# Inductive coupling - please settle argument ...

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

1. ### ZymurgyGuest

All,

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
up.

Paul.

2. ### Robert BaerGuest

Ther is some coupling from AC wires that are conducting current and
less (but stilll some) from AC wires that just have voltage.
BUT.
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. ### JimmyGuest

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 BaerGuest

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. ### ZymurgyGuest

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 !

Cheers,

Paul.

6. ### Robert BaerGuest

*** 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. ### JimmyGuest

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 GriseGuest

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.

Cheers!
Rich