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

Thomas Richards Manager, Fire Alarm Division - Viking Fire

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 Notification circuit for continuity, a process known as wiring supervision.

If a wire to wire short circuit occurs on a Notification circuit at any point, the affected section of the circuit is no longer supervised. A short circuit fault could affect different makes and models of fire panels differently. In some case the Notification circuits may have short circuit protection which would prevent the circuit from operating altogether during an alarm condition. Understanding how fire alarm systems supervise field wiring is essential for effective troubleshooting.

Ohms Law and Circuit Supervision

Fire alarm systems use the principles of Ohm’s Law to supervise field wiring. Notification circuits use a supervisory current, established 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 in a fire alarm bell. The diode operates as a current gate by blocking the supervisory current which in turn flows through the entire circuit, providing the circuit supervision. See figure 1

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.

A short circuit can be caused by a few scenarios. A wire to wire short is an obvious one. A notification device wired in reverse polarity can also present itself as a short circuit. See figure 3 below. In both scenarios the actual reading of the supervisory voltage would be reduced to almost zero and the supervisory current would in turn be much higher than normal.

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
  • 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 building owner and 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.
  • Notification Zone Trouble LEDs: When a short circuit occurs on a conventional notification circuit, the corresponding Yellow LED illuminates. *Note that these indicators may differ depending on make and model of fire alarm control panel. In this scenario we will assume there is simply one common notification circuit LED.

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. Repeat this step until the trouble clears. Note that there is always a possibility of having a short circuit fault on multiple circuits at once. If that is the case it is important to be thorough during this part of the investigation. Having a notepad available to take notes would be helpful.
  • 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 4.

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

  • Short circuit faults can be harder to diagnose visually. Look for potential signs of work that may have been performed. Devices removed and re-installed improperly could result in a reverse polarity.

In this scenario, we are unable to see any clear signs of work performed, therefor the next step is to use our multi meter as a diagnostic tool to help us look for the short circuit fault.

Investigation with a Multimeter

  • Try to pick a device that is installed at the approximate halfway point of the circuit. In this scenario the horn/strobe in the elevator lobby could be close to the halfway point as it is in the center of the floor space. Based on the resulting readings you can then work towards the short circuit fault which could be in the direction of the EOL or the FACP. See figure 5 below.
  • Disconnect the device, remove the wires from the terminals and measure voltage and resistance for each wire pair. See Figure 6 below.
  • It is important to only isolate one section at a time so that you do not get any false readings.
  • Disconnect the circuit from the panel before taking your resistance reading.
  • Document readings carefully. As per figure 6
  • A reading of 2.2KΩ confirms continuity to the EOL.
  • A reading of 0.83VDC confirms continuity to the FACP. But notice it is significantly lower than a “normal” supervisory voltage.
  • With the wires disconnected from the NAC circuit at the FACP the meter reads 10Ω. A reading on a circuit without a fault would not indicate any continuity.

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 we would look at the devices in Unit 202. When removing the horn at stair A you notice that the wires were connected in reverse polarity. Perhaps the device had been replaced and re-installed incorrectly.

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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