Jim,
I think your writeup is excellent. The only point I would disagree with you
about is your assertion that "ground loop" refers only to magnetic coupling.
This is just a matter of nomenclature, of course, which can mean different
things to different people, but to me the phrase includes any situation
where current can flow between different pieces of equipment because of a
difference in AC (or DC) potential. This can happen when there are two or
more ground paths connecting the equipment.
The situation certainly includes magnetic induction, but can also be caused
by other things. The most extreme example for hams probably is caused by not
using a single point ground for their station equipment. If lightning
strikes their tower, then destructive currents can flow along the coax
shield from their antenna, to their $5000 transceiver, into the green wire
on their power cord and to their service entrance ground. A single point
ground, by contrast does not have this problem, since there is no path for
the return current. The entire station just rides up to high voltage during
the strike, but no damage is done.(That's the theory, anyway. When hundreds
of KV are involved, all bets are off!)
Jim W8ZR
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Jim Brown
> Sent: Friday, June 07, 2013 4:49 PM
> To: amps@contesting.com
> Subject: Re: [Amps] The Pin One Problem - Common Impedance Coupling
>
> On 6/7/2013 12:54 PM, Roger Parsons wrote:
> > I think that a technically correct generic name for the 'Pin 1 Problem'
would be unwanted
> 'Common Impedance Coupling', and a bit of a search shows that this term
has been quite
> frequently used.
>
> No, it is NOT the correct description.
>
> Joe hit the nail on the head, and Paul got a lot of it right.
>
> Old time broadcasters who "never saw these problems in big consoles"
> were almost certainly working on gear built in the old days, when a jack
> was screwed down to the chassis, and the shield went straight to the
> chassis. Pin One Problems began to abound when mfg got too efficient
> for its own good, with connectors that mounted to the circuit board, but
> forgot that shields had to make contact with the chassis.
>
> Now I'm going to really arouse folks -- there is no such thing as a
> "ground loop!" When power line "buzz" couples into unbalanced wiring,
> it is because the chassis of two pieces of equipment being connected are
> at different potentials, and that potential difference is added to the
> signal. The fundamental cause of the potential difference is nothing
> more or less than LEAKAGE current from the AC power system. The path
> for this current is capacitance between the "high" side of the AC power
> line and the chassis, which must, by law, be connected to the "green
> wire" of the AC line, that current returns to its source (the power
> system) via the green wire, and the current produces IR drop in the
> green wire. That IR drop is what we hear.
>
> The leakage current consists primarily of the HARMONICS of 60 Hz, which
> is why we hear "buzz" rather than "hum" (pure 60 Hz). Why harmonics?
> Because nearly all equipment we connect to the power line as a load has
> a capacitor-input power supply, which causes current to flow in
> relatively short pulses at the top and bottom of each cycle, which in
> turn makes the current rich in harmonics.
>
> As if that weren't enough, in three-phase systems, "triplen" harmonics
> (any harmonic number divisible by three) will ADD both in the neutral
> and in the ground. Few of us have 3-phse in our homes, but the vast
> majority of homes are fed by 3-phase distribution systems. As a result,
> almost all of us will see a lot of triplen harmonics if we do an
> spectrum display of the power line, and of the voltage between one
> chassis and another of equipment plugged into different outlets.
>
> So what we incorrectly call a "ground loop" and do stupid things to
> avoid, is really AC leakage current, and the cure is VERY simple.
>
> 1) Minimize the voltage difference between one chassis and another by
> getting power for everything that will be interconnected from the same
> AC outlet, or from outlets that share the same green wire, or from
> outlets that are bonded together by short fat copper.
>
> 2) Bond with short, fat copper from chassis to chassis of every piece of
> gear that has an unbalanced connection.
>
> #1 takes the IR drop of the long "green wire" from the outlet to the
> panel out of the picture, so the remaining IR drop is only the short
> (typically 6 ft) line cord. #2 helps two ways. First, shorts out the
> remaining potential difference by brute force. Second, it diverts that
> leakage current (and most of the RF that might be picked up on a short
> cable) to the chassis, so it also cures any Pin One Problems that might
> be present.
>
> In the tutorials I do for pro audio/video trade shows, I call this
> technique "local bonding," emphasizing the fact that it only works where
> cable lengths are short enough that the resistance of the bonding
> conductors is low.
>
> As I see it, there is one, and only one, good time to use the word
> "loop" in a discussion of noise coupling, and that is when the coupling
> mechanism is the MAGNETIC field. With all magnetic coupling, the
> strength of both the field and the sensitivity of a system to a magnetic
> field is the LOOP AREA of the coupling path. In our stations, we
> commonly see magnetic coupling in three situations.
>
> 1) Leakage flux from big power transformers close to our equipment
> and/or its wiring.
>
> 2) Wiring errors in the power system that establish a magnetic field.
> Perhaps a neutral is bonded to the green wire at more than one point, or
> a load is connected between "hot" and "green," or an outlet is mis-wired.
>
> 3) We feed a long wire antenna that ends in the shack, and thanks to the
> length of the antenna, there's a current peak near our equipment. That
> current produces a strong magnetic field. As has been observed, a cable
> shield provides NO magnetic shielding at power frequencies, and TWISTED
> PAIR is VERY effective at rejecting magnetic fields.
>
> 73, Jim K9YC
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