Conflict-Monitor Trips Every Afternoon After a Head Replacement
A four-legged intersection drops to flashing almost every afternoon around 14:00 on warm days. Three weeks ago a contractor replaced a span-mounted three-section head and its cable at the far-side pole. The conflict monitor event log shows repeated field-check faults on two channels and one voltage-monitor event. An automatic restart puts the intersection back in stop-and-go each time, so drivers see only a brief flash. The crew reused the existing terminal block in the pole handhole, left 6 in of extra conductor coiled in the handhole, and the handhole cover gasket is missing. The cabinet is a NEMA TS-2 Type 2 with a 170 controller. The agency needs a diagnosis before the next afternoon peak.
1. Describe the fault-isolation sequence that separates field wiring from load switches and monitor.
Read the worked answer
Work from the log outward; do not swap parts first.
- Capture the evidence. Download and save the full monitor event log and the controller's event log with timestamps and channel numbers before anything is reset. Note which two channels fault and whether they belong to the same head, the same pole, or the same multi-conductor cable. That relationship is the whole investigation.
- Correlate with conditions. Note the time of day and weather. Afternoon-only failures on a warm day suggest thermal expansion, moisture, or a marginal connection that opens when it heats - not a hard failure.
- Test the field/controller split. Put the controller in a safe test state, remove the field wiring from the terminal facility, and exercise each channel into a test load while watching the monitor. If the fault follows the field harness, the problem is outside the cabinet. If it follows the channel with the field removed, the load switch, the terminal facility, or the monitor programming is implicated.
- Then, and only then, interrogate the field. Insulation resistance test the suspect cable with the circuit isolated, inspect each conductor termination in the handhole and at the head, look for nicked insulation, pinch points at the handhole throat, and standing water.
- Confirm and document. Re-terminate, dry, seal, and re-test; then run a soak test through the same time-of-day window that produced the failures.
2. Given repeated field-check faults on two channels, what is the most probable defect, and how would you confirm it in the field?
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Most probable: a conductive path or a wiring error in the field circuit, not a failed LED module.
A field-check fault means the monitor saw voltage on a channel it did not command on, or failed to see voltage on a channel it commanded on. The two classic causes are:
- A short between conductors - typically a chafed conductor against the pole wall or a pinch at the handhole throat, with moisture completing the path. The missing handhole gasket is a strong supporting fact, and the afternoon pattern fits condensation and heat cycling.
- Cross-connected conductors in the reusable terminal block when the crew re-terminated the new cable - the same fault appears on two channels at once because two circuits are tied together or swapped.
Confirmation steps:
- With the circuit de-energized and isolated, measure resistance channel-to-channel and channel-to-ground on the suspect cable. Any finite reading where you expect open circuit confirms the short.
- Insulation resistance test the cable at the agency-specified test voltage; expect a high reading (megohms) on a sound cable. A low reading is a failed cable.
- Inspect the handhole and head interior for water, corrosion, and scuffed insulation; check the head's internal wiring and sockets.
- Verify the terminal assignments against the cabinet wiring diagram - two channels faulting together is the signature of a swap.
Repair: replace the damaged conductor or cable section, install the missing gasket, dress slack so it does not sit in water, re-test, and run a 24-hour soak test.
3. What documentation and follow-up would you require before closing the work order, and how do you verify the monitor was reprogrammed correctly?
Read the worked answer
Close the work order on evidence, not on the absence of a repeat failure for one afternoon.
Required records:
- Saved monitor and controller event logs from before and after the repair, with timestamps showing the fault stopped.
- Insulation resistance and continuity readings with the meter used, the test voltage, and the technician's name.
- Photographs of the handhole interior before and after re-termination, and of the gasket installation.
- The soak-test interval and result, covering at least one full afternoon cycle.
Verifying the monitor configuration after any replacement or reprogramming:
- Compare the monitor's channel assignment, channel-type settings, and enable/disable table against the intersection's cabinet wiring diagram, line by line. A monitor that is correctly programmed but assigned to the wrong channel type will either nuisance-trip or fail to protect.
- Confirm the conflict and clearance tables match the intersection's phase sequence, including any overlap or protected left-turn arrangement.
- Confirm the automatic restart setting is what the agency intends; an auto-restart can hide an active fault by returning the signal to stop-and-go with no one on site.
- Put a copy of the verified configuration in the cabinet and in the maintenance file. Configuration drift after a repair is one of the most common findings on later inspections and the hardest to reconstruct.