What are pathway survivability and circuit pathway classification in regards to life safety systems?
Pathway classification is an extremely important component of life safety system function, covered extensively in Chapter 12 of NFPA 72: National Fire Alarm and Signaling Code, to define levels of protection required for the connections between life safety system components and their control panels. Before we get into the specifics, it is important to understand the significance of pathway survivability in detecting fires and other hazards and for life safety systems to maintain their integrity and continue to function operationally in order to notify building occupants of a hazard before it becomes life threatening.
In smaller, normal use occupancy buildings (i.e. businesses, educational and healthcare facilities, etc.), a high level of pathway survivability typically isn’t necessary given the size of the buildings, and limited active hazards to the life safety system’s infrastructure. For example, it is highly unlikely that a school or doctor’s office is going to have a fleet of forklifts regularly moving pallets within striking distance of life safety detection and notification devices. So, to truly understand the need for pathway survivability, we will focus on larger-scale commercial and industrial facility applications.
In high-activity commercial and industrial buildings, there are often an abundance of situations where an accident could disable a life safety system’s ability to notify occupants of a fire, or other potentially life-threatening hazard. Even worse, the system’s ability to detect a fire or other hazard could be impaired, if still functioning at all! Going back to the forklift example, if an operator were to misjudge a maneuver, and damage the first device on a life safety system circuit, without proper pathway survivability in place, all other devices on that circuit would no longer be operational until repaired. While notification of this type of issue at the panel would be transmitted to the building’s alarm monitoring center, if this occurred as a small fire started elsewhere in the building, it could spread out of control before anyone knew what was happening. When a system is so intricately interconnected, the lack of pathway survivability has the potential to result in loss of assets, property, or even lives.
Thankfully a potentially devastating situation such as this can be avoided by installing the system using a higher class of pathway survivability. Pathway classes are arranged into six performance designated classes, in order starting with the class with the highest pathway survivability requirements they are, X, A, B, C, D, and E. Typically, classes C, D, and E, are below most accepted installation standards, and are more specific to certain equipment, or environments where a typical installation would not work.
Class B is typically found in smaller, normal use buildings, where a single circuit is used with no redundant path, but is supervised for conditions that affect the operation of devices on that circuit. This would include issues of that would cause short circuits, open circuits, etc. The circuit shall also function with a single ground fault condition active, but ceases function with a single open.
Class A includes a redundant path for the circuit to return to the control panel, thus giving it the ability to function even if the circuit is broken. Each side of the break will have its own pathway, thus remaining functional while alerting of the problem at the control panel. There are also requirements for separation of the redundant pathway from the outgoing pathway, which lessens the chance of a single event damaging both pathways, rendering it non-functional.
Class X adds another layer to Class A’s requirements by requiring that the life safety system be able to continue operating with multiple issues occurring at the same time, typically through isolating a segment of the circuit. This could occur in a situation where there is an open fault, and ground fault occurring simultaneously, the overall circuit will continue operating by isolating the affected segment of the circuit.
The NFPA has these stringent classification codes in place to safeguard building occupants, as pathway survivability could potentially ensure that someone gets home safe to their family after a fire, gas leak, or other hazards. This is why understanding and ensuring that the pathway survivability is in place in your building, if necessary, is so important.
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