being shown in phantom the 2n configuration offers many levels of power supply redundancy crus may draw power equally from either rail a or rail b each rail is energized by a separate pair of bulk power supplies the bulk power supplies themselves have independent power sources in the event that ac power to both pdus is terminated both rails will remain independently powered by the two pairs of batteries in the n 1 configuration power supply redundancy is somewhat reduced but still significant crus may still draw power from either rail a or rail b rail a is powered by at least three bulk power supplies thereby permitting one bulk power supply to be disabled without affecting the power supply to the rail rail b is powered by at least three battery units thereby permitting one battery unit to be disabled without interrupting power supply to that rail the pdus may be tandem part no 112309 or similar units the dc bulk power supply units may be tandem part no 112314 or similar units the battery units may be tandem part no 112316 or similar units fig3 shows pdus installed in slots a and b in the 2n configuration each pdu has a four.