When 2 circuit breakers are in series on a Main Incomer - WHY would they need to be COORDINATED ?

[bdn2004]

We've got a 12.47kV Utility feeder to a switchgear that has a main circuit breaker. And there are 5 feeder breakers. There is bus differential protection on the switchgear bus.

In order to properly coordinate any of the feeder breakers with the Main Breaker there's supposed to be at minimum 0.15 seconds between the relay settings (Main Breaker vs. feeders) at the fault current they both see on a fault. That's the way I understand it.

Upstream of the main is the Utility's circuit breaker. I understand there needs to be coordination with the feeder breakers and the Main Breaker. Any feeder needs to trip before the Main Breaker does. But I don't see the need to delay - as in coordinate the Utility with the Main Breaker. The only time these two would be in the circuit alone and a fault - is if there were a bus fault, and the bus differential will react and trip the Main Breaker. If the bus differential didn't work, the Main Breaker and the Utility's breaker would back it up.

What I'm suggesting is to set the Main on the switchgear to the exact same settings as the Utility.
Is this the way this is generally done? If not why?

 

[Palletjack]

With that new information,I wouldn’t worry about the coordination.
And I work for a utility.