600V Ungrounded Delta and 480V Wye

[stevebea]

One of my co-workers was telling me about a local business (industrial) where he was working and telling me they have two services to the building, 600V Ungrounded Delta and 480V Wye.I started thinking about this scenario.... If both services are bonded to building steel then isn't it probable that if there was a fault to ground on the Delta service that the fault current would would follow the path back to the XO on the Wye service and trip the GFP on the Wye Main and leave the Ungrounded Delta service energized? I can find out more details if needed.

Steve

 

[shepelec]

I doubt the GFP main would trip as the CT's on the phases would not see the fault current.

 

[Smart $]

One of my co-workers was telling me about a local business (industrial) where he was working and telling me they have two services to the building, 600V Ungrounded Delta and 480V Wye.I started thinking about this scenario.... If both services are bonded to building steel then isn't it probable that if there was a fault to ground on the Delta service that the fault current would would follow the path back to the XO on the Wye service and trip the GFP on the Wye Main and leave the Ungrounded Delta service energized? I can find out more details if needed.

Steve

A ground fault on only one line of an ungrounded system has very little current. The fault is to [grounded] conductive material isolated from the electrical supply. The grounded parts provide no "solid" return path to the electrical supply. At maximum, there would only be enough current to compensate for reactive coupling plus any line-to-ground current introduced through the fault monitoring system.

But that is only for the first fault. A ground fault on a second line would be just like a grounded system, because the ground fault on the other line provides the return path.

If you need a better understading, draw out the secondary windings with all connections including grounding and some load-side conductors. Then draw a ground fault on any one of the ungrounded system's lines. Now if you try to determine a pathway for the ground fault current to get back to the winding(s) from which it came... there is none.