c) Starting from DC External Power
A Ground Power Unit (GPU) can be connected to the DC external power receptacle (Figure
12.5-16) on the left side of the forward fuselage. The GPU can supply DC power for the
aeroplane EPGDS and for engine starting (Figure 12.5-17). Contractors connecting the main and
aux batteries to the main buses are closed automatically upon selection of engine start, as the
main and auxiliary batteries assist in the start. The standby battery is diode isolated from the left
main, to ensure an acceptable level of voltage on the ESS Busses during the starting process.
When the starting process is terminated the power source is still the external power. While DC
external power is connected to the aeroplane, the generators connections to the main buses is
inhibited by the EPCU. The main and auxiliary batteries stay connected to the main buses and
the standby battery is reconnected.
Once the DC EXT PWR switch is turned off after engine start, the generators will come on line if
the GEN switches are in the ON position. Both vertical bus ties connecting the main to the sec-
ondary buses will remain closed until DC power is available from the TRUs.
The EPCU incorporates external DC power protection from too high or too low supply of external
DC power voltage. If the external voltage is more than 31+0.5/-0.75 VDC, or less than 22 ± 1
VDC, an over/under voltage condition will cause the external ground power to stop supplying
electrical power to the aeroplane. If the external power over/under voltage is rectified, the exter-
nal power source can be reselected by moving the DC EXT PWR switch to OFF and then to EXT
PWR.
Dash8 - Q400 - Electrical
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