Fire Alarm System Troubleshooting Guide: Diagnosing and Repairing Open Circuit Trouble on Initiating 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 an initiating zone at any point, the affected section of the circuit is no longer supervised. As a result, if an initiating device activates in the unsupervised portion, the FACP will not initiate an alarm. 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. See figure 1 Below.

Fire alarm systems use the principles of Ohm’s Law to supervise field wiring. 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. Conversely, an open circuit interrupts the supervisory current, causing a fault condition as shown in figure 2.

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 and the site.

The tools typically required for troubleshooting power supplies 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:

  • 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 initiating 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 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:

  • Observing Smoke Detector LED’s
  • Blinking LED’s indicate operating voltage and circuit continuity.
  • If not all smoke detectors LED’s blink, the break is likely between the last blinking detector and the first non-blinking one. In this example, only the smoke detector at Apt. 204 is blinking.
  • Spot Testing
  • Spot testing devices. Manual stations can be operated and reset easily.
  • Activate and reset manual stations to check for continuity.
  • If Some stations do not trigger an alarm, the fault lies between the last functional station and the next one.
  • In this example all 3 manual stations were operated, one at a time. Stair A & C manual stations trigger an alarm, Stair B manual station did not.

Based on the field investigation tests, we assume the open 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 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 4.7KΩ confirms continuity to the EOL.
  • A reading of 24VDC 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 apartment (e.g., Apt. 206), request access to inspect the wiring.
  • Upon removal of a the heat detector in Apt. 206, a broken wire is observed, confirming the open circuit fault. See figure 6.

Step 4: Repair

Wiring Repairs:

  • Remove any small broken wire fragments from the device terminals.
  • Strip the broken wire and reinstall under the appropriate terminal.
  • Check the FACP to confirm the trouble LED has cleared.

Retesting:

  • Test all reinstalled devices, including those before and after the fault.
  • 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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