Thicknet/10base5 Test Segment: The Cable is In!

[Eric Smith]

On Thu, Jun 28, 2018 at 7:18 AM, Paul Koning via cctalk <
cctalk at classiccmp.org> wrote:

> > On Jun 28, 2018, at 4:52 AM, Peter Coghlan via cctalk <
> cctalk at classiccmp.org> wrote:
> > On a slightly different point, didn't the thickwire spec call for the
> outer
> > conductor of the cable to be earthed at exactly one point, presumably for
> > safety reasons in case the cable contacted something at high voltage?
>
> Yes, Ethernet spec section 7.6.3.  Also for static discharge, though it
> doesn't say that explicitly.
> [...]
> Looking at 802.3, it says that a Thinwire segment MAY be grounded at one
> point, but not at multiple points.  It also requires a static discharge
> path at each transceiver, 1 Mohm to ground.
>

In case it may not be obvious to some readers, the reason you should NEVER
ground an Ethernet cable (of any kind) at two points is that the ground
potential at two different points is unlikely to be the same, so that will
cause a DC current flow through the cable.  If there was a DC flow of a
only a few mA, it might not be a big problem, but in practice the resulting
DC flow could be _many_ amps, and could both damage equipment and be a
safety hazard for personnel.

Of course, this isn't specific to Ethernet. It can happen with e.g.
TIA/EIA-232 (formerly RS-232) as well, and in fact that is even more common
in practice, because Ethernet is transformer-isolated at each station, but
TIA/EIA-232 is usually not.

It's perfectly fine to have the 1 Mohm static discharge path at multiple
points, because the 1 Mohm resistance prevents there from being any
significant current. If the cable is not grounded at all, having at least
one such static discharge path is important to ensure that no static charge
builds up on the cable.