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Thread: Backfeed

  1. Default Backfeed

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    We have a three phase tap (about 3-4 miles long), going to a couple of 277-480 pad mounts at the end. (several single phase taps pull off of it) It is protected by 3 single phase hydraulic ocrs. It seems that a tree fell against one of the outside phases, openning up that one ocr. Is it possible to re-energize this phase from the pad mounts at the end of the tap?


    P S What happen to dbbrown? I learned a lot from this guy. Very knowledgable.

  2. #2

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    Quote Originally Posted by mrgrowl791 View Post
    We have a three phase tap (about 3-4 miles long), going to a couple of 277-480 pad mounts at the end. (several single phase taps pull off of it) It is protected by 3 single phase hydraulic ocrs. It seems that a tree fell against one of the outside phases, openning up that one ocr. Is it possible to re-energize this phase from the pad mounts at the end of the tap?


    P S What happen to dbbrown? I learned a lot from this guy. Very knowledgable.
    it would depend on what type of bank it is. iff its a y-y then it will not backfeed (unless theres a generator hooked up) because all transformer coils are grounded, but as soon as you throw in a delta, high or low side everything is hot. might not be full line voltage but it will still be hot because there is not a ground on all coils
    Hemingray Insulators
    work smart, go home every night

  3. #3

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    277/480 is Y-Y aint it. or at least Y low side?

  4. #4

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    Quote Originally Posted by Daddyof2 View Post
    277/480 is Y-Y aint it. or at least Y low side?
    your right, 277/480 is a wye connection but he didn't say what the primary connections were..................don't know if it's wye also or not.
    Hemingray Insulators
    work smart, go home every night

  5. Default Backfeed

    On our 277-480 pole mounted banks, all the h2 bushings are tied together plus tied to the pole grd. I'll check on this today as far as our pad mounts go.

  6. #6
    Join Date
    May 2005
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    Default Well lets see

    Yes , you can heat the primary back up through the pad mounts , clear the trees off of it first ! But it would not be a good power supply.

    Koga
    Last edited by Koga; 12-27-2007 at 07:06 AM.

  7. #7

    Wink Why?

    Why would you want to back feed the line? Or are you asking would the line have back feed on it?

  8. #8
    Join Date
    Oct 2006
    Location
    South East Texas
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    3,278

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    Quote Originally Posted by mrgrowl791 View Post
    We have a three phase tap (about 3-4 miles long), going to a couple of 277-480 pad mounts at the end. (several single phase taps pull off of it) It is protected by 3 single phase hydraulic ocrs. It seems that a tree fell against one of the outside phases, openning up that one ocr. Is it possible to re-energize this phase from the pad mounts at the end of the tap?


    P S What happen to dbbrown? I learned a lot from this guy. Very knowledgable.
    Ok If your 3 phase line goes to a 277/480 xfmr or xfmr's there are several ways it could backfeed the most likley being if a 3 phase motor was running it would induce voltage back on the secondary windings of the failed primary xfmr, In turn the transformer would step that up to primary voltage. I have encountered this only once and the primary voltage on one of those taps was very low but enough to power a single phase transformer to put out 65 or so volts on each leg.Needless to say that will kill you just as dead. And yes D brown is a very sharp guy.

  9. #9

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    There'd also be the back-feeding of the "dead" conductor through the windings of the pad mount transformers themselves. This might be one of those occasions that they thought the tripped line was still powered because they read the back-feed. If so then I wonder how long it took them to discover the REAL problem.
    Portable defibrillators were first invented to save the lives of linemen. Where's yours?

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  10. #10

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    A good rule of thumb is to "always" isolate all 3 phases before you test and ground in the overhead and the undergound.Trying to keep some customers in lights might seem like a good idea but backfeed potential should always be on our minds. I like the question because it reminds us all of the dangers we face and keeps us sharp.Good luck and work safe!

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