The LWF Blog

Fire Safety Engineering for Design – Fire Detection & Alarm Systems – Part 145

August 14, 2023 10:18 am

LWF’s Fire Safety Engineering blog series is written for Architects, building designers and others in the construction industry to highlight and promote discussion on all topics around fire engineering. In part 144, LWF touched on the NFPA 72 guidance for fire alarm systems in private dwellings, as well as examining the types of fire detection systems. In part 145, we continue to discuss conventional monitored fire detection systems and addressable systems (including analogue addressable).

In the case of a conventional monitored fire detection system, the circuits are constantly monitored and so there is not a technical need for the wiring to be fire-rated. Any circuit break would be detected by the control panel as a change in the steady state.

When there is a fire, a detector would become activated and its operation would also alter the steady-state resistance of the circuit to which it’s connected and the alarm would be raised.

With this type of system, the location of the fire may only be isolated to the circuit area which will have a number of detectors connected. When planning and installation of the system is ongoing, it is good practice to allocate one radial circuit to each fire zone, therefore the activation of a fire alarm on that circuit will indicate on the fire panel which zone it refers to.

It is not possible to further narrow the location of a fire without the area being searched.

The sounders on a conventional monitored fire detection system will be wired on separate circuits with fire-resisting cable. This is to ensure that the sounders will continue to operate in the event of a fire.

Addressable Systems (including analogue addressable)

An addressable system is similar in most respects to a conventional monitored fire detection system. The detectors work on the same premise, but they are connected to loops rather than radial circuits. Each detector has its own identification used as an address during the commissioning process. This means that in the event of a fire, the location of the triggered detector is sent to the control panel, narrowing down the actual location of the fire.

It works by allowing the control panel to receive information about the steady-state of individual detectors rather than a whole circuit. The benefit of connecting to a loop rather than radial circuits is that damage to part of the circuit can be isolated allowing the system to continue operating. In order to achieve this, zone isolator unis are placed in the loop between zones.

In part 146 of LWF’s series on fire engineering, we will continue to discuss addressable systems (including analogue addressable). 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 since 1986 to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact the LWF office on 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.