earth Bonding Considerations: If the generator neutral is not bonded correctly or an incorrect earthing scheme is used, fault protection may not function as expected. Careful coordination is required between generator earthing and main MET connections.

    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 

    This table follows the chronological sequence of power failure, generator startup, and emergency load activation.

    Event (Cause) Action (Effect) Interlock (Timing & Sequencing Rule) Time (Seconds)
    Mains Fails
    ATS detects power loss & signals generator to start.
    Generator must start within 15s (per BS EN 12845).
    t₀
    ATS Start Signal Sent
    Generator cranks, accelerates to rated speed.
    No load applied until voltage & frequency stabilize.
    t₁ – t₇
    Generator at Rated Speed
    Generator holds stable voltage & frequency.
    ATS waits for stability before load transfer.
    t₇ – t₉
    ATS Closes to Generator Bus
    Generator supplies power to life-safety switchboard.
    Load is applied only after ATS transfer.
    t₉
    Load Applied
    Sprinkler pumps, lifts, and smoke fans start as per interlocking sequence.
    No simultaneous high-inrush loads (e.g., pumps & fans).
    t₉ – t₁₀

    Load Activation & Interlocking (After Power Transfer)

    Once the generator is running and loads are transferred, life-safety systems activate in a controlled sequence to prevent overload.

    Event (Post-Transfer Cause) Action (Effect) Interlock (Load Control Logic) Time (Seconds)
    Fire Alarm Activates
    Fire panel signals relevant systems.
    Sprinkler pump, fans, lifts activate only in the fire-affected zone.
    t₁₀+
    Sprinkler Pump Starts
    Fire pump initiates (DOL/Star-Delta/VFD).
    Second pump cannot start unless duty pump fails.
    t₁₀+
    Smoke Detected in Zone
    Zone smoke fans start.
    Other zones remain inactive unless their own smoke alarms trigger.
    t₁₀+
    Fire in Fire-Fighting Lift Zone
    Fire-fighting lift switches to emergency mode.
    Other lifts remain disabled to reduce SC impact.
    t₁₀+
    Car Park Fire Requires Ventilation
    Car park smoke fans operate (Star-Delta/VFD to limit inrush current).
    Sprinkler pump & fans must not start simultaneously—timing sequence applies.
    t₁₀+
    Mains Power Restored
    Generator runs until ATS signals shutdown.
    Loads return to mains, generator enters cool-down phase.
    t_restart

    Key Takeaways:

    • Sequencing prevents simultaneous high-inrush loads.
    • Fire systems activated only in affected zones, reduce generator load.

    Online Generator Load Calculator

    Click here for Online Generator Load Calculator

    Use our Online Generator Load Calculator to:

    • Enter equipment load values.
    • Simulate startup sequences.
    • Generate a PDF report with peak load values.

     

    Alternatively, download the Excel version to calculate generator load requirements.


    🔗Sign-up and select  ProDesign to get the Generator Load Calculator Excel File 

    Generator Maintenance & System Testing

    Ensuring long-term reliability involves routine testing and load assessments:

    • Weekly: Test ATS functionality & check fuel levels.
    • Monthly: Conduct full load bank testing.
    • Annually: Simulate a full-scale power failure test.
    • Every 3 years: Review fuel quality & replace if necessary to prevent degradation.
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