[802.3BA] Standards for immunity of shielded copper links to power-frequency ground loops
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