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  1. #31

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    Quote Originally Posted by lewy View Post
    A couple of different things here, yes with a 240 delta you can add a 120/240 service to 1 transformer, but this does not give you 120/208 3 phase like the OP was asking. Secondly as far as dead bushings in different locations, on an additive transformer the dead bushing would be on the left and on a subtractive transformer the dead bushing would be on the right and you could use both transformers in the same bank.
    The dead bushing would be the one that the man who split the pot decided it would be X3 X2 or X1, they are split differently depending on where you go. As far as banking additive and subtractive pots together I agree it can be done, but on more than one occasion I have seen people who didn't realize that they had an additive and subtractive banked together in an open delta configuration and changed out the bad transformer which happened to be the subtractive pot with an additive, then built it back to company specs and after closing the bank in burnt up some stuff on the customers side, I realize that voltage should have been checked prior to leaving the call but in these 2 cases it apparently was not, we try to bank additives with additives and likewise with subtractives.

  2. #32

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    Quote Originally Posted by Lineman North Florida View Post
    The dead bushing would be the one that the man who split the pot decided it would be X3 X2 or X1, they are split differently depending on where you go. As far as banking additive and subtractive pots together I agree it can be done, but on more than one occasion I have seen people who didn't realize that they had an additive and subtractive banked together in an open delta configuration and changed out the bad transformer which happened to be the subtractive pot with an additive, then built it back to company specs and after closing the bank in burnt up some stuff on the customers side, I realize that voltage should have been checked prior to leaving the call but in these 2 cases it apparently was not, we try to bank additives with additives and likewise with subtractives.
    It can be done,you would roll a highside or lowside.But I may have seen that done once if any.We,like you try to bank the same with the same.Although there was a time we banked 3 19.9 transformers.277/480.Two had two highside bushings and one had three.Took a bit to figure all that out but we did.That was years ago and by now I forgot all the details involved.....I just wanted to ask if anyone checks voltage on changed-out transformers before hooking up the secondaries,its a pain but Ive seen that done here.

  3. #33
    Join Date
    Feb 2007
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    Ontario Canada
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    1,284

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    Quote Originally Posted by Lineman North Florida View Post
    The dead bushing would be the one that the man who split the pot decided it would be X3 X2 or X1, they are split differently depending on where you go. As far as banking additive and subtractive pots together I agree it can be done, but on more than one occasion I have seen people who didn't realize that they had an additive and subtractive banked together in an open delta configuration and changed out the bad transformer which happened to be the subtractive pot with an additive, then built it back to company specs and after closing the bank in burnt up some stuff on the customers side, I realize that voltage should have been checked prior to leaving the call but in these 2 cases it apparently was not, we try to bank additives with additives and likewise with subtractives.
    I would think when splitting a pot for 120 phase to ground they would leave X2 as the ground, then it is just a matter of wether you use X1 or X3 as your live phase. We were always taught that H1 and X1 should have the same polarity so on an additive bank you would be using X1 on the right and on a subtractive bank you would be using X1 on the left, this is all assuming you are feeding the transformer on H1.

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    Sorry, made a vague statement and went away for a few days. I meant to say that you cannot bank two three phase banks of different polarities. Like if you wanted to upgrade one bank to a higher KVA, but not have the customer lose power, you could bank that bank to another bank the next pole of the same voltage for temp power if there happened to be one of the same voltage. I made a statement without really thinking it out, but you can possibly have the same polarity if the dummy bushings were in different spots on two different banks. Trying to point out that you could possibly change the polarity of the bank by moving bushings around. I don't know why you would but you could run into the scenario where a contractor that is from another part of the country put up a bank that is normal way for them, but not for you.

    Just something to look out for.. banking banks is almost never worth the hassle though and we would just get a generator or do it at night Just thought I'd throw it out there in case someone came across it. Obviously an odd scenario, but you never know.

    I have seen personally what happens when you bank two polarities together with step-downs... let's just say there is a lot of clean up.

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