As For the Ram Air Turbine (RAT)
A Ram Air Turbine (RAT) serves as an emergency source of electrical and
hydraulic power. It has no operating time limits, airspeeds, or altitudes.
Nothing to suggest it won't power the brakes on the ground if the battery isn't working AND then somehow the aircraft is then run out of fuel. (I think someone asked about this highly unlikely scenario)
Hi Cool Guys,
Apologies for answering your question by click and pasting,
Permanent Magnet Generators
Three engine driven Permanent Magnet Generators (PMG) are the primary source
of power for the flight control electronics. These power sources are independent
from the main airplane electrical system, and are also independent from each
other.
A secondary source for flight control power is provided by the airplane’s 28 Vdc
bus distribution system, and the main battery. In addition, a backup system is
provided by dedicated batteries to assure positive flight control operation during
temporary power interruptions.
Permanent Magnet Alternators
Two engine driven Permanent Magnet Alternators (PMA) (one per engine) are the
primary source of power for the EEC. PMAs are independent from the main
airplane electrical system, and are also independent from each other.
A secondary source for EEC power is provided by the airplane’s 115v AC bus
distribution system. During engine start, initial EEC power is provided by the
airplane until the PMA is able to provide power. The airplane also serves as a
backup EEC power source if a PMA is inoperative.
So engine driven is the short answer. Like most aircraft, the methods of producing electrical power are Ground external, APU, Engine and Batteries and RAT.
As for you second question about flight instruments if there were no battery and then the RAT deployed? Remember the RAT is only required if
• both engines have failed (and in your scenario there wasn't an APU battery available to start the APU or fuel to run it)
• loss of all electrical power to captain’s and first officer’s flight
instruments
In this (again) extremely unlikely example, I expect there might be a short power interruption until the RAT came online. That would be seconds rather than minutes though.