Intermittent SLC Trouble After a Plenum Extension

1. Cable type in a plenum. A ceiling used as a return-air plenum requires plenum-rated cable or a raceway. NM-B is not permitted in that space. The correct product is a plenum-listed fire alarm cable, typically FPLP, installed per the fire alarm article of the electrical code.

2. Unsupported and inaccessible splices. The box is lying on a tile, unsecured and effectively inaccessible once the ceiling is closed. Splices must be made in listed enclosures that are secured and accessible, using connectors listed for the cable and the circuit.

3. Circuit integrity and supervision. A Class B circuit is only as good as its terminations. Wire nuts on small-gauge fire alarm conductors in a vibrating, thermally cycling plenum are a foreseeable failure point, and the installation does not preserve the supervised path the panel expects.

4. Cable support and securing. The extension must be supported and secured per the fire alarm article and the manufacturer's instructions; a cable resting on ceiling tile and grid is neither.

These are not paperwork issues. The plenum cable rule exists because a fire in that concealed space spreads through the building's air path, and the splice rule exists because a hidden connection is a connection nobody will ever find again.

Both symptoms point at the same physical location.

Trouble window. The unit runs in the afternoon, the plenum heats up, conductors and terminals expand, and a marginal splice opens momentarily. On a Class B circuit an open produces a trouble, and when the connection remakes itself the trouble clears. That is the signature of a high-resistance termination, not a failing device.

False alarms. A poor connection at the end of a run can corrupt device communication or produce a transient the panel interprets as an alarm. Construction dust pulled into the detector by the unit, and a detector sitting in a warm airstream, compound it.

Test sequence:

• Pull the panel's device history and confirm which addresses are involved, not just the circuit.
• Measure loop resistance end to end and compare with the expected value; a significant increase localizes the bad splice.
• Test for grounds and shorts with the circuit isolated, remembering that a single ground on Class B gives a trouble while a second ground can produce alarms or loss of supervision.
• Thermally stress the suspect splices and watch the panel, or wiggle-test them with the system in test mode.
• Inspect the end detector for dust loading and airflow exposure, and check its reported sensitivity.

Do not clear the history and walk away; nine events in 30 days is a pattern, and the next event may be a missed alarm rather than a nuisance.

Physical fix:

1. Remove the NM-B entirely; repair rather than reuse it.
2. Install plenum-rated fire alarm cable or run the circuit in raceway through the plenum.
3. Replace both splices with listed connectors in a secured, accessible enclosure with a cover.
4. Support and secure the new cable per code and the manufacturer's instructions.
5. Relocate or shield the end detector as needed and clean or replace it.
6. Retest the full circuit device by device, verify the ground fault detection is functional, and confirm the panel is free of trouble with the unit running.

Obligations beyond the wire:

• The as-built drawings must be updated; the current set does not reflect what is in the ceiling.
• The owner and AHJ should be notified that a concealed, non-compliant installation was found and corrected. Concealing this creates liability for you and a surprise for the next inspector.
• The work is a change order against whoever directed or performed the original extension; identify the responsible party before the ceiling closes.
• Record the test results so the next inspection cycle has a baseline.