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Thread: 250.21(B) - ground detectors required?

  1. #11
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    Quote Originally Posted by Wire-Smith View Post
    grounding the wye primary opens up the possibility of problems when ground faults on the primary circuit or secondary(if secondary is also grounded) occur. not grounding the wye primary opens the possibility of problems if you loose a primary leg or if you close one leg at a time when energizing, which your probably not doing that at those voltage levels, that issue comes up in old distribution systems, which the solution is an extra cutout to temporarily ground the primary during initial energizing and then open the ground connection after the other legs are closed.

    https://apps.geindustrial.com/publib...388B%7Cgeneric

    the whys of wyes by GE

    Some lovely reading for a Sat afternoon. Thanks.

  2. #12
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    Note however that the ground reference for the ungrounded wye conductors is relatively firmly provided at the source end of the circuit (POCO secondary).
    It is true that the voltages applied to the wye windings may be a bit odd (but generally lower rather than higher) when the wye point of the wye primary is left open and one of the three ungrounded conductors is opened.

    Sent from my XT1585 using Tapatalk

  3. #13
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    Quote Originally Posted by Isaiah View Post
    Agreed completely. But what would be the negative impact of solidly grounding the XO on the primary side of first transformer?
    Just want to clarify a few things as multiple transformer winding configurations and setups have been thrown around and IMO a few things werent necessarily clear:

    The best setup is a 208 delta to 277/480 wye for the step up, and a 480 delta to 120/208y for the step down. The high side wye point of the first transformer would be grounded. The second transformer doesnt need a neutral, but does need an EGC.

    Secondly, note that the "leaving the xo floating" issue is for a wye-delta (typically resulting when a delta-wye transformer is used in reverse because its what someone had lying around) and does not apply to wye-wye transformers (which are quite rare in the low voltage world anyway).
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  4. #14
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    Quote Originally Posted by electrofelon View Post
    Just want to clarify a few things as multiple transformer winding configurations and setups have been thrown around and IMO a few things werent necessarily clear:

    The best setup is a 208 delta to 277/480 wye for the step up, and a 480 delta to 120/208y for the step down. The high side wye point of the first transformer would be grounded. The second transformer doesnt need a neutral, but does need an EGC.

    Secondly, note that the "leaving the xo floating" issue is for a wye-delta (typically resulting when a delta-wye transformer is used in reverse because its what someone had lying around) and does not apply to wye-wye transformers (which are quite rare in the low voltage world anyway).
    Agree.
    "Electricity is really just organized lightning." George Carlin


    Derek

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