The objective of an L1 system is to provide the earliest possible warning of fire in order to offer the longest available evacuation period from the building. Typically, detection will be cited in all parts of the building with only small areas such as cupboards and low risk areas such as toilets not requiring automatic detection.

L2 fire detection systems are installed to achieve the objectives set out in a category L3 system, with the added consideration of areas of a building that represent a higher level of risk of fire. Rooms considered by an L2 system might include Kitchens, boiler rooms and rooms where hot works are undertaken. Guidance should be sought from your Fire Risk Assessor if an L2 system is specified.

A category L3 system is designed to give a warning of fire at an early enough point that escape routes within building are not yet compromised. This typically means that automatic detection is cited on all escape routes, and the rooms leading directly off them.

L4 systems require automatic fire detection installed within the parts of the escape routes compromising of main circulation areas such as corridors, stairwells and routes to a final exit.

A category L5 system is used to protect areas or building needs where the location of detectors cannot be achieved using a L4-L1 system. This might be a design based on the localised need for fire detection within a building, to areas where the greatest level of risk is present. More recently, larger apartment buildings across the UK with exterior cladding installed with a demonstratable fire risk has required the installation of local detectors in the vicinity of the exterior walls only as an example of this type of system in action.

These types of Fire Prevention Systems provide the earliest possible warnings of fire to minimise the delay between detection and the arrival of firefighters.

P2 Systems are used in areas of a building where the risk of ignition is high or, the contents of said area are valuable.