Melting Ground Wire

[KRG9729]

I was working in a building and came across the attached damaged junction box.

I suggested that the building owner contact the electrician who installed it as the building has been remodeled somewhat recently. That electrician is unavailable and the owner would like for us to troubleshoot the problem.

The panels have been changed however there is a Federal Pacific sub-panel but it seems unlikely that the circuit is fed from it.

One idea was lightning but there were no other signs of that.

Anyone suggestions on what would cause this would be appreciated.

 

[Jgreagori]

I would try to trace the source of the circuit. Turn it off. Check for ground continuity. Very possible it is fed from the Federal panel as they are known to not work well as circuit breakers but more like a switch. Try tracing the actual wiring to find the source of the short. Very doubtful that it is lightning.
Happy hunting.

 

[K8MHZ]

I am confused as to how there can be potential between the ground screw and the adjacent side of the box.

If I didn't see a bonding screw, I would likely guess the box became energized and shorted out against the bare grounding wire.

ETA: Is it possible the bonding screw and jumper was added after the fact?

 

[KRG9729]

I am confused as to how there can be potential between the ground screw and the adjacent side of the box.

If I didn't see a bonding screw, I would likely guess the box became energized and shorted out against the bare grounding wire.

ETA: Is it possible the bonding screw and jumper was added after the fact?

The bonding jumper appears to be burnt up as well so I would assume it was there originally.

 

[Ponchik]

NM cable or conduit installation?

Can you verify if it just happened hence you are working on the circuit? Or you happen to come across it?

 

[kwired]

NM cable or conduit installation?

I'd like to know that answer as well. Next comes continuity of the equipment grounding conductor. Is connections in that wire nut and/or the bonding screw secure? How about continuity back to the grounded conductor at the service/separately derived system?