meter main breaker tripping instead of sub main breaker.... (3 phase)

[Electron_Sam78]

We had a strange situation in when i was in the Air Force. There was a building on base that lost power so we rsponded and found a blown fuse cutout on the primary side of a pole mounted transformer. A preliminary inspection didn't indicate a problem so we loaded a cutout and closed it. That immediatley blew. After a search the only thing we found was a branch circuit inside the building that had shorted. We corrected that and tried the cutout again and it held. We never had any problems after that. No one coud understand why a branch circuit short would blow a primary fuse and not the trip the 20 amp branch circuit OCPD or the service disconnect OCPD (200a) - and they weren't old FP breakers either. These were relatively new. I believe we changed out both of those after that incident.

 

[SG-1]

Exchanging the two 200A breakers may give you the selective coordination desired.

If the inrush current is high enough it may be causing the conductors to move around in the raceway. If this has be going on for years the insulation may be thin in some spots, especially if a burr exists inside the raceway. If the movement is just right a bare spot could come into contact with the raceway. This could explain why the upstream breaker is tripping.

The simpler answer is that the load has developed a problem.

 

[kwired]

Maybe the fault is between the two breakers. If that is the case the downstream breaker never sees the fault.

Meg the feeder.

Hot spot on the upstream breaker is another possibility, and it is tripping because of thermal activity. Ambient temp at each breaker location though maybe not too likely to be a problem, may be just enough to make a difference.

Is there any undersized conductors for the load carried? Especially at the breaker that trips.

You really need to try to determine if this is a short circuit/ground fault, or thermal overload causing the tripping to start with.