Fire Alarm System Troubleshooting Guide: Diagnosing and Repairing Short Circuit Trouble on Conventional Initiating Circuits

Thomas Richards Manager - Fire Alarm Division, Viking Fire Protection

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.

A short circuit condition can be harder to diagnose as it replicates an activation of a conventional initiating device and will display as an alarm condition on the fire alarm panel. If a short circuit is caused by anything other than a conventional initiating device it would be considered a nuisance alarm. The initial procedure would be to investigate the source of alarm, clear the alarm event and reset the fire alarm control panel. In this case, the short circuit will prevent the fire alarm panel from being reset and will remain in alarm if it is present.

Ohms Law and Circuit Supervision

Fire Alarm Systems use the basic principle of Ohms law to supervise the field wiring. See figure 1 Below.

Each input circuit on an FACP has a low supervisory current, typically ranging from 2mA to 6mA, regulated by the end-of-line (EOL) resistor at the circuit’s end. The EOL resistor varies by manufacturer, but its role remains consistent: maintaining the circuit’s supervisory current.

The example in figure 1 has a supervisory current of 5mA.

A decrease in resistance, such as a short circuit caused by a closed contact on a pull station or heat detector, results in increased supervisory current, which the panel interprets as an alarm condition. A short circuit fault will mimic an alarm condition and therefor you will see similar readings.

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)

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, the site and local jurisdiction.

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 input circuit fault issues include:

  • Input Zone Trouble LEDs: When a short circuit occurs on a conventional initiating circuit, the corresponding zone will show an alarm state.

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 initiating zone.
  • Reset the fire alarm panel.
  • If the alarm condition clears you have ruled out faulty panel hardware and can assume the issue is in the field.
  • Reconnect the field wiring before proceeding with field investigation

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

Based on the 2nd floor zone shown, there are 3 manual stations, 2 smoke detectors and 7 heat detectors. The smoke detector at top of stair C is connected to a separate zone.

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

Field investigation Methods:

  • Visual Inspection
  • Confirm manual stations have not been operated and are not damaged.
  • Confirm detectors are not in alarm state by observing the status LED’s
  • Confirm that heat detectors are not damaged
  • Measure Voltage of conventional initiating zone in alarm. 0Vdc is indicative of a short, reduced voltage would indicate a conventional smoke detector has been activated.
  • Spot Inspection with Fire Panel
  • Pick an easily accessible device that is approximately at the halfway point of the circuit like a manual station.
  • Remove the device from the circuit.
  • Try resetting the fire alarm panel.
  • If the alarm condition does not re activate, it means the short circuit is between the removed device and end of line resistor.

Based on the field investigation tests using the fire alarm panel, we assume the short circuit is between the manual station at Stair C and the EOL.

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 accessible testing points—such as a nearby smoke detector or manual station. See figure 4 below.
  • Disconnect devices one at a time 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
  • Reminder to never measure resistance (or impedance) n an energized circuit.
  • Document readings carefully. As per figure 5
  • A reading of 10Ω confirms the direction of the short circuit.
  • A reading of 24VDC confirms there is no short circuit between that section of wire and the FACP

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 direction, in this case between the smoke detector and the EOL.
  • Continue the process of elimination by breaking the circuit again at the halfway point between the removed device and the end of line by moving in the direction of the short circuit.
  • In this scenario the readings and process of elimination indicates the wiring between the pull station and EOL is the source of the short.

Step 4: Repair

Wiring Repairs:

  • Replace the damaged section of wire. (This will most likely require the assistance of an electrical contractor)
  • Ensure the correct type and gauge of wire is used for the replacement.
  • Make new terminations at either end of the new section of wire.
  • Check the FACP to confirm the system is clear of faults and that the alarm condition does not re-initiate.

Retesting:

  • Test all reinstalled devices, including those before and after the fault.
  • Verify EOL resistor and manual station.

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.
  • If the short circuit condition cannot be immediately repaired, a fire watch will need to be implemented.

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 short circuit troubles on conventional fire alarm systems, ensuring system reliability and safety.

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