Douglas Krantz - Technical Writer - Describing How It Works

Why do we Need an EOL Resistor in an Addressable System?

By Douglas Krantz | Descriptions

Why do we Need an EOL Resistor in an Addressable System?

Why do we Need an EOL Resistor in an Addressable System?

Greetings Douglas,

I have a question concerning an Open Trouble. I wonder why we need to add an end of line resistor to an input module like a SIGA-CT1 (Manufactured by EST) when we are connecting it to a waterflow switch. It seems like the flow switch is close to the module, so the wires are short.

Thank You, GT

Short Answer

I know that the fire alarm system is an addressable system, but the addressable part is the Signaling Line Circuit (SLC) loop. The input circuit to the input module (the SIGA-CT1) that is connected to the waterflow switch is a conventional loop. The conventional loop has to be wired conventionally, including an end of line resistor.

Also, the manufacturer doesn't know whether the wires are short, like less than a meter or a few feet, or if the wires are long, like hundreds of meters or feet. All the manufacturer can do is design a One-Size-Fits-All for the module, so it is designed for the long wires.

Long Answer

The end of line resistor doesn't affect the waterflow switch at all. With or without the end of line resistor, the alarm signal is sent from the waterflow switch to the addressable input module, or the control panel. The end of line resistor is there because people are afraid; afraid that a system they're depending on to save their lives might not work.

The real parts to a Fire Detection and Alarm System (FDAS) are the devices that detect fire and the devices that sound the alarm if there is a fire. Everything else in the FDAS is support for the fire detection and alarming.

If you compare a stand-alone residential fire alarm to a 12 panel, 3000 smoke detector, thousand horn / strobe fire alarm system, the active system parts, in both cases, are the fire detection devices and the alarming notification devices.

Really Basic Fire Alarm System

Basic fire alarm system uses a water flow switch as the input, and a bell as the output. There is no supervision, so the only way anyone will know if a wire has broken or come loose is for the system to fail when it is tested, or when there is a real fire, whichever comes first.

This is a basic Fire Detection and Alarm System. Fire is detected when water starts flowing to suppress the fire, and turns on the bell to sound the alarm. In this case, the purpose of the alarm bell is to tell the arriving firefighters that water is actually flowing and where to connect their hoses.

The system here is electrically identical to a light switch on the wall with a ceiling light fixture.

A drawback to this system is that if a wire is cut, or the circuit breaker power is turned off, or if a wire comes loose in the system, just like what happens to the light on the ceiling, the Fire Detection and Alarm System doesn't work.

Yes, there is a regular testing of the flow switch. However, if the system doesn't work, no one will know that it doesn't work until the next annual inspection, or until a fire starts, whichever comes first.

Conventional Class B Wiring

You don't always see the words "Class B" in the addressable module's installation sheet, but the wiring between the waterflow switch and the module is a conventional Class B fire alarm circuit. Another way of wording it is the circuit is an Initiating Device Circuit (IDC).

Whether the conventional Class B circuit is connected to a Fire Alarm Control Panel (FACP) zone input, or the conventional Class B circuit is connected to an addressable zone input module, the wires of a Class B circuit is the same.

The input to an addressable fire alarm input module is usually a Class B input system, with an end of line resistor.

This is similar to many of the diagrams shown on the installation sheets that come in the box with an installation manual.

There's the End of Line Resistor, waterflow switch, the wires of the Class B circuit, the input module, and the signaling line circuit.

Supervising the Wires of the Class B Circuit

To supervise the wires, the panel performs a continuity check of the wires. The continuity check includes an and of line resistor because most conventional input devices are too dumb to indicate that they are not connected


By always running a small current through all the circuit wires in a series, the wires in a conventional fire alarm system are checked for continuity; they are supervised.

The end of line resistor is there to severely limit the current, while still allowing the small current through.

Of course, the devices like switches or detectors aren't supervised by the input module; a switch, for instance, doesn't have the circuitry inside it to be able to indicate that it has failed.

Because conventional fire alarm systems were originally designed before the advent of the transistor age, very few conventional devices can show a trouble when the fail.


If there is a fire, the switch shorts out the end of line resistor. The panel translates this to mean that there is an alarm.

To send an alarm to the panel, a switch shorts out tne end of line resistor, letting a large amount of supervision current through. A smoke detector doesn't short out the resistor, but it does allow a larger amount of supervision current through.

Because the current increases, the addressable fire alarm panel detects an alarm.

Open Circuit

If a wire breaks or comes loose from a connection, the current stops, and the panel interprets this to meana there is a trouble on the circuit.

This is the part requiring an end of line resistor at the end of the line. If a wire breaks or a wire comes loose from a connection, anywhere in the circuit, the supervision current stops. Because the supervision current stops, immediately the addressable input module sees trouble, and using the SLC, sends trouble information to the Fire Alarm Control Panel.

Douglas Krantz
Life Safety
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