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Automatic Transfer Switch (ATS)
In a Higher-Risk or life-safety building, the firefighting and smoke-control plant is only as resilient as the device that switches it onto standby power. The automatic transfer switch (ATS) transfers the critical load between the normal mains and a secondary mains or generator supply on loss of the primary source, automatically and without ever connecting the two in parallel (BS 7671:2018+A4:2026, Reg 551.6.1). For life-safety and firefighting applications the ATS must additionally satisfy BS 8519:2020, BS 9991:2024 and the guidance in BSRIA BG 70/2017, and be built to BS EN 60947-6-1. The sections below cover its location, specification, bypass strategy, remote indication and fire-resistant cabling, and the design decisions a competent designer must be able to justify.
Regulatory basis: BS 7671 Regulation 551.6.1
Whatever the application, the cardinal rule for a standby ATS is that the alternative source and the distributor’s (DNO) supply must never be connected in parallel. BS 7671:2018+A4:2026, Regulation 551.6.1, requires precautions complying with Chapter 46 and Section 537 so that “the generator cannot operate in parallel with the system for distribution of electricity to the public“, and lists acceptable means:
- An electrical, mechanical or electromechanical interlock between the operating mechanisms or control circuits of the changeover switching devices;
- A system of locks with a single transferable key;
- A three-position break-before-make changeover switch;
- An automatic changeover switching device with a suitable interlock; or
- Other means providing equivalent security of operation.
For life-safety and firefighting supplies this baseline is layered with the more onerous requirements of BS 8519:2020 and BS 9991:2024 set out below.
ATS Requirements
The automatic transfer switch transfers the critical load between the normal mains supply and the secondary mains/generator supply where the rated voltage does not exceed 1000 V AC (BS EN 60947-6-1). For life-safety and firefighting duty the device should:
- Conform to BS EN 60947-6-1 and be based on switch technology (PC classification), not built up from circuit breakers or contactors (BS 8519:2020, Cl. 9).
- Be a single type-tested assembly with an integrated controller, from one manufacturer, not an assembly of parts from different manufacturers (BS 8519:2020; BSRIA BG 70/2017).
- Be specified to utilisation category AC-33A or AC-33B (motor / mixed loads incl. motors, resistive loads and up to 30% incandescent). AC-33A is preferred for its more onerous test regime, given that critical-plant ATS are seldom exercised (BSRIA BG 70/2017).
- Meet the PC short-circuit requirement: capable of making and withstanding , but not breaking, short-circuit current. Appropriate protective devices must therefore be provided upstream of the ATS on both the primary and secondary supplies for protection coordination (BS 8519:2020, Cl. 9).
- Have an interlocked operating mechanism to prevent simultaneous connection to both supplies, with clear indication of ‘Normal’ / ‘Off’ (where provided) / ‘Alternative’ positions.
- Monitor voltage and frequency of both supplies, transfer to the alternative automatically on mains failure, and return to the primary supply once it has been restored and is stable, manually or automatically after an adjustable time delay (BS 8519:2020, Cl. 9 Note 2; BSRIA BG 70/2017).
- Provide remote indication of ATS status to the main fire alarm panel / fire control room.
- Be served by a Category 3 fire-resistant control cable from each ATS to the generator for the start signal and status indication, mechanically protected where exposed (BS 8519:2020).
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]
Bypass arrangement
Where occupation of the building is conditional on the life-safety / firefighting equipment being available, a single or dual bypass should be provided so the ATS can be inspected and maintained without loss of supply. The choice, and whether the bypass is on the primary, secondary or both supplies, is set by a design risk assessment for the specific application, taking account of the manufacturer's maintenance requirements (BS 8519:2020, Cl. 9; BSRIA BG 70/2017). A single bypass is taken on the primary 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
Sizing the standby source behind the ATS? Use our Generator Load Calculator to establish the load profile before you specify the switch.
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
- 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, 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
Designing a life-safety power supply to BS 8519 or BS 9991? Talk to us about your project. We design standards-compliant electrical services for HRB and critical systems.
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