Old Lathe Electrical Question

[Julie Moriarty]

Julie Moriarty, this anecdote doesn't make sense. If there was a ground fault somewhere, then yes, this could happen. And if there was some other issue, maybe. But if the foreman was standing on a ground (zero volts) and touched the knob (grounded, zero volts) then no shock. But because he did get a shock, then there was some fault condition, broken ground, mis-wiring, etc.

(BTW, I'm a degreed electrical engineer with 50 years of experience. I've gotten my share of shocks, but it was always my screw-up, lack of attention, or something not grounded).

I realize it doesn't make sense but here's what happened...

After testing the voltage from the ironworker's metal stair railing to their trailer door knob (the two were not mechanically bonded) and finding 50v, I immediately thought the trailer must not be grounded. So I walked around the trailer looking for evidence of grounding. I found two locations. One bonded the shell of the trailer to a ground rod. The other bonded the electrical panel to a second ground rod. Granted, the trailer bonding did not necessarily mean the entire frame of the trailer was also bonded. The door was on aluminum hinges but if it was live, so was everything bonded to the doorknob. The metal stairs were resting on wet ground. (I could never determine how voltage leaked over to make it to the doorknob because the focus was get the job up and running again.)

But it gets better. Inside the building, which was mostly a concrete shell at that point, temporary lights were either very bright, dim or out completely. The man lift (skip) had 480v going to it, all legs read 277v, but would not operate. One guy said he plugged in his radio and it fried. I went back to our trailer to report to our general foreman. As I was approaching the trailer I saw the door fly open, heard a scream and saw a smoking telephone fly out the door. The GF said he was talking on the phone when it started smoking.

I traced the problem to a temporary 120/208 3ph transformer. It had lost a leg. The only thing I could figure is we had a lightning strike overnight which caused the damage (there was a big thunderstorm overnight). I've seen lightning strikes do strange things.

We replaced the transformer and things were back to normal. But the skip had to have the 120v controller replaced as it was fried. Maybe the lightning strike. Maybe over-voltage from backfeed caused by the lost leg.

And we had to get a new phone.

[Maurice Mcmurry]

I had a great experience working with electricians and engineers from The UK, Europe, Australia, and New Zealand. There is an entertaining, ongoing, debate as to whether 120 volts is even a real circuit.

Getting back to the lathe, maybe you eliminated a short while checking the wiring. We had a device that shorted because a cover screw was too long.

[Julie Moriarty]

Getting back to the lathe, maybe you eliminated a short while checking the wiring.

Most likely