The new construction building I'm working on had a problem the other night. The concrete finishers were working late and were the only ones left on-site so some of the details are unclear.

First off, the site's electricity is supplied by a diesel powered, 3-phase, 208/120V Y generator. There is a piece of cab-tire feeding a temporary panel inside the building. The panel is a single phase FPE.

Now to the problem. What the site super told me happened is that the concrete finishers had 2 portable 1000 W M.H. fixtures operating on a single extension cord plugged into a 15 A cct. The GFCI plug that the extension cord was plugged into is at the temporary panel. Somehow the plug managed to melt down and catch fire. The fire was contained to the single plug, and completely destroyed 1/2 of the receptacle...you could look right through where that half of the receptacle was and see the back of the box.

I show up yesterday morning and the super tells me what had happened the previous night, so I check out the damage. The first thing I did was to use my voltage sensor to be sure that the conductor feeding the destroyed plug is dead. However, I couldn't get the sensor within 6 or 8 inches of the plug or panel without it beeping and flashing.
This got me wondering because that's kind of the way my voltage sensor reacts to 600/347V.

I pulled off the panel cover to take some meter readings and things get really weird. While I was removing the panel cover I could see slight arcing a couple times in the burnt-out box. Once the cover was off I could clearly see that the wire from the fire-plug went to a breaker that had been turned off. So then I'm wondering what's causing the arcing, the neutral or ground? I metered things out and here's what was measured:
A phase - Neutral--------------120V
B phase - Neutral--------------120V
A phase - B phase--------------208V
so far so good...but wait
A phase - Ground---------------140V
B phase - Ground---------------333V
Neutral - Ground---------------266V

I guess that explains the arcing I saw...the neutral was arcing to ground.

So how is this possible?

The generator has a digital display that displayed normal voltages (for L-N and L-L) and frequency.

There was a ground wire going from the generator to a ground rod pounded at the generator. I checked the ground connections at the generator and the rod and they were good.

I metered the cab-tire to see if there were any opens/shorts but it was good.

The only thing I could see that might cause the situation was a possible problem with the generator so I recommended that the super have a service technician come out and take a look at it. He decided to have it picked up and taken off-site yesterday. Today the supplier calls and says that the generator is fine. What gives??

I'm completely stumped and really want to know what caused this to happen.

If any of you have any input/suggestions/ideas, I'm all ears.


Thanks for listening.


Shawn.