Reference · Testing & Inspection
End-of-Line DeviceEOL
The resistor (or diode/capacitor) installed at the electrical end of a Class B circuit. The panel measures the small supervisory current through it; an open removes the current (trouble), a short increases it (alarm or trouble depending on circuit type).
NFPA 72: Chapter 12 (Class B pathway supervision)
Section numbers vary by edition — confirm in the edition your AHJ has adopted.
Read more in these guides
Class A vs Class B (and Class X) Fire Alarm Circuits, ExplainedClass A, B, and X describe how much a circuit can survive before it stops working. Here's what each designation actually buys you.Commercial Fire Alarm Troubleshooting: Opens, Shorts & Ground FaultsTrouble signals on a commercial system almost always come down to opens, shorts, or ground faults. Here's a repeatable way to isolate them without guesswork.