Re: [802.3BA] Standards for immunity of shielded copper links to power-frequency ground loops - update
For the record, I did find some relevant standards, which I am documenting
here, in case someone in 802.3 is interested.
The most pertinent is MIL-STD-461 method CS109, which can be used on cable
shields, not just the boxes mentioned in CS109. RS101 may also be useful.
This recommendation comes from a member of the committee that developed
CS109 in the 1970s, and worked on the US DoD study published in 2000
comparing MIL-STD-461 to various commercial standards to see which
commercial standards could be accepted as equivalent to 461.
At 50/60 Hz, CS109 requires that the system under test tolerate at least
one amp rms flowing through shield and/or equipment case.
According to this study, the most relevant commercial standards are EN
61000-4-8, EN 61000-4-16, and EN 55103-2. I have not yet read these, as
they are rare and expensive, and according to the study CS109 is the
better requirement. MIL-STD-461 has the advantage of being free, and very
well proven.
Joe
Joseph M Gwinn <gwinn@xxxxxxxxxxxx> wrote on 12/17/2008 05:00:08 PM:
> First the background:
>
> In some ethernet 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 copper enet link is to
pull
> a large current through the shield, so there is a significant voltage
> between the ends of the link. 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 offset voltage, perhaps more. This offset often
> contains significant harmonics of the power frequency, up to the seventh
> harmonic, not just the fundamental.
>
> If the cable is (shielded) twisted pair, 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 signal
> voltage, and if the offset isn't too large the link still functions.
>
> (If the cable is fiber, no problem, but that is not the question here.
> While fiber is used wherevere possible, there are always some copper
> cables.)
>
>
> And now the question:
>
> What standards exist governing required immunity of signal
> ports to these ground-loop induced voltages?
>
> All the conducted suseptability standards I've found cover only
> frequencies exceeding 10 KHz, not power frequencies and their harmonics.
>
>
> Thanks,
>
> Joe Gwinn