Maximum sensitivity with minimum false alarms

HM Fire Service Inspectorate Statistics indicate that there were 449,000 false alarms in 2004. The introduction of the Regulatory Reform (Fire Safety) Order 2005 (RRO) and the Fire and Rescue Services Act 2004 has resulted in a major focus on the reduction of false alarms. Owner-occupiers of premises with frequent false alarms may see either a reduced attendance by Fire Services or higher premiums from their insurance company or both! This may result in a single fire fighter attending on a motorbike to examine the scale of the fire before calling for reinforcements!

False alarm activations can occur for a variety of reasons; dust, insects that get inside sensors, fumes from cooking and even steam from bathroom showers, etc can often be the root cause. However, there can be situations where the sensor has been so badly positioned, that the site could either end up with more false alarms, or perhaps more concerning still, no alarms whatsoever.

For new systems, responsibility for providing an effective and compliant fire detection and alarm system lies with all parties involved. The designer should recognise potential risks and problems. The installer and commissioning engineers should flag up other difficulties during the installation phase, such as the air-handling units being too close to the sensors. Ultimately the owner-occupier having carried out a Fire Risk Assessment specifying the level of protection required (M, L1-L5 etc) should ensure all documentation supports the requirements of the Risk Assessment.

At the design stage, formal consideration of the potential risk of false alarms should include:

• Siting and selection of manual call points.

• Selection of system type, albeit Conventional or Analogue, incorporating single or multi channel sensors.

• Protection against electromagnetic interference.

The designer should also monitor the performance of newly commissioned systems. Many of today’s fire control panels include numerous software-configured options that are under utilised; these include the disable/enable feature that can help filter potential real fires from potential causes of false alarms.

Generally the choice of sensor falls between a single point smoke or heat sensor, although there are other types. Alternatively, multi-sensors, that combine a number of different sensing elements, can be the best choice as they can switch between different sensitive states, at any given time. This allows the sensor to avoid false alarms during specific times of the day, but to be ready to react quickly to a real fire at all other times.

To help select the correct sensor for a specific application, the operating characteristics must be understood to ensure its suitability.

The heat sensor – the simplest choice?

Selecting a heat sensor rather than a smoke sensor, to overcome a potential false alarm has been common practice for years. There are potentially 12 different choices, all of which meet the requirements of standard BS EN54-5. Not surprisingly each has a different set of temperature characteristics and the majority offer two operating elements; one for a fixed temperature limit, the other a Rate of Rise.

Therefore when choosing a heat sensor for a specific application, one should also specify the grade or operating temperature characteristics that go with them.

The smoke sensor

Over the last twenty years there have been two main choices, optical and ionisation, and more recently a third, the Carbon Monoxide (CO) sensor.

Before selection, it’s worth spending some time understanding the type of fire that is likely to occur. For example if you are protecting a store containing flammable materials where a high energy free burning fire is expected, then an Ionisation smoke sensor is ideal as it looks for the invisible particles of combustion. In contrast, the Optical smoke sensor reacts to the abundance of visible particles of smoke present in a slow smouldering fire. Finally the chemical cell contained within a CO sensor, measures levels of CO, a by-product of a deep-seated smouldering fire.

Conventional or analogue detection?

Conventional detectors have been around since the 1960s and generally only have a single sensing element that operates immediately the operating characteristics have been exceeded. Analogue sensors first appeared during the 1980’s provided different operating levels such as alarm, pre-alarm, contaminated or faulty. These analogue techniques later became more sophisticated using time comparison or pattern recognition techniques that enabled the fire control panels to recognise the signals from real fires as opposed to potential false alarms.

Analogue – multi-sensors

Further improvements in technology occurred during the 1990s and we saw the first dual angle optical sensor produced, that combined with heat and CO sensing elements could differentiate between smoke particles and a whole range of contaminants such as steam, white dust, cigarette smoke etc. The S-Quad multi-sensor from Gent by Honeywell offers this variety of sensing elements, different combinations providing a large selection of different sensor states to maximise real fire detection and minimise false alarms.

Correct detector positioning

Sensor spacing is based on the products of combustion first rising as a plume and then rolling along the surface of the ceiling. To take advantage of this, detectors must be mounted within 600mm of the ceiling for smoke detectors and within 150mm for heat detectors, but no closer than 25mm. Due to the stagnant air space where the wall meets the ceiling; a detector should not be mounted closer than 0.5 metres to a wall or partition. For further advice on sensor siting we recommend you consult the British Standard BS5839-1:2002 although the Pocket Design Guide published by Gent by Honeywell covers the essential design recommendations.

Enhancing performance

All modern fire control panels designed to comply with EN 54 part 4 should be fitted with some form of verification feature that allows the user to accept the alarm and introduce a time delay whilst further investigations take place. This will prevent any unnecessary evacuations although the operation of a second detector or call point will override this condition and reactivate the alarms.

Most panels also incorporate an enable/disable feature that can be programmed to automatically switch sensors from one state to another. For example in a restaurant below a bedroom, the detector can be programmed as a heat only detector during the day and a combined smoke and heat detector at night.

It is also possible to change the sensitivity levels of a detector – e.g. to increase sensitivity of the smoke sensors within a bedroom when the occupant is asleep.

A double knock or coincidence sensing feature can also reduce unwanted alarms by ensuring the control panel gathers information from more than one sensor within the same room. The information can be combined, enabling the system to react more quickly to real fires while ignoring potential false alarms from a single device. This technique has been common practice in computer rooms for some time but surprisingly is rarely used for more general applications.

Finally, another technique, using a combined sensor sounder, is to programme the sounder so it only operates when its own sensor is activated. This feature is extremely useful within a hotel bedroom or students bed-sit, where the occupant is alerted to a potential emergency situation, while other guests are left undisturbed.

Striking the balance

The measure of a good system design is not only a minimal number of false alarms but also ensuring the effectiveness of the system to detect real fires is not impaired.

Understanding the type of fire you can expect and choose the correct sensor to detect that fire quickly is key to a good design, as well as recognising any ambient condition that could give rise to a false alarm.

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