Conflict Monitor Flash and Detection Faults at Turn-On
At a new eight-phase intersection, the controller is powered and the conflict monitor goes into flash whenever phases 2 and 6 receive simultaneous vehicle calls. The contractor's electrician has meggered the home-run feeder to phase 2 and reads 0.5 MΩ to ground; the other phases read above 100 MΩ. The loop lead for the phase 2 detector is 1,100 ft of #14 AWG, run in the same conduit as the signal conductors. The contractor wants to turn the intersection on using recall on phases 2 and 6 and fix the detection later. The agency's turn-on checklist requires all detection and conflict-monitor tests to pass.
1. Why does the conflict monitor flash when phases 2 and 6 receive simultaneous calls, and how do you isolate the cause?
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A conflict monitor flash means the monitor detected a fault, such as conflicting green outputs, a short, or a misprogrammed channel. Simultaneous calls alone should not cause a flash if the phases are compatible, so check whether both green load switches are actually energized. Use the monitor's event log and channel status, then isolate by removing field wires one at a time. Look for miswired load switches, a field short, a crossed conductor, or a monitor configuration error. Do not simply reset the monitor; find the fault and correct it before turn-on.
2. The phase 2 home run measures 0.5 MΩ to ground. What does this indicate and what should be done?
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A reading of 0.5 MΩ to ground is far below the usual minimum insulation resistance and indicates damaged insulation or moisture in the conductor or conduit. The fault is likely at a pull box, a splice, a conduit entry, or a nick in the jacket. Isolate the conductor, inspect the run, and repair or replace the damaged section. Do not energize the circuit until it meggers at an acceptable value and the inspector has reviewed the repair. A low megger reading is a defect, not a normal field variation.
3. The loop lead is 1,100 ft; what problems can this create and what are the options? Can the intersection be turned on before detection is repaired?
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A 1,100-ft loop lead of #14 AWG has significant resistance, roughly 3.4 Ω for the loop, which may exceed the detector's lead-length or resistance limit and cause weak or unstable detection. Options include using a larger conductor, relocating the detector closer to the loop, or using a detector designed for long leads. Twisted pair may reduce electrical noise, but it does not eliminate the resistance. The intersection should not be turned on before detection and conflict-monitor tests pass if the agency's checklist requires it. Recall is an operational mode, not a repair; using it to mask a failed detection circuit would leave a known defect in service and should require written owner approval.