Life Safety Generator

Life Safety Generator - selection guide

A Life Safety Generator must meet stringent fire safety and power resilience criteria. BS 8519:2020 provides key recommendations, including:

Independent Primary & Secondary Power Supplies: The secondary supply should be completely independent of the primary supply, preferably an automatically started standby generator.
Fire-Resistant Enclosures: Generators should be housed in a 120-minute fire-resistant enclosure (EI, ESa, or REI, as per BS EN 13501-2:2016).
Minimum Fuel Storage Requirements (BS 8519, Table 2):
- 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.

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.

In this guide:

Key Steps in Life Safety Generator Selection

1. 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

2. 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 power rating (kW)
Starting method (Star/Delta, Soft Start, Direct-On-Line)
Full load current & Starting current (A)
Locked rotor current & hot burn-out time (s)
Starter fuse selection

TB210 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.

3. 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).

Example Calculation (TB210 Compliance): Sprinkler (Primary) LRA: 199A
Sprinkler Starting Current (SC): 66A
Required Generator Amps: 265A (199A + 66A)

4. Apply Load Reduction Strategy

Reducing peak demand allows for a smaller, more efficient generator.
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 Delay: Lifts likely accelerate one at a time.

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

5. 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 Emergency Standby Power (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).

6. Life Safety Generator Sizing Considerations

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 staged loading to achieve these optimal load factors.

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.

(ℹ) Load Shedding: Only required when the generator is supplying both life safety and non-life safety loads from the same switchgear system i.e. LV switchboard. If a dedicated switchboard/panelboard supplies only life safety loads, load shedding is not required.

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.

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.

7. Automatic Transfer Switch (ATS) Requirements

The primary and secondary power supply cables should be connected to a changeover device (automatic transfer switch) located in one of the following:
A plant room housing life safety, fire-fighting, or other critical system equipment.
For a firefighters’ lift, within the fire-fighting shaft outside the lift well or within a fire-protected enclosure directly adjacent to the fire-fighting shaft.
The changeover device must automatically switch from the primary to the secondary power supply if the primary supply to the life safety, fire-fighting, or other critical system equipment fails.

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.

Automatic Transfer Switch (ATS) – BS EN 60947-6-1 Compliance
The primary and secondary power supply cables must terminate at a changeover device (ATS).
The ATS should be housed within the same plant room as the life safety equipment.
For fire-fighting lifts, the ATS must be inside the fire-fighting shaft or an adjacent fire-protected enclosure.
The ATS must conform to AC33A or AC33B classification, ensuring it can handle motor loads.

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.

8. 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.
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 as appropriate) 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.

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.
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

9. Fuel Storage & Containment Best Practices:

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.

10. 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.

11. Parallel Operation of Generators

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

12. 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.

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 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.

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

13. 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.

14. 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

15. Generator Maintenance & System Testing

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

Life Safety Generator - selection guide

Key Steps in Life Safety Generator Selection

1. Identify Life-Safety Loads

2. Determine Starting Inrush & Locked-Rotor Current

TB210 Requirements for Sprinkler Pumps

3. Burnout Time Considerations:

4. Apply Load Reduction Strategy

5. Select Generator Type & Rating

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

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

6. Life Safety Generator Sizing Considerations

7. Automatic Transfer Switch (ATS) Requirements

8. Cabling , Containment and Fixings

9. Fuel Storage & Containment Best Practices:

10. Location & Fire Compartmentation

11. Parallel Operation of Generators

12. Earthing Considerations

Earthing Considerations for Life Safety Generators in TN-S Systems

TN-S System Approach

IT Earthing for Life Safety Generators

13. Final Design Validation

Sample Cause-and-Effect Matrix

Load Activation & Interlocking (After Power Transfer)

14. Online Generator Load Calculator

15. Generator Maintenance & System Testing

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