Had a unusual one today....

The hole appears to be directly over one of the switch contacts. If, for some reason there was an electrical arc inside the switch that burned through the Balelite and pierced the metal on the switch, it shouldn't have gone any further. The switch should have been grounded to the box by virtue of its mounting screws and the arc would have gone to ground. I can't see how it would continue to make a hole in the cover plate.

A more likely scenario is that a hole was drilled in the cover plate and the switch providing a path to ground and possibly damaging the electrical circuit within the switch. I have seen something similar with a lightning strike where lightning came down a television antenna lead and blew a half dollar sized hole in a breaker box. There was an associated fire that totaled the business though.

You didn't say what type of pump you had. If it is a deep well submersible, they are an excellent earth ground and attractive path for a lightning strike. I would agree with Flyinfool. A pump should have a dedicated circuit.
 
Sounds to me like a spontaneous EDM machine grew inside that switch. All you need to do is figure out how to harness it!
... either that, or you've invented a new kind of plasma cutter :)
 
one beautiful constant in electrical work, electricity is always seeking the easiest path to ground.

as i see it....
your switch failed through heat as seen by the ball like anomaly on the end of the contact bar as seen in the last photo.
carbon was created as the bakelite decomposed, making a normally insulated switch a conductor
the carbon created an easier path to ground through the metal front cover of the switch as well as the metal switch cover, which were undoubtedly in close proximity.
since there was some resistance, there was no direct short to ground.
it may have been rather slow decay not unlike electrolysis
but the end result was undeniable
 
one beautiful constant in electrical work, electricity is always seeking the easiest path to ground.

as i see it....
your switch failed through heat as seen by the ball like anomaly on the end of the contact bar as seen in the last photo.
carbon was created as the bakelite decomposed, making a normally insulated switch a conductor
the carbon created an easier path to ground through the metal front cover of the switch as well as the metal switch cover, which were undoubtedly in close proximity.
since there was some resistance, there was no direct short to ground.
it may have been rather slow decay not unlike electrolysis
but the end result was undeniable
What I don't understand is the hole in the cover plate. The cover plate is grounded through the switch body which should be connected to a ground wire. #Even if the ground wire were connected to the box, the path would be through the switch body. Why would a hole blow in the cover plate?
 
Thats interesting, I never knew bakelite degraded into carbon. You Google it, and there has been several studies on converting old bakelite into carbon.


Sounds to me like a spontaneous EDM machine grew inside that switch. All you need to do is figure out how to harness it!
Sounds like you are darn close.
 
Thats interesting, I never knew bakelite degraded into carbon. You Google it, and there has been several studies on converting old bakelite into carbon.
Actually it's quite common. Back when we still had distributors in cars, a little moisture or dirt would create a leakage path between terminals and the arc would create a permanent carbon path. Older printed circuit boards used phenolic board instead of the glass or polyester boards used now. A bad electrical connection would generate heat which would cause the phenolic resin to decompose creating a carbon short.
 
The ground wire was grounded to the box. When I flipped the breaker it looked like a small torch shooting flames and sparks out of the little hole. Looking at the insides I cant see anywhere where anything touched to make a short. So I guess it had to be a carbon trail.
 
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