In aClass B circuit, supervision of wiring integrity is accomplished using anEnd-of-Line Resistor (EOLR)at thelast devicein the circuit. This allows the control unit to detect open faults.
NFPA 72 (2022), Section 12.3.4.1:
“Initiating device circuits shall be monitored for integrity by placing an end-of-line device at the last device on the circuit.â€
If the wiring is cut or disconnected before the last device, the panel detects it as a trouble condition.