ASD Comes Of Age

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An aspirating smoke detector (ASD) is a smoke detector which provides area protection by drawing air from a protected area into a detector unit. It does this via a network of 'sampling' pipes and capillaries, where sampled air is passed through the ASD and analysed for the presence of combustion products.

ASD systems usually draw sampled air from multiple holes spaced around a protected area, the cumulative effect of smoke entering more than one hole results in the ASD being more responsive than if combustion products entered only one sampling point. The detection element inside these devices is often (but not exclusively) based on laser technology that utilizes the optical scatter principle seen in more conventionally used optical type point smoke detectors. Other detection methods employed by ASDs include cloud chambers and even embedded EN 54 - 7 optical point type smoke detectors.

Traditionally aspirating smoke detection systems are used for applications where early warning of fire is needed, and protection of property assets is key, for instance business critical computer server or telecoms installations, where it is crucial that a fire is identified before it can cause disruption and jeopardise business continuity. Of course early warning also allows additional time for safe evacuation. Other applications include where building aesthetics are of concern or where the environment presents challenges with respect to maintenance access or harshness, or areas where high airflows are present.

STANDARDS OLD AND NEW

LPCB has been approving aspirating smoke detection devices to a CEA document (Comité Européen Des Assurances) number CEA 4022 for some years now. This standard is now set to be superseded by a new European 'EN' document which will need to be adopted by all European member states as part of the Construction Products Directive (CPD).

The soon to be published EN 54 - Part 20 - Requirements and test methods for aspirating smoke detectors, mirrors a large proportion of the requirements set out in the earlier CEA 4022 document, with some key changes and enhancements with respect to the requirements and test methods.

The key changes incorporated within EN 54 - 20 concern airflow monitoring and fire testing. For airflow monitoring (the method by which the integrity of airflow through the sampling pipe network is monitored), the requirement has become more stringent. Previously the CEA document required that a change in airflow of +/-50% be detected within a given time and indicated as an airflow fault. EN 54 - 20 requires the criteria to be met upon a change of +/-20%. This is a clear step change in the requirement which could present some engineering challenges for ASD manufacturers, but should result in greater system reliability.

For fire testing, EN 54 - 20 has introduced 'diluted' fire tests and a fire sensitivity classification system incorporating classes A, B and C. The fire tests are based upon the 'TF' series of test fires previously seen in EN 54 - 7 for optical point smoke detectors and earlier ASD standards. Class C is concerned with the usual TF fires and serves to demonstrate 'Normal' sensitivities. Classes A or B, high sensitivity and enhanced sensitivity respectively, employ fires with less initial fuel and also utilise a smoke 'stirring' mechanism in the fire test room during the tests to mix the smoke and air within the room effectively producing a diluted smoke mixture.

It is anticipated that the formal published version of EN 54 - 20 will be available later this year. It is intended that the published version will incorporate an Annex ZA outlining the Construction Products Directive (CPD) mandate requirements. Based on formal availability of the standard in late 2006, this will indicate that CE marking in accordance of the CPD will be mandatory for ASD in 2009.

BFPSA COP FOR ASDs

In December 2005, the British Fire Protection Systems Association (BFPSA) published issue 2 of its Code of Practice for Installation, Commissioning & Maintenance of Aspirating Smoke Detector (ASD) Systems. This code draws on the type test performance requirements set out in EN 54 - 20, together with some of the ideas and concepts within the automatic fire alarm Code of Practice BS 5839-1. It also develops the concept of viewing aspirating sampling points when working with BS5839-1.

For a full, up to date list of approved aspirating smoke detectors, please visit www.RedBookLive.com.

For further information, please contact LPCB:

T: +44 (0)1923 664100
F: +44 (0)1923 664994
E: fdetenquiries@bre.co.uk.

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