TL;DR

Nuisance trips with measured current well below the trip setting point to a trip unit fault or CT calibration drift. Verify with a secondary injection test and replace the trip unit if it fails.

What you might see

breaker trips at current well below its overload setpoint
trip indicator shows overload but current is normal
trips occur at consistent times regardless of load level
replacing the trip unit with a known-good unit stops the trips

Likely causes

Tap to mark as ruled out

Trip unit electronic module drifted out of calibration, triggering at incorrect current levels
CT (current transformer) feeding the trip unit degraded, sending an inflated current signal
Trip unit damaged by a transient or moisture, giving erratic trip behavior
Trip unit setpoints incorrectly adjusted after a recent maintenance event

Required tools

Clamp ammeter (true-RMS, log-on-trip capable if available)
Secondary injection test set
Replacement trip unit (correct frame size and rating)
Arc-flash PPE
LOTO kit

Safety first

Secondary injection testing requires opening the breaker control wiring with the bucket in TEST position. Follow lock-out procedure and use insulated tools.
Never bypass a tripping breaker to keep a motor running. A trip unit that appears to be nuisance-tripping may be detecting a real fault.

Procedure

7 steps · ~60 min

1
During the next trip event, immediately clamp an ammeter on the load wire and record the actual current at the moment of trip. Compare to the breaker's overload setpoint.
2
Check the trip unit setpoint dials or digital settings against the motor nameplate FLA and the overload protection design basis.[1]
3
If setpoints are correct and current is below the trip threshold, perform a secondary injection test on the trip unit. Inject a calibrated current through the CT terminals and verify the trip unit responds within specification.[1]
4
If the trip unit fails the secondary injection test, replace it with a new Square D trip unit of the correct rating for the breaker frame size.[1]
5
If the trip unit passes the injection test, inspect the CTs for damage, loose connections, or shorted secondary windings. A shorted CT delivers inflated apparent current to the trip unit.
6
After replacement, run the motor at normal load for 30 minutes and confirm no spurious trips.
7
Document the failure mode and the replacement in the MCC bucket maintenance record.

Sources

Square D Model 6 Motor Control Center Instruction Bulletin, Square D (Schneider Electric)
Square D Model 6 MCC installation and maintenance instructions, trip unit calibration and secondary injection test procedures (general)
View source

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