ASM Online Member Community

A question for you all.

The story, I had a 7,200-volt underground distribution cable feeding the transformer in my front yard fail. This was an old (~40 years) unjacketed UD concentric neutral cable, with tinned copper neutral conductors wound around the outside of the insulated core conductor. After the utility pulled this old cable out of the plastic conduit, I picked up some pieces of the outside Cu neutral. When I bent them and they broke in a brittle manner with limited ductility. I left these wires outside for a couple of weeks and looked at them again. This time they bent without breaking. What would be a mechanism for Cu becoming brittle at room temperature and then later ductile again? I might have guessed a form of corrosion-based hydrogen embrittlement, except Cu isn't supposed to subject hydrogen embrittlement at room temperature. The Cu wires should also have been under minimal stress. The Cu wasn't exposed to "clean" air long enough for any hydrogen to escape. More strikes against hydrogen embrittlement. The Cu is presumable a soft annealed OFHC copper. These pieces have signs of significant corrosion in sections, with the tin coating remaining in other sections. The location is damp but not immersed. Oh, and I don't know the cause of the line failure.