Fire Alarm System Troubleshooting Guide: Diagnosing and Repairing Power Supplies

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

The power supply is an integral component of a fire alarm system. Its malfunction can lead to a complete loss of system functionality, depending on the severity of the issue. Diagnosing power supply troubles is not always straightforward, as these issues can manifest in various conditions displayed at the panel. Some problems are clear-cut, such as an illuminated “Battery Trouble” LED or the absence of a green “AC On” light. However, others can be misleading, presenting as generic trouble indicators, open circuits on field wiring, or even false alarms.

Before diagnosing a power supply issue, it is essential to understand its role in a fire alarm system and the various types of voltages involved. Note that power supplies may differ slightly depending on the make and model. This article provides a foundational understanding of troubleshooting power supplies in fire alarm systems. Always refer to the specific equipment documentation for detailed guidance.

Common Voltages in Power Supplies

Most power supplies operate with multiple voltages. The typical voltages for most systems are as follows, though these may vary slightly:

  • Source Power – 120VAC: Supplied directly from an electrical panel. Breaker information should be documented at the panel and in the verification or inspection report.
  • Transformer Output Power – 27VAC: For accessible transformers, this is the voltage on the secondary winding.
  • DC Output Voltage – 24VDC: This is the supplied output voltage to the main fire alarm CPU or field circuits, including initiating zone circuits and resettable or non-resettable power circuits.
  • Battery Charging Voltage – 26-27VDC: The charging voltage at the terminals connecting the power supply to the batteries. This must exceed 24VDC.
  • Battery Voltage: A fully charged “healthy” set of batteries connected in series should read approximately 25VDC. For individual 12VDC batteries, a full charge typically measures 12.3 to 12.6VDC.

Troubleshooting Steps

This 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
  • Long-sleeve shirt (natural fiber)
  • Safe Work Practices:
  • Identify the breaker providing the power source for the power supply.
  • Remove jewelry, including rings, bracelets, and watches.
  • Adhere to Lockout/Tagout (LOTO) procedures.

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

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 power supply issues include:

  • AC On LED: The green “AC On” LED should always be illuminated if source power is present. If not, this indicates the FACP is not receiving power.
  • Battery Trouble LED: This indicates a battery-related issue, such as a fault in the batteries or the charging circuit. Some panels may display a separate “Charger Fault” indicator.
  • Other Trouble LEDs: Issues like ground faults, open circuits, or false alarms may initially appear as field wiring issues. If these are ruled out, the power supply should be investigated.

Step 3: Investigation

Investigation methods vary depending on the identified issue:

  • AC On LED Not Illuminated:
  • If the AC On LED is off, verify the presence of source power using a multimeter rather than relying solely on the LED.
  • If source power is absent, check the circuit breaker. Ensure the circuit is dedicated to the FACP. A tripped breaker may indicate other branch circuits or wiring issues, requiring an electrician’s intervention.
  • Battery Trouble LED Illuminated:
  • Inspect batteries for visual signs of failure, such as swelling or electrolyte leakage. Replace faulty batteries immediately.
  • Remove batteries from the charging circuit. Measure the voltage of the batteries in series to ensure the supplied voltage is within an acceptable range, and then individually. Significant voltage differences between batteries indicate a fault.
  • Verify the power supply’s charging voltage (26-27VDC). Low or absent charging voltage points to a power supply issue.
  • Other Trouble LEDs:
  • Ground Fault: If the Ground Fault LED remains illuminated after disconnecting all field wiring, the power supply may have an internal ground fault.
  • Open Circuit: A loss of supervisory voltage from the power supply can cause an open circuit condition.
  • False Alarm: Fluctuating or abnormally high voltage from the power supply may trigger a false alarm.

Step 4: Repair

The type of repair depends on the findings during the investigation:

  • Wiring Repairs:
  • Engage a licensed electrician for wiring-related issues. Provide detailed findings, including photos or videos, to expedite repairs.
  • Battery Replacement:
  • Use batteries of the correct size and type. Always replace in pairs with batteries from the same manufacturer.
  • Confirm charging voltage and charging current readings fall within the manufacturer's specifications.
  • Power Supply Replacement:
  • Turn off the breaker supplying power to the system before replacing the power supply.
  • Ensure all wires are reconnected accurately before restoring power. Reconnect batteries last, only after source power is restored.
  • Consult the manufacturers installation manual for proper installation configurations. (Dip Switches/Jumpers/Wire Terminations etc.)
  • Re-verify battery readings, a failed power supply charging circuit could have damaged the existing batteries. Replace existing batteries as required.

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 power supply issues in fire alarm systems, ensuring system reliability and safety.

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