Detector Options for Down-stream System Performance
Choosing a fire detection solution is not as simple as it used to be. Of course, if the base price is all you are focussed on and you need nothing more than a system to sound an alarm, then there are plenty of choices, but buyer beware, not all simple systems are the same, or in the end remain good value.
The cheapest system may still cost you more in the end, due to higher labour costs, poor quality and repeated nuisance alarms. Just one return trip to a site to solve a simple problem could cost more than the initial saving.
As fire, safety and living standards around the world improve and building technologies change, so the fire detection system needs to keep pace. The overall objective of any system is to protect people and assets and in order to do this effectively; systems need to interface with the building and its technology. More and more is being asked of detection and alarm systems.
When designing a system, the choice of the correct detection principle and technology is always a first step. We often see designs with very little thought put in up-front and it causes only compromises and problems downstream. In some cases, bad detection choice can lead to a breach in compliance to standards. A modern fire system will often cover multiple buildings and environments and needs to work with a range of detection principles from optical smoke point detectors to beam and aspirating systems, thermal, multi-sensors and flame detectors. A large system will often have a mix of detection from a short list of common contenders.
The most common choices are point detectors and these work very well in most installations. The high performance devices are addressable. They are usually installed easily and offer many detection options, the most common being optical smoke and multi-sensor (combined heat and optical smoke) but also include CO and even flame detection. Optical smoke and multi-detectors are the mainstay of any fire system and cover most environments. Devices can be wireless or wired, be supplied with integrated isolators, beacons and sounders or of modular construction allowing a combination of detection, sounder and beacon to be chosen.
Where ceilings are very high and spaces wide and fitting a point detector would not comply with local standards, or where maintenance is difficult or impossible, beam detection is a good choice. Beam detection uses smoke obscuration of a beam of light (usually infrared) that bounces between a transmitter and a receiver or an integrated transmitter and receiver via a reflector.
Aspirating detectors use an array of pipes with small holes that suck in air (and any smoke) from the environment to a (usually optical) specialist detector. They are highly sensitive and can be used in all manner of environments and configurations, but are often specified for high, wide spaces or hard to reach and service areas, such as basements and lift shafts. The pipes can provide detection while the detector can be located in an easily accessible space.
Flame detectors optically sense the flame through naturally dusty or obscured environments. They are often used in hazardous environments.
Performance Regardless of Detection Method
Modern detectors and the algorithms used in them, can supply all manner of options and benefits, but not considering how detector information can be used as part of a wider system can severely compromise overall performance.
Installing a simple smoke detector that cannot perform multi-stage warning with cause and effect logic, will often mean that the first you know of a potential problem is an alarm situation. However, if a device that has the ability to communicate potential problems early is used, then fires can be detected earlier, problems can be dealt with before they get out of hand, and cause and effect programing can put the building into an alert stage, initiating actions such as ventilation control.
In point detection, the next option could be whether to use a multi-criteria detector in place of a smoke detector. Again not all of these devices perform the same. Their big advantage is the ability to detect a broader spectrum of fires early and reduce false alarms. Better devices allow the sensing elements to be used independently and can use a combination to confirm a potential fire.
Assuming the fire system is sophisticated enough to utilise the various detection elements to trigger different cause and effect, many opportunities exist to maximise performance. A good example is in systems protecting large residential apartment complexes where alarms are frequent:
- A fully adjustable independent pre-alarm smoke threshold can be linked to cause effects. A high sensitivity can be used to activate a specific self-resetting local alarm to alert the occupant of a potential problem. This helps the occupant deal with the problem before it is escalated to an alarm condition.
- With the alarm threshold fully adjustable (within local standards), the cause and effect can be localised so that a second alarm tone is generated, notifying the occupant that the problem has escalated.
- In a high performance device, the point at which an alarm threshold is reached, allows a timed sequence of events to be activate the following possibilities:
– A designated re-evaluation time period starts.
– If the alarm has subsided after the time period has expired, then the system is reset automatically.
– The option of a resample from the device at a higher sample/algorithm setting can determine if the situation has deteriorated.
– An alarm acknowledgement (AlarmCalm) device can be utilised to enable the occupant to deal with the problem locally or signal that there is no obvious fire. This is a device that is mounted within the apartment and when the alarm is activated the occupant can press a button and obtain more time to deal with the problem, if he or she believes the alarm is not the result of a genuine fire. Once the button is pressed a further delay is activated and after the predetermined time period expires, the system will go into fire mode if the detector is still above the alarm threshold. If the signal has been cleared the system will reset.
- It is important to note that in any of the above conditions, the thermal element of the device is enhanced, so that even a small movement in temperature increase would generate an alarm and override any other confirmation processes.
- Should the system go into an alarm condition then the local sounders would generate a different alarm tone to warn the occupant that there is a potential genuine alarm.
- If all time periods have elapsed and the sensing element is still indicating a positive signal then the alarm tone would switch to an evacuation tone.
- In an alarm condition it should be possible to activate the warning system in another tenant’s premises, so that they are aware there is an alarm in the building.
- As there are multiple tones used in these scenarios, it is important to have synchronised sounders to ensure there is no confusion of tone types.
- It should also be possible to incorporate the cause and effect into any mass evacuation system or public address warning system.
As systems become more complex, the communication between system components becomes more crucial. Time is of the essence and systems can no longer wait to be interrogated on a polling basis. Detection devices need to be capable of monitoring their own environment and alert other components immediately of a status change. Fast accurate information is the key to ensuring the correct decisions are made quickly. It is no longer acceptable for systems to take many minutes to respond to events, when it is now possible for systems to respond in less than three seconds no matter how large the system is.
Throughout the overall system, the dependence on a single point of reference for cause and effect should be reduced. Distributed decision making is crucial, so that the system can continue to operate even when parts of the system have been compromised. This is true peer to peer communications.
Short circuit cabling protection down to device level has long been available and in a life safety system the system should not be compromised by a single cable fault. Devices need to be operational at all times or for as long as possible to ensure the system can do what it was designed for. From a practical point of view, if problems occur with cabling during the life of a building, then these problems can be identified quickly if short circuit isolation protection is incorporated in every device i.e. the fire panel will identify where the problem is.
Detection technology needs to embrace existing buildings, system upgrades and difficult site conditions. Often the existing cabling infrastructure cannot be changed and in some cases new cabling cannot be installed. It is important to have a system that is flexible enough to cope with these situations without any compromise to performance.
Turning conventional systems into fully addressable systems without alterations to cabling – and delivering all the performance advantages of analogue detection discussed here – is no longer a problem and if cabling is not possible, then it is possible to do everything wirelessly.
The heart of any fire detection system is the fire panel, which today interfaces with all kinds of critical and important building systems. The power of a modern analogue system adds pressure on detectors and detector choices. As the start of a cause and effect path, a detector that is appropriate and supplies detailed data is vital.
Cause and effect capabilities that are limited to purely the fire detection and warning system are a thing of the past. Fire systems now provide a direct link between detection signals and control ventilation systems, air pressurisation and smoke control systems, control fire doors, security, lighting and interface to building management systems.
As a life safety system it is important that the fire system is stand alone, but in many countries, the fire system needs to take overall control of these systems in a fire situation. The system needs to have the logic and interfaces to achieve this. Fire systems now need to be able to:
- Control extract and supply air systems.
- Monitor and react to external smoke ingress.
- Provide fire brigade controls to control the above.
- Offer graphical display and control of a site.
- Interface with BMS and other building control services.
- Provide multiple location control in real time (seconds not minutes).
- Offer building managers clear and concise operating terminals throughout a building with graphical displays and touch screen operation.
- Provide secure and dedicated paging services.
- Provide mass evacuations systems (public address or audio emergency warning systems), which are controllable from multiple locations.
- Provide distributed systems covering multiple sites.
- Offer web access for diagnostics and control.
So, as we can see, as our built environment becomes more complicated the choice of detection and the system that runs it becomes more important than ever. To offer real performance and more importantly protection a designer must understand both, not only in terms of standards and appropriate use but also detailed technical capabilities that can be carried right the way through a fire system and into the third party systems it integrates with. This is the power of the right detection choice; it leads to a choice of the right system and ultimately the right level of public and asset protection.
For further information, go to www.advancedco.com