Wet Were fires
back when the Lpckheed Jetstar came out, they had some peculiar fires involving the wiring. The installers were saving time in routing wires through clamps by using a bent set of needle nosed pliers to pull wites though. The teeth damaged the insulation leaving small pinholes.
Iworked at Lockheed New York and we viewed some films the investigators took through microscopes. of similiarly damage wires. The wires were charged to 28V DC and high humidity introduced. Under the microsope you could see, obviously in time lapse photography, a slow growth of copper crystall form between the cinductor, through the damage and work its way to the airframe which was at ground potential. The small current leak, using the moisture as a conductor was forming a thin film heater which in time grew to the point where it caused a fire. Lockheed New York rewired a number of Jetstars including some Air Force VIP aircraft.
The talk here of condensation and humidity coupled with a constant electrcal potential from a battery makes me wonder if the same mechansm might be at play in the 787.