[time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
Brian Kirby
kirbybq at bellsouth.net
Wed Jan 7 03:54:41 UTC 2009
During my experiences involving audio/phone, video and data
transmission, we were taught to ground the shield at one end only so we
would not cause a ground loop.
I ran into problems everywhere I went with this and as much as folks
disdain transformers, they are your friend in this type of problem.
Don White Consultants/Interference Control Technology published a whole
series on EMI, Grounding, and EMC for the military. They are located in
Gainesville, VA.
Brian
Joseph M Gwinn wrote:
> First the background:
>
> In some timing distribution applications, the primary source of
> interference comes from different ground voltages in different parts of
> the facility, such as a ship or a megawatt radar.
>
> The effect of differing ground potentials on a shielded cable is to pull a
> large current through the shield, so there is a significant voltage
> between the ends of the cable. No matter how good the shield is at RF,
> one consequence is that the same power-frequency offset voltage appears on
> the conductors within that shield, because the skin depth at 60 Hz vastly
> exceeds the thickness of any reasonable shield. Unshielded twisted pair
> will suffer the same common-mode offset voltage, perhaps more. This
> offset often contains significant harmonics of the power frequency,
> nominally up to the seventh harmonic, not just the fundamental.
>
> If the cable is shielded twisted pair, such as twinax, the offset appears
> as a common-mode voltage on the two conductors, and (if not too large) is
> eliminated by the CMRR of the receiver.
>
> If the cable is coax, the offset voltage appears added to the timing
> signal voltage, and if the offset isn't too large the signal receiver will
> be sufficiently immune to this conducted EMI.
>
>
> And now the question:
>
> What standards exist governing required immunity of signal ports to these
> ground-loop induced power-frequency (hum) voltages?
>
> All the conducted suseptability standards I've found cover only
> frequencies exceeding 10 KHz, not power frequencies and their harmonics.
>
>
> Thanks,
>
> Joe
>
>
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