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Thread: 2500KVA xfmr service entrance not grounded - ?

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    2500KVA xfmr service entrance not grounded - ?

    Hi,

    At our site, we have two 2500KVA service transformers (12470v primary, 480v secondary). X0 terminal is not grounded and not brought inside the building.

    On one of the transformer, the X0 terminal is connected to the frame of the transformer. This transformer is a wye-wye transformer.
    On the other of the transformer, the X0 is not connected at all. This transformer is a delta-wye.

    For many years, the folks here thought that these are delta ungrounded systems on secondary side. There is no ground fault detector in either of the transformers. My questions are:

    1. Is this a violation of the NEC code.?
    2. Should we have the X0 grounded and bonded to a ground electrode? What would be the impact to existing three phase systems.?
    3. Do we need to install ground fault detector? There are no main CB on either transformer (secondary side), but there are several conductors leaving the secondary and wired to indivisual CBs (i.e. no bus or switchboard) mounted on walls. Is this allowed under NEC?
    4. For many years, one phase could go to ground and noone would know it until the next phase go to ground. This created havoc with the equipment. I am looking for suggestions on how to improve safety of these systems. The site is over 60 years old and we just recently came upon this.
    5. What would be the impact of not grounded the X0 as related to short circuit and arc flash.

    Thank you in advance for your help and guidance.

    Regards,

    FK19

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    Quote Originally Posted by francis.key19 View Post
    Hi,

    At our site, we have two 2500KVA service transformers (12470v primary, 480v secondary). X0 terminal is not grounded and not brought inside the building.

    On one of the transformer, the X0 terminal is connected to the frame of the transformer. This transformer is a wye-wye transformer.
    On the other of the transformer, the X0 is not connected at all. This transformer is a delta-wye.

    For many years, the folks here thought that these are delta ungrounded systems on secondary side. There is no ground fault detector in either of the transformers. My questions are:

    1. Is this a violation of the NEC code.?FK19
    See 250.24(C) and 250.20
    Where your XO has been grounded at the transformer, 250.24(C) would require a grounded conductor. (or not have XO bonded)

    Quote Originally Posted by francis.key19 View Post
    2. Should we have the X0 grounded and bonded to a ground electrode? What would be the impact to existing three phase systems.?FK19
    If XO is bonded, you would need a neuitral to each service disconnenct.
    Many folks feel there are advantages to a grounded system. I can see little advantage to an ungrounded system unless you have ground detectors

    Quote Originally Posted by francis.key19 View Post
    3. Do we need to install ground fault detector? There are no main CB on either transformer (secondary side), but there are several conductors leaving the secondary and wired to indivisual CBs (i.e. no bus or switchboard) mounted on walls. Is this allowed under NEC?FK19
    NEC 250.21(B) requires detectors. Also, the advantage of an ungrounded system relies on your bring able to detect that 1st fault to ground.
    As far as feeding multiple disconnects, NEC 230.71 basically limits you to six mains and they need to be grouped.
    230.2 makes allowances for large buildings and services over 2000 amps. More details would be needed.
    (with that size transformer, you most likely meet one or more requirements for multiple locations)
    Quote Originally Posted by francis.key19 View Post
    4. For many years, one phase could go to ground and noone would know it until the next phase go to ground. This created havoc with the equipment. I am looking for suggestions on how to improve safety of these systems. The site is over 60 years old and we just recently came upon this. FK19
    again..ground detectors

    Quote Originally Posted by francis.key19 View Post
    5. What would be the impact of not grounded the X0 as related to short circuit and arc flash.

    FK19
    Not knowledgeable enough to advise
    Quote Originally Posted by francis.key19 View Post
    Thank you in advance for your help and guidance.

    Regards,

    FK19
    Last edited by augie47; 01-24-14 at 07:17 AM.
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

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    Quote Originally Posted by francis.key19 View Post
    5. What would be the impact of not grounded the X0 as related to short circuit and arc flash.
    The advantage of ungrounded system in your case is that there is no short circuit or arc flash hazard for any one phase to ground fault unlike a solidly X0 grounded system where such problem can be to an extreme level. But for phase to phase short or arc flash, both ungrounded and grounded systems can have the same level of hazard.

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    Quote Originally Posted by Sahib View Post
    The advantage of ungrounded system in your case is that there is no short circuit or arc flash hazard for any one phase to ground fault unlike a solidly X0 grounded system where such problem can be to an extreme level. But for phase to phase short or arc flash, both ungrounded and grounded systems can have the same level of hazard.
    Thank you for help.

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