Data communications link with alternate path and false isolation modules.
Fire alarm devices and control panels known as “addressable” are distinguished by their ability to identify each active fire alarm field device. This technical innovation meets the requirements for the electrical supervision of field devices and it provides the means of identifying the point in a building where an alarm has been initially activated.
Addressable fire alarm systems have real advantages. In addition to reducing the amount of fire alarm wiring needed, they have the benefit of providing the annunciation of alarms beyond what is required by the building code. They give a superior means of identifying the type and location of faults on the system.
The data communication path is the central nerve of the system
Addressable fire alarm systems are dependent, however, on the data communication path: the central nerve that interconnects the fire alarm devices. This technology allows for the design of a system where all the fire detectors, manual stations and signal controls can be connected on a single pair of conductors threaded throughout the building. It is the vulnerability of the communication path that is at the heart of the issue about the reliability of new fire alarm technology. Without precautions, the technology courts a disaster scenario: failure of the system when it really counts during a fire.
Surviving a fire is the new concern
The new concern is over how well a fire alarm system will survive during a fire. There is an expectation that life safety systems should continue to provide some level of performance during a disaster. In a significant fire, components will be quickly destroyed and failure of the fire alarm wiring backbone can disable the fire alarm system in minutes unless it is adequately protected. A fatal fire in 1996 at 2 Forest Laneway in Toronto’s North York, where six people died, demonstrated this vulnerability. Both the conventional fire alarm and voice communication systems were destroyed within the first few minutes of the fire. The same destructive result can occur in an addressable fire alarm system. This survivability issue is considered in installation requirements mandated by the Standard for the Installation of Fire Alarm Systems CAN/ULC S524-04.
New criteria were developed to keep faults isolated by fire zone
Born from the concept that an open circuit or ground fault on one circuit will not affect any other circuit in a conventional fire alarm system, new fundamental criteria were developed for fire alarm systems that use data communication links (DCLs) and addressable devices. These criteria first occurred in the 1991 edition of the CAN/ULC S524 standard.
The criteria establish that a fault in one fire zone should not result in the inoperability of the fire alarm input and output devices in any other fire zone. The fault might not only be any of the usual electrical faults –open circuit, ground fault or short circuit — but also the destructive faults caused by a fire. While open circuits may disconnect some components on a circuit and possibly permit other devices to remain operable, a short circuit is the most destructive fault. The reality is that in a fire, short circuits will occur in unprotected DCLs, and a short circuit will kill all data transmissions. Fire rated DCL cable cannot provide the solution since a short circuit failure is possible at any point where a device is connected. Any device exposed to a fire can be the source of a short circuit.
Fault isolation modules provide protection
A fault isolation module, i.e. a circuit device that monitors the data communication line between installed isolators, provides protection. In the event of a short circuit fault that disables communication, isolators disconnect the bad segment of the link and allow data communication on all of the remaining segments through an alternate path until the fault is cleared. Where a DCL circuit interconnects addressable fire alarm devices on more than one floor, fault isolation modules must be used to meet the survivability requirements of Section 4.2 of the CAN/ULC S524-04 fire alarm installation standard.
Fault isolation modules may not be required on all DCL circuits. There is an alternate means of achieving the performance requirement. If the DCL connecting addressable devices can be limited to one fire zone with the fire alarm control unit or transponder safely located in a fire-separated electrical room, the failure of the DCL will not affect the functionality of any other fire alarm zone. In most cases, however, fault isolation modules are part of the solution on DCLs with addressable devices.
Alternate path also requires protection
But fault isolation modules alone do not provide the solution. The alternate path is critical to achieving the performance and the alternate path itself requires some protection. The alternate path, after all, is the secure route by which the data is directed to the remainder of the addressable devices. The DCL must be configured as a loop connecting all of the addressable devices and must return on a different path to connect to the transponder or control unit. This independent route of the DCL cannot pass unprotected through the same fire zone since a fire in that zone could disable the primary segment of the loop as well as a section of the alternate path (see figure). The alternate path must lead through separate fire compartments or be appropriately fire-protected by methods such as a fire rated cable, either mineral insulated or circuit integrity cable.
Isolation modules must be used at transitions between zones
Data communication fault isolation modules must be installed at the point of transition each time the DCL leaves one fire zone and enters another zone. The section between isolators defines the segment that may be lost in the event of a fault and that segment must not exceed one fire zone. This provision of fault isolation modules at transitions between zones is necessary to achieve the performance survivability objective required to limit the effect of a fault to one fire zone.
Two isolators necessary to meet performance standard
The dilemma is where the fault isolation modules are to be located. In order to meet the objective, the fault isolation module is usually located at the fire separation barriers or floor slabs. Ideally, the fault isolation module should be in a fire separated electrical service room. Since each isolator has the data link wiring for two fire zones connected to it and protects two zones, exposure of the isolator to a single fire event could result in the data link wiring for two fire zones being compromised. The application of two isolators is therefore necessary to meet the performance level of the installation standard: one at the last wired location prior to the data line leaving the fire zone and one at the first wire location entering the next fire zone.
The exception to requiring pairs of fault isolation modules occurs when an area is considered to be more than one fire zone by virtue of its size, for instance in a large warehouse. In this case, only one fault isolation module mounted at the division of the fire zones is sufficient.
Careful design needed for isolators integral with smoke detectors
As an active fire alarm component on an addressable circuit, fault isolation modules must be compatible with the fire alarm system. Fault isolation modules are available in two versions. One is a separate module. The other is integral with a detector such as a smoke detector or built into the detector mounting base. If the isolator is integral with a detector there is no need to provide a separate fault isolation module. However, such systems require careful design. In addition to taking precautions to have a loop configuration, taking care to route the return path and have isolation devices in pairs, the designer must also ensure that the locati
on of the integral isolators meets the spacing and mounting requirements for the detector.
New verification and inspection standards
The critical role that fault isolation modules play in addressable fire alarm systems is recognized in both the 1997 edition of the Standard for the Verification of Fire Alarm Systems CAN/ULC S537 and the latest edition of the Standard for the Inspection and Testing of Fire Alarm Systems CAN/ULC S536. The verification procedure mandates that when a DCL serves more than one floor, the testing at each floor should involve imposing a short circuit on the DCL and two conditions must be met: confirmation that the required trouble signal is received and that an alarm can also be received from another floor. As part of CAN/ULC S536 each fault isolation device must be tested annually in a similar manner. Because of these testing requirements, it is prudent to install the fault devices where they are visible and accessible. The next edition of CAN/ULC S524 will require this.
By understanding the above issues, consulting engineers will be able to ensure that these fire alarm system designs provide durable communication, system survivability and optimum performance.
Dave Goodyear is president of D. Goodyear Fire Consulting of Toronto and is a member of several Underwriter Laboratories of Canada fire alarm standard committees.