I was out riding in the desert over the weekend with a friend/colleague of mine who set up a panel at the pumphouse on the property for a 3ph 480 open delta. Dunno what got us talking about this, but after breakfast he said that with an open delta a wiggy would not read any voltage to ground, only phase to phase. Sounded interesting so I broke out my wiggy and we opened the cover for the disconnect and found: 480v between brown & yellow, no voltage registered on the wiggy between brown/orange or orange/yellow, so he was partly correct- at least between ph1 & ph3 the wiggy registered 480, but why was the center phase registering nothing? Also checking any of the phases to ground caused the plunger to "pogo" violently off the scale. He couldn't explain what it might be and gave a vague explanation that "maybe because the panel was supplying a load or something.." At the time the panel was supplying a 480-to-120/240 transformer which was in turn powering a house on the property and his motorhome for the weekend, a pump for the well and a small florescent light in the pumphouse. Maybe the answer belongs in the theory section, but can anyone explain this one to me? Or was my friend wrong on this one? I am not very well versed in 3 phase so any information would be interesting...
Using the word "pumphouse" has me worried... Many well set-ups have corner-grounded Delta set-ups. These show "0" volts to ground from ONE phase, and full voltage between all phases. They are not intended to do anything but power a motor....with a tiny control circuit transformer the only exception. Part of the reason is that, with this set-up, you, in effect, fuse & switch the neutral.
So what would make it go nuts? First of all, I'm not sure it's a good idea to have that second transformer in the system. I'd begin by making sure that the installation was done correctly, and that the secondary is truly a "separately derived" system. My next guess is that the grounding of the delta is improper. or damaged. There might actually be two legs grounded...perhaps resulting in "dueling transformers."
Sounds to me like an open phase on the primary side of the 480 delta bank resulting in 2 of the phases going ground return across the ground connection between primary and secondary, and paralleling the pole ground wire and the service drop. Is the 480 delta bank a wye primary (e.g. one bushing on the HV instead of two)?