Definition of an EOLR in a Fire Alarm System

By Andrew Erickson

August 6, 2023

When you're researching the complex world of fire alarm systems, the term "EOLR" can pop up quite often.

You might be wondering: What exactly is EOLR? Why is it so important? Let's unravel this technical term together.

Defining EOLR: End-of-Line Resistor

"EOLR" stands for End-of-Line Resistor. It might sound like a small electronic component (and it is!), but its role in fire alarm systems is pivotal. The fundamental function of an EOLR? To supervise and ensure the integrity of your alarm circuits.

The Role and Relevance of EOLR

Think of each possible alarm in your fire alarm system as an electrical circuit, because that's exactly what it is. It's a continuous loop of wire from your fire panel to the sensor/detector and back again.

An EOLR acts as a tiny guardian at the end of this loop. By being there, it ensures that every segment of your alarm circuit remains intact and operational.

Each end-of-line resistor changes the default electrical flow to help your fire alarm control panel (F A C P) detect problems. This ensures timely detection and rectification of any potential issues, safeguarding the efficacy of your entire system.

The Electrical Physics Behind EOLR

How does an EOLR function? Let's back up a step to make this more clear.

Your fire alarm panel constantly measures electrical resistance in each connected circuit. In a standard "contact closure" used for monitoring purposes, there are only two states: an "alarm" state and a "clear" state.

In a typical "Normally Open" (NO) circuit, an alarm is indicated by a closed circuit. This is detected by the fire panel as very limited resistance. In other words, a tiny bit of detection electricity is flowing quite freely through the wires.

Except during an alarm state, any normally open circuit is, you guessed it, open. That creates effectively infinite resistance (no electrical flow) that is easily detected by something like a fire panel. The problem? What if a wire gets cut? You would never see any alarm on that circuit if one were to happen. The circuit will be "stuck open", even if the initiating device (ex. smoke detector) has been activated.

Now, a "Normally Closed" (NC) circuit is a lot better, since a cut wire will register as an alarm. While this is a lot better than total blindness, it can create wasteful unnecessary fire responses when the problem is a wire fault and not a raging inferno.

Fire alarms demand higher reliability than this standard circuit type can offer. For that reason, we use EOLRs to add a third possible state to the mix so we can monitor wire integrity in addition to alarm states. Let's look at these three states now...

Understanding the Three States Indicated by EOLR

An EOLR effectively communicates the system's status through three distinct states: normal, alarm, and wire fault. Each state tells a unique story about the system's health and operational status. Let's delve into each of these states. We'll also look at the underlying electrical physics behind their communication.

1. Normal State

When everything is running smoothly and there are no disturbances in the system, the EOLR keeps things in the "normal" state. If the resistance measured by the fire panel matches the EOLR's predefined value, it means the loop is complete, the wiring is intact, and no alarms have been triggered. Essentially, "all is well" with this fire alarm.

2. Alarm State

The "alarm" state, as you might guess, indicates that one of the sensors (like a smoke or heat detector) has detected a potential threat. When this happens, the detector effectively short-circuits or bypasses the EOLR. This change drastically drops the resistance in the circuit to near zero. This is how your fire panels detect any alarm condition that uses conventional communication.

3. Wire Fault

The third state is the "wire fault." If there's a break in the circuit or a malfunctioning connection, the resistance in the loop will spike to an infinite value, indicating an open circuit. The EOLR identifies this anomaly and alerts the system about the fault, ensuring that any issues are swiftly addressed to maintain system integrity.

A Clever Understanding of Physics Makes This Possible

So, there you have it. Now you know the secret of: How does a single wire convey these states? At its core, this involves a clever use of electrical resistance values.

A single wire can only naturally convey two states: current flowing (low resistance) or no current flowing (infinite resistance). This binary system corresponds to the "normal" and "wire fault" states. The genius of the EOLR system is its ability to artificially create a third state, the "alarm" state, by manipulating the resistance value to something between "low" and "infinite".

In other words, while the wire's natural properties allow it to indicate the presence or absence of current, the EOLR enables varying resistance levels to communicate three distinct states through a single wire loop.

EOLR: A Necessity in Conventional Fire Alarm Systems

Without EOLRs, a fault in your alarm circuit might go unnoticed. The result? A compromised safety net.

With an EOLR in place, you're ensuring that the fire alarm system is always at its optimal performance, ready to alert you at the first sign of danger.

The EOLR's ability to translate the nuances of electrical resistance into meaningful system states is a testament to its importance in fire alarm systems. By understanding these states and the electrical principles that underpin them, you're better equipped to appreciate and maintain the safety infrastructure that protects your buildings.

Of course, there are many modern fire alarm technologies that use protocol communication to send small messages between devices. These detect a lack of connectivity without the need for end-of-line resistors.

To the extent you have traditional fire alarm systems in place, you need to understand how EOLRs work.

Talk to a Digitize Engineer for More Help

At Digitize, we're equipment manufacturers and experts in this industry. If you have a question, just call us and ask. We're happy to help.

Call Digitize at 1-800-523-7232 or email

Andrew Erickson

Andrew Erickson

Andrew Erickson is an Application Engineer at DPS Telecom, a manufacturer of semi-custom remote alarm monitoring systems based in Fresno, California. Andrew brings more than 17 years of experience building site monitoring solutions, developing intuitive user interfaces and documentation, and...Read More