tryinghard
Senior Member
- Location
- California
Is 480 wye (secondary) ungrounded system ever used?
If so is it compliant, seems to be in light of 250.20(B)?
If so is it compliant, seems to be in light of 250.20(B)?
Ungrounded System & 250.20(B)
- Thread starter tryinghard
- Start date
- Status
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
I have seen, and touched, several 1500 and 2000kVA 480Y/277V transformers that were ungrounded (their X0 terminal was floating). They were installed in a paper (converting) mill in the early 70's. The customer wanted 'run-of-the-mill' transformers. All 4-wire loads were fed by individual 480-480Y/277V.
The current owner (a meat processor) is considering changing the un-grounded system to high-resistance ground instead.
The current owner (a meat processor) is considering changing the un-grounded system to high-resistance ground instead.
tryinghard
Senior Member
- Location
- California
Are these at least bonded at the source?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
The X0 bushing is not connected to anything, that is why the system is called ungrounded. Yes, all of the 'steel' is correctly bonded to ground.
charlie
Senior Member
- Location
- Indianapolis
It sounds like your system is indeed correct. As long as the center point of the 480 wye is not connected to anything and the neutral point is not brought out to be used, it will look like an ungrounded delta system and should be treated like one. In accordance with the 2008 code, it is to have ground detectors installed. Also, it could be corner grounded in the same manner as a delta system.
If your customer wants a high resistance grounded system, this should be relatively easy to convert.
If your customer wants a high resistance grounded system, this should be relatively easy to convert.
tryinghard
Senior Member
- Location
- California
Jim, don't they have to connect the center point of the wye to get the 277V, assuming they have a common conductor?
Otherwise this would simply be a 480Y if nothing is connected to the center point, is this correct?
Last edited:
tryinghard
Senior Member
- Location
- California
The system I was curious about ended up being a 240/480V delta. It's not corner grounded so there is a stinger 415V; weird. :roll:
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
On their substation transformers they have absolutely no connection to the X0. 277V loads are fed from smaller 480-480Y/277V transformers.
tryinghard
Senior Member
- Location
- California
It sounds like your system is indeed correct. As long as the center point of the 480 wye is not connected to anything and the neutral point is not brought out to be used, it will look like an ungrounded delta system and should be treated like one. In accordance with the 2008 code, it is to have ground detectors installed. Also, it could be corner grounded in the same manner as a delta system.
If your customer wants a high resistance grounded system, this should be relatively easy to convert.
Thanks, sure helps to have other opinions to validate
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