Series Vs. Fully rated

[don_resqcapt19]

MO_EE,
Under the conditions you stated, the fault being right at the load side of the second breaker, isn't it very possible that the fault current would be in the instantanuous trip range of both breakers making selective coordination impossible even with a fully rated system?
Don

 

[jim dungar]

The system must be analyzed as if the fault is directly on the load side of the downstream breaker.

I have never seen this recommendation before. Do you have a reference or this is your opinion?

The vast majority of faults in existing sytems are arcing faults to ground. I find no data that says they are likely to occur immediately at the load side of a breaker. Of course it would be extremely likely, if the breaker is being worked on while energized.