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Watching the detectors

Oct 06, 2006
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Peter Cook takes a close look at the latest generation of conventional fire detectors, which offer enhanced safety levels through the use of innovative self-check technology.

The fire detection system is without doubt the most important safety system in any building. Early warning of a fire not only allows safe evacuation before escape routes become impassable but also enables fire fighting to commence without delay, thus minimising the consequent damage.

There is a lot of research which demonstrates that significant fire damage often has a catastrophic effect on the ability of a business to continue to trade.

Because of the safety-critical nature of the fire detection system, there is a mandatory requirement for it to undergo planned and regular testing to ensure that it is always capable of functioning correctly and will therefore fulfil its purpose of protecting life and safeguarding property.

Such testing regimes are well established practice and provide building operators with peace of mind that they have complied with their legal responsibilities and, more importantly, that they have taken every reasonable step they can to ensure the well-being of employees. Unfortunately, the regular testing of a fire alarm system may not actually pick up faults in the detectors themselves.

Analogue systems

Most large sites are equipped with ‘analogue addressable’ fire detection and alarm systems, which offer a high level of technical sophistication utilising ‘intelligent’ sensors within a software-based communications system. Each fire-detecting sensor or call point is electronically coded with a unique identification or ‘address’, and the analogue system uses a low-power pulsed signal to interrogate each detector in turn, comparing the reply with data representing a normal, healthy condition within individually programmed limits.

Therefore, if any of the detectors within an analogue addressable system were to develop a fault, this would normally be picked up by the fire panel, enabling the appropriate remedial action to be taken without delay.

Conventional systems

However, in many smaller and medium-sized installations, the higher initial equipment cost of an analogue system cannot be justified, and ‘conventional’ fire detection systems are used instead. These systems are based on automatic two-state binary detectors, which function like switches - activating the fire alarm when a predetermined smoke or heat threshold is reached. The automatic detectors are a vital part of the fire alarm system and must be working properly at all times for the system to function correctly.

Although testing of a fire detection system may be carried out frequently, it is important to bear in mind that individual detectors are typically only tested once per year, and so a detector could remain non-functional for some considerable time without being discovered. Admittedly, modern
fire-alarm panels do incorporate monitoring circuits that generate a warning signal if an automatic detector is either accidentally or maliciously removed, but these circuits are only designed to verify that the detectors are installed and that the external wiring is intact; they do not check whether or not the detectors are actually working.

If for any reason a traditional conventional detector stops working, this will only become apparent when the detector is manually tested or, worse still, if there is an actual fire.

Self-check technology

Now a new type of detector has been developed to address this situation. The patented JSB self-check range of detectors have been designed to monitor both their own circuitry and background smoke/heat readings to verify that they are functioning correctly. In effect, this innovative type of detector tests itself several times a second and, if it discovers a fault, it triggers a visual alert and a signal to the fire panel, warning the system user of the need for maintenance. Because the fault activation technology is failsafe, even if a detector completely ceases to function, a fault condition will still be flagged at the control panel.

If the detector finds an internal fault, the panel will display a fault alarm on the affected zone circuit. In addition, the bi-colour LED status indicator on the detector itself will illuminate amber to show which detector has a fault.

In the unlikely event of a detector failing completely, the panel can be set to flash the LEDs of all the other healthy detectors that are connected to a particular zone of the panel instead, thus enabling the faulty unit to be identified. In the event of a break in the external wiring, the panel can be set to flash the LEDs of all detectors between the panel and the break, thereby greatly simplifying the fault-finding process.

If a detector should at any time be triggered into a fire condition, the LED on the detector will illuminate red, avoiding any possible confusion between a faulty detector and one in alarm mode.

Drift compensation

With traditional conventional detectors, the gradual accumulation of dust within the detector over many months or even years can adversely affect performance, causing false alarms or possibly even loss of sensitivity. To avoid this common problem, the self-check smoke detectors incorporate drift compensation technology, which monitors and adjusts the detector sensitivity in response to levels of dust contamination. This technology maintains peak detector sensitivity while at the same time offering maximum protection against false alarms.

The self-check detector is available in a variety of models to suit the requirements of most applications. As well as an optical conventional detector, the range includes a combined photo/thermal conventional detector and three conventional heat detectors for 77degC, 92degC and rate-of-rise operation.

In conclusion, the new patented self-check conventional detectors increase safety levels by providing an immediate warning of any malfunction, rather than allowing it to remain undiscovered until scheduled manual testing of the system or, worse still, resulting in a failure to detect a fire.

Biography

Peter Cook is product marketing manager at Cooper Lighting and Security, with responsibility for product development and marketing of the company’s Menvier and JSB ranges of fire detection systems, emergency lighting and central battery systems. He has over 20 years’ experience with the company.