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Thread: system grounding

  1. #11
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    Quote Originally Posted by Strathead View Post
    Unless I am mistaking what is being said or terminology, it would be a grounded conductor, but not a neutral and the reason is exactly why the code uses the terms the way they do. A neutral would have to be a tap point in the center of the transformer coil as drawn. I don't see how it could be referenced as dual voltage when there is not a center tap. if this is just a habit which it seems like electrofan and smart$ are saying then it is a bad habit. This is all dependent on whether I actually understand what smart$ was saying which is very possibly a false assumption.
    You see this often on POCO transformers, 2400/4160 rating on a single phase two terminal unit (one terminal may effectively be the case). I don't know why they do that, bottom line is the unit has a 2400 volt rated coil on the high volt side.

  2. #12
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    Wow! Thanks for all the replies. It seems to be confirmed that the bonding strap should be removed for this setup. That was my conclusion but I just wanted to get some opinions.

    I think everything posted is spot on.

    Quote Originally Posted by cadpoint View Post
    I've never seen a set up like this.
    I did go to Duke Energy
    https://www.duke-energy.com/_/media/...nual.pdf?la=en
    Pages 85, 86, 87.
    This isn't for a NC installation but I still will read through this.

    Quote Originally Posted by winnie View Post
    And the above voltage difference implies that the neutral of the wye is at about 138V relative to the neutral of the single phase coil. You will either see high current or high voltage between the two 'grounds'.

    -Jon
    I am trying to sort out system grounding rules and the implications of an incorrect installation, or in this case if we hadn't thought about removing the strap that comes standard from factory. You are basically saying a ground fault would have been created between the two transformers and some amount of current would circulate between the transformers? The amount of current would mostly depend on resistance in the earth connection?

    Quote Originally Posted by Smart $ View Post
    Electrofelon is correct if the OP is reverse wiring a typical pole pot. It's just how MV equipment is designated.
    Correct. Reverse wiring this one. Just how they are designated. I believe there is an ANSI standard governing this designation.

    Quote Originally Posted by kwired View Post
    Many of those transformers are a single bushing on one side of the HV coil and bonded to the case on the other, so at very least we can call it a grounded conductor whether it is a true neutral or not.
    Correct. If ordered with the 4160GY/2400 designation, one side of the HV coil is bonded to case by factory, internally. This means you could use a bank of three of these to develop a three phase service.

    Please see attached pictures of step up transformer with bonding strap.
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  3. #13
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    Quote Originally Posted by electrofelon View Post
    Note that is how these single phase transformers are typically designated. The transformer high side is 2400, line to neutral of a 4160 wye system.
    Typically a single phase utility system with a G/ in its voltage, like 4160G/2400, means that the utility system is 4160 L-L and 2400V L-G it does not necessarily say the circuit is 3-phase or 1-phase.
    As I remember, it does tell the 'line workers' that a single phase transformer winding with one insulated terminal and 1 grounded terminal will be 2400V while a single phase transformer with (2) insulated terminals would be 4160V.
    Just because you can, doesn't mean you should.

  4. #14
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    Quote Originally Posted by robeward View Post
    And the above voltage difference implies that the neutral of the wye is at about 138V relative to the neutral of the single phase coil. You will either see high current or high voltage between the two 'grounds'.

    -Jon
    I am trying to sort out system grounding rules and the implications of an incorrect installation, or in this case if we hadn't thought about removing the strap that comes standard from factory. You are basically saying a ground fault would have been created between the two transformers and some amount of current would circulate between the transformers? The amount of current would mostly depend on resistance in the earth connection?
    Essentially yes. If you put a meter between the neutral of the wye and the center point of the single phase transformer, you would measure a pretty stiff 138V. Connecting these two points would cause large circulating currents in the transformer(s), and if you ground these two points the current flow would depend on the ground electrode resistance.

    My assumption is that the single phase transformer is most commonly used for step down operation, where the secondary is supposed to be grounded by the strap...but when this transformer is used for step up operation the strap needs to be removed.

    -Jon

  5. #15
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    Quote Originally Posted by jim dungar View Post
    As I remember, it does tell the 'line workers' that a single phase transformer winding with one insulated terminal and 1 grounded terminal will be 2400V while a single phase transformer with (2) insulated terminals would be 4160V.
    Jim, if you are saying that a two bushing transformer designated 4160y/2400 takes 4160, that is not the case. I thought that was the case myself and started a thread on that topic last fall. These "utility type" designations are silly imo.

    The op needs to be careful in getting the correct number if bushings and the correct voltage. As drawn and designated, he would have 2400 not 4160.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  6. #16
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    Robeward,

    May I inquire into the application? Is this a step up because of a long wire run?
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  7. #17
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    Quote Originally Posted by kwired View Post
    You see this often on POCO transformers, 2400/4160 rating on a single phase two terminal unit (one terminal may effectively be the case). I don't know why they do that, bottom line is the unit has a 2400 volt rated coil on the high volt side.
    Ahhhhh!


    I know what I don't know, and I know where to go to find it!

  8. #18
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    Quote Originally Posted by robeward View Post
    I am trying to sort out system grounding rules and the implications of an incorrect installation, or in this case if we hadn't thought about removing the strap that comes standard from factory. You are basically saying a ground fault would have been created between the two transformers and some amount of current would circulate between the transformers? The amount of current would mostly depend on resistance in the earth connection?
    There are several things that can happen many of them are a threat to life. First remember the basis of current flow. Current takes the path of least resistance. Draw out the circuit with the primary coil of the transformer, the center tap and the source. You will see that current can basically flow two ways. From L1 through the coil and back to L2 or from either L1 or L2 through half the coil and back along the ground through the bonding point of the neutral and the ground and the along the neutral to the source.

    Remember that the ground is not allowed to intentionally carry current with VERY few exceptions in the code. Anything that causes a higher impedance in one side (half) of the the transformer coil will cause some current flow on the ground. A fault (open) on either L1 or L2 will cause current flow on the ground that is equal to the current on the line wire. Now picture the ground being insufficient, (loose connection, broken ground wire. All exposed metal between the point where one failed to remove the strap and the break point would be sitting there waiting for a person to complete the circuit to another grounding point.

    I am putting this is simple concepts. One could delve quite a bit deeper in to it with exceptions additional problems, etc. But it is far more than having current travelling between the transformers. (btw, that would also technically be "between coils")


    I know what I don't know, and I know where to go to find it!

  9. #19
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    Quote Originally Posted by electrofelon View Post
    Jim, if you are saying that a two bushing transformer designated 4160y/2400 takes 4160, that is not the case.
    You are correct, the simple presence of two bushings does not necessarily indicate the voltage.

    The terminology used by POCOs, and the equipment manufacturers focused on them, is not always directly interchangeable with the terminology of the NEC, and the premises wiring industry.
    Just because you can, doesn't mean you should.

  10. #20
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    Quote Originally Posted by jim dungar View Post
    You are correct, the simple presence of two bushings does not necessarily indicate the voltage.

    The terminology used by POCOs, and the equipment manufacturers focused on them, is not always directly interchangeable with the terminology of the NEC, and the premises wiring industry.
    So would it be fair to say that the utility designation of 4160/2400 is just indicating that it can be used with either a single ended 2400 source or with a 4160Y/2400 source as long as in either case it is connected across a 2400V potential?
    In the second case that would require an L-N connection to the wye source.
    If the transformer has two insulated primary bushings, it could also be connected line to line IF the line to line voltage is 2400.

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