Coincidence Connection of Detectors for Triggering Extinguishing Systems

Guidance for Coincidence Connection of Detectors for Triggering Extinguishing Systems

This guide is a collation and explanation of the existing recommendations provided on the use of coincidence connected detectors with a particular focus on their application for triggering extinguishing systems. It defines a scheme for describing the different approaches to coincidence connection and in particular considers potential replacements for ionisation detectors.

Although predominantly aimed at assisting those involved in the automatic release of extinguishing systems, this FIA guide also provides information in situations where coincidence of detectors is needed for the actuation of other fire protection measures.

Review of Standards

There is a common misconception that BS 7273-1 Clause 5.2.3 calls for coincidence detection using two different types of detector, typically ionization and optical smoke detectors to trigger fire suppression systems. This is not correct.

5.2.3 Type of detector used

The selection of detectors should be in accordance with the recommendations given in BS 5839-1 and, where applicable, BS 6266. In some circumstances, fire detection considerations might dictate the need for use of two different principles of detection (e.g. optical smoke detectors and ionization chamber smoke detectors) to ensure the earliest warning of fire. In such circumstances, an even distribution of each type of detector should be provided throughout the protected space.

Where coincidence is used, normally it should be possible to achieve coincidence from two detectors of the same operating principle. In these cases, if, for example, two independent circuits are used to achieve coincidence, there should normally be an approximately equal number of detectors of each principle connected to each of the independent circuits. For example, where four detectors are required to protect the space and these comprise two optical smoke detectors and two ionization chamber smoke detectors; there should be one optical smoke detector and one ionization chamber smoke detector on each circuit.

However, it is not always necessary to use two different principles of fire detection. For example, given the type of fire anticipated and the speed of detection required, it might be acceptable to use detectors of a single type.

BS 7273-1 does not require the use of detectors using two different principles of operation:

·        Clause 5.2.2.1 suggests that co-incidence detection is one method of minimising the possibility of false discharge.

·        Clause 5.2.2.4 clarifies that co-incidence detection requires alarm signals from two independent detectors (whether the same type or not).

The second paragraph of clause 5.2.3 (above) was originally a continuation of paragraph 1 and as a separate paragraph is now confusing. The intent is to clarify that where mixed types are used, it is not normal to require both types (e.g. optical and ionisation) to have indicated an alarm before the extinguishant is released. In other words extinguishant may be triggered from any two independent detectors e.g. two ionisation detectors or two optical detectors or an ionisation and an optical detector.

Clause 5.2.3 only includes as examples the traditional combinations of optical and ionisation detectors. There are many other technologies which may be used, for example flame detection, multi-sensor devices, aspirating smoke detectors (ASD) which can be used to mitigate the risk of inadvertent discharge of extinguishant.

BS 7273-1 refers back to BS 5839-1 and BS 6266 for advice on the selection of fire detectors. Both these standards have been revised since BS 7273-1 was published and provide useful guidance:

·        BS 6266:2011 Clause 8.3

·        BS 5839-1:2013 Clause 21

Options for Coincidence Detection

Co-incidence detection is when at least two independent detectors are used to initiate the release of the extinguishing system. These can be of the same type or of two different types.

The type(s) chosen should be selected dependant on the fire risk and on the objectives of the fire system.

Given that ionisation smoke detection is in obsolescence, the traditional ionisation / optical combinations as exemplified in BS 7273-1 is losing relevance.

Some common possible alternative combinations include:

·        Optical & Optical

Traditional scatter type optical smoke detectors

o Optical smoke detectors must pass a range of fire tests including smouldering and flam-ing types and have proved to be suitable for many applications requiring co-incidence

o Flaming fires will be detected; however response may be slower than ionisation detec-tors

·        Optical & Optical-Heat (or Optical-Heat & Optical-Heat)

o Optical-Heat detectors speed the response of a standard optical smoke detector to a flaming fire by responding to heat

o Some Optical-Heat detectors may respond more slowly to a smouldering fire than a traditional optical detector

·        ASD & ASD (or ASD & other)

o  Very early warning of incipient fires is possible using Class A & B ASD systems which can

prompt early intervention and avoid automatic discharge of the fire suppression system

– whether to a real fire threat or an unwanted event (false alarm)

o    ASD & ASD requires two separate detectors to achieve true co-incidence detection

o    Inputs to the extinguishing system must be carefully selected (e.g. Class C only)

o    See also FIA Aspirating Smoke Detectors CoP

·        Optical & Flame (or Flame & Flame)

o    Fast detection of flaming fires

o     Flame detector requires unobstructed line of sight to area of coverage

o    Used only in special applications where flame is a particular risk

·        Multi-sensor & Multi-sensor (including dual-optical)

o   Many different types available with many different modes of operation therefore careful consideration needs to be made of the configuration of the detectors

o   Often designed to mitigate nuisance alarm risk and enhance fire detection

·        Heat & other

o   The use of heat detectors only is not recommended as heat detector response is very slow in comparison to a smoke detector

References and Applicable Standards

BS 5839-1:2013, Fire detection and fire alarm systems for buildings - Code of practice for design, installation, commissioning and maintenance of systems in non-domestic premises

BS 6266:2011, Fire protection for electronic equipment installations. Code of practice

BS 7273-1:2006, Code of practice for the operation of fire protection measures. Electrical actuation of gaseous total flooding extinguishing systems.

Fire alarm installation practices and workmanship

Fire alarm installation practices and workmanship

The nature and quality of the fire alarm installation work needs to be such as to maintain the integrity of the fire alarm system and minimize the duration and extent of disablement of the system during maintenance or modifications. Installation practices and workmanship need to conform to the requirements of BS 7671. 

Penetration of construction (e.g. for the passage of cables, conduit, trunking or tray) need to be “fire stopped” to avoid the free passage of fire or smoke, regardless of whether the construction has a recognized degree of fire resistance.

Recommendations:
The following recommendations are applicable.
1) The entire electrical fire alarm system installation should conform to the requirements of BS 7671. In general, the recommendations of this standard supplement, but do not conflict with, these requirements. Where any such conflict is considered to exist, the recommendations of this standard should take precedence.
2) Cables which are directly fixed to surfaces should be neatly run and securely fixed at suitable intervals, in accordance with the recommendations of the cable manufacturer. Cables should not rely on suspended ceilings for their support.
3) The installer should ensure that all wiring complies with, at least, 26.2f) to 26.2o).
d) Joints in fire alarm cables, other than those contained within the enclosures of equipment, should be avoided wherever practicable [see 26.2g)].
4) So far as is reasonably practicable, cable routes should be selected to follow the recommendations . Care should be taken to ensure the electrical continuity of electromagnetic screens, including metallic sheaths of cables.
5) Where new conduit, trunking or tray is installed, its capacity should be in accordance with the recommendations given in BS 7671.
6) Where a cable passes through an external wall, it should be contained in a smooth-bore sleeve of metal or other non-hygroscopic material sealed into the wall. This sleeve should slope downwards towards the outside and should be plugged with a suitable non-hardening waterproof compound to prevent the entry of rain, dust or vermin.
7) Where a fire alarm cable passes through an internal wall, a small clearance hole should be provided. If additional mechanical protection is necessary, a smooth-bore sleeve should be sealed into the wall.
8) Care should be taken to ensure that the ends of any sleeves are free from sharp edges which might damage cables during installation.
9) When a cable passes through a floor, the considerations a sleeve should extend as far above floor level as is required for protection of the cable it is to carry, but never less than 300 mm.
10) Where cables, conduits, trunking or tray pass through floors, walls, partitions or ceilings, the surrounding hole should be as small as reasonably practicable and made good with fire stopping materials that ensure that the fire resistance of the construction is not materially reduced. Spaces through which fire or smoke could spread should not be left around the cable, conduit, trunking or tray.
11) If cables or conduits are installed in channels, ducts, trunking or shafts that pass through floors, walls, partitions or ceilings, barriers with the appropriate level of fire resistance should be provided within the channels, etc. to prevent the spread of fire unless, in the case of ducts and shafts, the construction of the duct or shaft affords equivalent fire resistance to the structure penetrated; in the latter case fire stopping need only be provided where cables pass into, or out of, the duct or shaft.