Fundamentals in wiring are the same for all aspects of a fire alarm system. This article applies the fundamentals of conventional initiation circuit termination. Initiation circuits include smoke detectors, heat detectors, and pull stations. When installing devices, your supervisor will inform you if your wiring a conventional or addressable system.
Conventional fire initiation circuits can contain one device or a series of devices. Each circuit is called a zone. The fire panel cannot determine which device initiated the fire alarm; it just knows a zone has a fire event. The wiring and termination of conventional initiation circuits are virtually identical to NAC circuits. If you have not read the Fundamentals of Terminating NAC Circuits, please read first then return here.
All conventional initiation circuits wire the same. The grounds are common, and each positive wire goes to its own termination point. When a fire event triggers the device, the device creates almost a dead short in the circuit, which triggers an alarm condition in the Fire Panel.
To monitor the integrity of the circuit, a resistor is installed at the end of the circuit. This resistor creates a voltage drop which is measured by the panel. If a device is removed, the circuit is broken, and the Fire Alarm Panel creates a trouble condition. If a wire is cut or comes loose from a terminal, the circuit is broken and a trouble condition is created. All the wiring fundamentals presented here are required to assure the initiation circuit operates properly and alerts the user in the event of a wiring fault.
The image below illustrates the correct wire connection on a conventional initiation device. When the device is installed it will complete the connection between the two + connections. When the device is removed, the circuit is broken.
When the insulation of the wire is placed under the terminal wire clamp, it can lead to a very poor or no connection. Locating these wiring faults can be VERY VERY difficult. Always make sure you can see a little copper.
The image below shows the last device and the End Of Line Resistor, commonly referred to as EOL. The resistor is parallel to the device connections and so long as the device is installed the circuit is complete.
It is not always easy to secure the resistor lead under the same terminal as the larger diameter wire. It is generally not a good idea to install two wires of different sizes under the same terminal. To help keep the resistor secure, it is acceptible to place the resistor lead under the larger wire. Fold the end of the resistor lead over (about 1/4 inch) and slide the resistor under the larger copper wire. Tighten the terminal and give the resistor a tug to verify it is secure. IT IS CRITICAL the resistor is firmly secure.
While it is tempting to wrap the resistor lead around the wire, DO NOT. Remember that the fire alarm panel must be able to monitor that every device is connected to the circuit. If you wrap the resistor around the wire and that wire becomes disconnected from the terminal, the circuit is not broken.
NEVER USE YOUR DRILL TO TIGHTEN TERMINAL CONNECTIONS. Doing so can and does strip the termination screws very quickly.