From a Practical Point of View
In all of this, remember, a fire alarm system is supposed to detect fires and alarm people at any time there's a fire.
When you understand the purpose of an end of line resistor, you'll understand why it's called an "End of Line" resistor.
An End of Line Resistor is a Circuit Completer
Devices used in a conventional fire alarm system can't tell the fire alarm control panel that they are not connected anymore. However, the control panel can at least run a continual continuity check of the wires (wire supervision).
The electrons travel through one of the wires, through the end of line resistor, and back through the other wire. If the current stops, the panel assumes that there's trouble on the circuit, and turns on its trouble light and buzzer.
The Continuity Test Includes All of the Wires
The continuity path for a properly wired conventional fire alarm circuit includes all wires in the circuit. It also goes through all connections, one at a time, so that all connections are included in the path.
The Continuity Test Detects when There's a Break in the Circuit
When a wire breaks, a wire comes loose from a connection, or someone deliberately removes a device, the continuity path is interrupted, and then the current stops. When the current stops, the panel says there's trouble on the circuit.
If the end of line resistor is placed at the beginning of the line, inside the panel, the wires are not supervised. When the wires are not supervised, any break in the circuit will be discovered at the time of the annual test (devices won't work), or when there's a real fire (devices won't work), whichever comes first.
From a Legal Point of View
As such, an end of line resistor isn't mentioned as much as it used to be in the National Fire Protection Association 72 (NFPA - a publishing house). Instead, the NFPA says that all fire alarm equipment must be "Listed for Use".
The NFPA, though, does not maintain a "List". . . at all. Instead, the lists are maintained by laboratories that have tested the equipment and found that it can be used for fire alarm equipment (Listed for Use). Incidentally, the laboratories that perform the tests are ones like UL, ULC, FM, CE, CCC, EN, etc.
To be placed on the laboratory's list, for equipment like fire alarm control panels, the equipment has to be completely wired-up, including the devices, and the end of line resistor. Only if the panel works, as it is properly wired up, can the panel be "Listed for Use" in a fire alarm system.
If it is not wired up according to the way it was Listed for Use, the panel is not Listed for Use.
To find out how the panel is supposed to be wired up, you can look up the exact way it was Listed for Use in the Installation Manual that came in the same box as the panel. Or as an alternative, see if a manual is available on the web, or contact the technical support team for the fire alarm panel you're working with.
The devices being used with the panel (like smoke detectors, pull stations, waterflow switches, etc.) also have Installation Sheets that come inside the same box that the devices came in. These sheets are also available on the web, or the technical support team can help.
Make sure that the work "Installation" is included in the title of the manual or sheet, or assume it's not accurate.
At the End of the Circuit
On a conventional fire alarm system, the Installation Manuals that come with the panels, and the Installation Sheets that came with the devices, all show that the end of line resistor is to be placed at the end of the circuit.
Douglas Krantz
