There are many reasons for using ASD systems. The detection sensitivity of such systems is frequently tailored to the particular application to achieve the performance capability desired without being susceptible to unwanted alarms.

This section describes a means to define any ASD system encompassing all the essential elements in such a way that any individual system can be described in a single unambiguous phrase. The definition takes into account:

• Class of the detector (as defined in EN 54-20)
• Type or method of sampling (e.g. primary or secondary sampling)
• Compliance route (either prescriptive or performance based) 
• Prime motivators for using an ASD system

Table 2 in this section is provided to assist in the correct specification of the ASD system to be deployed and relates to the performance tests presented in Appendix A.
Several typical examples ASD definitions are given in 5.4 

5.1 Sensitivity Classes:

For the purposes of definition there are considered to be 3 sensitivity classes which relate directly to those specific in EN 54-20:2006

Note: The sensitivity class relates to the “ASD system” (see section 3.2) and includes any dilution due to the “sampling device”. It is not the sensitivity of the detector without dilution.

Class A - Very High Sensitivity

An ASD system with very high sensitivity that is capable of providing very early warning of a potential fire condition. Such systems are particularly relevant for high- risk areas where staged responses to the multistage alarm conditions are justified to ensure minimum down time of the protected area that may result from any fire related incident

Class B - Enhanced Sensitivity

An ASD system with enhanced sensitivity, for applications where an additional degree of confidence is required for the protection of a particular risk. The enhanced capability of such systems is often required to compensate for other risk factors in the protected area such as unusually high ceilings or significant air flows.

Class C - Normal Sensitivity

An ASD system designed to give equivalent performance to standard point detection systems meeting the requirements of EN 54-7For the nominal bands of sensitivity for these classes see Table 2.

 

Primary Sampling is arranged to sample from specific locations in the protected area where smoke is most likely to travel / accumulate. Most typically this is at the air intake grilles of Air Handling Units (AHUs) or pressure relief vents (PRV), but sampling may be arranged wherever any smoke is likely to travel. This type of system is usually regarded as supplementary to other forms of detection where its response capability is dependent on external systems such as the air movement provided by AHUs. However, where such conditions exist it is generally acknowledged that ASD systems arranged for Primary Sampling provide the earliest possible warning of a fire condition. Given the fact that Primary Sampling is generally installed to provide early warning or to overcome the challenge of air movement in a particular area it is recommended that Class A systems are used with this sampling method. However, in some small or “dirty” applications a Class B system may be specified.

 5.2.2 Secondary sampling

Secondary Sampling is arranged such that the air sampling points are sited and spaced as an alternative to point type smoke detectors. Sampling holes are therefore positioned in accordance with prevailing national or international Standards but it should be noted that these are maximum spacing requirements which are normally bettered when using ASD as a result of Performance Based Design. Some example Standards are shown in Table 1.

Where such systems are installed and intended to be in accordance with the appropriate Standard, it is important to ensure that the installed performance of each hole (or group of holes if more than one hole is specified to be equivalent to a point detector), is equivalent to (or exceeds) the minimum performance requirements of a point detector. 

The sensitivity of each individual air sampling point (assuming that all sampling points have been designed to provide a balanced system) can be estimated using the following simple calculation:

Individual Sampling Point Sensitivity = Smoke Sensing Element Sensitivity x Number of Sampling points

For example, a detector with a 0.1%obscuration/m sensitivity smoke sensing element and a total of 40 balanced sampling points, can be estimated to have a sensitivity at each sampling point equivalent to a 4%obscuration /m point type smoke detector.

Where each sampling point is protecting separate compartments, the response of the ASD system may (in crude terms) be considered to be equivalent to a 4%obscuration/m point type smoke detector.

However, in open areas where smoke can enter into more than one sampling point, then the response of the ASD system would be significantly better than 4%obscuration/m. This is known as the cumulative effect and is an inherently beneficial feature of ASD systems.

Note: The estimation method given above is not sufficient to assure the performance of the ASD system as being equivalent to a point detector. Such assurance can only be given through product testing and approval by an appropriate authority against the relevant Standard. 

5.2.3 Localized sampling

For Localized Sampling, the pipework and air sampling points are arranged to monitor specific pieces of equipment within an open area. The method of design can only be established by ASD specialists and test simulations. The sensitivity of such localized systems can only be determined during the full design considerations by the specialist / manufacturer. However, it is generally the case that the risk will justify the use of an enhanced Class B system or very high sensitivity Class A system.

5.2.4 In-cabinet sampling

For In-cabinet Sampling, the pipework and air sampling points are arranged to monitor specific pieces of equipment and is distinct from the use of Localized Sampling systems in that the protected volume is small, generally self contained and has some distinct challenges and risk associated with the cooling of critical electronic and electrical equipment.

Specific performance tests for such systems are given in this document (See10.3) and it is generally considered that enhanced Class B or very high sensitivity Class A systems are appropriate to meet these performance tests.

 5.2.5 Duct sampling

In some cases the ASD can be arranged to sample from within a duct. There are two basic applications for such an approach:

 

• Duct Smoke Detection (DSD) provided to trigger a control system to prevent smoke being transferred through a building;
• Duct sampling applications, where the detection of smoke in a duct is used to provide early warning of fire in the area (or equipment) from which the air is being extracted.

The underlying reason for smoke detection in the duct should be clearly stated.

In either case, earlier warning using a higher sensitivity detector is possible using an ASD system and is often cited as a reason for using an ASD system as an alternative to passive type duct detectors.

ASD systems can be used for DSD installations dependant on the risk and application. They do not rely on the duct air-flow to pass any smoke samples through the detector because of the integral aspirator or fan.

Note: See the FIA Code of Practice for Duct Smoke Detector (DSD) systems for further information In areas that are less than 100m2 (e.g. detention cells) duct sampling may be at a similar smoke

sensitivity to replace point smoke detectors i.e. ~4.0%obscuration/m.

In larger areas the smoke sensitivity would have to be increased to cater for either the increased duct airflow or the larger risk area. Manufacturer’s recommendations should be sought to determine the optimum position in the duct for the intake and exhaust pipes and the maximum duct airspeeds (See 10.7 for specific application guidance).

When used in a duct sampling application, the maximum floor area covered by one duct mounted ASD should not exceed 2000m2 or one smoke detection zone under BS 5839-1: 2010 Section 2:13.

Note: When an ASD is used in a duct sampling application to provide early warning of a fire condition the area protected should also be monitored by secondary detection to provide protection in the event of the duct airflow shutting down. 

 5.3  Route to Compliance (Prescriptive or Performance Based)

The decision as to whether performance tests should be conducted during commissioning depends on the classification of the ASD system being deployed. Generally, there is only one situation where a performance test can be omitted and that is when; an approved ASD system is deployed (and is fully compliant with the specific requirements of the product approval), with sampling hole spacing that falls within the full requirements of the relevant prescriptive code (e.g. BS 5839-1: 2008),. This situation is highlighted in the table on page 19. In all other situations it is recommended that a suitable performance test is specified and carried out during commissioning to verify the system. Refer to Section 14.

5.4 Summary and Examples: