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Signalling a warning

Neal Porter, sales and marketing director at E2S, discusses the key considerations for fire system designers when specifying warning devices.

Signalling a warning

In the majority of automatic fire systems, the end result of the detection of a fire is the initiation of audible and visual warning devices that alert the occupants to the danger. Given the complexity and variation of different types of buildings and structures warning devices with differing performance characteristics will be required. The key consideration for the system designer is how to produce clear, unambiguous warnings throughout the protected areas in the event of an emergency.

The relevant Code of Practice, BS5839-1, states that the evacuate tone should contain frequencies within the range 500Hz to 1000Hz - no specific tones are defined.

In normal commercial environments such as offices, hotels, hospitals and public buildings, the ambient background noise will typically be around 65dB, and most individual areas are relatively small. Such environments may be covered with multiple sounders with typical outputs of approximately 100dB(A) at 1m; even the traditional 6in bell is effective in small installations, although not particularly compatible with today's low-current control systems. BS5839-1 states that the effective distance of a sounder is when the calculated dB(A) is at least 5 dB(A) above the known ambient background noise.

In high background noise industrial environments, higher output devices are required, although there is always the danger of installing units with too high an output. High output sounders should not be used in low ambient noise areas or as a means of ‘drenching’ the area in sound. Alarm systems that are too loud may be dangerous, cause panic and discomfort and make communication difficult, impeding evacuation procedures. The overall alarm level should be a maximum of 10 to 15 dB(A) over the ambient background noise.

Voice alarms
Voice alarm systems have suffered adversely from historic confusion with Public Address Systems, enshrined in most people's minds as the source of the muffled and incomprehensible announcements. However, modern technology and the introduction of robust standards mean that this perception is now no longer relevant. Voice alarm loudspeakers will typically combine pre-recorded standard or custom messages with a choice of pre- and post-message tones, enabling, for example, phased evacuation instructions to be broadcast in larger buildings.

It is becoming increasingly common in both commercial and industrial applications for visual signals to be required to reinforce the primary audible warning device. A visual signal should never be used by itself as part of a life safety system, although they are widely used in industry on a stand-alone basis to indicate machine state or environmental condition. Advances in lighting source technology have generated a number of alternatives to the traditional Xenon tube as the basis for strobes. In particular, high output brilliant white or monochromatic LEDs provide the benefits of low current draw, long life and simple electronic configuration and control.

Hazardous areas

Hazardous areas are defined as areas where concentrations of flammable gases, vapours or dusts may occur, either constantly (Zones 0 and 20), under normal operating conditions (Zones 1 and 21) or unusually (Zones 2 and 22). Hazardous areas are found in a range of manufacturing industries, far beyond the obvious petrochemical plants. Food, pharmaceutical and cosmetic manufacture all involve processing potentially explosive substances, while the problems of explosions in grain silos and sugar-processing plants are well documented.

Wide area coverage
Wide area sounders, with an output at 1m in excess of 140dB, significantly higher than the human threshold of pain, have an effective warning range of between 500 and 750m depending on the atmospheric conditions. Used in quarries, on large industrial and petrochemical sites and for civil defence requirements, electronic wide area sounders will normally generate multiple internationally recognised alarm tones including fire, security, civil defence, alert, COMAH (SEVESO II) toxic gas alarms and disaster warnings for flood, tsunami, tornado and other severe bad weather conditions.

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