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Inductive coupling - please settle argument ...

Discussion in 'General Electronics' started by Zymurgy, Nov 19, 2003.

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  1. Zymurgy

    Zymurgy Guest


    There's a bit of a debate going on over in uk.d-i-y about ac line
    (mains) voltage being induced into unterminated adjacent wires.

    We've split into 2 camps, some of us are saying that the inductive
    coupling of the AC is inducing a low AC current into the unterminated
    (i.e. 'floating') line.

    The others are saying this is capacitive coupling due to the metres of
    dielectric that separate the two wires.

    Can one of the AC Current theorists on here, please debunk either or
    both theories with some reasonably technical mumbo jumbo to back it

    Thanks in advance

  2. Robert Baer

    Robert Baer Guest

    Ther is some coupling from AC wires that are conducting current and
    less (but stilll some) from AC wires that just have voltage.
    Open wires cannot have current flowing thru them, only an AC voltage
    would be found due to capacitive coupling.
    Closed wires may have a very small current flowing (due to transformer
    effect) but zero voltage; this, as far as i know, has not been measured
    or observed - most likely do to the level is below noise.
  3. Jimmy

    Jimmy Guest

    I am an electronic tech by trade and recently the company I work for has
    insisted that we electronic types learn something about power generation and
    air conditioning, so it is off to school I go. In our equipment lab the
    phenomenom that you mention is demonstrated by measuring voltage on open
    circuited wires run adjacent to wires that have voltage applied to them.
    The voltage on the open circuit was nearly as high as the one s connected to
    an AC source when mesured with a High impedance voltmeter. The gurus who
    teach the coures attribute this primarily to capacitive coupling without
    giving any real explanation as to why capacitance and not inductive though I
    am sure both play a part.
  4. Robert Baer

    Robert Baer Guest

    The key here is *open* circuit, meaning zero or very low current.
    If one were to make this other line into a loop, then it may act as a
    secondary of an air core transformer. If any "useable" current were
    found in that loop, then one could specifically state that ther was
    magnetic coupling.
  5. Zymurgy

    Zymurgy Guest

    Can you ask for more clarification then, i'd appreciate some sort of
    technical theory !

    I don't know how much capacitive coupling (if any measurable) you'd
    get if the wires weren't closely adjacent. I suppose if you glued 2
    wires to a piece of glass 1" apart (glue not touching), you'd have to
    say any voltage induced would be down to inductive coupling.

    Can you get capacitive coupling between air separated wires ?

    I think this is a particularly prevalent phenomenom with ac mains
    wiring, in that you have tightly coupled conductors sheathed together
    for many metres. Despite there being megohms of resistance between the
    wires, it would make for a decent dilectric.

    As has been said previously, you'd measure a coupled voltage, but no
    current in an unterminated wire, which is what i've measured on
    floating earths (26VAC from an adjacent 240VAC feed)

    The HVAC 3 phase AC theory i've done has only been to do with magnetic
    field & reactive losses on adjacent wires and all a long time ago !


  6. Robert Baer

    Robert Baer Guest

    *** Absolutely - even seperated by a number of feet.
    Radio Shack had sold under their Micronta brand (maybe they still do)
    a wide "pen" that will detect powered wires.
    When turned on (push button on it) and held near a power line, one
    sees the LED flash; brighter as one gets closer.
    I have another by A.V. Sperry (1.5 by 0.8 cross-section) that has
    adjustable sensitivity that is far more sensitive, and one not only sees
    the LED flash, but one also hears the hum!
    The pickup is strictly capacitive, and a wire near and semi-parallel
    with a powered line will also pick up a voltage as you mentioned.
    And this is strictly *air* as a dielectric.
  7. Jimmy

    Jimmy Guest

    Hadnt really thought abought it that much till now but with the open wires
    the primary method of coupling would have to capacitive.
  8. Rich Grise

    Rich Grise Guest

    Technically, there will be both under any circumstances; it's
    just that usually one or the other is practically negligible.
    Think of the wires as an antennas - a very small fraction of
    a wavelength at 50~60 hz, but still an antenna. The kind of
    matching you'd have to do to connect it to a receiver would
    tell you whether you're getting more inductive or capacitive
    coupleng. :)

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