Pathway classes in plain English: Class A, B, C, D, E and X
The class tells you what a circuit does when something goes wrong with it. Here is each letter, and the field checks that catch a mislabelled one.
NFPA 72 Chapter 12 describes the wiring and communication paths in a fire alarm system by class. The class tells you what the pathway does when something goes wrong with it: whether it keeps working past a fault, and whether it reports the fault. The letters get used loosely on submittals, so here is each one in plain English.
The six classes
| Class | What it does on a fault |
|---|---|
| A | Has a redundant path. Keeps operating past a single open and past a single ground fault, and reports each as a trouble. |
| B | No redundant path. Stops at a single open, so everything past the break is dead, but the open is reported as a trouble. Keeps operating through a single ground fault. |
| C | One or more pathways verified by end to end communication. The individual paths are not monitored; the loss of end to end communication is what reports as a trouble. |
| D | Fail safe. No fault is reported, because a pathway failure causes the intended operation. |
| E | Not monitored for integrity. Nothing about a fault is reported. |
| X | Has a redundant path. Keeps operating past a single open, a single short circuit, a combination of open and ground, and a single ground fault, and reports each as a trouble. |
Class A: both halves are required
A Class A pathway has to keep working past the fault and report it. Both are required, not either.
From the field
Look for the return conductors actually landed back at the panel. A Class A circuit wired out and not back is just Class B with extra wire.
Class B and the end of line device
The end of line device is how Class B supervision is usually achieved, but it is not part of the class definition itself. What matters is that a single open is reported.
From the field
The end of line resistor belongs at the far end. Moving it back to the panel to clear a trouble leaves everything beyond it unsupervised.
Class X, and Classes C, D and E
The short circuit case is what separates Class X from Class A. On a signaling line circuit that is what drives isolation at each device, which is why some loops have isolator bases on every device.
Class C is how network and IP based pathways get supervised. With Class D the failure is the operation, so there is nothing to report. Class E simply is not monitored.
How fast faults are reported
NFPA 72 Chapters 10 and 12 require a single open or ground fault on installation conductors to be detected and reported within 200 seconds. A fault on one notification appliance circuit must not knock out another notification appliance circuit for more than 200 seconds. That is the reason a single shorted horn should not silence a whole building.
Annunciation zones
Under NFPA 72 Chapters 10 and 23, each floor of a building is a separate zone for alarm annunciation. Where a floor is divided by fire or smoke barriers into several zones and the fire plan lets occupants relocate from one zone to another on the same floor, each of those zones is annunciated separately too. Where one system serves more than one building, each building is annunciated separately.
Visible annunciators have to be able to display every zone in alarm. Where one cannot show them all at once, it displays the zone of origin, with a separate indication that other zones are also in alarm.
From the field
A two story building annunciating as one zone tells responders nothing useful.
Keep reading
Smoke alarms and dwelling unit systems: what NFPA 72 Chapter 29 requires
Every sleeping room, every level, all interconnected. Plus the power figure people quote wrongly and the replacement date people read from the wrong label.
Class K extinguishers and the placard beside an automatic system
The portable and the hood system work as a pair, in a set order. The placard that says so is small, cheap and almost always missing.
Duct smoke detectors: what they are for and what they are not
A duct detector protects the air handler from spreading smoke. It does not protect the room, whatever the owner believes.
Supervising station monitoring: communication paths and timing
Which kind of station is actually under contract, how fast the signal has to arrive, and why the router in the closet matters as much as the panel battery.