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

[Joseph M Gwinn]

Poul-Henning,


time-nuts-bounces at febo.com wrote on 01/07/2009 04:25:04 PM:

> In message <OFADE54B4F.D29DBA7A-ON85257537.00086866-85257537.
> 000907CA at mck.us.ra
> y.com>, Joseph M Gwinn writes:
> 
> >The effect of differing ground potentials on a shielded cable is to 
pull a 
> >large current through the shield, [...]
> 
> The correct enginering solution is to use twinax, ground the shield
> in one end only and transformer-couple the signal at least in the
> other end from the grounding.

Yes, I know of this.  Shielded twisted pair is also widely used in audio, 
for the same reasons. 

But my system is coax and was that way before I arrived.

I know of some similar but ship-board systems that use twinax for time 
reference distribution, as you suggest.  Don't know if they use 
transformers though.  Could be a differential TX and RX.  I recall that 
they send a RS422 signal.  I imagine that the shield is grounded at both 
ends, if only for safety reasons.

Fortunately, my system is not so noisy as a ship.

If I had it to do over, I might well use multimode fiber.

 
> Look at IBM's 5250 terminal hookup for an school book example of getting
> it right.

A blast from the past - shades of the 1970s!

A parallel story:  Some years ago I was working on shipboard systems that 
used 10BASE5 ethernet over thick coax (nominally RG8).  The problem is 
that there is no real ground on a ship, and there can be 7 volts 
difference between bow and stern because the hull is used as the power 
system neutral.  Well, 10BASE5 ethernet uses 2-volt signals, so 7 volts 
offset would prevent communications.  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.  For safety, this 
connection had to be able to handle 1,000 amps, to ensure that breakers 
would pop before ground links opened.  (One of our young engineers was 
going to use a AWG #30 wire-wrap link.)  The outer shield stopped at the 
cabinet I/O panel, with only the inner shield and center conductor 
continuing (as a bit of RG58) to the etherent transceivers.  This worked 
flawlessly.


Joe