A Class of Its Own: Class X
By: Wayne Aho
Isolation features in Signaling Line Circuits used in today’s commercial fire alarm systems have evolved over the past few years. Their performance has greatly improved and now can better address the requirements of true Class X operations – while saving owners significant material and labor costs.
Signaling Line Circuits, also known as SLCs, are foundation elements of an addressable fire alarm system with positive and negative conductors interconnecting the central fire alarm control unit (FACU) to each of the installed peripheral devices. Smoke detectors, manual alarm stations, HVAC duct smoke detectors, heat detectors, control outputs, and monitored inputs are some of the more common peripheral devices found in a modern commercial building.
Two SLC conductors supply both device operating power and bi-directional communications between the FACU and the peripheral devices. In Class A wiring schemes, the SLC is completed with two additional conductors returning (or closing the loop) from the last addressable device in the circuit back to the control panel.
Varying levels of SLC performance are defined in national fire alarm installation standards such as NFPA 72 ®. [1] These provide SLC performance options to fire alarm system designers, based on expected risks in a facility and other factors such as local code requirements.
Common Causes of Wire Faults
- Could disable a device or circuits
- Could disable the SLC or prevent notification to other zones (floors)
- Construction - open, short, or ground condition
- Tie wrapping to all threat supports
- Junction boxes and metal covers
- Vandalism
- Corrosion of wires, insulation, or components
- Water leaks
In many cases these types of faults are considered to be a minor problem, but timing of the occurrence could escalate the potential risk to life and property damage if the minor fault is not corrected quickly.
Losses can be even greater if the fault occurs during an emergency and limits the system’s ability to warn occupants of increasing danger.
However, Class X solves that issue by maintaining 100% system reporting in the presence of a wiring fault – providing additional peace of mind to building owners, operators, and occupants.
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Early Circuit Isolation Design
The term “circuit isolation” was introduced early in the history of addressable fire alarm systems to protect the FACU’s ability to maintain the supply of power and communication to the system’s interconnected devices. Conventional initiating device circuits are inherently constrained in potential coverage area by the quantity of supported initiating devices and initiating device circuit (IDC) wire length. However, addressable SLCs extend far greater distances with significantly more devices connected – thus increasing the amount of potential threats to normal system operation.
Wiring fault conditions such as “wire to wire shorts,” “ground faults” (short to earth ground), and “open circuits” (breaks in the wiring) can all negatively impact how the interconnected devices within a fire alarm system communicate with one another, resulting in either sections of or the whole building being without adequate fire protection.
Early SLC isolation solutions significantly increased the amount of required equipment to provide full circuit isolation for each device on the circuit. In most systems, the increase was two circuit isolation devices for each addressable device installed. In most projects, this created significant increases in system material and installation costs. This led to SLC isolation becoming a little-used protection solution, with end users often balking at the feature’s added costs without regard for its potential life-saving value.
Revised NFPA 72 ® SLC Standards
The industry’s concern about having to install so many isolation devices in order to create “true” Class X SLC performance in legacy systems spurred NFPA technical committees to make numerous adjustments to the NFPA 72 ® SLC technical fire alarm system performance requirements over the past several code cycles.
Many such revisions attempted to balance cost and SLC performance within buildings. For example, a “hybrid situation” entailed isolation devices protecting numerous addressable devices within building “zones” rather than protecting each individual device. This zone model of isolation would protect one zone from wiring faults occurring in other zones of the building – by “isolating” each zone of devices from one another and thus limiting the potential for loss of normal system operation.
Initially, this appeared to be an acceptable compromise. But over time, these isolation zones resulted in system installations having large numbers of addressable devices in a single zone that could increase the overall risk of compromising normal operation of the entire zone. (Think of Christmas light strands from long ago – and how when one bulb blew, none of the other lights on that strand would work.) To counter this scenario, the NFPA 72 technical committees proposed another compromise that allowed for a maximum of 50 devices between isolators on the SLC, regardless of building area or zone.
Although the 50-device requirement limited the potential risk to a site, it was not a “true Class X” solution. This again proved to be another compromise to the premium solution where every device on the SLC would be protected and remain fully operational in case of a single wiring fault.
Achieving True Class X
One fire equipment manufacturer recently created an entire line of addressable fire sensors and modules offering “true Class X” performance without using expensive external isolation modules adjacent to each device. This “built-in” circuit isolation feature delivers superior “true Class X” performance while also removing the requirement for extraneous devices to protect each addressable fire alarm device connected to the circuit – thus eliminating the negative financial impact of circuit isolation.
To better illustrate how these devices might work in an application, let us look at a modest-sized commercial facility that has a fire alarm system with 150 devices of all kinds to fully protect the facility under local fire code. If installing devices with a built-in true Class X circuit isolation feature, the end user would just need the 150 fire alarm devices. This design specification would offer increased system survivability, lower overall risk, and reliable protection – at no additional cost over the purchase and installation of the devices.
However, if the same commercial facility were to follow the traditional design approach for isolation of the SLC and its attached devices, this fire alarm system design would require an additional 300 isolation devices (150 devices x 2 isolators per device) to implement a true Class X circuit operation. Totaling all the additional costs across the system – circuit isolation device cost, additional equipment installation labor cost, additional electrical mounting hardware and additional system programming time/cost – it becomes clear how the economics of the situation have tended to work against Class X performance being widely implemented.
Wayne Aho is with Siemens Smart Infrastructure USA
References
- NFPA, "NFPA 72®, National Fire Alarm and Signaling Code®," Qunicy, MA, 2019.