Transformer Secondary

[templdl]

Same reasoning you'd want any ungrounded system.

I can't recall any ungrounded wye applications though. If there were a reason to do so there would be examples of such an application.

 

[hurk27]

I can't recall any ungrounded wye applications though. If there were a reason to do so there would be examples of such an application.

I have a seen ungrounded 480/277 banks used on industrial, most utility's stock the 277 volt WYE can's but you have make sure they do not bond the X/0 to the MGN if it is going to be an ungrounded system or there will be a shock hazard, I have never seen a 208 WYE ungrounded, as 208 can cause problems with high torque motor applications such as air compressors if the motors are 240 volt rated, the only ungrounded systems I have ever seen was a 240 delta, or 480 WYE, or 480 delta, and one 600 volt delta.

The purpose of an ungrounded system is so it takes two faults to ground before the system shuts down, with an ungrounded system there is no reference to Earth, and when the first fault happens it will sound a ground fault alarm that give the operators time to do a scheduled down time to make the repairs instead of bringing the production line down with a first fault, so critical operations that could be made more dangerous if this were to happen is a good reason to have an ungrounded system, but there can not be any reference to earth on the secondary side of the transformer or there will be a very high shock hazard if the first fault happens as all the grounding in the building will be at the potential of the connection that was grounded such as 277 on a WYE or 480 volts on a corner grounded delta, so one must make sure the utility does not make the mistake of bonding the secondaries at the pole or pad transformer.

If you chose a grounded system or its all the utility will supply, then the NEC requires the grounded conductor to be brought to at least the service disconnect, even if you don't need it for any loads where it is bonded to the EGC's and grounding electrodes, from there it becomes the EGC to all loads, this is to provide a low impedance fault path that allows the breakers protecting the circuits to open in the event of a ground fault.

I have many lift stations that are supplied by ungrounded deltas with ground fault alarms that are sent through a SCADA system to a central computer that call the maintenance director who can scheduled a down time as they don't want the lift stations to stop working just because one phase went to ground, this is the kind of operation that make sense to use an ungrounded system.

 

[templdl]

I have a seen ungrounded 480/277 banks used on industrial, most utility's stock the 277 volt WYE can's but you have make sure they do not bond the X/0 to the MGN if it is going to be an ungrounded system or there will be a shock hazard, I have never seen a 208 WYE ungrounded, as 208 can cause problems with high torque motor applications such as air compressors if the motors are 240 volt rated, the only ungrounded systems I have ever seen was a 240 delta, or 480 WYE, or 480 delta, and one 600 volt delta.

Thanks, My experience has always been with ungrounded deltas in industrial facilities where an automatic trip as a result of a GF would be catastrophic to say the least. I had to address everything from the correct OCPD rating to various GF indicators.
It,s strange that I never ran across a similar application with an ungrounded wye which does make sense.