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    Lost leg on Delta Transformer?

    Recently, I looked at a 30kVA, 3Ph, 480 - 240, 3wire Delta transformer that apparently had lost a leg about ten years ago. I measured the voltage at the panel being supplied by the xfmr:

    A-B=240v
    A-C=240v
    B-C=240v
    A-Grnd=240v
    B-Grnd=0v
    C-Grnd=240v

    Can someone explain how these readings are possible? Obviously, something happened to B phase, but wht would cause these voltages?

    #2
    Originally posted by Psychlo View Post
    Recently, I looked at a 30kVA, 3Ph, 480 - 240, 3wire Delta transformer that apparently had lost a leg about ten years ago. I measured the voltage at the panel being supplied by the xfmr:

    A-B=240v
    A-C=240v
    B-C=240v
    A-Grnd=240v
    B-Grnd=0v
    C-Grnd=240v

    Can someone explain how these readings are possible? Obviously, something happened to B phase, but wht would cause these voltages?
    B-phase was purposlly connected to ground.

    This is technically called a corner grounded system, but a lot of people call it a Grounded B Phase.

    There is nothing wrong with your installation.
    Just because you can, doesn't mean you should.

    Comment


      #3
      240 corner ground would be a rare bird around here, but then again we 240/208/120 high leg deltas everywhere.


      Nothing wrong with what you have unless you need 120, You might even find you can change the taps and make it into a high leg if you need 120
      Proverbs 31;10

      Comment


        #4
        Originally posted by Psychlo View Post
        Recently, I looked at a 30kVA, 3Ph, 480 - 240, 3wire Delta transformer that apparently had lost a leg about ten years ago. I measured the voltage at the panel being supplied by the xfmr:

        A-B=240v
        A-C=240v
        B-C=240v
        A-Grnd=240v
        B-Grnd=0v
        C-Grnd=240v

        Can someone explain how these readings are possible? Obviously, something happened to B phase, but wht would cause these voltages?
        If you go to the point where the GEC terminates, probably the transformer in this case, you should see the connection between the B phase and the supply side bonding jumper.
        Rob

        Moderator

        All responses based on the 2014 NEC unless otherwise noted

        Comment


          #5
          Originally posted by Joethemechanic View Post
          Nothing wrong with what you have unless you need 120, You might even find you can change the taps and make it into a high leg if you need 120
          Most likely this is not a feasible recommendation.
          The equipment is probably all rated for corner grounded 3W and not 4W, so it is possible the entire facility would need to be gutted and rewired.

          The system has been running for 10 years, according to the OP. I am pretty sure they already have a method of getting 120V where it is needed.
          Just because you can, doesn't mean you should.

          Comment


            #6
            What could perhaps be the purpose of such an installation?

            Comment


              #7
              There was only one 3-phase load in the panel, and that piece of equipment no longer exists. The rest are 1-phase loads (heaters, A/C, etc.).

              The owner said that it lost a leg about 10 years ago and that they have "gotten by" ever since. Don't know what makes him think he lost a leg, but apparently something is not as it was originally.

              Comment


                #8
                Originally posted by jim dungar View Post
                Most likely this is not a feasible recommendation.
                The equipment is probably all rated for corner grounded 3W and not 4W, so it is possible the entire facility would need to be gutted and rewired.

                The system has been running for 10 years, according to the OP. I am pretty sure they already have a method of getting 120V where it is needed.
                Originally posted by OP
                that apparently had lost a leg about ten years ago
                I took it to mean the transformer was deemed to be defective by someone who was unfamiliar with a corner ground. It sounds to me like this trans has been abandoned for 10 years and now he is starting from scratch. If a transformer had "lost a leg" would you have continued to keep it in service for an additional 10 years after the diagnosis (although incorrect) was made?
                Proverbs 31;10

                Comment


                  #9
                  The transformer has been in use the whole time. As far as I can tell, the 1-phase loads have remained unaffected. This is an older fab shop that has had a lot of mickey-mouse repairs and add-ons over the years. That is why I didn't trust the "grounded B system", having had no prior experience with them.

                  The crux of the problem: He wants to add another 100 amp panel off of this transformer for some 3-ph, 240v loads for his new tenant. The existing panel is 150 amp. Location prohibits taking the new loads from there.

                  That being said, does a guy leave it as a corner grounded system and add the new panel? Or change it over to a standard Delta configuration (since existing loads are all 1-phase anyway)?

                  Comment


                    #10
                    Not to mention, no need to change the rest of the building if the 120 loads are comparability small.
                    Attached Files
                    Proverbs 31;10

                    Comment


                      #11
                      This is a good reference for corner grounded transformers.
                      http://www.google.com/url?sa=t&rct=j...zVaRQWqgJFVpbQ
                      Ron

                      Comment


                        #12
                        The 120v loads are all served by another 1-phase panel and transformer, so that is not an issue. Also, the new loads will not require a neutral.

                        Comment


                          #13
                          Originally posted by Psychlo View Post
                          The transformer has been in use the whole time. As far as I can tell, the 1-phase loads have remained unaffected. This is an older fab shop that has had a lot of mickey-mouse repairs and add-ons over the years. That is why I didn't trust the "grounded B system", having had no prior experience with them.

                          The crux of the problem: He wants to add another 100 amp panel off of this transformer for some 3-ph, 240v loads for his new tenant. The existing panel is 150 amp. Location prohibits taking the new loads from there.

                          That being said, does a guy leave it as a corner grounded system and add the new panel? Or change it over to a standard Delta configuration (since existing loads are all 1-phase anyway)?

                          Is the new panel only going to supply 240V loads?
                          Proverbs 31;10

                          Comment


                            #14
                            Originally posted by Psychlo View Post
                            The 120v loads are all served by another 1-phase panel and transformer, so that is not an issue. Also, the new loads will not require a neutral.

                            Then I'd leave it alone and just grab your 240 3 phase from A, B &C

                            In a hacked up shop like that, you could be opening up a can of worms by changing grounding points.

                            I've been in places like that with a 3 phase trans and all single phase equipment hooked up on romex and SO cord. You can end up with a project on your hands
                            Last edited by Joethemechanic; 01-19-12, 05:13 PM.
                            Proverbs 31;10

                            Comment


                              #15
                              Originally posted by Psychlo View Post
                              The crux of the problem: He wants to add another 100 amp panel off of this transformer for some 3-ph, 240v loads for his new tenant. The existing panel is 150 amp. Location prohibits taking the new loads from there.

                              That being said, does a guy leave it as a corner grounded system and add the new panel? Or change it over to a standard Delta configuration (since existing loads are all 1-phase anyway)?
                              Time to start over.

                              You gave us voltage readings that conform with a corner-grounded system.
                              You talk about 1-phase loads, are they all 240V? Heaven forbid, have people been running 120V loads off this existing transformer?
                              You want to add a new panel, will it need 120V?


                              A corner grounded 240V system is a 'standard delta', you are just more familiar with center-tapped systems.
                              Changing the system from a corner-grounded 240V 3PH 3W to a center tapped 240/120 3PH 4W may create a need for a major rewire job based on how the grounded conductor was previously installed (it should have been white even though it is a phase conductor). It is possible that all of the protective devices are 2-pole instead of 3-pole.

                              If three phase is no longer needed I would consider replacing this transformer with a 120/240V single phase unit.
                              Just because you can, doesn't mean you should.

                              Comment

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