What is the significance of end-of-line resistors in conventional fire alarm circuits, and how does their absence affect system operation?

End-of-line (EOL) resistors are crucial components in conventional fire alarm circuits because they allow the fire alarm control panel (FACP) to monitor the integrity of the wiring and detect any faults that could compromise the system's ability to respond to a fire. In a conventional fire alarm system, initiating devices (such as smoke detectors and pull stations) are wired in a loop or a series circuit back to the FACP. The EOL resistor is placed at the end of this circuit, furthest from the FACP. The FACP sends a small current through the circuit and monitors the resistance. The presence of the EOL resistor ensures that the circuit has a specific, known resistance value under normal operating conditions. The FACP is programmed to recognize this resistance as a normal state. The absence of an EOL resistor or a change in the circuit's resistance indicates a fault condition. If the wiring is cut or disconnected (an open circuit), the resistance will increase to infinity, and the FACP will detect a trouble condition. Similarly, if the wiring is shorted to ground, the resistance will drop to zero, and the FACP will also detect a trouble condition. The absence of the EOL resistor prevents the FACP from distinguishing between a normal condition and a fault condition. Without the EOL resistor, the FACP would only be able to detect a fire alarm condition (a short circuit) but would not be able to detect an open circuit, such as a cut wire or a disconnected detector. This means that a significant portion of the fire alarm system could be rendered non-functional without anyone knowing until a fire actually occurs. For example, if a wire is accidentally cut in a zone without an EOL resistor, the detectors downstream of the cut will be disconnected from the FACP and will not be able to activate the alarm system in the event of a fire. Therefore, EOL resistors are essential for ensuring the reliability and integrity of conventional fire alarm circuits, allowing the FACP to detect and report both alarm and trouble conditions.