Applicable Systems
MNS-MCC
Symptoms
The motor feeder breaker trips immediately on start-up. The red trip indicator on the breaker front panel pops out and the overload relay status LED turns solid red.
Possible Causes
- Locked rotor condition due to mechanical jam or seized bearing
- Severe overload exceeding the breaker's instantaneous trip setting
- Short-circuit fault downstream between breaker and motor terminals
- Incorrect breaker rating selected for the motor full-load current
Troubleshooting Steps
- Step 1: Verify the motor shaft rotates freely by hand; inspect coupling and driven equipment
Shaft turns smoothly with no binding or unusual resistance - Step 2: Measure motor insulation resistance with a 500V megohmmeter
Phase-to-ground and phase-to-phase resistance exceeds 1 MOhm - Step 3: Check the breaker instantaneous trip setting against the motor locked-rotor current
Setting is within 8-15x motor FLA per IEC 60947 - Step 4: Inspect cable terminations at the motor and breaker for signs of arcing or damage
All lugs are tight, insulated, and show no carbon tracking
How to Reset
Reset the breaker by pressing the blue reset button on the overload relay after the cooling period elapses. Close the feeder breaker only after confirming the fault cause is resolved.
Common Replacement Parts
- 1SFA619550R1001: ABB TA75 thermal overload relay (Verify coil voltage and motor FLA range before ordering)
Frequently Asked Questions
- Q: What is the cooling period for the TA75 relay?
The TA75 requires approximately 5-10 minutes of cooling after a severe trip before the bimetal resets enough to allow manual reset.
- Q: Can I increase the breaker size to prevent nuisance trips?
No. The breaker must protect the cable and motor. If nuisance trips occur, investigate motor or load issues rather than oversizing protection.
Source: ABB MNS-MCC Technical Manual, Edition 04/2022
Category: Electrical