Applicable Systems
MNS-MCC
Symptoms
The main circuit breaker has failed to close after a trip. The operating mechanism does not latch and the breaker handle springs back to the tripped position.
Possible Causes
- Breaker mechanism spring broken or fatigued, preventing latching
- Trip unit internal fault causing the breaker to immediately re-trip on close
- Mechanical binding in the linkage due to lack of lubrication or corrosion
- Breaker contacts welded from interrupting a large fault current
Troubleshooting Steps
- Step 1: Attempt to charge the breaker spring mechanism using the manual charging handle
If the mechanism does not charge, the spring is broken; replace the breaker - Step 2: If the mechanism charges but does not latch on close, inspect the trip linkage
Look for bent, worn, or missing linkage components - Step 3: Use the breaker test function to attempt a no-load close
If the breaker trips immediately, the trip unit may be faulty; try resetting the trip unit - Step 4: If the contacts are welded, the breaker cannot be safely reused
Replace the breaker; do not attempt to force welded contacts apart
How to Reset
Reset: After the mechanism is repaired or the breaker is replaced, reset the trip unit and close the breaker.
Common Replacement Parts
- Circuit breaker: Molded-case or air-frame type (Match frame size, interrupting rating, and trip unit)
- Breaker lubricant: OEM-specified grease (Do not use petroleum-based grease on plastic components)
- Trip unit: Microprocessor or thermal-magnetic (Match to existing breaker model)
Frequently Asked Questions
- Q: How many operations is a breaker rated for?
Molded-case breakers typically rated for 10,000 mechanical operations. Air-frame breakers: 5,000. Track operation counts for predictive maintenance.
- Q: What causes welded contacts?
Interrupting a fault current larger than the breaker's interrupting rating, or slow clearing time allowing the arc to persist and weld the contacts.
- Q: Can I service a breaker mechanism?
On air-frame breakers, yes, with OEM service kits. On molded-case breakers, the mechanism is sealed and the breaker must be replaced.
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Source: OEM Technical Manual
Category: Electrical
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