Fire Alarm System Troubleshooting Guide: Diagnosing and Repairing Open Circuit Trouble on Notification Circuit

Thomas Richards Project Sales Executive, SECUR - A Fire Safety & Protection Company

Fire alarm wiring is a crucial component of a fire alarm system, serving as the means of communication between the fire alarm control panel (FACP) and field devices. Therefore, fire alarm systems are designed to continuously supervise each circuit for continuity, a process known as wiring supervision.

If an open circuit occurs on a Notification circuit at any point, the affected section of the circuit is no longer supervised. As a result, during an alarm condition, notification devices downstream of the open circuit will not operate as they will not receive the operation voltage. Understanding how fire alarm systems supervise field wiring is essential for effective troubleshooting.

Ohms Law and Circuit Supervision

Fire Alarm Systems use the basic principle of Ohms law to supervise the field wiring.

Fire alarm systems use the principles of Ohm’s Law to supervise field wiring. Notification circuits use a supervisory current, regulated by the end-of-line (EOL) resistor at the circuit’s end. The supervisory voltage and EOL resistor value varies by manufacturer, but their role remains consistent: maintaining the circuit’s supervisory current. Though the basic principles of Ohm’s Law are used for supervision is the same for both initiating and notification circuits, there are differences that should be noted.

Notification devices rely on voltage to operate, therefore if the panel produces a constant supervisory voltage, why is it that the devices only operate during an alarm condition? Notification devices use a component called a “blocking diode” installed in series with the audible generating component such as a coil. The diode operates as a current gate by blocking the supervisory current which in turn flows through the entire circuit, providing the circuit supervision.

The example in figure 1 shows the direction of supervisory current and the blocking diode preventing it from passing through the coil.

During an alarm state, the FACP reverses the polarity of the notification circuit which allows the current to flow through the notification device allowing the sound generating components to operate. See figure 2.

The example in figure 2 shows the change in direction of the alarm current allowing it to pass through diode and the coil.

Troubleshooting Steps

The article outlines four basic steps for troubleshooting fire alarm systems, which will be detailed below.

Step 1: Preparation

Before beginning work on a fire alarm system, identify potential hazards. Safety is paramount in this field. Ensure you have the proper personal protective equipment (PPE) and tools:

PPE Requirements:

  • Hand protection
  • Eye protection
  • Fall protection
  • Hand protection
  • Long-sleeve shirt (natural fiber)
  • Hearing protection

Safe Work Practices:

  • Identify any confined space (if applicable)
  • Adhere to working from heights procedures (if applicable)
  • Ensure safe work practices around high voltage.

Follow additional safety procedures established by your company and the site.

The tools typically required for troubleshooting include:

  • Digital multimeter
  • Multi-driver
  • Insulated Terminal driver
  • Ladder or Elevated work platform
  • Various types of end of line resistors. Common values used in fire alarm: 2.2K, 3.9K, 4.7K, 10K, 20K, 47K

Notify the monitoring station to place the system on test and inform the fire department before proceeding to the next step.

Step 2: Identification

Begin troubleshooting by examining the information provided on the fire alarm control panel (FACP) display. Common indicators of notification circuit fault issues include:

  • Common System Trouble LED: Illuminates for any system trouble.
  • Input Zone Trouble LEDs: When an open circuit occurs on a conventional initiating circuit, the corresponding Yellow LED illuminates.

Step 3: Investigation

Start at the FACP to rule out panel hardware faults.

Confirm a Field Wiring Issue:

  • Disconnect the field wiring from the terminal of the affected notification zone.
  • Place the appropriate EOL resistor on the terminals.
  • If the trouble condition clears, the issue lies in the field wiring between the panel and the EOL.
  • Reconnect the field wiring before proceeding with field investigation

For this article, assume the affected notification circuit covers the 2nd floor of the building as shown in figure 3.

There is 1 class B notification circuit per floor. Based on the 2nd floor shown, there are 9 horns and 1 horn/strobe.

If the circuit pathway is visible, follow it. If not, assume logical starting points based on the layout. For this case, we can assume the first device is the horn at Stair A, opposite the EOL location on that floor.

Field investigation Methods:

  • Visual Inspection
  • Visually inspecting the notification devices is an easy way to spot an issue. A device removed for painting, can be easily observed and repaired.
  • Activating Notification Circuit
  • If permitted, activating the notification circuit is an easy way to identify an area or section of the circuit that is past the break.
  • If one or more notification devices do not operate, the break is likely between the last operating device and the break. In this example, the horns in the Unit 201 including the horn in the storage closet are non-operational.

Based on the field investigation tests, we assume the open circuit is between the horn/strobe in the elevator lobby and the end of line at Stair B.

Since the raceway is not visible, we can use our multimeter for further investigation.

Investigation with a Multimeter

  • Use the data gathered during field investigation to identify multiple testing points such as a horn/strobe in the elevator lobby and the horn in the storage closet. Selecting a device closer to the EOL can help as you can then work your way towards the break from both ends of the circuit. See figure 4 below.
  • Disconnect devices and measure voltage and resistance for each wire pair. See Figure 5 below.
  • It is important to only isolate one section at a time so that you do not get any false readings
  • Document readings carefully. As per figure 5
  • A reading of 2.2KΩ confirms continuity to the EOL.
  • A reading of 5VDC confirms continuity to the FACP.
  • A reading of 0V and ∞Ω suggests a break in the wiring.

Confirming the Fault Locations

  • Identify the direction of the fault by observing the entry point of the wire in the electrical box.
  • If readings and observations indicate an issue in a specific area, continue removing devices and taking note of the voltage and resistance readings.
  • In this example the only 2 other locations could be the office and the boardroom in Unit 201. When removing the horn in the boardroom you notice that one of the wires is no longer connected. Perhaps the device had been removed for painting and improperly re installed upon completion.

Step 4: Repair

Wiring Repairs:

  • Terminate the wires accordingly and re mount the device. (Make sure to mount all devices that had been removed during the investigation)
  • Check the FACP to confirm the trouble LED has cleared.

Retesting:

  • Operate the notification circuit and ensure that all devices operate.
  • Verify EOL resistor.

Additional Considerations:

  • If a large section of cable is damaged beyond repair, replacement may be necessary and would require an electrician.
  • Extensive wiring repairs or repairs to raceway will require an electrician.

For all repairs, consult your office and the local Authority Having Jurisdiction (AHJ) to ensure compliance with proper procedures. Perform post-repair testing as required by applicable codes and standards. Document all work for inclusion in the site file for your office and the customer.

By following these steps, technicians can effectively diagnose and repair open circuit troubles on fire alarm systems, ensuring system reliability and safety.

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