Wye/Wye Distribution problems. 1

[BRCampbell]

I have a small facility fed from three 2.5 Mva 480/480-277 transformers. The secondary feeds gear housing three 1500 amp Pringle fused switches, each of which are protecting a load. which are 1rocess machinery, 2:facilities equipment, and 3: a lighting Panel board.

When a ground fault occurs on a small motor everything in the whole operation shut down. . We are considering installing a High Resistance Grounding system where we would lift the primary "center tap" ground connection to the grid and route the secondary "center tap" connection thru the HRG circuit then back to ground.

We are also considering putting a transformer between the gear and the Panelboard to isolate the single phase loads.

Does this sound like a reasonable solution to our Ground Fault dilema?

 

[davidbeach]

If you have any 277V loads you cannot use an HRG system. Far and away the easiest solution would be a second level of ground fault trips and a coordination study to determine where to set each trip. I don't know how you define small motor, but if every thing is set properly, ground faults on small circuits, say 30A and less, should be able to be cleared by the branch breaker before the ground fault trip at the Pringle switch picks up and trips, but not if the ground fault is still at its minimum pickup, fastest trip as it came from the factory.

 

[JJHorak]

Why do you state that if you have any 277V loads you cannot use an HRG system? Under what conditions is an HRG system allowed?

 

[davidbeach]

NEC 250.36

 

[dpc]

It's allowed when there are no 277V loads, basically for 3-wire systems with no active neutral.

 

[BRCampbell]

Our intention was to route the feeder to the single phase distribution panelboard through a delts/wye transformer and develope a neutral specifically for these loads.

The gear is 2o+ years old and the Pringle switches are LBFs.

 

[davidbeach]

Are the ground fault trips on the Pringle switches set where the coordination study says they should be set?