On Thu, Jun 28, 2018 at 7:18 AM, Paul Koning via cctalk < cctalk@classiccmp.org> wrote:
> > On Jun 28, 2018, at 4:52 AM, Peter Coghlan via cctalk < > cctalk@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.