[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


Poul-Henning,


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 
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.


> >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: 
<http://en.wikipedia.org/wiki/Triaxial_cable>.

The terms are very similar.


> But yes, double shielding works great, provided you don't have morons
> with screwdrivers around.
> 
> Poul-Henning
> 
> (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?


Joe





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