I came across this in an engineering book on substation design, and I have to admit I do not see the purpose, but the book seems to indicate that 4 disconnects per bay are required for a single breaker, double bus substation. My understanding is that only 3 disconnects are needed per bay, as the disconnect between the substation breaker and other two bus (selection) disconnects ends up being purely redundant.
However because the book is based with IEEE/ANSI systems in mind, I am lead to think that the other knows something that I do not (ie, NESC requirement).
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