Global Solutions for the fire and security industry . . .
Copyright @ 2014 ITS Independent. All rights reserved
Analogue Addressable Systems
This section on Analogue Systems describes the most common form of addressable fire system currently in use. These systems have been developed from the earlier non-analogue addressable system technology to enhance their capabilities dramatically giving a powerful & adaptive system suited to many complex applications. The two primary enhancements include analysis of the amount of smoke / heat or other phenomena in the form of an analogue value, and the addition of fire alarm sounders powered and controlled by the addressable loop. As the underlying technology is similar to the earlier Addressable Systems, the basic technology is not described here so you may need to read the Addressable Systems page first.
The meaning of the term Analogue in reference to a fire alarm system is often misunderstood and assumed to relate to older electronics industry analogue era technology. What actually happens is that each sensing (not detection) device converts an analogue measurement of its measured smoke or heat level into a numerical value. This numerical value is then collected by the control panel for analysis and possible alarm triggering: ‘the data communications used to collect the data are usually digitally based’.
The Control Panel microprocessor and memory are sized to allow rapid analysis of each sensing devices’ data to compare instantaneous, historical and user required sensitivity (threshold) data. This means that slow changes in value may not necessarily cause an alarm to be triggered and may be used to re-calibrate a sensor threshold due to possible contamination (dust etc) and to generate a contamination warning. Partial alarm phenomena may be used to give an early warning or ‘Pre-Alarm’.
This combination of features allows the analogue system's sensing device's to be configured to exactly suit site conditions, to continually adjust each sensor and to give warning to maintenance staff of any devices requiring attention.
The use of a suitably screened fire cable became more important with Analogue Systems to protect the increased loop comm’s traffic from interference and the corrupting effect of EMC. Often mistaken as a safety earth, this ‘data protecting screen’ must be connected to ground at the panel only: not at any other point. Some early systems were simple enough to handle multiple earth connections of the screen and were installed using unsheathed (bare copper) mineral insulated cable. However, beware if attempting to upgrade the system as this cable type is rarely suitable for more modern protocols.
All devices on the addressable loop circuit must be suitably compatible devices to allow correct communication. Hence addressable smoke detectors, heat detectors and manual call points must be utilized and more traditional resistance monitored conventional devices are not directly compatible without some form of interface module.
Around the time that Analogue sensing technology was introduced, manufacturers were working to allow alarm sounders to be connected directly to the addressable communications circuit. Adding addressing and communications capability to each alarm device and managing the higher current levels & data corruption experienced achieved this end. Early attempts would allow only a few sounders per circuit but as the technology progressed some systems will now allow over 100 loop powered sounders per circuit.
A well engineered control panel should provide basic interfacing circuits including: conventional sounder circuits, resistance monitored programmable inputs, Fire & Fault relay contacts and Repeat Control &Indication circuits. These circuits allow interfacing to the wide variety of 3rd party equipment encountered in real world applications. Many interfacing requirements may be loop based using loop powered addressable modules, but there are many important functions that should be carried out from the control panel to allow clearer visibility to engineers and to give more ad-hoc flexibility.
Different design approaches may be taken to cater for the requirements of larger sites: either to allow additional addressable device loops, or to network a number of control panels together. Both approaches have their own merits. The current EN54 equipment standards state that no more than 512 detection devices (detectors and manual call points) may be controlled by single processor, often limiting systems to a maximum of 4 detection loops per control panel. Extreme care should be taken when designing or specifying such a large system to avoid problems later.
These systems are ideal where system programming and set-up flexibility is essential and where complicated detection conditions are present. Apart from very small systems, savings in cable installation and ongoing maintenance will often justify the increased purchase cost of the equipment.
Good system design is essential with this sort of system and practical experience will ensure that many of the common pitfalls are avoided. Remember that quite substantial costs are involved in implementing a large analogue addressable system and practical preparation is time well spent.
The experience of the Installation Contractor’ and the ‘Commissioning Engineer’ must also be taken into account. A high level of experience is essential when setting up and commissioning an ‘Analogue Addressable System’ and the system manufacture may limit access to programming software and technical support depending on training and/or business affiliation. However, with suitable guidance and support many electrical installers are capable of installing these systems ready for a more experienced commissioning engineer to take over towards the end of the installation.
In assessing the suitability of any type of system for your application, often the practical experience of an established engineer will give a lot more insight into daily practicalities than the manufacturers ‘Marketing’ information.
|2 Wire Systems|