The LWF Blog

Fire Safety for Facilities Management Personnel – Fire Detection & Fire Alarms – Part 185

January 23, 2023 12:53 pm

Lawrence Webster Forrest (LWF) is a specialist fire engineering and fire risk management consultancy whose aim is to give information on best practice in fire safety for facilities management personnel through this blog series. In part 184, LWF began to look at the components of fire alarm installations. In part 185, we continue to look at the elements of a fire alarm installation by discussing automatic detectors.

Automatic fire detectors are usually one of four types – heat detectors, smoke detectors, flame detectors or combustion gas detectors, such as carbon monoxide.

Heat and smoke detectors may be either point detectors or line detectors. A point detector covers a single point in the volume of the protected space, a line detector covers a defined line within the space.

A line detector may be either integrating type or non-integrating type. An integrating heat line detector would respond to a low-temperature increase over a long length, as well as a high temperature increase at a point along the line. A non-integrating type will only respond to the high temperature increase at a single point along the line. Therefore, it can be helpful to think of non-integrating line detectors as a series of point detectors.

A heat detector can also be divided into two categories – fixed temperature devices (behaving like a thermostat) and fixed temperature/rate of rise devices (respond to a rapidly rising temperature or trigger at a pre-determined fixed temperature limit).

In the past, point heat detectors were usually electromechanical and comprised bimetallic strips. Modern devices are normally either pneumatic or electronic (thermistor based).

Line-type heat detectors may be pneumatic, but more commonly are electrical. Non-integrating line heat detectors involve current-carrying cable in which the insulation melts at a defined temperature resulting in a short circuit which triggers the alarm. Integrating line heat detecting cables are similar in that the capacitance and/or resistance of the insulation changes with temperature.

Heat detectors are useful only in certain circumstances. They may be used for property protection, but usually smoke detectors are faster to react to a fire and operate. A heat detector may allow a fire to grow to the point where flames are a third of the way to the ceiling before triggering. As the ceiling height increases, therefore, so does the potential fire size to a possible factor of five or six.

In part 186 of this series, LWF will continue discussing the types of detector and what the circumstances they are best suited for use in. In the meantime, if you have any queries about your own facilities or wish to discuss this blog series, please contact LWF on freephone 0800 410 1130.

 

Lawrence Webster Forrest is a fire engineering consultancy based in Surrey with over 35 years’ experience, which provides a wide range of consultancy services to professionals involved in the design, development and construction and operation of buildings.

 

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.