The main bus supplies the other electrical consumers through circuit-breakers.
Normally the main bus and the essential bus are connected via the MAIN TIE circuit-breaker,
the tie relay, and the ESS TIE circuit-breaker. In the event of electrical failure during flight, the % pilot can set the ESS BUS switch to ON to open the tie relay and separate the essential bus % from the main bus. When the ESS BUS switch is set to ON, the only the electrical consumers
which are listed above will be supplied with power.
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Electrical Power
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AVIONIC
VOLTAGE REGULATOR MAIN BUS
BUS
Figure 2: DC Electrical Load Distribution with Essential Bus (simplified)
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B. Avionic Bus and Essential Avionic Bus
The normally-closed contacts of the avionics master relay connect the main bus to the avionic bus. A 25 amp circuit-breaker protects the system. A 2 amp circuit-breaker protects the control circuit for the relay. The control circuit of the avionics master relay connects to the avionics master switch.
When the electrical system has an essential bus, then it also has an essential avionic bus. The normally-closed contacts of the essential avionics relay connect the essential bus to the essential avionic bus. A 25 amp circuit-breaker protects the system. A 2 amp circuit-breaker protects the control circuit for the relay. The control circuit of the essential avionics relay connects to the avionics master switch.
C. Battery Bus
The external power unit relay, the starter relay and the battery relay connect to the battery bus. The battery supplies power to the battery bus through the battery relay. Power from an external power source can be supplied to the battery bus through the external power relay.
D. Hot Bus
The airplane battery supplies power to the hot bus from the battery side of the battery relay coil. Items of electrical equipment which are connected to the hot bus are protected by fuses.
E. Circuit-Breakers
Circuit-breakers protect the wiring in each circuit from too much current. The circuit-breakers are
% located on the instrument panel, see Figures 3 and 4. You can open and close all the circuit-breakers manually. If too much current flows in a circuit, the related circuit-breaker opens automatically.
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% Figure 3: Circuit-Breaker Locations for Larger Instrument Panels
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3. Operation
When the ALT/BAT switch is set to ON, it gives a ground to the battery relay coil. The battery relay closes and connects the battery to the main bus through the battery bus and the battery circuit-breaker.
When the AVIONICS MASTER switch is set to OFF, the control circuit is closed. If there is power on the main bus, the avionics master relay energizes. The master relay contacts open. The avionics bus disconnects from the main bus.
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% % % % % % % % % % % % % % % % % % % % % % Figure 4: Circuit-Breaker Locations for Smaller Instrument Panels
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Trouble-Shooting
1. General
Make sure that the alternator is supplying power and that there is 28 VDC on the Main Bus. Refer to Section 24-30. For faults on a piece of equipment, or a system, refer to the applicable Chapter. For example, for no oil pressure indication, see Chapter 77 Engine Indicating. The table below lists the defects you could have with the DC load distribution system. If you have the trouble detailed in the Trouble column, read across to the Possible Cause column. Then do the repair given in the Repair column.
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