This is me, having too much free time and deciding to learn how the electrical system of the Space Shuttle worked. So, because I couldn't find any "easy to read" diagram online, I built a high-level diagram of the electrical buses by using the SCOM as a reference:
Full size available here.
So, let's recap what we see in the diagram:
- 5 possible sources: 3 fuel cells, 1 Ground Support Equipment, 1 power from ISS
- 23 DC buses: 3 main buses, 9 control buses, 3 essential buses, 6 payload+cabin buses, 2 pre-flight buses
- 2 DC buses for the 120V payload (OBSS and the ISS payload)
- 3 AC inverters
- 5 AC buses (3 output of the inverter + 2 AC payload buses)
Most DC buses have a triple-redundant source (for instance ESS 3AB can be powered by fuel cell 3, main bus A, and main bus B).
As a comparison:
- An Airbus A380 has only 4 DC buses
- A Boeing 747 has only 8 DC buses (including 2 APU buses)
What surprises me in the space shuttle is the extreme high number of DC buses. Why nine (9!) triple-redundant control buses? Why 6 different payload buses?
Obviously, one may think "well the Shuttle was complex". Sure it was, but even an A380 or a 747 is very complex and still a safety-critical machine. Moreover, the higher the buses the higher the number of components such as relays, switches, etc., and the higher the design and maintenance costs, increased complexity, weight, ...
It's very difficult to imagine why 9 different control buses, all triple redundant, are needed. Why not just 3? Do NASA engineer supposed more than 2 simultaneous failures? I don't think so (fuel cells and AC buses are just 3 each and still critical...).
Probably there is not a "simple answer", as they have been multiple design choices. But maybe someone have some insights on why more than 20 DC buses rather than 4-5 of other safety-critical aerospace systems (such as aircraft).