Supervisory Signal on a Class B Initiating Circuit After a Renovation
A 40,000 sq ft warehouse was renovated: a new storage mezzanine was added and two smoke detectors were relocated on the same conventional Class B initiating device circuit (IDC). The panel is an addressable panel using conventional zone modules. Two weeks after turnover, the panel logs intermittent 'open circuit' supervisory troubles on Zone 7 that appear during the afternoon and clear overnight. The zone module supervises with an end-of-line resistor at the last device. During a site walk, you find the relocated detectors were spliced with wire nuts inside a junction box mounted to the underside of the new steel mezzanine, and the circuit conductor is run in flexible conduit across a doorway that gets heavy forklift traffic. All devices in Zone 7 test normal when individually activated at the detector. The building has no sprinkler system. The owner reports the trouble began when the mezzanine racking was loaded.
1. What is the most likely cause of the intermittent supervisory trouble, and why does it clear overnight?
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An intermittent open on the IDC, not a device failure. The zone module only reports normal when the full supervised loop is continuous through the EOL resistor; a marginal splice or a pinched/strained conductor in the flexible conduit will read open when the circuit is disturbed.
Reasoning:
- Every device tests normal individually, so the detectors and module are functioning. The fault is in the wiring between devices, which is exactly what Class B supervision is designed to detect.
- The trouble correlates with forklift traffic at the doorway and with loading the mezzanine racking. Mechanical flexing of the flexible conduit run and vibration at the wire-nut splices under the steel mezzanine are consistent with a conductor that opens under movement.
- It clears overnight because traffic and vibration stop; the conductor re-contacts and the EOL supervision reads normal again. Intermittent + load-correlated + location-correlated is the signature of a mechanical/wiring defect, not an electronic one.
This is a judgement call: the temptation is to replace detectors, but the pattern points to the raceway and splices.
2. The installer proposes simply re-twisting the wire nuts and adding a junction box cover. Evaluate this and state what you would require before signing off.
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Re-twisting is a cosmetic fix that does not address the root cause and will likely recur.
What I would require:
- Open and inspect the splice under the mezzanine. Wire nuts on solid circuit conductors are acceptable only if the conductors are properly twisted and the connection is mechanically and electrically sound; a splice that has been flexing is suspect and should be remade or replaced with a listed connector.
- Relieve the mechanical strain at the doorway. Flexible conduit crossing a forklift doorway is being flexed every pass. The run should be re-supported and protected (rigid raceway, guard, or rerouting) so the conductors are not repeatedly bent. Verify no conductor is pinched or nicked.
- Re-verify supervision end to end after repair: confirm the EOL resistor is present and correct at the last device, then confirm the panel reports the zone normal and that a deliberate open at the far end produces a trouble.
- Document the repair and the retest, and note the cause so a recurrence is recognized quickly.
The key point: restoring continuity is not the same as correcting a defect that will fail again under the same traffic.