The distribution of DC and AC power throughout the system  is accomplished through the use of power distribution buses.  These “buses” as they are called are actually common  terminals from which individual electrical circuits get their  power.  [Figure 14-9] Power distribution buses are protected from short circuits  and other malfunctions by a type of fuse called a current  limiter. In the case of excessive current supplied by any  power source, the current limiter opens the circuit and thereby  isolates that power source and allows the affected bus to  become separated from the system. The other buses continue  to operate normally. Individual electrical components are  connected to the buses through circuit breakers. A circuit  breaker is a device that opens an electrical circuit when an  excess amount of current flows. Buses are usually named for what they power (avionics  bus, for example) or for where they get their power (right  generator bus, battery bus). The distribution of DC and AC  power is often divided into functional groups (buses) that give  priority to certain equipment during normal and emergency  operations. Main buses serve most of the airplane’s electrical  equipment. Essential buses feed power to equipment having  top priority.  [Figure 14-10] As previously stated, a turboprop airplane flies just like any  other piston engine airplane of comparable size and weight.  It is the operation of the engines and airplane systems that  makes the turboprop airplane different from its piston engine  counterpart. Pilot errors in engine and/or systems operation  are the most common cause of aircraft damage or mishap.  The time of maximum vulnerability to pilot error in any gas  turbine engine is during the engine start sequence.  Multiengine turboprop airplanes normally have several power  sources—a battery and at least one generator per engine. The  electrical systems are usually designed so that any bus can be  energized by any of the power sources. For example, a typical  system might have a right and left generator buses powered  normally by the right and left engine-driven generators.  These buses are connected by a normally open switch, which  isolates them from each other. If one generator fails, power is  lost to its bus, but power can be restored to that bus by closing  a bus tie switch. Closing this switch connects the buses and  allows the operating generator to power both. Turbine engines are extremely heat sensitive. They cannot  tolerate an over temperature condition for more than a very  few seconds without serious damage being done. Engine  temperatures get hotter during starting than at any other time.  Thus, turbine engines have minimum rotational speeds for  introducing fuel into the combustion chambers during startup.  Vigilant monitoring of temperature and acceleration on the  part of the pilot remain crucial until the engine is running at a  stable speed. Successful engine starting depends on assuring  the correct minimum battery voltage before initiating start or  employing a ground power unit (GPU) of adequate output. After fuel is introduced to the combustion chamber during  the start sequence, “light-off” and its associated heat rise  occur very quickly. Engine temperatures may approach the  maximum in a matter of 2 or 3 seconds before the engine  stabilizes and temperatures fall into the normal operating  range. During this time, the pilot must watch for any tendency  of the temperatures to exceed limitations and be prepared to  cut off fuel to the engine. An engine tendency to exceed maximum starting temperature  limits is termed a hot start. The temperature rise may be  preceded by unusually high initial fuel flow, which may be  the first indication the pilot has that the engine start is not  proceeding normally. Serious engine damage occurs if the  hot start is allowed to continue. A condition where the engine is accelerating more slowly  than normal is termed a hung start or false start. During a  hung start/false start, the engine may stabilize at an engine  Figure 14-9.  Typical individual power distribution bus. 14-9 Operational Considerations GEAR WARN TRIM INDICATOR TRIM ELEVATOR TRIM AILERON STALL WARNING ACFT ANN-1 L TURN & BANK TEMP OVRD HP EMER L & R FUEL QUANTITY L ENGINE GAUGE R ENGINE GAUGE MISC ELEC LDG LT MOTOR BLEED L WSHLD L LIGHTS AUX FUEL FLOW Power Distribution Bus 5 5 3 5 5 5 5 5 5 5 5 5 5 5 5 3 3 5