Fact File 66. Guide to Beam Smoke Detectors

Transcription

1 Fact File 66 Guide to Beam Smoke Detectors

2 Guide to Beam Smoke Detectors INTRODUCTION... 3 TERMINOLOGY... 4 PURPOSE OF BEAM DETECTORS... 5 PROS AND CONS OF BEAM DETECTORS... 6 FUNDAMENTAL PRINCIPLES OF BEAM DETECTORS... 7 FEATURES OF BEAM DETECTORS... 8 FUTURE TRENDS...12 REFERENCES AND APPLICABLE STANDARDS of 13

3 INTRODUCTION This Fact File describes the terminology, purpose and operation of beam detectors and then explains common features and the latest technology of beam detectors. In simple terms, a beam detector monitors for a drop in light received from a transmitter as a result of smoke getting in the light beam path and signals a fire alarm when the reduction exceeds pre-set levels. 3 of 13

4 TERMINOLOGY Attenuation (in db) The reduction in intensity of the optical beam at the receiver (see EN for full definition). End to end beam A beam detector configuration that has a transmitter and a receiver on opposite sides of the area to be protected. The receiver can be connected to a control unit installed at ground level for easy maintenance. Obscuration (in %) The % reduction in light intensity at the receiver (I) compared to a clear path (I 0 ). Obscuration is directly related to attenuation - refer to graph below showing for example, 3dB attenuation = 50% obscuration and 6dB attenuation = 75% obscuration. Reflective beam A beam detector configuration that has the transmitter and receiver in the same housing (a transceiver) directed towards a reflector on the opposite wall. The reflector is prismatic so that it will reflect the beam straight back even if it is not mounted perpendicular to the transmission path. Response value (in db) The level of attenuation when an alarm signal is produced (see EN for full definition). Sensitivity A setting which configures how sensitive a beam detector is to smoke. It is often expressed as the % obscuration, but may also be expressed as an attenuation or as a discrete setting (eg level 3 ). However, there are other factors that can determine exactly when an alarm is signalled (such as rate of change, delay times, etc) which depend on the sophistication of the particularly beam detector. Stratification layer A layer of warmer air (usually towards the ceiling) under which cooler smoke can be trapped spreading a horizontal layer of smoke across the space. 4 of 13

5 PURPOSE OF BEAM DETECTORS In principle, all beam detectors are required to detect the same level of smoke that point detectors are required to detect and within the prescribed area. This principle is reflected in both product standards and installation codes, but is outside the scope of this Fact File to present full details. Suffice to say that a beam detector mounted near the ceiling is required to detect the same test fires as a point detector (see EN 54-7 & EN 54-12) and are installed using the same spacing principles i.e. that no location on the ceiling is more than 7.5m from any point or line of detection (see Clause 22 of BS :2013). However, as beam detectors typically are used over a long path length, they integrate the total smoke level over the path length of the beam and therefore will signal an alarm when low density smoke is spread out along the path length, or when high density smoke crosses a small section of the path length. Thus beam detectors are often used in large open areas. Remember, smoke is only detected when it intercepts the beam path and is of sufficient density. Just as smoke must travel to a point type detector, or the sampling hole of an aspirating smoke detector before it will be detected, it must travel to and obscure the path of a beam detector. 5 of 13

6 PROS AND CONS OF BEAM DETECTORS Beam detectors are: Suited to large area coverage per detector since the transmitter and receiver (or transceiver and reflector) can be spaced far apart. More sensitive to widespread, low density smoke than point smoke detectors, as they respond to the smoke density along the entire beam path Suited for use in spaces where there is more opportunity for smoke to spread, such as high ceiling spaces or areas with moderate airflows Able to detect smoke in open volumes away from the ceiling. Hence, they may be used as supplementary detection at an intermediate level to detect a rising plume, or at an angle to detect smoke at multiple heights, for example when a stratification layer might occur Able to provide a low aesthetic impact, eg they can be wall mounted or concealed Functionally tested without the need for smoke or test aerosols, eg using filters Not reliant on local air movement to ensure smoke enters a sensing chamber, hence beam detectors are likely to detect very low energy fires faster than point smoke type detectors Less susceptible to different smoke types, eg different smoke colours, than most optical point smoke detectors (which detect scattered light), since beam detectors respond to obscuration Suited to providing detection in areas with limited access, such as above machinery where access to ceiling mounted detectors would be difficult However, beam detectors: Need to be aligned to operate correctly Must be rigidly mounted for correct operation Require a consistently clear line of sight along the beam path Are vulnerable to unintended total obscuration which will render them ineffective, although beams are usually designed to give a fault signal Can be vulnerable to partial obscuration sources that cause an unwanted alarm, eg airborne dust, steam, diesel fumes Can be susceptible to excessive building movement, whereby the alignment is compromised Can be susceptible to interference from other light sources (visible and non-visible), eg sunlight or crosstalk in multiple beam installations Are difficult to functionally test with smoke or a test aerosol in applications where a functional test using filters is deemed unacceptable Often require the use of height access equipment to carry out maintenance 6 of 13

7 FUNDAMENTAL PRINCIPLES OF BEAM DETECTORS 1. The detector transmitted light beam can be understood best by comparison with a torch (see figure below). Like the torch, the intensity of light falls with distance from the transmitter. The intensity also falls as you move away from the axis of the light beam. Thus, the light beam is generally conically shaped. However, unlike the torch beam, beam detectors usually operate with non-visible light - most commonly in the infra-red region. In a similar manner, while the receiver is sensitive to light falling anywhere within its field of view, its sensitivity generally falls as you move away from the axis of the light beam. Thus it is also generally necessary to point the receiver axis directly towards the transmitter. The principle is the same for reflective beam detectors. Both end-to-end and reflective beams are only responsive to smoke entering the direct "line of detection" as illustrated in the figure below. 2. It is essential to align the receiver to the transmitter so that sufficient light is received. By achieving a well centred alignment, the greatest tolerance to movement in any direction is achieved. In general, central alignment is the goal, how important that goal is depends upon the technology chosen. 3. The receiver and transmitter must be matched to suit the distance to be protected. That is, the light output of the transmitter is such that it is sufficient, but not excessive, for the satisfactory operation of the receiver given that the light intensity falls rapidly with distance (using an inverse square law). 4. Smoke is detected when there is a drop in the amount of transmitted light received at the receiver. In simple terms, an alarm is signalled when the received light falls below a pre-defined threshold (see the definition of sensitivity above). 5. As with all fire detector types, beam detectors must be functionally tested. One method is to introduce appropriate optical filters into the line of detection. 7 of 13

8 FEATURES OF BEAM DETECTORS 1. Features required by EN There are several features common to all beam detectors, since they are required by EN 54-12, for example: An Individual alarm indication. The intention of this feature is so that the unit in alarm can be easily identified. Where the indicator is not visible from the ground, a remote visual indication of alarm or another means of locating the device in alarm should be provided A code or special tool is required to adjust the response value (sensitivity) and any non-approved settings must be identified. Note that it is important to refer to manufacturers instructions to ensure the sensitivity is suited to the application, e.g. path length, height, ambient lighting and the likelihood of contamination) 2. Power Supply Every beam detector requires power. Whilst EN does not specifically require that the detector is powered from an EN 54-4 compliant power supply. However EN 54-1 and many installation codes, including BS , expect it. Power may come directly from a dedicated, local supply or from a centralised power supply. Power may be derived from the Control and Indicating Equipment (CIE), either directly or via an analogue loop; or via conventional zone wiring. Recent systems have benefitted from the low current demands of modern electronics, which has enabled increasing numbers of beam detectors to be powered from a single loop/zone/power supply. Some recent systems use a battery to power the transmitter on an end-to-end system, thus eliminating the need to wire power to it. Such systems are typically designed to provide power for several years and to raise a fault or warning before the battery requires replacement. 3. Initialisation Since it is essential to align the receiver to the transmitter (whether end to end or reflective type) all beam detectors include features to achieve this. There are three key elements needed an adjustment mechanism, some way of knowing where you are pointed during alignment, and a locking mechanism. There are many ways of achieving and combining these three elements, for example: Manual adjustments with locking mechanisms, eg thumbwheels/gears, universal brackets, clamping ball and socket. Any of these could be combined with, for example, alignment indicator LEDs or laser pointers Visual aids for visually pointing elements/devices towards each other, eg visible laser, optical sighting systems Automatic adjustment systems using motors (see 7 below) As described above, the receiver and transmitter must be matched to suit the distance to be protected. This can be achieved in different ways, for example some systems adjust transmitted power and/or receiver gain automatically, some adjust these parameters manually and some achieve matching by selection of elements/devices, including where relevant, the reflector. Thus the light received is sufficient but not excessive, for satisfactory operation. The alignment and matching processes are related. Different approaches are possible, for example: Alignment is first performed followed by a matching step Aligning and matching occurs iteratively until optimal 8 of 13

9 Matching is achieved as a first step by selecting components and/or settings specified for the separation intended and alignment is done at installation At the end of the alignment and matching process, all beam detectors establish a baseline clear path received intensity, eg 100%, from which any future obscuration will be judged. 4. Compensation Beam detectors typically contain a means for compensating for slow changes in the clear path received intensity, which may occur over long periods. This is most often attributed to a build-up of contamination on optical elements of the system and/or a slight deviation in the alignment due to, for example, gradual building movement. These are changes that occur over several hours, days, or more, rather than minutes or seconds. Compensation is automatic and necessary. For example, if a beam detector did not use compensation, a gradual build-up of contamination would cause the system to report a false alarm when the signal drops below the fire threshold. There are different techniques for compensating small changes in signal, for example time-averaging of received intensity, automatically adjusting receiver or transmitter gain (commonly called automatic gain control, AGC), or other software techniques. The rate of compensation is limited by EN 54-12:2002 clause 5.7, so that there is no substantial reduction in the detector s sensitivity to a fire which develops over four hours. 5. Wavelengths typically used Single-wavelength beam detectors typically work in the infrared region, often at wavelengths of 800nm to 900nm. Infrared wavelengths are used since: The light is attenuated and diffused by smoke particles The light is invisible to the human eye (which can see wavelengths between 380nm to 750nm 1 ), and therefore is not intrusive when installed Infrared light sources and receivers are readily available Infrared receivers can be daylight filtered to increase immunity to ambient light Some detectors use visible or ultraviolet wavelengths, though these are less common. Some recent beam detectors use two wavelengths and are able to make use of the fact that smoke attenuates shorter wavelength (violet region) light more than it does longer wavelength (red region) light. In contrast, both wavelengths are attenuated more equally as a result of contamination, or misalignment, or partial blockage, eg by insects. Thus dual wavelength beam detectors are less prone to false alarms due to such phenomena. 6. Functional test All beam detectors should be functionally tested at commissioning and during maintenance to confirm correct signalling of alarm and fault conditions. Most commonly this is achieved by introducing appropriate optical filers into the line of detection to create the attenuation levels expected for an alarm and a fault condition. Some detector have features which permit functional testing without direct access to the line of detection. For example: a) A temporary reduction in transmitted light or received signal to simulate an alarm/fault condition b) Automatic/remote introduction of optical filters in the line of detection 1 figure of 13

10 7. Motorisation A motorised beam detector includes a means to change the direction that the transmitter and/or receiver is pointing using a motor driven mechanism. Motorisation allows the beam detector to rapidly, automatically align using an appropriate algorithm and also enables the beam detector, in conjunction with AGC, to automatically compensate for changes in alignment due to gradual building movement. 8. Features for avoidance of crosstalk When light from another beam detector falls on a receiver, this is often called crosstalk and this can cause false alarm or fault conditions. Crosstalk is more likely to occur when beams are closely spaced, eg when used as supplementary detection at an intermediate height and can often be avoided by correct positioning of the beam components in accordance with the manufacturer's instructions. Typically, in an end-to-end system, the receivers and transmitters are alternated, whereas on a reflective system the transceivers are installed on one wall and the reflectors on the opposite wall. As it is not always practical to avoid crosstalk by positioning of the components, most system use signal processing techniques which pulse and/or modulate the transmitter so that signals from different transmitters do not interfere. There are different levels of sophistication in how such signal processing is realised, but some examples are: Set frequency technique, in which adjacent beams can be manually set to operate at different operating frequencies so that crosstalk does not occur Dynamic frequency allocation, in which beams automatically select an operating frequency so as not to interfere with adjacent beams A unique pulse sequence or signature is given to each transmitter which the receiver can automatically recognise and subsequently distinguish each transmitter Other techniques of pulsing and synchronising 9. Effects of strong light sources Saturation by strong infrared light sources, such as the sun, can be a problem for beams as they can saturate receivers - particularly when the sunlight shines directly into the receiver. This is usually mitigated by careful positioning of the receiver, taking into account the position of the sun and its movements throughout the day and seasons. For example, it may be mitigated by pointing the receiver below the horizon, or by adding accessories that shadow the receiver. Beams are made more tolerant to ambient (or indirect) sunlight by discriminating between the light coming from the intended source, ie the transmitter, and that from the ambient sunlight, using signal processing techniques similar to those described in Features for avoidance of crosstalk above. 10. Imagers Most beam detectors monitor a single line of detection (see figure on page 7). More recently, it has become possible for a single receiver to monitor the attenuation from several transmitters, thus providing several lines of detection across the space to be protected. This is achieved by using an imager (dedicated camera) in the receiver which can simultaneously monitor the attenuation from several transmitters. One key advantage of such an approach is that alignment of the receiver is not critical as long as a transmitter is in the field of view of the imager/camera it can be identified as a valid source and subsequently monitored for any attenuation due to smoke. Moreover, any change or drift in alignment during operation can be tolerated by using image 10 of 13

11 stabilisation techniques common in many digital cameras. Thus the use of an imager as the receiver is also advantageous for single line of detection applications too. 11. Delay times Some beams include a feature that delays the time to signal a fire or fault from when the condition occurs. The delay time may be a fixed value or user selectable and it may be possible to set different delay time for fire as opposed to fault. The benefit of delay times is to cater for temporary obstructions such as large flying birds, or when there is a temporary puff of unwanted smoke, such as from a passing vehicle. In addition to these user configurable delays, most beams have filtering delays which are embedded into the alarm and fault algorithms. 12. Anti-condensation devices Beam detectors, being optical items, are not generally specified for use in condensing environments. However, some technologies or accessories exist to mitigate the effects of condensation reducing the received signal and potentially causing a false alarm or fault signal. Examples include heaters and anti-fog coatings. 11 of 13

12 FUTURE TRENDS Beam detectors have been successfully applied in many applications for many years and can provide reliable, cost effective, fault free operation as long as they are appropriately positioned, securely mounted and suitably set up and maintained. Recent developments and advances have simplified the challenges associated with aligning beams and further improvements in this regard can be anticipated. Moreover, increasing awareness and understanding through training, in combination with recent techniques such as; the use of motorised alignment, features to avoid crosstalk and strong light sources, dual wavelength beams, algorithms, anti-condensation techniques, imager based receivers, and battery powered transmitters, etc have increased the reliability and ease of use of beams and reduced their installation and operating costs. Beam detectors are becoming more sophisticated; capitalising on the lower power consumption and cost of micro processing and the increasing availability of raw data. This data can be logged or transmitted for enhanced detection and for more informed troubleshooting. In addition, beams interfaced to wireless systems are becoming available. Undoubtedly manufacturers are developing further technology, so this document is but a snapshot in time. 12 of 13

13 REFERENCES AND APPLICABLE STANDARDS BS : 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 EN 54-1: 2011, Fire detection and fire alarm systems. Introduction BS EN 54-4: 1998, Fire detection and fire alarm systems. Power supply equipment BS EN 54-7: 2001, Fire detection and fire alarm systems. Smoke detectors. Point detectors using scattered light, transmitted light or ionization BS EN 54-12: 2002, Fire detection and fire alarm systems. Smoke detectors. Line detectors using an optical light beam NOTE: EN is currently under revision with an expected publication date in DISCLAIMER The information set out in this document is believed to be correct in the light of information currently available but it is not guaranteed and neither the Fire Industry Association nor its officers can accept any responsibility in respect of the contents or any events arising from use of the information contained within this document. Tudor House, Kingsway Business Park, Oldfield Road, Hampton, Middlesex TW12 2HD Tel: +44 (0) of 13