[time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
Joseph M Gwinn
gwinn at raytheon.com
Wed Jan 7 18:34:19 EST 2009
time-nuts-bounces at febo.com wrote on 01/07/2009 05:56:19 PM:
> In message <OF56303512.93B049A7-ON85257537.0079CDE3-85257537.
> 007CC84F at mck.us.ra
> y.com>, Joseph M Gwinn writes:
> >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.
> >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.
> 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.
> >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.
> 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?
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