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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.


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 
> a ship or a megawatt radar. 
> The effect of differing ground potentials on a copper enet link is to 
> 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, 
> consequence is that the same power-frequency offset voltage appears on 
> 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