also connect to the battery bus and eventually to the battery.  The essential DC bus (top left) can be fed from DC bus 1  or the essential TR. A diode prevents the essential DC bus  from powering DC bus 1. The essential DC bus receives  power from the essential TR, which receives power from the  essential AC bus. This provides an extra layer of redundancy  since the essential AC bus can be isolated and fed from any  main generator.  Figure 9-105  shows generator 3 powering  the essential AC bus.  A split-parallel bus basically employs the best of both split-  bus and the parallel-bus systems. The split-parallel system  is found on the Boeing 747-400 and contains four generators  driven by the main engines and two APU-driven generators.  The system can operate with all generators in parallel, or  the generators can be operated independently as in a split-  bus system. During a normal flight, all four engine-driven  generators are operated in parallel. The system is operated  in split-bus mode only under certain failure conditions or  when using external power. The Boeing 747-400 split- parallel system is computer controlled using four GCU and  two BPCU. There is one GCU controlling each generator;  BPCU 1 controls the left side bus power distribution, and  BPCU 2 controls the right side bus power. The GCUs and  BPCUs operate similarly to those previously discussed under  the split-bus system.  Figure 9-106  shows a simplified split-parallel power  distribution system. The main generators (top of  Figure 9-106 )  are driven by the main turbine engines. Each generator is  connected to its load bus through a generator control breaker  (GCB). The generator control unit closes the GCB when the  pilot calls for generator power and all systems are operating  normally. Each load bus is connected to various electrical  systems and additional sub-buses. The BTB are controlled  by the BPCU and connect each load bus to the left and right  sync bus. A split systems breaker (SSB) is used to connect  the left and right sync buses and is closed during a normal  flight. With the SSB, GCBs, and BTBs, in the closed position  the generators operate in parallel. When operating in parallel,  all generators must be in phase.  If the aircraft electrical system experiences a malfunction,  the control units make the appropriate adjustments to ensure  all necessary loads receive electrical power. For example, if  generator 1 fails, GCU 1 detects the fault and command GCB  1 to open. With GCB 1 open, load bus 1 now feeds from  the sync bus and the three operating generators. In another  example, if load bus 4 should short to ground, BPCU 4 opens  the GCB 4 and BTB 4. This isolates the shorted bus (load  bus 4). All loads on the shorted bus are no longer powered,  and generator 4 is no longer available. However, with three  remaining generators operational, the flight continues safely.  As do all large aircraft, the Boeing 747-400 contains a DC  Split-Parallel Systems Figure 9-106.  Split-parallel distribution system.  GEN 1 GEN 2 GEN 3 GEN 4 GCB 1 BTB 1 AC LOAD BUS 1 GCB 2 BTB 2 AC LOAD BUS 2 LEFT SYNC BUS GCB 3 BTB 3 AC LOAD BUS 3 GCB 4 BTB 4 AC LOAD BUS 4 APU 1 EXP APB External power 1 RIGHT SYNC BUS APU 2 EXP APB External power 2 SSB 9-64