[time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
magnus at rubidium.dyndns.org
Thu Jan 8 03:47:46 UTC 2009
>>> Could be a differential TX and RX. I recall that they send a RS422
>> 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 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.
>> 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.
>>> If I had it to do over, I might well use multimode fiber.
>> Yes, never roll copper more than 100m or between buildings if you
>> can get away with installing fiber.
>>> The solution was to use triax. The
>>> outer shield was grounded at both ends. The inner shield and center
>>> conductor together formed the ethernet media. The inner shield was
>>> connected to the outer shield in exactly one place.
>> That's technically speaking not triax, that's double shield. Triax
>> would have the conductors and one shield.
> No, I think that's twinax: <http://en.wikipedia.org/wiki/Twinax_cable>.
> Triax is a center plus two concentric shields:
> The terms are very similar.
I have some triax cables and connectors, but not twinax...
>> But yes, double shielding works great, provided you don't have morons
>> with screwdrivers around.
>> (Who once lost all ethernet interfaces, the access control system
>> and a few minor computers when a moron first created and then cut
>> a 600+ A ground loop).
> 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.
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