corner grounded delta.

[gentech]

I was just curious as to how common these systems are. We
sent a generator out for rental and the customer requested
a delta connected 120/240 3 phase output. The particular
unit we had available(based on the customers kw needs) could not be connected 120/240 delta. Because his needs
were for the 240 three phase output only( they did not have
any single phse load) we connected a low wye configuration
139/240 3 phase. What the customer neglected to mention was the corner grounded delta. Another condition that existed was we overlooked the fact that a small wire had been connected to generator ground and the neutral bar.
The resulting condition was that the generator trailer was reading about 137 volts trailer to ground. I would be grateful if anyone could explain how this could occur.

 

[CurtJ]

Since the corner of the 139/240 volt delta was grounded, the neutral of the low-wye would be at 139 (or in your case 137) volts to ground. You can remove the strap between the neutral bus bar and the generator frame (trailer) ground and then ground the generator frame. Caution!: Some people may assume that the neutral is still grounded. Cover it with insulation, tag it, do whatever it takes to make sure that no one touches it without realizing that it IS NOT at ground potential. Floating neutrals are not common, but they do exist. They can be a safety hazard if not marked and protected properly.

 

[jbartos]

Some suitable system grounding scheme (e.g. high-resistance grounded neutral, grounding transformer, etc.) is suggested for various beneficial reasons.