747 classic bus #3 isolation
 

With DC essential power switch moved to position 1,2 or 3, the #3 isolation relay opens automatically, (but the #3 switch does not move). Does this leave only DC #3 and essential to run the show while #'s 1 and 2 are isolated. If so may I request enlightenment as to why this is?

Power supplies are isolated, but independantly functional in LAND mode ( coupled ). I'm not certain that this is what you are asking but it may answer your question.

Inept, there is no DC essential power selector switch on the Classic. There is however an AC Essential power switch which selects the source for the Essential AC bus and is usually selected to Normal position. DC bus isolation relay opens automatically if the ESS AC is selected to Gens 1, 2 or 3 and forms the DC buses into two parallel pairs. You are getting confused with the three DC Bus Isolation switches, one of which is the Essential (DC) bus isolation switch.

roger that
 

Thanks to the both of you for replying. I was looking at page 10.30.04 in the old 100 manual and saw the word DC Essential, when in fact it is only DC isolation.Sorry about that.
Hence "Inept".. I cant find a schematic for the individual DC busses but would it be safe to assume it is for Pilot/ FO instrumentation redundency and such.
thanks...........