Alarm Systems
1.INTELLIGENT FIRE ALARM SYSTEMS
1.1. INTRODUCTION
Conventional fire alarm systems provide an adequate and cost effective fire alarm system for many small buildings. In larger, more complex buildings however, more sophisticated ‘intelligent’ fire alarm systems tend to be used. These systems offer benefits in speed of detection, identification of the location of a fire and easier maintenance. Intelligent systems also offer tolerance to faults in the system wiring, which allows a single pair of wires to be used to connect up to 198 devices to the system, allowing cost savings in the wiring of large systems. In larger installations, the benefits of improved maintenance and reduced cabling cost are overwhelming. Currently, the point at which an intelligent system becomes economical is around 6 zones in the UK.
This guide is intended as an introduction to the technology used in intelligent fire alarm systems. For more information on conventional systems, refer to System Sensor’s ‘Guide to Conventional Fire Systems’.
1.2. INTELLIGENT SYSTEM TYPES
There are two methods commonly used for implementing intelligent fire systems:
The most common type of system is “Analogue”. In this case the detector (or sensor) returns a value to the panel representing the current state of its sensing element(s). The control panel compares this value with the alarm threshold in order to make the decision as to whether a fire is present. Note that the term analogue, used to describe these systems does not refer to the communication method (indeed many “analogue” fire systems use digital communications) but to the variable nature of the response from the detector to the control panel.
In “Addressable” type intelligent systems, mainly used to meet the requirements of the French market, detector sensitivity is programmed to each device by the control panel or is preset in the factory. The detector compares its current sensor value with the configured threshold to make the alarm decision, which is then transmitted to the panel when the sensor is interrogated.
Intelligent Fire
FIRE ALARM SYSTEM OK 28 January 2004 14:01
SYSTEM OK
FIRE ALARM
FAULT
SYSTEM RESET
INTELLIGENT FIRE ALARM CONTROL PANEL
ISOLATOR |
CONTROL |
MODULE |
CONVENTIONAL |
ALARM ZONE |
ISOLATOR |
MONITOR |
MODULE |
CONTACT |
(E.G. SPRINKLER |
SWITCH |
ISOLATOR |
EOL
EOL
In many systems the features offered by the two detection techniques are so similar that it is not particularly relevant which technique is used to make the alarm decision. It is better to select a system based on the features offered by the system as a whole.
1.3. COMMUNICATION PROTOCOL
Intelligent systems use the same pair of wires both to supply power to the loop, and to communicate with devices on the loop. The communication language, or protocol used varies from manufacturer to manufacturer, but generally comprises switching of the 24V supply voltage to other voltage levels to achieve communication.
+24V | Panel to detector | Detector Response |
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Figure 1.1.1 Intelligent Fire Alarm Systems
Figure 1.1.1 demonstrates an example of a single loop intelligent fire system layout. The wiring is looped, and connects to the control panel at each end. All detectors, call points, sounders and interface modules are wired directly to the loop, each having its own address. The control panel communicates with each device on the loop, and if an alarm or fault condition is signalled, or if communications are lost with one or more detectors, the appropriate response is triggered. The loop can be powered from each end so that if the loop is broken at any point, no devices are lost. In addition the use of short circuit isolators minimises the area of coverage lost in the case of a short circuit.
Detector | Control | ErrorCheck | Device | TestStatus | Sensor | Other Info | |
Type | Value | e.g. drift | |||||
Address |
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Figure 1.3.1 Typical Protocol Configuration
A typical basic protocol comprises two main parts (See Fig 1.3.1): A query or poll of a device by the control panel including the device address and control information, and a response from the device giving its status and other information. Precise details of the information transferred will depend on the manufacturer, but normally will include:
Poll: Control Panel to device:
•Device address
•Control of device LED - blink to indicate polling, switch on when device is in alarm
•Control of device
•Control of module output
•Error detection for example parity bit or checksum Response: Device to Control Panel
•Device type (e.g. optical detector, heat detector, multi- sensor detector, module)
•Analogue Signal - i.e. the current sensor value
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Note: This document is based on the recommendations of BS5839 Part 1: 2002. It is intended only as a guide to the application of fire detection systems.
Reference must be made to relevant national and local standards.
