[time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops

David I. Emery die at dieconsulting.com
Thu Jan 8 20:38:59 EST 2009


On Thu, Jan 08, 2009 at 08:51:45AM +0000, Poul-Henning Kamp wrote:
> In message <49657762.5060504 at 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 at 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."





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