Interest Check: Belden Thicknet 10base5 Ethernet Coax

Grant Taylor cctalk at
Fri Feb 2 17:07:11 CST 2018

On 02/02/2018 03:50 PM, Paul Koning via cctalk wrote:
> Yes, exactly.  And if the cable crosses between buildings, which at least 
> for 10Base5 is plausible, they might have different ground systems. 
> If so, grounding both ends might produce a LARGE current through the 
> cable, possibly enough to be hazardous.


> (Somewhat different but similar: there's a story about the DEC building 
> at Marlborough, which apparently had two separate power sources at the 
> two ends, from different external supplies.  Each was grounded at the 
> service entry, but the two were not bonded together (a code violation). 
> One of the machine rooms had branch circuits from both.  One system had a 
> string of large disk drives, RP04 or the like, some fed from one branch, 
> some from the other.  As required by the book, the drives were bonded 
> together by substantial braided wire jumpers.  One of those got hot, 
> possibly enough to melt it, because the two grounds were at different 
> voltages and the "sneak current" was many amps.  I'm not sure if the 
> story is true, but it sounded somewhat plausible.)

I've heard similar stories about single power sources with bad 
insulations or near large magnetic fields inducing voltages which would 
alter references.  Frequently things would float too far one way or the 
other, causing sneak current, and frying ports.

That's one of the reasons that I like to use optical connections between 
buildings and / or electrically noisy environments.

> Many coax connectors have the shell connected to the chassis.  But 10Base2 
> Ethernet connectors are required to be insulated: if you look closely 
> you will (should) see a plastic sleeve between the jack shell and the 
> mounting flange.  I don't remember for sure, but it may be that 10Base2 
> repeaters ground that end, or have an option to do so.  That would make 
> sense because you usually have just one repeater on a 10Base2 segment, 
> so grounding there is a logical thing to do.

Now that you say something, I do recall seeing that.

I also recall seeing BNC terminators with a short ball chain with a 
screw loop on the end to ground the segment at /one/ end.

> The requirement for controlling grounding is also why 10Base2 connectors 
> are often made with insulating sleeves.  For example, the ones DEC sold 
> had plastic shells as an integral part of the connector assemblies (T 
> connectors too).   Similarly, you might find plastic shells covering 
> 10Base5 barrel splices or terminators (those were separate from the 
> connectors themselves).

I was not aware that there was an insulator around the connectors to 
avoid grounding in undesired locations.

I guess that 10Base2 should only have T connectors at the cards and all 
of the coax in between would be shielded.  Thus the entire length would 
be isolated, safe for the ground point, along the entire run, if 
properly installed and nothing grounded the terminators unexpectedly.

Thank you for the clarification Paul.  :-)

Grant. . . .
unix || die

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