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Fault in isolation transformer

  1. Dec 30, 2011 #1

    Maybe this is a stupid question, but I've been wondering about this all day.

    In my book, they only explain the isolation transformer really brief.

    As far as I understand, the main of the transformer has a neutral which is grounded, if you touch that, the current will flow through you (you act as the ground).

    Now the secondary of the transformer isn't referenced to ground, so if you touch one wire, there will be no current.

    This is the part I don't understand... How come no current will flow?
  2. jcsd
  3. Dec 30, 2011 #2
    Firstly, the neutral on the primary side is not connected to ground, so don't go touching it!

    The secondary coil is free to float up and down in voltage. Suppose you have a simple isolated circuit with a battery and a resistor. You would expect that it makes no difference to the current flowing round the circuit if you ground it at the bottom of the resistor. However it also makes no difference to the current if you ground the circuit at the top of the resistor, effectively grounding the positive terminal of the battery. What happens is that the 0V part of the circuit, the battery negative terminal, shifts down by the battery voltage to compensate. If the battery was replaced by a floating AC source, as in the isolation transformer, a similar thing would happen. The voltage on the other terminal would oscillate up and down relative to the grounded point. If you were the grounded point then no current would flow through you, it would just flow round the circuit as before. If you touched both terminals then you would be electrocuted!
  4. Dec 30, 2011 #3

    jim hardy

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    one can learn a lot from the humble flashlight battery.

    hold a battery (C or D or AA or 9v) up in front of you and contemplate this::::

    electrons are pushed out of the negative end and sucked back in the positive end.
    motive force is electrochemistry inside but that's irrelevant - it could as easily be a motor driven electron pump (imagine a miniature aquarium pump - this is just a thought experiment.)

    electrons pushed out the negative end must get back to the positive end, else the battery would run out of electrons almost immediately.
    the internal electron pump must have a constant supply of electrons to push around the loop, just as an aquarium pump must have water molecules to push around..

    that is the basis of Kirchoff's current law - electricity must get back to where it came from.

    NOW - your transformer winding is not a lot different from that flashlight battery.
    its motive force is electromagnetic not chemical
    and the pump reverses direction 120 times per second

    but every electron that comes out one end of that transformer winding
    must get back to the other end of same winding

    so - if you touch the winding at one end only
    and there's no path through you to other end of winding
    there's no reason for electrons to go through you. they'll only take a path that gets them back home.
    since the isolated secondary winding has no connection to earth ground
    its electrons have no reason to go there. they only want to get back home.

    that's oversimplified, but not by much.

    alternating current is, at any instant, DC. that's a handy thought tool when figuring things out.

    old jim
  5. Dec 30, 2011 #4


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  6. Dec 30, 2011 #5

    jim hardy

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    Great link. Bill Beatty is one of my favorite writers of science for the common man.

    from nsaspook's link:

    "Batteries and generators do not create these electrons, they merely pump them, and the electrons act like a pre-existing fluid which is always found within all wires."

    the analogy to water works provided one is rigorous and keeps his thinking straight.

    what is risky is to think the electrons are moving fast.

    they aren't. if they approach 1m/sec they'll melt the wire.

    bill beatty has essays on that subject too.

    edit it would have been more correct for me to say "charge must get back to where it came from"
    as bill's essay points out, 'electricity' is really an undefined term.
    Last edited: Dec 30, 2011
  7. Dec 30, 2011 #6


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    As long as we say electrons carry charge I'm on the boat but charge is not energy and the direction of electron flow has nothing to do with energy flow. With AC, electrons drift to and from the load with no net movement but energy only moves forward. Electrons/protons (charge carriers) confine and modulate the flow of photons/EM fields that carry electrical energy.

    The isolation transformers transfer of energy with magnetic fields is a good example of charge (in the wire) and energy being separated.
    Last edited: Dec 30, 2011
  8. Dec 30, 2011 #7

    jim hardy

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    """charge is not energy and the direction of electron flow has nothing to do with energy flow. ""

    no disagreement with that.

    i was taught in high school ca 1962 charge is coulombs and energy is joules and volts is joules per coulomb.

    a few years later in thermo i was taught mass is pounds and energy is btu's and enthalpy is btu's per pound.
    i really liked the parallels.

    i guess i was one of the lucky ones . i missed "new math" too.

    i still dont understand what is meant by "electron energy flow" .
    sounds like an oxymoron to me.

    maybe growing up on vacuum tubes was a blessing.

    i try to explain things in words that lead a beginner to a valid mental picture of what's physically going on..
    electric current is mostly electrons drifting along.
    it's different than wave propagation which approaches c.
    like people on an escalator - they move slowly but a rumor will traverse the length of the escalator immediately. they're different phenomena.

    in semiconductors the texts speak of "hole current"
    but even that is just valence spots in p doping atoms getting occupied briefly by electrons en route to the next hole - just like in chinese checkers it's not the holes in the board that move but the marbles.
    the atoms in p material pretty well stay put.

    if i missed something, or said something misleading, please correct me.

    old jim
  9. Dec 30, 2011 #8


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    What's usually missed in the beginner lesson is how the 'rumor/energy' travels. They hear electron electron electron and start to assume that electrons move at c instead of photons without it being made clear in simple terms the basis of electrical energy.

    As Bill has written:

  10. Dec 30, 2011 #9

    jim hardy

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    thanks mr spook.

    today i held one end of grandson's Slinky, had him hold the other and we stretched it out about 4 feet.

    i had him "tap" it repeatedly until he became aware of the wave traversing mr slinky's full length and bouncing off the ends. he was fascinated.

    he's eight. i'm laying groundwork now.

    in that last link you gave to Bill,

    he said
    " I kept discovering new ways in which I'd MISunderstood it myself, and I kept adding to the growing pile. The misconceptions list became large, and soon I also was discovering parts of electricity that the general public invariably found misleading, or parts that were universally explained badly even in physics textbooks."

    it seemed an uncanny parallel to this, from Lavoisier's Introduction to his treatise on Chemistry:

    ""Instead of applying observation to the things we wished to know, we have chosen rather to imagine them. Advancing from one ill founded supposition to another, we have at last bewildered ourselves amidst a multitude of errors. These errors becoming prejudices, are, of course, adopted as principles, and we thus bewilder ourselves more and more. The method, too, by which we conduct our reasonings is as absurd; we abuse words which we do not understand, and call this the art of reasoning. When matters have been brought this length, when errors have been thus accumulated, there is but one remedy by which order can be restored to the faculty of thinking; this is, to forget all that we have learned, to trace back our ideas to their source, to follow the train in which they rise, and, as my Lord Bacon says, to frame the human understanding anew.""

    the more things change, the more they stay the same.
    i was profoundly affected by Lavoisier's essay when first read it thirty-odd years ago, and always keep it handy.

    Lavoisier speaks to the importance of unambiguous language in explaining things, and indeed that's the hardest part.


    i know i've posted the Lavoisier piece before, but old men repeat themselves.....
    if you're really with nsa you will see plenty of Lavoisier's compounding of errors it's inherent in bureaucracy.
    ( i used that paragraph about 'ill founded suppositions' in more than one inter-department memo. )
    May i also recommend Parkinson's "Law of Delay" ? Even Gorbachev quoted him...


    old jim
  11. Dec 30, 2011 #10


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    The OPs question is not stupid because there seems to be a problem with the teaching of electricity starting at the most basic levels that hasn’t been addressed and is causing immense confusion for technicians working on machines that control energy in forms other than simple power circuits. I have lots of very sharp young guys who know all the math (Just like the Mayans calculating the rise and fall of Venus) but have a very jumbled idea of what it really shows and are unable to use that information to make an educated guess at problems.

    When I was a member of the San Diego Community College "Electronics Technician Advisory Committee" almost 25 years ago reviewing Vocational Education we had the same problems of books spending a huge about of time discussing + to - electron flow or - to + flow but with only a passing reference to joules, charge and energy.


    Happy New Year.
    Last edited: Dec 30, 2011
  12. Jan 11, 2012 #11
    Sorry for the really late response, but I was on holidays.

    I read all your replies, and I have to agree with this the most:

    "The OPs question is not stupid because there seems to be a problem with the teaching of electricity starting at the most basic levels that hasn’t been addressed and is causing immense confusion for technicians working on machines that control energy in forms other than simple power circuits."

    I study these machines or difficult problems, where I have no trouble understanding how they work (usually I find it relatively easy), but somehow I always get stuck thinking about the basics... I figure out the entire machine, and all that is left are the basics of Thermodynamics, Electricity, you name it. While this may be a flaw from myself (I don't really 'accept' things that quickly and spend hours trying to figure out every detail), I 'blame' it on my teachers back in my early educational days too.

    So thanks again for the insight, and the helpful links.


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