The one-point connection od the 10 Base- coax shield is specified by IEEE standard 802.3. In my (2000) revision, it is e.g. in paragraph 8.7.2.2., page 158.
"....to prevent the building up of voltages that may result in undue hazard to connected equipment or to persons." However, when twisted-pair (differential) interface is used in 10/100/ and 1000 Mbps Ethernet, the cabling may be unshielded twisted pair (UTP) or shielded twisted pair (STP). There isn't such a requirement to connect the shield at only one side in that case. E.g. section 41.4.2.2 of the same standard describes it, and it says that if the shielded cable is used, then "....the shield may be grounded at both ends according to local regulations and ISO/IEC 11801, 1995, and as long as the ground potential difference between both ends of the network segments is less than 1 Vrms." Neven > > Rich, > > I think you must be correct. I do have anecdotal evidence of persons being > "knocked on their butts" by grabbing hold of an Ethernet coax in a large > facility and trying to connect it to a computer. > > My experience is in EMC and the 10Base2 Ethernet (coax) is a persistent > offender. (Actually, the twisted pair stuff has its own set of EMC > problems.) > > Regards, > > Jim > > > Jim Knighten, Ph.D. > Teradata, a Division of NCR http://www.ncr.com > 17095 Via Del Campo > San Diego, CA 92127 > USA > Tel: 858-485-2537 > Fax: 858-485-3788 > [email protected] > > -----Original Message----- > From: Rich Nute [mailto:[email protected]] > Sent: Wednesday, March 05, 2003 4:31 PM > To: Knighten, Jim L > Cc: [email protected]; [email protected] > Subject: Re: Ethernet coax connection > > > > > Hi Jim: > > > It is a potential shock hazard if the coax run is long and runs from > > building to building (for instance) where the ground potentials may be > > different in the different buildings. One can develop a large potential > on > > the shield of the cable, so that if you put yourself between the cable > > shield and ground you may get a strong shock. That is the reason for > > isolating the shield from more than one direct connection to ground. It > is > > a real issue. > > I don't believe the issue is that of electric shock. I don't believe the > current and resistance of the neutral are high enough to develop 30 volts > (the shock hazard limit). > Instead, I believe the problem is that, with two connections, the shield is > in parallel with the neutral. That means that some of the neutral current > will pass through the shield, and will likely open a PWB trace somewhere. > In the process, the PWB trace heating may start a fire. > I believe the circuit is: > (You may not be able to read this schematic if your mail reader is not set > for plain ASCII text.) > > > L Bldg A > +---------+----------------------------+ > | | | > | | | > | | | | > (~) 120 V | | | load > | | | | > | | | > |N | | > +-------------+--------+---------+-----+ > |utility | | | Bldg A | > |ground | | | ground | > |rod | | | rod |PE BNC shield > ----- | | ----- +----->>-------+ > --- | | --- | > - | | - | > | | | > | | | > | | | > | | L Bldg B | > +----------------------------+ | > | | | > | | | > | | | | > | | | load | > | | | | > | | | > | N | | > +--------+---------+-----+ | > | Bldg B | | > | ground | | > | rod |PE | > ----- +----->>-------+ > --- BNC shield > - > > > You can see from the schematic that the shield is in parallel with both the > neutral and the earth-ground. Being in parallel, it will carry some of the > neutral current, depending on the respective resistances. > Because of both the neutral and the earth-ground, the voltage should never > approach 30 volts (1/4 of the mains voltage). But, there can be a very high > current in the shield of the BNC cable. > > Best regards, > Rich > > > > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ > > To cancel your subscription, send mail to: > [email protected] > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Ron Pickard: [email protected] > Dave Heald: [email protected] > > For policy questions, send mail to: > Richard Nute: [email protected] > Jim Bacher: [email protected] > > Archive is being moved, we will announce when it is back on-line. > All emc-pstc postings are archived and searchable on the web at: > http://www.ieeecommunities.org/emc-pstc This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
