Life Safety Generator

Created: 2025-02-27
Jarek Wityk
Category: How To
Updated: 2026-08-13

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

[BS 8519:2020, Table 2]

Best Practice: If a generator is roof-mounted, the roof slab must also provide 120 minutes of fire separation in accordance with BS EN 13501‑2:2016.
[BS 8519:2020, Section 6.]

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 and 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

Burnout Time Considerations:

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. Run this case in the free calculator: the worst case is the default, and the staggering advisor quantifies the saving.

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]

Approval Needed In Life Safety Systems, Interlocking must be approved by the Fire Strategy Consultant & Building Control.

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, capped average, 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

Performance class G2: the numbers the selection must meet

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% (indicative; the set manufacturer declares the actual reverse-power acceptance) 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 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. Once power is restored, the firefighter’s lift must be operational and ready for use within 60 seconds. [BS 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 a fault-study convention; confirm against the protection study). 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.

Life safety Load type Smoke Vent Fans: Typically linear unless they have a VFD for speed control.
Sprinkler Pumps: Typically linear DOL started.
Lifts: Almost always non-linear in modern installations because of VFD or regenerative drives.

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 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]

Bypass arrangement If the building's occupancy depends on the availability of life safety and fire-fighting equipment, a single or dual bypass arrangement should be incorporated to allow maintenance of the changeover device without disrupting power to critical systems.
If a single bypass is required, it should be installed on the primary power supply.

Typical ATS sizes based on Mertech

SBP – Single bypass
DBP – Dual bypass

Frame Size (Amps)	Height (SBP \| DBP)	Width (SBP \| DBP)	Depth (SBP \| DBP)
45	600 \| 800	800	210
63	600 \| 800	800	210
100	600 \| 800	800	210
125	600 \| 800	800	210
160	800 \| 1200	1200	300
250	800 \| 1200	1200	300
350	1000 \| 1600	1600	300
400	1000 \| 1600	1600	300

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)

Neutral Separation at ATS The ATS is often the only point where parallel neutral paths can occur between the generator and the mains supply.
Solution: A four-pole ATS should be used instead of a three-pole ATS to ensure the neutral is switched between mains and generator. This eliminates the risk of unintended neutral paths and prevents circulating currents.

Remote indication of Life Safety Generator status

Remote indication of generator status is required by BS EN 12845, BS 8519, and BS 9999. The remote indication (mimic panel) should be positioned to allow rapid identification and response by the fire service on arrival, as mandated for higher-risk buildings under the Building Safety Act and Gateway 2 compliance.

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 utilise 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 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

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.

120 min fire-resistant building fabric enclosure Generators and their associated distribution equipment may be installed within the same 120 min fire-resistant building fabric enclosure, but separate from the primary supply equipment, provided the equipment is suitable for the environment in which it is to be installed and system resilience is maintained

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.

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.

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+A4:2026 wiring regulations and BS 9999 fire strategy.

Generator Earthing to the Main Earth Terminal (MET)

In TN-S systems, the generator’s earth is typically connected to the MET to maintain a common reference point.

Potential Issue: If the generator is operating in IT mode (isolated neutral), connecting its earth to the MET can create an unintended return path for fault currents.

Solution:

If using IT earthing, the generator’s neutral must be completely isolated, and its frame can still be earthed to the MET through a high-impedance grounding resistor to limit potential differences.
If not using IT earthing, standard connection to the MET is acceptable, provided there are no neutral bonding conflicts.

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 7671:2018+A4:2026 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.

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.

Key Takeaways:

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

Online Generator Load Calculator - Do this in minutes!

Generator Maintenance & System Testing

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

Weekly: Test ATS functionality & check fuel levels.

Periodic on-load testing to the set manufacturer’s regime (BS 8519:2020 sets no interval); load bank testing where building load cannot exercise the set.

Annually: Simulate a full-scale power failure test.

Every 3 years: Review fuel quality & replace if necessary to prevent degradation.

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