12.5.6.5 Fault Tolerant Operation
The DC Power subsystem is extremely tolerant to power source failures. It has four separate
sources of DC power in flight. They are the two generators and two TRUs. When all DC power
sources are operational, each source powers its own dedicated bus. When one or more source
failures are detected, the EPCU will automatically reconfigure the power flow.
The EPCU monitors the system for failures. Once a failure is detected, the EPCU will cause the
bus system to reconfigure and ensure mission completion with minimal effect for the EPGDS and
for the utilization equipment power supply.
Four bus tie contactors are controlled by the EPCU that connect the appropriate feeder buses
together when there are one or more inoperative DC power sources. The EPCU ensures opera-
tion of all DC buses with loss of up to two power sources. Automatic load shedding will occur only
after more than two DC source failures.
The four contactors connect the following feeder buses:
• L Main to R Main
• L Secondary to R Secondary
• L Main to L Secondary
• R Main to R Secondary
For example, if one DC generator fails, the L to R Main bus tie closes and the other generator
then powers both main feeder buses. If one TRU fails, the L to R secondary bus tie closes, and
the other TRU then powers both secondary buses. If both DC generators fail, the vertical bus tie
close, and the TRUs supply DC power to both the secondary and main buses. If both TRUs fail,
the vertical bus tie close, and the DC generators supply power to both secondary buses.
The EPCU operates in emergency mode when:
• the aeroplane is in air and
• both DC Generators are not available and
• at least one TRU is not available
When the EPCU operates in emergency mode, it automatically disconnects the batteries from
the main buses. The main buses are not powered in this mode. In case of emergency operation
the batteries are the main energy sources for the aeroplane. If an engine start attempt is made
during an emergency condition, the batteries will automatically be reconnected to the main buses
for the duration of the start attempt.
12.5.6.6 Bus Fault Protection
The EPCU and the DC GCUs protection function protects the DC subsystem against short cir-
cuits on the main and secondary buses. The selectivity and protection coordination is ensured by
the logic, detection and by reaction timing in these units.
Dash8 - Q400 - Electrical
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