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Thread: Lost leg on Delta Transformer?

  1. #41
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    Quote Originally Posted by jimbo123 View Post
    I don't know much about this type of transformer and would like to ask a question if i may.
    What if a service is in place with a 480v corner ground and a generator is needed for what ever reason . How do you spec out the correct generator for a corner grounded system ? Will generator service know the differrence?
    A few things need considered in this case. If the grounded phase happens to pass through three pole breakers for all loads, one could use a generator with a 277/480 Y output much easier as all wiring would be same for either supply.

    If you have fuses in the existing system you can not put them in the grounded phase - this would be a problem with a 277/480 Y generator. You may be able to float the neutral and ground a phase and then it will be ok.

  2. #42
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    Quote Originally Posted by Psychlo View Post
    I just meant in regard to the preference of a 120/240v vs. 240v corner ground. This Schneider link makes it clear that they think corner grounds are obsolete and quite inferior. Other material I have read confirms that opinion. So they started to sway me that way. But you fellas have brought me back around to thinking it is best to leave the system as is, if possible.

    If I have to upgrade the trans, then perhaps I'll consider changing it all over. There are really only six or seven loads in the panel - easy enough to check.
    Inferior and obsolete - do not always equate to unsafe.

    Yes grounded B-phase is 'obsolete' for new services serving entire premises, but it still has its place for feeding small areas like a group of machines in a machine shop, and especially for a single machine.

    I don't know if I would call it inferior, as it has some definite advantages over ungrounded systems. But, it does require 'non-routine' equipment (well at least equipment approved for it).

    Because it it so un-common corner grounded installations often lead to consternation on the part of electricians and misunderstanding by 'lay' people. When you expect to see 120V L-G and it isn't there you may start to question your sanity.

    If you only have 6-7 loads connected single phase 2w, it probably isn't a big deal to re-wire them to a center tapped 240/120V supply instead. The only ones which might have wiring issues are those using the grounded phase conductor (anything connected to only A and C must be wired correctly or there would be a short circuit).

    But it sounds like you have reached a reasonable solution for your project.
    Just because you can, doesn't mean you should.

  3. #43
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    Here, just for reference are the common polyphase XF connections from American Electrician's by Croft & Summers

    http://s168.photobucket.com/albums/u...20connections/
    Proverbs 31;10


  4. #44
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    [QUOTE=Joethemechanic;1366811]Here, just for reference are the common polyphase XF connections from American Electrician's by Croft & Summers

    http://s168.photobucket.com/albums/u...F connections/[/Don't assume

    Don't assume the ground is intentional... The owner said it went bad ten years ago to find the ground go back to basic's
    Disconnect the main just like you did in the beginning is the ground on the primary side? if yes go that direction to find it. If not it means a device like a motor or connection to one of the machines can be the cause. Drop off each disconnect for each machine including the transformer until you get a voltage reading from b to ground. Have had this exact type of problem in several plants that I have had to rewire. Hope this helps.

  5. #45
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    Well, after all this time, I finally was able to get back in there and test the transformer thoroughly. It turns out it is certainly a grounded B phase system.

    X3 was tied to ground.

    So I guess I will never know why there was thought to be an issue with a "lost leg". Classic case of being unintentionaly misled by the owner. But it has been a real learning experience just the same. I appreciate all of the comments on this post. It definitely saved my bacon this time.

    Thanks guys.

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