Douglas Krantz - Technical Writer - Describing How It Works
Get the Book Make It Work - Convetional Fire Alarms
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Why Use Pigtails or a UL Approved End-of-Line Resistor?

A normal resistor has smaller wires than the zone wires for the device. Because the resistor wire is smaller, it does not tighten properly under the screw plate. No matter how tight the screw, either the screw will distort the wires, or tarnish will build up, causing troubles on the system.

Terminals at the end of a Class B Circuit where the installer had twisted the wires of the end-of-line resistor around the zone wire. On the right side, the twisted wire slid down before the screw was tightened but he installer didn't notice.
The resistor wire was twisted around the zone wire. It slid down before the installer tightened the screw, but the installer didn't notice.

By Douglas Krantz

Many times, a fire alarm system installer will put the leads of an end-of-line resistor (22 to 24 gage - very thin) under the same screw plate as the zone wire (16 to 18 gage wire - by comparison very thick).

Because both a thick and a thin wire are under the same screw plate, the plate, when screwed down on the wires, cannot press against both wires evenly. One wire is always looser, and the loose wire shows up later as a trouble on the panel.

To try to get around the problem, one method some fire alarm system installers use is to wrap the resistor wire around the zone wire. They then insert this twisted pair of wires under the screw plate. The screw plate does tighten down on both wires.

There are two issues with this approach, though.
  1. Even though the end of line resistor may still be connected to the zone wire, if the screw plate ever comes loose, the panel won't be in trouble but the device itself may not work. Besides being against NFPA Code, this is just plain bad practice for Class B wiring.

  2. The resistor wire itself might slide up the zone wire as it is inserted under the plate, not quite getting under the screw plate itself. (I've see this happen quite a few times.) When the installer is through wiring, the fire alarm system is normal. However, only the zone wire is tightened under the screw plate. Months or even years pass, and tarnish builds between the zone wire and the resistor wire. Eventually the electrical contact between the wires fails. When the panel goes into trouble (sometimes intermittently), the fire alarm service technician is going to have to troubleshoot to find and fix the problem.

Instead of trying to secure the thin resistor wire and the thick zone wire under the screw plate, use either a UL approved end-of-line resistor (it has spade connectors crimped to the wires), or make pigtails for the end-of-line resistor with the same thickness wire as the zone wire.

Using pigtails or a UL approved end-of-line resistor, the connection won't fail, and the system (at least for that problem) doesn't have to be serviced later.
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