Differentiate NFPA 72 Class A and Class B circuits and provide an example of where each would be used.

• A. Class B circuits have a single fault path and may lose signaling if a circuit fails; Class A circuits have a continuous path back to the power source; open circuits still signal on fault; Class A is preferred for life-safety signaling; Class B may be used where wiring simplicity or cost is prioritized.
• B. Class A circuits have a continuous path back to the power source; open circuits still signal on fault; Class B circuits have a single fault path and may lose signaling if a circuit fails; Class A is preferred for life-safety signaling; Class B used where wiring simplicity or cost is prioritized.
• C. Class A circuits operate wirelessly; Class B circuits are wired.
• D. Class A circuits are only for hardware; Class B for software.

Answer: (B)

The main idea here is redundancy and signaling reliability in fire alarm circuits. NFPA 72 distinguishes how Class A and Class B circuits handle faults so occupants and responders remain informed even if part of the wiring has a problem.

In a Class A circuit, there are two or more independent paths that can carry the signaling back to the power source and control panel. Because of these parallel paths, if a fault occurs in one segment—such as a short or an open—the other path keeps the circuit complete and signaling can continue. Open circuits are still able to signal on fault because the system can route the signal along the alternate path. This makes Class A circuits highly reliable for life-safety signaling, where continuous notification is critical.

In a Class B circuit, there is a single signaling path from devices to the panel. A fault along that path can disrupt signaling because there isn’t an alternate route back to the control equipment. This makes Class B less fault-tolerant, but simpler and cheaper to install, which is why it’s chosen where wiring simplicity or cost is a priority.

An example of Class A usage would be a large building or campus where you want the system to continue signaling even if a section of wiring is damaged—think multi-floor or complicated layouts where uninterrupted alerts are essential. An example of Class B usage would be a small office or a single-area installation where a straightforward, cost-effective loop suffices and the highest level of redundancy isn’t required.

So the best description is that Class A provides a continuous path back to the power source with open circuits still signaling on fault, while Class B has a single fault path and may lose signaling if a circuit fails; Class A is preferred for life-safety signaling, and Class B is used where wiring simplicity or cost is a priority.