Making Fire Panels Even More Reliable
24.11.2011 - Creeping shorts and creeping opens are a nightmare for every operator of critical transmission lines, in particular in fire alarm systems. They can not only trigger false alarms, m...
Creeping shorts and creeping opens are a nightmare for every operator of critical transmission lines, in particular in fire alarm systems. They can not only trigger false alarms, much worse they can inhibit alarm transmission and/or notifications that would otherwise warn residents of a fire. Improved supervision methods of transmission paths now considerably increase fire alarm system reliability.
European standard EN 54 Part 13 requires that transmission paths must be continuously supervised for creeping opens and creeping short circuits, reducing the risk of inhibited alarms and increasing overall safety levels. Bosch has developed a patented technology that continuously monitors all transmission paths for such creeping short or open conditions.
Once installed, every technical system is exposed to both environmental influences and human error. Fire alarm systems are no exception here. In a fire alarm system, sensors such as manual call points or fire detectors and actuators such as sounders and strobes are distributed in a building and connected to a fire alarm control panel via a wired bus. The bus is typically a 2-wire cable to transmit data and energy between the fire alarm control panel and the peripheral elements. For peripheral elements with high power consumption a 4-wire cable is typically used whereof 2 wires just transmit data and the other 2 wires transmit energy.
These ‘transmission paths' have to be continuously supervised: even in air-conditioned environments, metallic contacts can corrode slowly over time, and gases, humidity and aging may reduce the insulating characteristics of plastic sheaths. Human error includes everyday incidents such as drilling into cables without entirely destroying them or not tightening screw terminals while installing the fire alarm system. Sudden incidents and long-term developments alike can easily lead to creeping shorts and creeping opens that dramatically reduce the reliability and the functionality of fire alarm systems, specifically the transmission paths. Here a creeping short circuit is understood to be a slowly developing reduction of the resistance between two wires, for example in two-wire lines. A creeping open is thus a slowly developing increase in the line resistance of one, a pair or more wires. Both creeping opens and creeping shorts can inhibit data transmission and thus represent very real threats that may result in the loss of human lives and/or valuable assets if not detected in time.
In order to detect such conditions before a fault occurs during an emergency situation, EN 54-13 introduced prophylactic measures to detect creeping shorts and creeping opens in transmission lines. As a system standard, EN 54-13 does not stipulate the mandatory features of individual products but rather assesses the compatibility of system components in fire alarm systems.
To begin with, it differentiates between the functions of fire detection and notification, which are however typically combined in today's fire alarm systems. According to EN 54-13, detection refers to identifying a fire at the earliest possible moment and triggering notification and controls so that suitable measures can be initiated. The fire alarm system typically triggers acoustic and/or visible alarms to warn people in a building who might be at risk from the fire. All essential parts of the fire alarm system are designated type 1 components in EN 54-13, while components that are not stipulated to be mandatory, such as a log printer for example, are type 2 components. The standard requires that all system components of type 1 must be compatible with each other and all type 2 components must be connectable. Here, compatibility means that type 1 components must work together with the alarm control unit within specified limits (i.e. by supplier and EN54 standards) and connectability means that type 2 components must not adversely affect the operation of the fire alarm system in any way.
EN 54-13 imposes very strict requirements on compatibility, specifically with regard to transmission paths. In particular, for a system to comply with the standard it must be ensured that all interconnecting wiring supplies the voltage that is necessary for the operation of all the connected actuators or sensors under proper load conditions. Transmission paths include both signal lines and supply lines. Instead of testing the line status only with standby current, as has been the case before EN 54-13, it must now be ensured that all functions of the transmission path are working under creeping open or short conditions even in alarm state where the peripherals typically have higher current consumption.
Aiming to increase reliability through early detection of creeping shorts and opens, EN 54-13 introduced defined procedures and criteria of acceptance for the functional tests of fault warning conditions in transmission paths. The test for a creeping open requires the introduction of serial resistance into the transmission path. Starting with the quiescent condition, the resistance must be increased until a fault warning condition is reached and a fault is signaled to the fire panel. Decreasing the resistance by 10% must then make the entire transmission path fully operational again.
This means that all elements that are connected to the tranmission paths remain fully functional, even if elements require higher current values in alarm state compared to the normal state. By the EOL modules it is assured that the line resistances on the tranmission paths are within the limits that are required for flawless functionality. The creeping short test is very similar, requiring parallel resistances. Here, the resistance needs to be reduced until a fault is signaled, and a subsequent increase of the resistance by 10% must bring the transmission path back to normal.
End of The Line
In order to comply with the requirements of EN 54-13, and thus be able to connect detector lines to the fire alarm system in a way that conforms to this standard, Bosch has developed a family of active end-of-line (EOL) modules. They act as line terminators that continuously test 2 and 4-wire transmission paths for creeping opens and shorts according to the procedures and criteria of acceptance as laid out in the norm. These EOL modules are simply installed on the end of a conventional detector line, an LSN stub or an LSN T-tap in parallel to the detectors. Being a closed loop, a 2-wire LSN loop does not require an EOL module. A 4-wire LSN-loop does require an EOL-module to supervise the auxiliary voltage.
Bosch offers EOL modules for both LSN stubs and T-taps as well as for conventional transmission paths. The LSN modules support both LSN ‘improved' and the classic variation of this bus. In the LSN improved mode, the address can be assigned automatically or manually. The modules work together with all Bosch fire panels that support LSN. EN 54-13 is a European standard, but compliance is not obligatory unless the respective national standards organizations have adopted it in their national application guidelines. In most European countries they have done so and others may follow at any time. Therefore it is certainly a good idea to comply with EN 54-13, even if it may not be required today - not least for the increased reliability that it brings.