Thanks Neil & Scott for getting the picture up.
Neil, you certainly found an interesting one.
From the clues you've shared:
- These people have lived in this home since it was new.
- Like I stated before his work looked fine and no signs of damage in the areas he worked.
- The wires did not burn right next to the receptacle or even into the jacket of the romex. Only on the exposed portions out of jacket.
- There is no sign of damage to any of the devices. Only to the wiring.
The situation is truly puzzling.
The conductors inside the cable sheath being unburnt, and the devices being unburnt is a show stopper.
I wonder about the original construction crew. . .who they might have been; what they had to deal with in the way of weather. Are any of the floors finished wood that would have required a floor sander? (Around here, they're wild cards. I've seen'em hook cheesy alligator clips on unfused buses). But all of this would have burnt to the device. . .
It's almost as if, prior to devicing, too much heat was blown or radiated into the roughed in boxes, melting the exposed insulation. The ends that were eventually stripped and connected to the devices were back inside and somewhat protected from the heat. The original installer felt the damage would "get by" and left it.
Following on this idea, you may well have had two problems that you fixed. Beside the tripped breaker, you found and replaced legacy circuit damage left from the original construction crew.