End of line resistors (EOLR) are resistors, of a known value, that are used to terminate protective loops or zones, such as those found in ESD or F&G systems. Microhm Electronics metal foil resistor PMRY series can be used as end of line resistors.
 
The purpose of EOLR's is to allow the control panel to supervise the field wiring for open or short circuit conditions. How the control system responds to each depends on the system's configuration, but generally speaking, a control system views an open circuit as a fault or alarm condition, and a short circuit as an alarm condition (if armed). The EOLR's allow the panel to differentiate between the two conditions by looking for a known resistance.
 Some manufacturers allow the use of double EOLRs, often referenced as DEOLRs. In this case, a second resistor is added across the terminals of the sensor, so that it is in parallel with the circuit. The second resistor adds the capability of differentiating between an open sensor and an open, or broken, circuit. If the sensor opens, the current passes through both the series and parallel resistors. The panel senses the reduced voltage and knows that the sensor is open. With this configuration, zero voltage can mean only one thing: a broken circuit. This constitutes the maximum supervision of loop wiring. The use of EOLR's is recommended when the field wiring is subject to damage or compromise, and is often referred to as "line monitoring".


 
An EOL resistor should always be installed at the last device and never inside the control panel. Placing resistors anywhere other than the end of the line does nothing to supervise the wiring, which is the reason for using resistors.
 
Fire zones usually use EOLRs. This is because fire sensors are "Normally Open" devices; they only close the circuit when they trip. Therefore, a fire zone in its normal state would appear the same to the system as one where the wire has been cut: namely, open. To prevent this, a resistor is used to close the circuit (with reduced voltage), so that it can be supervised.