Since the condition comes and goes, you likely have a floating delta power distribution transformer. Since there is no reference anywhere to ground, the system floats. That means that whatever leakage there might be can "tip" the system one way or another with respect to ground. It sounds like something somewhere in your plant intermittently takes one phase to ground and you just happen to be measuring it when that happens. You should try to find where this is and fix it as the system is not really intended to operate with one phase grounded.
These systems have one nice feature. As you found, you can keep operating with a single ground fault without problems. Unfortunately, that benefit comes with a heavy cost in the safety department.
First, since there is no control of voltage to ground, it could be 4000VAC or 10000VDC and you won't know it until you and your CAT 3 meter gets blown away. So you have to approach the power network with great care and caution.
Second, if the network is grounded on one phase, that means that, instead of 277VAC to ground, you now have two phases 480VAC to ground. In a perfect world with 600V insulation, there is no problem. But, in the real world, dust (sometimes conductive) and moisture are everywhere. This means that, when you open a disconnect, the dust around the disconnect sees 480V instead of 277V. Dusts rarely ionize at 277V but often do at 480V. That ionizing dust begins to conduct AROUND THE OPEN CONTACTS and the disconnected load lines begin to float up to 480V. The conductors you thought were dead are now at 480V waiting for you to touch them. Fifteen years ago, I did touch one of them and almost lost my life! That's right---with an open disconnect! The bottom line: if you want to be safe in a floating delta power environment, you must carry with you a four-legged ground clamp wire set. After opening the disconnect, attach one leg to a known good ground and then the other three legs to the three disconnected phases. Now they are dead and you can be sure about that. This is the same procedure the utility people use for protection on their ungrounded delta high voltage systems.
Third, you cannot operate any DC drives/motors on floating power as you will destroy the motor. You must place a drive isolation transformer ahead of the DC drive and ground the center of the wye secondary. Now you have 480/277 grounded power for the drive. AC drives can operate on floating power but not well. You have to turn off the output ground fault and short circuit fault features or, every time the network shifts with respect to ground, you will get a nuisance fault on your AC drive. Or, worse, you could have an Allen Bradley PowerFlex drive (any model from 4 thru 700). On those, the ground fault and short circuit fault detection simply stops working and the drive is destroyed if any output grounds or shorts occur. Using an isolation transformer on AC drives is also highly recommended for these reasons.
Basically, I hate to see floating delta power in industrial and commercial facilities as I regard it as unduly hazardous. When you have an opportunity to recommend one over the other, always recommend grounded wye power. And, don't be seduced by these characters who want to take a perfectly good grounded wye network and add resistance in the ground bond so that system can now operate corner grounded just like the floater. What a travesty! Just be very careful. It's your life!
