Alternator output to bus Alternator output to bus Left alternator A+ F Generator control A+ F Right alternator Paralleling switch To voltage regulator and circuit breaker To voltage regulator and circuit breaker Paralleling coil Paralleling coil Voltage coil Voltage coil Left voltage regulator and alternator Right voltage regulator and alternator Figure 9-100.  Vibrating point system used for paralleling alternators. The primary power supplied for this aircraft is DC, although  small amounts of AC are supplied by two inverters. The  aircraft diagram shows the AC power distribution at the top  and mid left side of the diagram. One inverter is used for main  AC power and the second operated in standby and ready as a  backup. Both inverters produce 26-volt AC and 115-volt AC.  There is an inverter select relay operated by a pilot controlled  switch used to choose which inverter is active.  contemporary aircraft. The different types of voltage  regulators were described earlier in this chapter.  The power distribution systems found on modern multiengine  aircraft contain several distribution points (buses) and a  variety of control and protection components to ensure  the reliability of electrical power. As aircraft employ more  electronics to perform various tasks, the electrical power  systems becomes more complex and more reliable. One  means to increase reliability is to ensure more than one  power source can be used to power any given load. Another  important design concept is to supply critical electrical  loads from more than one bus. Twin-engine aircraft, such  as a typical corporate jet or commuter aircraft, have two  DC generators; they also have multiple distribution buses  fed from each generator.  Figure 9-101  shows a simplified  diagram of the power distribution system for a twin-engine  turboprop aircraft. The hot battery bus (right side of  Figure 9-101)  shows a direct  connection to the aircraft battery. This bus is always hot if  there is a charged battery in the aircraft. Items powered by this  bus may include some basics like the entry door lighting and  the aircraft clock, which should always have power available.  Other items on this bus would be critical to flight safety, such  as fire extinguishers, fuel shut offs, and fuel pumps. During  a massive system failure, the hot battery bus is the last bus  on the aircraft that should fail.  If the battery switch is closed and the battery relay activated,  battery power is connected to the main battery bus and the  isolation bus. The main battery bus carries current for engine  starts and external power. So the main battery bus must  be large enough to carry the heaviest current loads of the  aircraft. It is logical to place this bus as close as practical to  the battery and starters and to ensure the bus is well protected  from shorts to ground.  This aircraft contains two starter generator units used to start  the engines and generate DC electrical power. The system  is typically defined as a split-bus power distribution system  since there is a left and right generator bus that splits (shares)  the electrical loads by connecting to each sub-bus through  a diode and current limiter. The generators are operated in  parallel and equally carry the loads.  Power Distribution on Multiengine Aircraft  9-58