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Thread: Neutral bond in transformer

  1. #21
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    Quote Originally Posted by templdl View Post
    You have a separately derived system whether your "load" requires or neutral or not.
    The X0 in bonded to the enclosure and you ground the X0 as directed by the NEC. The line conductors and a EGC is pulled to the load. Should there be a L-G fault the fault current will flow back to the X0 causing the OCPD to trip. Since the secondary is a 208Y/120 any L-G ground fault will be 120v regardless if there is a neutral pulled to a load or not. To even consider grounding a phase conductor in leu of the X0 does not make sence. If one elects to ground one of the 208v lines a ground fault will now be 208v. What anvantage is that? Is that better than 120v? When there is no choice other than grounding a phase such as corner grounding a 240 or 480v delta I find it unreasonable to even consider grounding one of the phases of a wye. One of the guys said "Just because you can doesn't mean you should."
    If you have a 480 volt corner grounded system and need a backup generator would you say it is unsafe to use a generator with wye connected windings, float the neutral and ground a phase? I would not have a problem with it.

    Why would the situtation with the transformer be any different.


    Alternating-Current Systems to Be Grounded. 250.20(B)(2):

    Where the system is 3-phase, 4-wire, wye connected in which the neutral conductor is used as a circuit conductor
    If the neutral conductor is not used as a circuit conductor it does not have to be the conductor that is grounded. It is usually wise to still use it as the grounded conductor if that is possible.

  2. #22
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    Quote Originally Posted by kwired View Post
    If you are using the neutral you must ground it. If you don't bond anything you have an ungrounded system. You do however need to use a ground detection system and still need to bond all non current carrying metal enclosures, raceways, etc. with the usual equipment grounding conductor and install a grounding electrode system.

    Grounding a phase on a wye secondary is probably not too common but could be done. One example of where you may do this is if you need to replace a transformer that was originally a corner grounded delta, but other equipment is not being changed.
    This is more to the question I'm asking. It was asked not to bond or ground the X0 terminal, not the enclosure. Still grounding and bonding the metal enclosure with a grounding electrode and bonding metal parts likely to become energized.

    Again the question is does anyone know why they would ask for this? Is it an ungrounded system? I don't think so. They did not ask to ground a phase either. How would the secondary recognize a fault? I would brush this off if this was one person saying this. But there is a few that asked for this. Problem is I can't ask any of them myself. It blew me away, I feel like I'm missing something.
    Mark

  3. #23
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    Quote Originally Posted by teco View Post
    This is more to the question I'm asking. It was asked not to bond or ground the X0 terminal, not the enclosure. Still grounding and bonding the metal enclosure with a grounding electrode and bonding metal parts likely to become energized.

    Again the question is does anyone know why they would ask for this? Is it an ungrounded system? I don't think so. They did not ask to ground a phase either. How would the secondary recognize a fault? I would brush this off if this was one person saying this. But there is a few that asked for this. Problem is I can't ask any of them myself. It blew me away, I feel like I'm missing something.
    If done as you say it is an ungrounded system. But ungrounded systems have to have a ground detection system installed in the system to indicate there is a fault. See 250.21(B) (2008 NEC)


    (B) Ground Detectors. Ungrounded alternating current systems as permitted in 250.21(A)(1) through (A)(4) operating at not less than 120 volts and not exceeding 1000 volts shall have ground detectors installed on the system.
    So in general you have to have either a solidly grounded system (any conductor of the system - if there is a neutral it is likely the conductor required to be grounded), a high impedance ground system (and associated fault detection equipment) or an ungrounded system (with associated fault detection equipment).

  4. #24
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    Quote Originally Posted by kwired View Post
    If you have a 480 volt corner grounded system and need a backup generator would you say it is unsafe to use a generator with wye connected windings, float the neutral and ground a phase? I would not have a problem with it.

    Why would the situtation with the transformer be any different.


    Alternating-Current Systems to Be Grounded. 250.20(B)(2):



    If the neutral conductor is not used as a circuit conductor it does not have to be the conductor that is grounded. It is usually wise to still use it as the grounded conductor if that is possible.
    With all do respect am I missing the intent of your reply?
    I am to understand that this is this how you yourself would install and ground a 208Y/120 transformer by leaving the X0 unconnected and grounding one of the line conductors as one would a corner grounded delta where the loads don’t require a neutral? Is this what you would recommend that the OP do in your option?
    If a qualified person were to run into an installation such as this he would strongly question the compentancy of the designer as it would not be customery, reasonable as one would never expect the a line conductor to be used as a grounded conductor one a wye transformer.
    Are you saying that 120 v L-G and 208v L-G are just as safe as one another and that it is easier to ground the line instead of the X0? For what practical reason would one ever ground the secondary of a wye transformer on this way?
    Interesting.
    Again, "Just because you can doesn't mean you should."

  5. #25
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    Quote Originally Posted by templdl View Post
    With all do respect am I missing the intent of your reply?
    I am to understand that this is this how you yourself would install and ground a 208Y/120 transformer by leaving the X0 unconnected and grounding one of the line conductors as one would a corner grounded delta where the loads don’t require a neutral? Is this what you would recommend that the OP do in your option?
    If a qualified person were to run into an installation such as this he would strongly question the compentancy of the designer as it would not be customery, reasonable as one would never expect the a line conductor to be used as a grounded conductor one a wye transformer.
    Are you saying that 120 v L-G and 208v L-G are just as safe as one another and that it is easier to ground the line instead of the X0? For what practical reason would one ever ground the secondary of a wye transformer on this way?
    Interesting.
    Again, "Just because you can doesn't mean you should."
    I would question the competence of someone that finds a system with no neutral and tries to connect a line to neutral load to said system. If a phase conductor has white insulation isn't that supposed mean the phase is grounded? Does it matter if it is a wye or delta transformer as long as correct number of phases and correct voltage is delivered? True it is not likely to run into this situation very often but it is certainly possible and code compliant if done correctly.

    This situation is more likely with a 480 volt system as you will not easily find a 240 volt system that is derived from a wye.

  6. #26
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    kwired -
    For 208/120, 250.20.B.1 requires that the neutral be grounded. 250.20(B)(2) does not apply. I can't think of a single case where it would be desireable or legal to ground one phase of 208/120.

    And I'm thinking you already knew that.

    ice
    Harmless flakes working together can cause an avalanche of destruction

  7. #27
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    Quote Originally Posted by teco View Post
    This is more to the question I'm asking. It was asked not to bond or ground the X0 terminal, not the enclosure. Still grounding and bonding the metal enclosure with a grounding electrode and bonding metal parts likely to become energized.

    Again the question is does anyone know why they would ask for this? Is it an ungrounded system? I don't think so. They did not ask to ground a phase either. How would the secondary recognize a fault? I would brush this off if this was one person saying this. But there is a few that asked for this. Problem is I can't ask any of them myself. It blew me away, I feel like I'm missing something.
    Teco,
    You are right to be concerned. If this is a 208 wye secondary transformer we are talking about here, X0 must be bonded/grounded- period, do not pass go, etc.-see NEC 2011 250.20 (B). And forget about grounding a phase conductor. If you guys have an instructor saying you can float X0, it mat be time for a new instructor.

  8. #28
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    Quote Originally Posted by texie View Post
    Teco,
    You are right to be concerned. If this is a 208 wye secondary transformer we are talking about here, X0 must be bonded/grounded- period, do not pass go, etc.-see NEC 2011 250.20 (B). And forget about grounding a phase conductor. If you guys have an instructor saying you can float X0, it mat be time for a new instructor.
    I should have added that the fact that you do or don't need a neutral for the load is not relevant, it still needs to be brought to the first disconnecting means, which as you know, is the fault return path.

  9. #29
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    Quote Originally Posted by iceworm View Post
    kwired -
    For 208/120, 250.20.B.1 requires that the neutral be grounded. 250.20(B)(2) does not apply. I can't think of a single case where it would be desireable or legal to ground one phase of 208/120.

    And I'm thinking you already knew that.

    ice
    No argument on whether or not it would ever be desirable to ground a phase of a 208 volt wye. Notice I said 208 volt wye and not 208/120 volt wye.

    In order for a wye connected set of windings to work you must tie one lead of each winding together to form the X0 point. Does this mean you have to use XO as a system conductor? Pole mounted transformers using 120/240 pots to make a delta bank have unused center taps on 2 of the pots and if corner grounded all three have an unused tap. Just because there is a tap does it mean it needs to be used? If you use the center tap for a circuit conductor on single phase or a wye then 250.20 makes it clear that it will be the conductor that is to be grounded.

    If a POCO replaces a transformer bank (for whatever reason) that was a 480 volt corner grounded delta and chooses to use 277 volt transformers and connect in wye configuration and not bond XO but instead bond a phase - there will be no difference in what is needed for service equipment or in voltage readings on system conductors. The neutral for all practical purposes does not exist as a system conductor - it is just a connection within the transformer bank that is necessary to make it work. A little like having extra leads that can be swapped for dual voltage purposes.

  10. #30
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    Quote Originally Posted by kwired View Post
    ... Notice I said 208 volt wye and not 208/120 volt wye. ...
    No, I didn't notice. And I don't understand the nuiance. How could you get a "208Y" and not have "208/120 Wye"? Color me clueless on this issue.

    Quote Originally Posted by kwired View Post
    ... In order for a wye connected set of windings to work you must tie one lead of each winding together to form the X0 point. Does this mean you have to use XO as a system conductor? ...
    Depends on what you mean by "system conductor". You don't have to have 120v loads. But you do have to bond this neutral to ground, per 250.20.B.1

    Quote Originally Posted by kwired View Post
    ... Pole mounted transformers using 120/240 pots to make a delta bank have unused center taps on 2 of the pots and if corner grounded all three have an unused tap. Just because there is a tap does it mean it needs to be used? If you use the center tap for a circuit conductor on single phase or a wye then 250.20 makes it clear that it will be the conductor that is to be grounded. ...
    Quote Originally Posted by kwired View Post
    ... If a POCO replaces a transformer bank (for whatever reason) that was a 480 volt corner grounded delta and chooses to use 277 volt transformers and connect in wye configuration and not bond XO but instead bond a phase - there will be no difference in what is needed for service equipment or in voltage readings on system conductors. The neutral for all practical purposes does not exist as a system conductor - it is just a connection within the transformer bank that is necessary to make it work. A little like having extra leads that can be swapped for dual voltage purposes.
    You left me confused with these last two. I don't see any connection to the OP.

    Where are you going with this?

    ice
    Harmless flakes working together can cause an avalanche of destruction

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