Because prisons are designed to keep people inside, by their very nature they pose special problems for fire safety. Colin Roberts, senior fire engineer at Arup Fire, examines available guidelines and some of the contentious issues involved.
Due to their very purpose, prisons offer a unique challenge in terms of fire safety. The very essence of people’s safety, enshrined in the functional requirements made under the Building Regulations, is that “the building shall be designed and constructed so that there are appropriate provisions for the early warning of fire, and appropriate means of escape in case of fire, from the building to a place of safety outside of the building, capable of being safely and effectively used at all material times.”
Indeed it is clearly stated in Approved Document B that “requirement B1 does not apply to any prison provided under section 33 of the Prisons Act 1952.”
Until recently, prisons constructed in the UK had the benefit of Crown Immunity from Building Regulations. This is no longer the case and compliance with Building Regulations, in particular with Part B, Fire Safety, is now required. Prescriptive guidance adopted for prison design in mainland UK is Prison Design Briefing System, PF 24, in England and Wales, and Fire Safety Technical Standards, DS 24, in Scotland.
There are many different functional buildings, or sections of buildings, in a prison including workshops, laundries, stores, education, sports, religious, administration, medical and the house-blocks. In the majority of these escape from a fire, while being controlled, is possible without requiring special consideration. It is the cell blocks (known as ‘house-blocks’ ) and potentially the medical unit, where prisoners are locked into their accommodation, that special consideration is needed.
Means of escape
One of the fundamental requirements, irrespective of any code, when designing a building in terms of fire safety is for the occupants to be able to move freely away from a fire. This is, of course, not possible when the purpose of the building is to keep the occupants secured. Like most fire safety objectives, the main purpose behind the guidance is to maintain a reasonable level of life safety in the event of a fire.
In most fire situations, the aim is to provide a way for people to escape to a safe location, normally outside the building. In the case of prisons, escape is not really the right term to be using, nor indeed should it be to outside the building, unless it is to a secure area.
Hence a slightly different approach is required. During the day it is normal for prisoners to be in ‘association’ areas, or in workshops, educational or sports areas. A fire at this time is unlikely to cause a major threat to life – people will be able to move freely away from the threat to a point of exit where an unlocking procedure will be in place.
The guidance considers that the worst time for a fire to occur would be at night when staffing levels are low and the occupants are asleep and locked inside their cells. A fire in a cell is designed to be contained in the cell in which it starts, and measures should be in place to:
– Provide sufficient time for staff to evacuate inmates from cells not involved in a fire before smoke enters these cells.
– Provide sufficient time to enable staff to remove the occupant of a cell on fire, before conditions of access to the cell become hazardous.
– Enable staff to attack the fire in safety.
– Prevent the spread of fire or damage by smoke and heat to areas beyond that immediately involved.
The spread of fire is normally achieved by the use of materials that have fire resisting properties. Walls and floors of 30 minutes fire resistance would be acceptable in a single storey building, going up to 60 minutes if the top storey of the building is 18-metres or less above ground. Longer periods of fire resistance would be used if the building were higher than this: 90 minutes if the top storey is 30 metres or less, and 120 minutes if the top storey exceeds 30 metres.
Smoke control
The other objectives are met by introducing smoke control into the building. For this to work a design size fire needs to be adopted, but there is a level of conflict between the two guidance documents. The Scottish document suggests that a design fire with a peak heat release rate of 4.028MW is used, while the one for England and Wales suggests that a 1.0MW or 1.5MW convective heat flux fire is adopted.
So a reasonable scenario upon which a design size fire can be based needs to be considered. Arup Fire has undertaken a fire load study on the typical contents of a double cell, which concluded that with stoichemetric burning (i.e. everything burns to its absolute potential) a peak heat release rate of 5.9MW would be achieved. The assessment was performed using the guidance given in the Society of Fire Protection Engineers Handbook of Fire Protection Engineering (2002).
But is the peak heat release rate achieved a realistic fire against which to design the smoke control system? Probably not, as any attempt to open a cell door would be extremely hazardous, with smoke temperatures in excess of 1500 degrees C expected to be flowing through the cell door opening.
So human intervention is required to reduce the risk to both inmates and prison officers. This tends to take the form of an addressable automatic fire detection and alarm system designed to L1 standard (as defined by BS 5839 Part 1) to ensure a prompt indication of a fire incident. Using such a system, the prison officer responding will know which cell is affected and will use the first aid fire fighting equipment provided in the form of self- contained breathing apparatus (if appropriate) and a fire hose reel with the nozzle deployed into the cell, via an inundation port. In this way, effective action can be taken without risk to the prison officer. The effect of the water discharge is to reduce the fire size to a level where the cell door can be opened and the prisoner extracted.
It is the smoke that is spilling through the open door during this scenario that the smoke extraction system is expected to control. The effect of the water being discharged through the inundation port will be to reduce the heat output.
Water has an approximate cooling rate of 2.6MW per litre per second. A single hose will be discharging approximately 0.7l/s (the hose-reel water supply being designed for more than one hose being used at one time) So this discharge should, in theory, deal with a fire of 1.8MW output.
It is important, then, to gauge the size of fire when the hose is deployed. From data supplied by a leading private company responsible for running UK penal establishments the following sequence is expected.
– Fire starts in a cell.
– In-cell detection activated, alarm raised and local alarm sounded in house block office. The fire at this point is in its incubation period, so timing sequence is T = 0 seconds.
– At this point the fire is assumed to have entered its growth phase.
– Alarm relayed to control room and fire procedures initiated: timing = T+10-seconds
– Officer identifies cell of fire origin at the alarm panel, confirms fire location and confirms fire to control room.
– Control room calls the fire and rescue service.
– Timing = T+1.5 minutes.
– At this time the fire will have reached approximately 0.4MW and it is at this point that the hose reel is deployed through the inundation port and, if safe to do so, the officer rescues the prisoner or awaits the arrival of the response team.
A safety factor would need to be applied to the 0.4MW figure, and a design fire size of 1.0MW convective heat flux could be considered reasonable.
Mechanical extract or natural smoke ventilation could be adopted, depending on the constraints of the building design. For a standard central association space surrounded by cells on two levels, natural venting would give a flexible approach. Natural venting tends to be self policing and the larger the fire the greater the smoke temperature, which makes the smoke less dense. The effect of this is for the smoke to flow more quickly through the vents. The result of this is that the smoke vents provided for a small design size fire can cope with larger fires, which adds a level of safety to the system.
Automatic fire sprinklers
Under the guidance document there is a strong recommendation to install sprinklers in certain areas – such as workshops and stores – but not throughout. It is currently not normal practice to fit sprinklers within house-blocks.
Sprinklers need to be assessed against the added level of safety that they bring to the building. If fire spread is controlled by passive fire resistance and spread of smoke is controlled by venting, then they may not be necessary. Standard sprinklers would also offer both a mischief device and a ligature point. Institutional sprinklers, designed to be less accessible, would reduce both of those risks. But these types of sprinkler have not been tested and approved by a UK test house, and this sometimes causes concerns to the approving authorities.
If the lives of prison officers or indeed prisoners not in the cell of fire origin are not threatened, then would the sprinkler actuate in time to save the life of the prisoner in the fire cell? This would be subjective and dependent on the rate of fire growth, the actuation time of the sprinkler and the toxins being produced by the materials being burnt. Critical dose levels can be calculated and an assessment of the loading of the typical cell has indicated that a critical dose could be reached after 86 seconds. Exposure to this dosage would be required for 5 minutes before death would occur, the problem however is that the dosage level would continue to increase and exposure for a lesser time period could result in death. Using a fast response sprinkler with a fast response element and a fast (t2) fire growth curve, the sprinkler would be expected to operate after approximately 110 seconds. There is, therefore, a distinct chance that the prisoner would not survive the fire.
Sprinklers are, however, a good fire control system which, if adopted early in the design stage, offer distinct advantages. Periods of fire resistance could be reduced and travel distances could be increased, leading potentially to a reduction in the number of stairs. Compartment sizes could be increased and smoke vent requirements removed.
Conclusion
With the prison population on the increase and existing prisons being overcrowded, there is an increase in the number of prisons and detention centres being built. Fire safety in these establishments is important but tends to be secondary to security. So fire safety measures need to be effective and affordable. If automatic fire sprinklers are adopted early in the design, they do offer a route for negotiation on other fire safety measures. Sprinklers have the potential to deal with fires both inside and outside the cells, and have the potential to deal with multiple fire sites. As long as it is realised that there will be issues with sprinklers they should at least be considered, ideally at the concept stage of the design.
