System categories specified in BS 5839-1:2017

Category M systems are intended for buildings that are fully occupied when fire detection is required. This is because the Category M system uses people as the fire detectors. The first person to find the fire, operates a manual call point, this activates the system and the alarm warns everyone in the building that there is a fire.

If a fire should start in an unoccupied part of the building, it could continue to grow undetected until it is discovered. If the time to discovery is significant the fire might have become very serious.

Category M systems are relatively low cost and they are reliable. The reliability is partly due to their simplicity, but mainly because people are very good at detecting fires (using sight, smell, heat and sound) and people are also excellent at assessing the fire. If trivial, it could be dealt with without alerting the whole building.

However, Category M systems should not be chosen where a fire on an escape route, could grow to the point where it blocks the safe evacuation of all, or part, of the building.

Category L4 systems are used to protect people by protecting their escape route. Optical smoke detectors are placed on all escape routes and in the event of a fire being detected, it is essential that building occupants react immediately, because at least one escape route might become unusable very quickly.

The optical smoke detector detects smoke when it is almost invisible, so at the point of detection the escape route can still be used. However, because smoke is already on the escape route, time is limited.

A category L4 system is intended to protect the lone worker, that is someone, or a group of people, who are working in an otherwise empty building and where fire would only be detected when it affects the peoples work space.

Category L3 systems are used to protect escape routes, where more time is needed to offset a greater risk. For example, bigger buildings, more people, higher risk of fire spread, or even sleeping risk. BS 5839-1 considers that a category L3 system is the minimum suitable to be considered for sleeping risk, because, in most cases, the system should provide adequate warning for all people to exit the building safely, other than those in the room of fire origin. Their safety will depend on the specific circumstances of the fire.

Optical fire detectors are located on escape routes, exactly as they would be for an L4 system and fire detectors are placed in the rooms opening onto escape routes.

In the early stages of fire, the fire will generally spread through doors if the building is in compliance with the building regulations. Also, fires do not, generally, start on escape routes. Fires generally are cause by faulty equipment, or accidents or causing a fire risk e.g. by placing a flammable garment over a heater. As the fire starts and then begins to spread, to get to an escape route, it must go through the rooms that open onto the escape route, so the extra fire detectors in the rooms opening onto escape routes will operate earlier, than the detectors on the escape route, thus the system gives earlier warning of the fire.

Category L2 systems are similar to Category L3, but areas of high fire risk are also considered and appropriate detectors are placed in those locations. For example, 80% of hotel fires start in the kitchen. So, a hotel L2 system would be fitted with detectors in the kitchen, as well as all the detectors that would be required for compliance with L3.

Category L1 systems have fire detectors in all rooms and voids deeper than 800mm. The whole building is protected, and the system is intended to give the highest possible level of protection for the safety of life. In fact, an L1 system is intended to protect the lives of those in the room where the fire starts. To achieve this, smoke detectors should be used everywhere, except those locations where smoke detectors would cause false alarms.

For example, a smoke detector in a kitchen would be prone to false alarms from cooking steam or fumes, where food has been overheated. Heat detectors are therefore used. The heat detector will not protect the occupants of the kitchen from fire, but it will warn of a fire in the kitchen before the fire spreads outside of the kitchen. If staff are in the room, they will know of the fire before the fire detection system responds, so early warning of the fire can be given by the operation of a manual call point.

All of the above systems have alarm devices throughout all accessible parts of the building, so as to alert all occupants in the event of fire. Manual call points are installed at all final exits, at story exits and on escape routes such that anyone in the building does not have to travel more than 45m to get to the nearest call point.

The final life safety category is L5. Category L5 systems do not fit into the hierarchy of the other life safety systems. L5 is defined as fire detection where specified, or in other words, the detection is to achieve a specific fire objective.

A category L5 system might consist of only one fire detector in a specific location, or it might require more detectors than an L1 system. A specification is needed, and a fire specialist will design a system to meet the customers objective. The specification might be simple or complex.

For example, a category L5 system might be in accordance with category L4, but with detectors at specific locations of high risk. A sort of mix between L4 and L2.

Manual call points are not required to comply with the specification of L5. However, if they are required, category L5/M would be specified. The ‘/M’ would require the fitting of manual call points in accordance with the recommendations for a category M system.

Property Protection Systems

There are just two property protection categories, P1 and P2.

Category P1 systems protect the whole building with detectors, located as they would be for L1. No alarm devices are required, unless specified, but it is usual for P systems to have an automatic facility to summon the Fire and Rescue Service, FRS. Normally this would be a link to a remote alarm receiving centre, who have an agreement to pass alarm signals to the FRS. Category P2 systems have fire detectors where specified. Generally, a P2 system is intended to protect something in the building, rather than the building itself. Property such as computer or telecom systems might be protected by a category P2 system.

Life and Property Systems

Most buildings have people in them, even if the main objective is to protect the property and many building owners will specify a dual purpose system. This would be described as a P1/M or P2/M system. The detection is designed for either P1 or P2 and a manual system (manual call points and alarm devices) is added onto the P1 or P2 system, using the same panel and cables, but with the extra components.

Similarly, a building might be equipped with a category L3 system and an insurer is content with this level of detection, but requires an automatic link to an alarm receiving centre, and possibly an enhanced power supply to meet the insurance requirements.

Got a question about BS 5839-1:2017? Need Fire Safety Compliance advice?  Get in touch today!

For over 45 years, Fixfire® has been providing quality products, systems and services for Life & Property Protection. Whatever your requirement, please call our Compliance Team for expert advice and a refreshingly different approach.

Freephone 08000 891999