Boeing Electrical System

Primary (Main) Power Source
A primary source is equipment that
generates electrical power from energy
other than electrical, and is independent
of any other electrical source.

Secondary Source
A secondary source is equipment that
transforms and/or
converts primary
source power to
supply electrical
power to either AC
or DC powered
equipment .

Emergency Power Source

In the event of a primary power source failure, emergency power is usually provided from independent auxiliary power unit (APU)-driven generator(s), ram air or hydraulically-driven generator(s), or batteries.
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