Standards sought for immunity of shielded cable links to power-frequency ground loops
On Thu, Jan 08, 2009 at 08:51:45AM +0000, Poul-Henning Kamp wrote:
In message 49657762.5060504@rubidium.dyndns.org, Magnus Danielson writes:
Was there a big bang? What was the source of the 600 amps?
I think there (with some delay) was some awfull scream of dispare.
The cost of Ethernet interfaces where much more significant back then.
The most expensive one we lost was in a UNISYS 2200, where three
microprocessors worked together to limit bandwidth to 100 kB/s.
I belive the sticker prices as $15k.
I'm somewhat confused about how this took out Ethernet
transceivers or interfaces... from the beginning even vampire tap RG-8
yellow cable Ethernet transceivers were ground isolated from chassis
ground of the computer system just exactly to avoid ground loops and
back path ground currents. Both power and transmit/receive and control
signals are isolated... and usually transformer coupled... and as I
remember it rather a substantial voltage difference between shield on
the cable and computer system ground had to be tolerated (hundreds of
volts at least)...
I guess, however, if someone grounded the yellow cable at more
than one point enough current could flow on its outer conductor to
induce substantial voltage between the shield and the center conductor
which could trash the driver/receiver/carrier sense chips or protective
clamp diodes ...
One was never, of course, supposed to ground the yellow cable at
more than one point...
--
Dave Emery N1PRE/AE, die@dieconsulting.com DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in
celebration of what could have been, but wasn't and is not to be now either."
Joseph,
time-nuts-bounces@febo.com wrote on 01/07/2009 10:47:46 PM:
Joseph,
Could be a differential TX and RX. I recall that they send a RS422
signal.
Depending on the speed, RS422 works fine with transformers.
Yes. It would be 10 MHz or 20 MHz, depending on coding. Or 5 MHz, so
the
transitions are at 10 MHz. I don't recall, or never knew.
RS422 does not imply any encoding as such so it would be 10 MHz but
naturally there is twice that many transitions, but it is the frequency
of the signal you are interested in for this case.
I know that RS422 is not the encoding. I cheated, and talked to the
relevant engineer.
That is to cheat! :)
For digital signals (1PPS, various triggers), it's RS422 over 100 ohm
twinax (fancy shielded twisted pair).
The 10 MHz sinewave is sent over a pair of 50 ohm coax links, with the
signals 180 degrees out of phase. This is acheived with a pair of hybrid
transformers which convert from one-cable to two-cable and then back to
one-cable, where all cables are 50 ohm coax.
OUCH! The trouble with that arrangement is that the coax cables MUST be
twisted or else H-fields will induce differential mode current. It will
induce current into both directions which through the 180 degree will
not cancel but add up. The 0/180 degree arrangement will save you from
common mode problems. You would prefer a twisted cable over a twisted
cable pair, as the later allows for installation procedure errors to
have huge impact and the twisting properties will not be as good either
and thus compromising the quality. A single ended coax is not as
sensitive to H fields to induce diffrential currents, but can have some
other problems.
I imagine that the shield is grounded at both ends, if only for
safety reasons.
That is actually a very unsafe practice, unless there is another
much thicker and reliable ground connection between the two domains.
There is a very heavy grounding grid, and such systems almost always
ground the (outer) shields at every connector.
Which would imply that if the signal passes through a connector jack or
through a wall, much of the current would be sent back to its EMF source
locally in the room. This does have its merits.
Yes, but that isn't the reason. It's really a safety and EMC rationale.
As suspected, but this is really just another of these EMC rationales.
But you should never let the screen float in the far end, you should
terminate it with a 10M resistor and a sparkgap in parallel to the
local ground.
The resistor takes care of static electricity and the sparkgap will
do lightnings.
I've done such things, but with a 100 ohm resistor (and a safety
ground to
ensure that the voltage doesn't get too large. But this was
a lab lashup.
The trouble with 100 ohm is that still can be a little low in relation
to ground loop impedances, it still allow some fair current to roll down
the cable. A capacitor in parallel would cut most of the transient
energy straight through and allow for a higher resistive path for the
low frequency energy.
The ground grid impedance between any two points is well less than one
ohm, so 100 ohms will pretty much abolish all ground loops. I've used 10
ohms in like labs, with success. I'll grant that this would not work with
long wires outside.
Should be sufficient then. But remember that capacitive coupling helps
you in the RF area and impulse protection.
By the way, I also finally talked to one of our most experienced EMI/EMC
engineers. He suggested using MIL-STD-461 test CS109, even though CS109
was developed for enclosures. It turns out he was involved in developing
CS109 when he worked for the US Navy.
Need to look it up. Never had to do any of the MIL-STD-461 stuff.
Cheers,
Magnus