Intermittent Red-Fail Trips on a TS2 Conflict Monitor
A NEMA TS2 cabinet at a six-phase intersection with LED signal heads has logged four conflict-monitor trips in three weeks, every one during or within a day of rain. The event log shows 'Red Fail, Phase 4' each time. The field crew swapped in a spare monitor; the trips stopped for eleven days, then resumed with the same fault on the spare unit. The original monitor, a 16-channel unit, is now on your bench with a work order that says test it and return it to service if it is good. The cabinet is a pole-mounted Type 1 with a 332 controller. Phase 4 is a protected-permissive left turn with a 12-inch LED head on a mast arm, and its red load switch is two years old. The cabinet has a history of water intrusion; the door gasket was replaced last spring, but the floor shows rust stains and the intersection is currently on flash.
1. What bench tests will you run on the returned monitor, and what result would make you condemn it rather than return it to service?
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Bench testing has to answer one question: can this unit produce the logged fault on its own? Work through it in a fixed order.
- Incoming inspection. Look at the edge connector for water tracks, green corrosion, or a pushed-back pin; check the cover gasket and the inside of the cover for condensation. A rain-correlated fault with a corroded pin is a bench finding, not a field one.
- Configuration check. Compare every DIP switch and jumper with the cabinet configuration sheet: red-fail enabled or disabled, 50/60 Hz, conflict flash rate, latching. A monitor set up for the wrong mode can trip on a healthy field circuit.
- Channel-by-channel test with a TS2 signal tester. For each channel, apply red, then remove it, and confirm the unit trips on red fail within the required time. Apply conflicting greens to confirm conflict detection.
- Soak and log check. Run it warm for several hours and compare event-log time stamps to a known clock.
Condemn the unit if any channel fails to detect or falsely detects, if the connector is damaged beyond a pin replacement, or if it will not hold configuration. A clean pass does not prove the intersection is fixed.
2. The monitor passes every bench test. What does the rain correlation tell you, and what field measurements do you request before it goes back in the cabinet?
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The pattern is the evidence. A spare monitor tripped with the same fault, so the monitor is probably not the cause; rain is. The bench pass and the field recurrence together point to moisture-related leakage or an intermittent open on the phase 4 red circuit.
Request from the field crew:
- Voltage at the monitor's phase 4 red input while the phase is red, measured at the MMU connector, and again at the load switch output. A sag or dropout under load identifies the load switch or the wiring.
- Insulation resistance on the phase 4 red conductor to ground and to neutral, power off, load switch and head disconnected, using a megohmmeter. Compare with the other phases; a reading far below the others is the defect.
- Visual inspection of the phase 4 head: gasket, terminal block, LED module connector, and water stains. Open the pole handhole too; water there can wick into the head.
- Cabinet environment: heater operation, door gasket, floor rust, and whether the cabinet drains. Moisture inside the cabinet can bridge the monitor connector.
Repair the field defect, then run the phase for a week before returning the bench monitor to that intersection.
3. Write the disposition for the monitor and explain the judgement behind it.
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Disposition: tag the monitor as passed bench, do not install at the phase 4 intersection; return it to spare inventory only after the field repair is verified. Record the bench results, the tester used, and the configuration settings in the maintenance file.
The judgement is that a passing bench test and a field fault are not contradictory. The monitor is doing its job by detecting a real field condition. Putting it back before the moisture path is repaired just restarts the trip cycle, erodes confidence in the signal, and may tempt someone to disable the red-fail function, which is the worst outcome.
If the field crew cannot find the moisture path with the requested measurements, escalate to a cabinet inspection and a night-time thermal check rather than swapping monitors again.