Substation LV Circuit Breaker Tripping when energizing load on adjacent Circuit

[Arentsch]

We have two 1600A adjacent feeder circuit breakers(CBA3 and CBA4)on a rooftop substation. Both CBA3 and CBA4 feed busducts BD3 and BD4 respectively. Last night CBA4 opened on an Instantaneous fault of 7kA. All loads off of CBA4 were de-energized as expected. However, we also found that a machine fed from a 500A busplug on BD3 had lost it's control power but still had its line power. CBA4 was reset and re-closed. All loads were brought back up one at a time with no issues. Approx. 5 minutes later CBA4 tripped again. The CBA4 trip unit was replaced and CBA4 was re-closed. Again all loads were successfully brought back on one at a time and approx. 5 minutes later CBA4 tripped again. This time it was decided to open and lockout the machine fed from the 500A busplug on BD3 (see the orange box on attached one-line). CBA4 was reset and re-closed and has not tripped again. We would like to get the offending machine back up and running. Any troubleshooting advice or plausible explanations are greatly appreciated. Attached is the one line.

 

[jghrist]

Can you re-energize the machine that you locked out and make some load recordings? Also, can you provide a simplified one-line? I can't find the breakers and buses that you refer to.

 

[JezNZ]

1. Reviewing the Single Line, I see some notes that give indication this drawing may be retrospective and has some possible inaccuracies?

2. Is it the conclusion that the control power on the machine is fed from the wrong source (BD4 instead of BD3)? Seems to me..

3. Does the machine have a start-up sequence that takes longer than 5 minutes?

4. How was it determined that the machine still had line power? The machine doesn't appear to have any large motors or drives that might backfeed power for several minutes, but it also has a 500A feed that doesn't seem to match the demand...

 

[RRaghunath]

"we also found that a machine fed from a 500A busplug on BD3 had lost it's control power"
There seem a few unknown things here and going in to more detail may bring out important clues.
1) What do you mean by 'we also found'. Do you mean the control power is not supposed to have been taken from BD-4?
2) If control power is taken from BD-4, suppose the machine stops on loss of control power when CBA-4 is OFF.
Fault current (recorded in CB trip unit) of 7 kA suggests there was a genuine fault. The CBA-4 was stable after said control power connection was disconnected suggests that fault is in that control circuit. But, why the protection (Fuse or whatever) is a question that needs to be explored.
Also, what does 1600/600A mean in the SLD for CBA-3&4?
To me, CB rating of 1600A with protection trip unit rated for 600A fitted.