The LWF Blog

Fire Safety for Healthcare Premises – System Technology for fire alarms – Part 144

September 7, 2020 12:11 pm

In LWF’s blog series for healthcare professionals, our aim is to give information on best practice of fire safety in hospitals and other healthcare premises. In part 143, LWF discussed addressable systems, and considered wiring, short-circuits and how home systems can transmit instructions to other devices. In Part 144, LWF will begin to look at the types of addressable system available – two state, analogue and multi sensor.


A two-state addressable fire alarm system is the simplest of the addressable systems. The detectors are responsible for indicating whether there is a fire or not. The main difference between these detectors and those in a conventional addressable fire alarm system is that when the addressable detector generates a fire signal, it also transmits its identity. The rate of unwanted fire signals (UwFS) experienced with a two-state system should be no different from a conventional system.


Most addressable systems used in healthcare environments and other buildings are not two-state, but analogue/addressable. An analogue/addressable system has detectors which do not decide whether or not there is a fire, but instead, transmit a signal level to the control and indicating equipment (CIE) indicating the amount of heat, smoke or flame being sensed. The decision of what those readings constitute is then taken at the CIE.

The simplest analogue system has ‘fixed thresholds’ which are applied to the signal level from each detector. This means that a pre-alarm warning can be given and the situation further investigated. A lower level warning might mean that there is a contaminant interfering with the detector, or a small fire which can be extinguished on site without outside assistance or evacuation of the area. A higher threshold, once reached, would trigger the fire signal.

A very low signal level might indicate that the detector has become insensitive and should be investigated, this would be indicated at the CIE as a fault.

This simple analogue system is known as a four-state, the four states being fire, pre-warning, normal and fault.

Some analogue systems are significantly more sophisticated and may help to eradicate more UwFS. The system may analyse the rate of rise of signal level and enable the early detection of fire and help in eliminating some types of false alarm.

In Part 145, LWF will finish discussing analogue systems and begin looking at multi-sensor systems. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.

Lawrence Webster Forrest has been working with their clients for over 25 years to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact LWF on freephone 0800 410 1130.

While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.


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