Why do we use End of Line (EOL) Resistor in Fire and Gas Systems?

Although the End-of-Line Resistor in fire alarm and security systems looks similar to a Terminating Resistor, the End-of-Line Resistor has an entirely distinct function.

It’s critical to realize that the “signals” utilized in fire alarm systems are DC (on or off), not AC (which carries information such as video or data).

There are no reflections from the end of the loop since there is no AC; the End-of-Line Resistor is used to pass DC current.

The only purpose is to pass a modest supervision current so that the fire alarm or security panel may “look” at the wire; if the supervision current fails, the failure can be reported quickly, and the problem can be discovered.

Alarm zones, Hooters, Manual Call Points (MCP), and Smoke Detectors are all current limiting dc circuits in an analogue fire and gas system. Depending on the type and model of alarm system, a volt metre connected across an unused zone might read somewhere about 12 volts. This voltage is used by the alarm system to identify the zone’s state.

There is an open circuit between zone positive and ground at 12 volts.

0 volts = Zone positive and ground are both shorted.

You’re probably aware that a short in an electrical circuit is a terrible thing, and in most cases, this is correct. An alarm panel zone, on the other hand, is built to handle a short with ease.

In reality, in a regular (non-tripped) zone, if end-of-line resistors are deactivated in programming, a short is exactly what the panel is searching for. The zone is considered secure if the panel detects zero volts (or near to it). It deems the zone faulty if it detects 12 volts (or near to it).

Resistors give the zone an extra layer of protection. Alarm companies create zones that check for a specific resistance on their circuits. On its hardwire zones, Honeywell, for example, utilizes 2000 ohm resistors.

With a 2000 ohm (2k) resistor inline, the Honeywell hardwire zone will read roughly 5 volts at its terminals.

As a result, we have:

There is an open circuit between zone positive and ground at 12 volts (the zone is faulted)

5 volts = The circuit is closed and the resistor is in good condition (the zone is clear)

0 volts = Zone positive and ground are both shorted (the zone is faulted)

By placing the resistor at the zone’s farthest device (the “end of the line”), you’ve created a supervised circuit that can’t be turned off by simply shorting its wire somewhere along the wire run between the control panel and the detecting device.

Note: The value of the EOL Resistor varies from model to model and make to make.

Hardwire Zone EOL Resistor Status

The alarm panel will be able to “see” the 2000 ohm resistor since the circuit has a continuous path through the closed contact. On the keypad, the zone will be normal (not faulted.)

Because the contact has opened and broken the circuit to the 2000 ohm resistor, this circuit will show as faulty on the keypad. The system would go into alert if the system was armed when the contact opened.
The zone is triggered in this way by a motion detector or a door/window contact.

This circuit illustrates a short across the 2000 ohm resistor; if the zone is unarmed, it will fault; if it is armed, it will sound the alarm.
Alarm resistors are named “end of line” resistors because the circuit can be checked for shorts by installing the resistor at the end device (motion detector or door/window contact).
Note that the value of the End of Line (EOL) Resistor varies from manufacturer to vendor.

Advantages of End of Line Resistor :

The end-of-line resistor is at the end of the loop: the last device, to allow the panel’s inbuilt ohmmeter to monitor the continuity of the wires (supervise the wires).

However, before we can discuss the necessity of the end-of-line resistor, we must first comprehend the significance of having a fire alarm system in the first place.

Fires are terrifying. Building fires kill people. Fires are dangerous.

People can escape the fire with the help of a properly equipped fire alarm system, however, there’s a dilemma. If something is wrong at the control panel or elsewhere in the building,

Simple Systems

Fire alarm systems are straightforward devices. They must be straightforward. Keep in mind that the more intricate a system is, the more things might go wrong.

A very simple fire alarm system is where the input wires begin conducting, similar to turning on a light switch. The fire horn is activated by the switch (pull station, heat detector, water flow switch, etc.). This is a basic system that works well, or at least it does for me.

Will anyone notice if a screw in a pull station comes loose before a fire starts? Pulling the lever on the manual pull station achieves nothing because the screw is loose. It’s too late to troubleshoot and rectify the electrical problem once there’s a roaring fire.

If a wire connecting the control panel and the fire horn breaks, the building will burn and smoke will escape.

Continual Testing of the Continuity of the Wires

A fire may erupt at any time. The problem is that no one will spend 24 hours a day, 365 days a year using an ohmmeter to ensure that the wires are always connected to all of the input and output devices.

Testing Continuity

The fire alarm panel constantly checks the continuity of the wires to ensure they are always connected and not broken. It supervises the wires by putting a little current across them.

If the current is interrupted, the panel’s trouble light and buzzer illuminate. This is how the building’s owner knows there’s an issue.

Current Limiting End-of-Line Resistor

We’re still dealing with a restricting issue. The electrical current is entirely turned on by the draw station. The fire alarm panel learns about a fire this way: full current to the panel indicates a fire.

We can’t fully turn on the current to prevent false alarms while monitoring the cables (supervise the wires), otherwise we’ll get a false alarm. We must somehow reduce the current to a minimal minimum, allowing jus

End-of-Line Resistor

The end-of-line resistor is used in this situation. The resistor keeps the current low enough so the panel doesn’t think it’s detecting a fire, but it also allows it to confirm that current is flowing through all of the wires.

The end-of-line resistor must be fitted at the end of the line to ensure that all cables are supervised for continuity. This ensures that the electrical current passes through all wires.

Of course, we can’t just twist the wires together at the end to allow the panel’s ohmmeter to check for continuity of the horn wires; we have to add a resistor there as well: to limit the current so the panel’s ohmmeter can check for continuity.

Continual Continuity Testing

Because fires might happen at any time, all of the building’s cables are constantly monitored (checked for continuity). The end-of-line resistor is there to allow the fire alarm panel to continuously test (supervise) the wiring without creating a false alert or shorting out the horns and strobes.

What is End of Line Resistors and do I need them on my alarm system?

End-of-line resistors (EOLR) are fixed-value resistors that are used to terminate protective loops or zones.

EOLRs are used to enable the control panel to monitor the field wiring for open or short circuit problems. The alarm’s response to each is dependent on the panel and system zone setting, but in general, an alarm considers an open circuit to be a fault or ala.

The goal of EOLRs is for the panel to be able to distinguish between the two states by looking for a known resistance.

Unless specific circumstances exist, EOLRs should be positioned at the last device on the loop, electrically speaking, rather than inside the control. Professional installers frequently discuss the merits of EOLRs on protective zones with completely concealed wiring, as well as EOLRs installed inside the control unit, negating their usefulness, as well as removing the EOLR feature and employing NC (normally closed) loops for zone definitions. The use of EOLRs is suggested, and it is especially critical when the field wiring is prone to damage.

End Of Line Resistors — The Basics

What are they, and why are they used?

To monitor the condition of doors and windows, early security systems relied on simple electrical circuits. The circuit was either closed or open, and as a result, the control panel received full voltage or no voltage at all; that was all the system cared about. Although such circuits are still in use today, the digital age allowed manufacturers to design systems that were more secure.

A resistor is a tiny semiconductor that prevents electrical current from flowing through it. The current is allowed to flow, but the value of the resistor reduces it. When a resistor is placed in line with a sensor on an alarm circuit, the control panel no longer shows full voltage across the circuit when the circuit is closed, but rather a lowered voltage.

The control can now measure three different conditions: full open-circuit voltage (if the circuit is open), decreased voltage (if the circuit is closed and secure), and no voltage (if the wiring is compromised). Because the current has a route back to the control if the two sides of the circuit make contact at some point between the control and the resistor, the current bypasses the resistor.

Where should resistors be placed?

Only if the resistor is placed at the end of the line will the procedure described above work. There have been numerous disputes regarding where the resistors should be placed on the control panel (or, as the pros say, “in the can”).

Although there may be practical reasons for doing so, it is important to understand that installing resistors somewhere other than at the end of the line does NOT supervise the wiring.

Why does the wiring need to be supervised? How can the circuit become shorted?

If a potential burglar gains access to the zone wiring, the wires can be intentionally shorted, allowing the burglar to gain entrance to the building at a later date.

Furthermore, a nail or screw (used to hang a picture, photo, or other items) could pierce both the outer and inner insulation of the wires, causing contact between the two conductors. While both of these scenarios are improbable, they are possible.

If a shorted zone is so unlikely, are resistors really necessary?

There is no clear answer to this question. There are liability considerations to consider if a pro installation asks, especially if the manufacturer advises or needs resistors. No one else can provide a good answer if a do-it-yourselfer asks.

When building a system in one’s own home, one must evaluate the possibility of faulty wiring and make an informed decision.

Finally, there’s the matter of whether a given system will even enable resistors to be removed. If it doesn’t, the only decision left to make is where to put them. If resistors are required but wire supervision is not required, they can be installed at the control.

Resistors seem like such a sensible idea. Is there any reason NOT to use them, and place them at the end of the line as required?

There are several causes for this. First, while placing a resistor inside a motion detector or glass break sensor is normally simple, attaching one to a magnetic contact, especially with recessed contacts, can be a difficult task. The resistor must be spliced to the wire in some fashion, and the splice must be forced back through the hole before the contact is put (ideally in such a way that the contact is pushed back through the hole).

The resistor and splice will be visible on a surface-mounted contact, which can be aesthetically irritating (depending on the installer’s expertise). Second, if the control panel is replaced at a later date, the resistors will not be the correct value if equipment from a different manufacturer is used.

Is there any way to place the resistor inside the can but still supervise the wiring?

Yes. The two free conductors can be used to extend the circuit from the sensor site to the can, via the resistor, and back to the sensor when utilizing a four-conductor cable for a two-wire sensor. The resistor would be at the very end of the line, and supervision would be possible.

Are there any other ways in which resistors are used?

Yes. First, even if the other zones do not, fire zones always employ resistors. Fire sensors are “Normally Open” devices, which means they only close the circuit when they trip. As a result, a fire zone in its regular state seems to the system to be the same as one where the wire has been severed: open. To avoid this, a resistor is used to close the circuit (at a lower voltage) and keep it closed.

Second, some manufactures allow double EOLRs to be used. In this situation, a second resistor is put across the sensor’s terminals to make the circuit parallel. A single resistor, as you may recall, allows the system to determine if the circuit is secure, open, or shorted. The second resistor adds the ability to distinguish between an open sensor and a circuit that is open or broken.

Third, certain manufacturers permit “zone doubling,” which involves connecting two zones, each with a different resistor value, in parallel to the same two terminals. When both zones are closed, the system sees the total of both lowered voltages because they are connected in parallel. If one opens, the voltage is eliminated, and the system knows which one is open because of the different resistors.

Although this practically doubles the number of zones available on the mainboard, it has no influence on the system’s maximum capacity.