Originally Posted by
BugBear
Looks entirely unrelated to batteries. Without seeing an incident report, I would say parallel arcing fault, and inadequately designed protection. Lack of differential protection alluded to in the last comment probably doesn't help, though it shouldn't really be necessary if you can make everything soft start and your circuits are small relative to the generator. It might not interact well with the DC portion of the large motor systems, depending on how that's implemented. Unfortunately, all the Boeing drawings are just big-black-box.
Fault containment on ground switchboards is heavy. I doubt the situation is too different on an aircraft. Steel or distance to segregate different portions of the switchboard so that a fault in one part cannot cause a fault in the other. Aircraft have long had separate left/right electrical panels because of this.
(edit: energy released from arc flash is almost entirely due to power available and the operation time of the protection; connected load is irrelevant)
You also often have reclosers set to reset 3-5x then not further close into a fault, but that doesn't seem to be a thing in aviation
Originally Posted by
BugBear
What powers (turns) the alternator?? If the generator(s) go off line, and the emergency batteries (Thales) are inop, how does the APU start? The batteries were destroyed when two generators failed in the United incident. Panel? EICAS??
Isn't the RAT intended to support descent and safe landing??
Not save a TO?
How resilient is the RAT to overload when in flow with arcing and shorting in the Main Generation system??
.
Thanks GF
You won't start an APU without a battery or large AC power (i.e. not RAT). This is part of why the 737 must have the APU running when in ETOPS conditions, because otherwise loss of two generators or engine+offside generator means you are relying on ~30min battery. Combined reliability of cold-soaked already-loaded battery + APU start is not great. A RAT negates this because you have instruments/radios indefinitely.
The general design of electrical systems is simply to isolate energy from faults. If there is a major bus fault, the generator and bus-ties to that bus should disconnect clearing the fault. You can't really do this internally to a battery which is why battery fires are harder to deal with.
The emergency bus(es) don't backfeed into the normal buses, so if a fault is present in a normal bus, switching the emergency bus to an alternate source of supply (offside or RAT/EMER GEN) will physically disconnect it from the original faulty supply.
If the fault exists in the emergency bus, you shed the emergency bus and rely on the redundancies present in the other two+ buses of that type.
For reference, here's the relatively simple setup on the A320:
