combiner circuit breakers

ws9876

I was looking at the wiring instructions for a CC and it displayed the various set ups. 2 panels,3 panels,2 series in parallel,etc.
I noticed that only the parallel strings diagram included 2 extra breakers in the combiner box. They were 15 amp breakers and that set up
was the only one that required those extra breakers. Other than that the wiring was basic and the same for all the rest of the possible set ups.
Why does the series/parallel combiner require those extra breakers.??   

vtmaps

Anytime you have three or more strings of panels, you must have a fuse or circuit breaker on each string.  If one string faults, the other strings could have their combined current flow through the faulted string and cause a fire.   Each string of panels has a 'series fuse rating' on the nameplate of each panel.

If you only have two strings, the circuit breakers are not necessary because the faulted string only receives current from one other string (which it can handle).  Although not necessary, it is tremendously convenient to have a circuit breaker on each of the two strings... the breaker serves as a switch for diagnostic or servicing purposes. 

--vtMaps