This is on one hand, good news that the RAT system will work to power the aircraft in an emergency. It could very well be all the extra electrical test equipment and test systems they have (extra) in the plane may be the cause of the problem, not the aircraft electrical systems themselves.
Not so, from my EE point of view. If a circuit feeding the cabin (and the test equipment) is overloaded then its breaker should trip. Under no circumstances a fire may be an expected, planned outcome - not any more than a similar fire in a house. That's what we use breakers for.
An electrical fire usually is caused by improper selection of a wire (too thin) or by a bad contact (assembly error.) Since they landed successfully, the cause won't be a mystery for long. They Must have already opened the panels and found what burned up; you don't want to leave anything smoldering there.