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Why doesn't a center tap from a transformer short when grounded?

  1. Jan 29, 2015 #1
    Hi everyone,
    Basic question here. Residential pole mounted transformers and many other transformers that produce a neutral have that neutral "center taped" from a current carrying coil in the transformer and bonded to ground. My question is why doesn't the current in the coil short threw that neutral to ground?

    I understand a neutral in a transformer before bonded to ground has equal potential with reference to the ungrounded/hot conductors and once it is bonded to ground it creates a conductor that has zero potential to its surrounding environment. This is all done in the name of safety in case an ungrounded/hot conductor comes in contact with ground or anything grounded it will trip an over-currnet protection device.

    Now Ive read if just one current carrying conductor is connected to ground their is originally no potential difference so nothing happens. That makes sense but theoretically their is not just one conductor connected to ground. At every transformer there is a neutral connected to ground. Is their too much resistance between the different neutrals to constitute a short or is it cause all the neutrals are in phase? Or am I totally off? Thanks in advance for the help. It is much appreciated.

    Gary
     
  2. jcsd
  3. Jan 30, 2015 #2

    davenn

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    hi Gary

    there is no short circuit because there is no complete circuit
    If one of the Phase lines touched the ground, then there would be a completed circuit and a short circuit ( or at very least ) a very low resistance path between the phase and neutral

    Dave
     
  4. Jan 30, 2015 #3
    Hi Dave, Thanks for taking the time to help me out with this one.
    Ive heard that theory before and believe it must be true. I just can't see how. So its said electricity wants to return to its source and nothing else. Its source in the case of a residential home would be the secondary of the pole mounted transformer it derived from.

    So if that statement is true we can take one neutral or any single phase conductor for that matter and ground it and their will be no potential difference because there is no complete circuit with only on phase conductor to ground.

    What about a neutral or any phase conductor that is grounded from the next pole mounted transformer down the block from the same utility. If the two phases are out of phase, wouldn't that constitute a short circuit due to the nature of the push and pull in a/c voltage? does the separation in the coil of primary and secondary in a transformer really change the game that much? If so Ive never tried this but if you put a volt meter from a primary phase of a trans former to different phase on its secondary will you have no voltage?

    Best,
    Gary
     
  5. Jan 30, 2015 #4

    jim hardy

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  6. Jan 30, 2015 #5
    Hi Jim,
    Once again your wisdom has helped me understand a topic I've been struggling with. The idea, all charge must find a path back to its source, and the fact that its one of kirchhoffs laws made it a turning point for me. When I was looking deeper into that idea I found this quote from you on another thread.

    "To have a circuit, all charge must find a path back to its source, whether that source be a battery in orbit around Pluto or a transformer on the pole in your back yard."

    That gave me a good laugh and although I don't completely understand the physics of it, that line has given me confidence in the idea and that confidence will help me to learn it. So because of that principal and the fact that in a transformer their is no contact from the primary to the secondary hence, the secondary is a source, "home", their will be no potential difference between conductors of the different secondary?

    best,
    Gary
     
  7. Jan 31, 2015 #6

    jim hardy

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    Thanks for the kind words......

    those links to AMASCI posted by nsaspook are excellent and will help you in your studies.



    There'll be no current return path between primary and secondary, so it's almost correct to say that but not quite.
    Reason is reality vs theory.
    To understand theory we exaggerate to perfection... then adjust backward to reality
    When you connect your meter from primary to secondary there's no return path for meter current , so the needle will not deflect, theoretically.

    In reality we always have minute amounts of stray capacitance between transformer windings because they're metal and close to one another
    and that minute capacitance will allow a tiny trickle of current to return,
    which might allow a sensitive voltmeter to honestly report a potential difference.
    Because the stray capacitance is a high impedance the voltage reported will be less than full transformer winding voltage.



    Point being - when you try this (as i know you will)
    use several different voltmeters.
    A good DMM will be ten megohms so it may report a goodly fraction of transformer voltage
    the inexpensive Wal-Mart meter i like so well has a lot fewer ohms so will report less voltage
    note it's only 2Kohms/volt which in the world of test equipment is downright insensitive

    k2-_ad6a4671-ccac-476f-85e0-0de9e313cc88.v1.jpg

    try it - connect a dmm, then watch its reading drop when you connect a cheap meter at same time.

    There are some applications where a cheap meter does better than a fancy one !

    old jim
     
  8. Jan 31, 2015 #7

    jim hardy

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    Have you looked up a definition of electric potential ?

    That's why we use potential difference always referred to some local place, for nobody is going out to infinity to get us a meter reading referred to there.
     
  9. Jan 31, 2015 #8
    Got it. Thanks Jim.

    Gary
     
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