Douglas Krantz - Technical Writer - Describing How It Works
Get the Book Make It Work - Convetional Fire Alarms
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Why is There an EOL Resistor on a Pressure Switch?

On an addressable fire alarm system, a pressure switch isn't addressable. The resistor is part of a conventional Class B circuit. The conventional Class B circuit connects the pressure switch to an Addressable Input Module, which is then connected to the Signaling Line Circuit.

An input module on an addressable signaling line circuit is actiually a Conventional Class B Zone Input

Greetings Douglas,

I have gone through your posts and really very much impressed.

I have one question. In an intelligent addressable fire alarm system, why do we need to put a resistor in a pressure switch? If a resistor is not provided, what will happen?

Thank you, AA

The pressure switch is a switch; the pressure switch is not "addressed". Electrically, there isn't much difference between a switch operated by pressure and a switch-on-the-wall operated by a human hand to turn on a light.

If the pressure switch were connected directly onto the fire alarm system's Signaling Line Circuit (SLC), when the switch turned on, the switch would not send an alarm, the switch would short out the whole fire alarm system.

Instead of connecting directly to the fire alarm system's SLC, the pressure switch is connected to an addressable input module. The addressable input module is the interface between a Conventional Class B Fire Alarm Circuit and the SLC.

Check out the diagram.

To get the proper wiring diagram, look at the installation sheet that comes with the input module. If the diagram is lost, get one from the technical support for the manufacturer of the fire alarm system.

Always use the installation sheets. The installation sheets are there for you, the installer.

Class B Conventional Circuit

In a fire alarm system, all devices have to be continually checked to make sure they never come loose. For conventional devices on Class B systems, like the pressure switch, the best the fire alarm panel can do is make sure the wires are always connected.

To test the wires (supervise the wires) the panel, or in the case of the pressure switch - the input module runs a continual continuity check of the wires. If a wire comes loose from a device, the continuity of the circuit is broken, and a trouble is shown on the panel.

The purpose of the End of Line (EOL) Resistor at the end of the circuit is there to connect the ends of the circuit together to complete the electrical path. If the ends weren't connected, the electrical path would be broken, and a trouble would show up at the panel.

The EOL Resistor has to be at the end of circuit because if it were installed at the beginning of the circuit, the wires wouldn't be supervised; the wires wouldn't be in the continuity path.

Douglas Krantz
Life Safety
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