BS 9991: What are the new changes?

By Carlsson Elkins
Friday 29th October, 20215 minute to read

Fire safety is incredibly important — this doesn’t need to be reiterated. We all know that compliance with fire regulations protects buildings and the environment, as well as being a case of life or death, so they’re not just a legal requirement. One of these essential best practices is BS 9991, which is a document produced by the British Standards Institution (BSI), the UK national standards body.

While BS 9999 regulates fire safety in the design, management and use of non-residential constructions, BS 9991 complements it for residential buildings. It lays out specifications and practices for suitable fire safety measures, including fire detection alarm systems and fixed pre-fighting arrangements.

However, BS 9991 is being rehauled at the moment in an attempt to reflect the many changes to buildings and construction in the years since 2015, when the latest update was released. So, if you’re an architect, fire safety engineer, fire risk assessor or any other professional who is concerned with fire regulations for flats and other dwellings, it’s vital that you keep updated.

Luckily, a draft of the proposed 2021 revision is out, so we have taken the time to point out the recent proposed changes, as well as some glaring omissions in the new text. Note that the document is likely to be amended further, though.

1. Systems and strategy

One of the biggest changes the new content introduces are to smoke ventilation systems and fire strategy.

Smoke ventilation systems (Section

Albeit rare on a stair core that is above 18 metres, the previous regulations allowed natural smoke ventilation systems on any height of building (excluding natural facade ventilation on buildings over 30 metres). If you’re wondering why this type of system is so uncommon in tall constructions, the answer is concern over smoke-logging — when smoke cools down and loses its buoyancy. As natural ventilation systems require the smoke to travel up a very long shaft, the chances of this increase, which will be counter-productive. It can significantly hinder the stack effect and result in the smoke persisting. In other words, failed ventilation.

The new changes spell this out and restrict the use of natural smoke ventilation to buildings up to 30 metres in height, as long as travel distances are below 7.5 metres (extended to 15 if sprinkles are installed). If the structure is taller or the distances are higher, BS 9991 states that a mechanical smoke ventilation system or pressure differential system should be used.

‘Stay Put’ strategy (Section 3.62)

The ‘Stay Put’ policy has been the subject of much controversy. This fire evacuation strategy, relevant mainly for blocks of flats, suggests that as long as fire and smoke have not reached a room, the dwellers are encouraged to shut the doors and windows and remain in the unaffected area. If and when the fire does reach their flat or common area, that’s when they should vacate the premises and seek help.

The new phrasing in BS 9991 explains that “all residents are always free to leave their flats if they wish to do so”, but that it might, however, “place them at greater risk than remaining within their flats”. Of course, this has always been the case — no one has ever forbidden anyone from leaving their apartment even in buildings with the strategy in place. Nonetheless, it being clarified in the Standard will bring comfort to those who are unsure about this policy and its use.

2. System control and control products (Annex A.2.1)

When it comes to rescue services, sometimes the need arises to fiddle with controls and fire systems. This is to allow firefighters to turn parts of the system on or off as the situation changes. However, some of these controls and overrides are either incredibly complex or unclearly labelled, which discourages emergency services from interacting with them for fear of worsening conditions. The new guidelines lay out how these systems should be structured to allow for maximal fire safety:

  • Automation: Systems should be fully automatic and never rely on the interaction of firefighters during the means of escape or firefighting phases. This does not apply to openable vents.

  • High-speed mode (AKA boost or firefighting mode):

    • Smoke control systems should start in high-speed mode at the outset if this is the requirement.

    • Variable speed mechanical systems should be controlled using pressure sensors or other means that will not require firefighters to operate controls to initiate a high-speed mode.

  • Overrides: Systems should not possess any overrides or manual inputs that may interfere with their automatic function. If overrides are absolutely necessary, the automatic system should be completely turned off — this is unlikely to occur until the incident is deemed ‘over’ by the firefighters.

  • Control panels: Control panels should be simple and logical to operate without relying on manuals.

The most significant change is the regulation stating that systems should start in high-speed mode from the get-go, as well as have no overrides or manual input to change their automatic function. These amendments will completely transform the way systems are used in a fire scenario. Fire and rescue services will have to be trained to understand these controls, while designers, architects and fire engineers will need to overhaul their approach.

3. Construction (Annex A.5.1.4(c))

Previously, the only shaft types with size restrictions were natural smoke ventilation systems. These have a minimum cross-sectional area of 1.5m² and a minimum dimension of 850mm in any direction. The new amendment to BS 9991 now asserts the same minimum dimension for mechanical smoke ventilation systems, too — although it still does not lay out a strict cross-sectional area requirement. Instead, it gives a range of 0.6m² and 1.0m². Granted, this will not make a huge difference in practice, as the vast majority of mechanical extract shafts are around 0.8m² anyway. However, it is now codified, which means designers will have to pay extra attention to it.

4. Product

BS 9991 also deals with a number of smoke control products and further clarifies their correct use. Notably, this is sometimes done in a contradictory manner to previous advice, which implies it is even more important to pay attention to these changes.

Smoke vents (Annex A.5.1 and A.

In contrast with Approved Document B, which states that “All vents should either be a fire doorset [...] or fitted with a smoke control damper [...]”, the new standards set in BS 9991 declare that automatic opening vents (AOVs) that open into shafts or ducts should be smoke control dampers. This is significant because, unfortunately, recent fires have taught us that fire doors acting as vents are not ideal. They have failed to operate, causing evacuees to be removed from buildings wearing smoke hoods. The new regulation makes it abundantly clear that this is no longer considered best practice.

Actuators (Annex A., Note 2)

In some buildings, site-installed actuators or ‘window actuators’ have been used as makeshift smoke vents. BS EN 12101-2 already clarified that this is an unapproved practice, but BS 9991 now strengthens this point even further, affirming that “the fitting of actuators to other components to ‘make’ natural smoke vents or smoke control dampers is not acceptable, as no performance has been tested.”

Smoke control dampers (Annex A.5.5)

The document now posits that smoke control dampers should be classified for reduced leakage S, upholding BS EN 12101-8 but departing from the SCA Guide. It also applies a minimum operations classification of C10 000. Both of these requirements ensure that the dampers are of good quality and are able to last for a long period of time, as they are used or tested weekly.

5. Power (Section 23)

Secondary power supply methods previously included an option to use two separate intakes into the building from the same external substation. This now appears to have been removed, limiting alternatives to a life safety generator, an independent utility primary network substation to the one feeding the primary supply or an uninterruptible power supply. As of now, it is unclear why this change has been made, however, independent queries will surely clarify this.

Moreover, a clause in Section 23 states that “the UPS battery equipment should be located adjacent to the equipment that it is to support […].” This is an interesting concept, but yet again ambiguous regarding its application to roof-mounted extract units, as at present it would be unusual to situate a UPS on the roof of a building.

6. Standards (Section, Note 1)

Interestingly, the BS 9991 gives us a sneak peak into a new Standard coming out soon: “When published, BS EN 12101-13 will take the place of the designs in the current BS EN 12101-6 and will refer to BS EN 12101-6 for kit and component requirements.” Designers and architects should keep their eyes peeled for that.

7. Maintenance (Annex J)

Annex J brings along many changes and clarifications, including:

  • Inspections (J.2.3): Smoke control systems and fire damper control panels should be inspected daily.

  • Cleaning (J.7): Ducting and shafts should be cleaned at least annually, and certificates of cleaning should be obtained.

  • Records (J.8): Each system and component of active and passive fire protection should have a record kept, with positive evidence and documentation of inspection — even where there are no faults.

  • Faults (J.9): A written risk assessment should be drawn up in case of a fault, including the measures that will correct it. Residents should be informed and advised.

8. Notable omissions, anomalies and required clarifications

Seeing as the document as it stands now is still in its draft stage, it is understandable that some parts require clarification, while others are lacking:

  • Aerodynamic free area: There is some uncertainty between EN 120101 and Approved Document B regarding declaration of free areas versus aerodynamic free areas. BS 9991, unfortunately, does not give a standardised answer to this question in Section 3.33.

  • Control equipment: The document seems to be confused on when to conform with BS ISO 21927-9. The Standard is mentioned in Annex A.2.2 regarding natural smoke and heat exhaust ventilators, whereas it should be dealt with when talking about control specifications.

  • Mechanical smoke ventilation systems: In Annex A.3.2.1, Note 1, the document lays out parameters for a test criteria. However, these are usually applied to pressure differential systems rather than mechanical smoke ventilation systems, so it seems conspicuous.

  • Depressurisation: When it comes to methods to stop excessive depressurisation of extract zones, pressure sensors are mentioned while reverse-hung doors aren’t. This is despite the fact they are far less likely to fail.

  • Smoke shaft: It has previously been acceptable to use the smoke shaft for cabling for the smoke ventilation system, however, in Annex A.5.1.3(j), it is mentioned that no service should be contained within the smoke shaft — including its control devices. It is unclear whether the guidance has changed on this.

  • Extract point: Section contradicts SCA guidance when it states that a mechanical smoke ventilation system that is installed in the protected lobby or corridor can be directly adjacent to the stairwell enclosure. The previous practice asserts that the extract point should be remote from the stair core.

BS 9991 is an incredibly important update to the 2015 Standard. It has many well-needed clarifications and elucidates key concepts and practices that may have been confused before, while taking into account the changes to construction and the lessons that have been learnt since. This draft still requires some fine-tuning, though, so make sure to follow us for more information, or get in touch with us if you need professional help.

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