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Thread: Nuisance breaker tripping on the primary side of a 2kva transformer

  1. #11
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    Quote Originally Posted by ptonsparky View Post
    Make this simple, keep putting in different breakers or fuses until the smoke is out of the transformer.
    How many transformers should one let the smoke out of before checking out the connected load?

  2. #12
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    Quote Originally Posted by ptonsparky View Post
    Make this simple, keep putting in different breakers or fuses until the smoke is out of the transformer.
    jumper around the cb and call it a day



  3. #13
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    Quote Originally Posted by ptonsparky View Post
    Make this simple, keep putting in different breakers or fuses until the smoke is out of the transformer.
    or, better, until code allowance for breaker over sizing limit is reached. What prevents OP from trying that? The OP over ambition to use primary protection of the transformer for over load protection of the secondary load does not work in his case.
    Last edited by Sahib; 11-13-17 at 05:01 AM. Reason: To add last sentence

  4. #14
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    This might be off topic. Have the transformer tested. There might wrong in the transformer's insulation.

    Or plot the fuse curve with the transformer's damage curve.

    Or put the transformer near an allspark it might transformer to an autobot..hehe

    Sent from my vivo 1606 using Tapatalk

  5. #15
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    Is the secondary a two wire system? If not you need secondary protection anyway.

  6. #16
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    To kwired's point, the NEC allows for primary only protection, because the NEC doesn't know what TYPE of transformer you have and is only concerned for safety, not functionality. It's not a design guide, it's a set of minimum standards.

    The REASON why you see MOST Control Power Transformers fused on both the primary and secondary is that by fusing the secondary, you are allowed to use 250% OCPD on the primary side and the magnetizing inrush current of CPTs is typically higher than on standard transformers. That then is because CPTs are designed specifically to not allow as much voltage drop on the secondary when a large coil inrush takes place, but the trade off is a higher inrush then initially energized. It doesn't blow every time because there are other factors in determining the peak inrush current, such as the point in the incoming sine wave at the exact moment of energization, but 167% is almost assuredly going to pop once in a while.

    So if this is not a control power application with inductive coils on the secondary side to worry about, change to a distribution transformer, or more simply, just put protection on the secondary side and a larger breaker on the primary.
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