The aircraft generator delivers a 3 phases system to A B and C busbars. Single phase equipments is fed from these respective busbars such that the power drawn through out the aircraft is evenly distributed across them.
Hence if you lose No1 generator A phase bus bar then all equipment on that bus bar is lost.
Throughout the aircraft there is 3 phase equipments fuel pumps, radio rack cooling fans etc. The effect of losing a single phase depends on the total current drawn by that equipment. If a cooling fan is running when a phase is lost it may still operate at a reduce output or trip the CB If it is not running an attempt to start it will result in a tripped CB due to an excessive starting current taken by the other 2 phases.
Hence the reason why if a a 3 phase CB pops you should reset all three
of course this is not applicable to fuel pump circuit breakers which only maintenance should reset
