I knew that I was headed for trouble when I responded to Ken Javor original question about EMI frustrations with 10Base2 Ethernet. I figured the safety guys would pummel me a little - and I accept the comments. However, I couldn't resist since I have experienced the same EMI frustrations.
Nevertheless, good EMI engineers are sometimes faced with an existing design that uses 10Base2 (legacy, if you will) and has the severe EMI problem that an experienced EMI engineer could anticipate. Ignoring it is unacceptable >from a regulatory point of view (in the case of commercial products), or >from other spec and/or contract reasons in the government world. Many, if not most of the practical solutions appear to violate either the letter or the spirit of the Ethernet 10Base2 spec. This puts the EMI engineer in the same seemingly untenable spot that Ken Javor appeared to be in yesterday. I've been there and I empathize. 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 jim.knigh...@ncr.com -----Original Message----- From: Robert Johnson [mailto:john...@itesafety.com] Sent: Wednesday, March 05, 2003 9:53 PM To: Knighten, Jim L Cc: emc-p...@majordomo.ieee.org Subject: RE: Ethernet coax connection << File: Robert Johnson.vcf >> If you have shock level differences between the grounds in two different buildings, either an entire building isn't connected to an earth electrode, or someone is paying a whopping electric bill to heat up mother earth. I suspect the shock situation you were talking about was a power cross condition, either to the cable or within equipment. I hope it was diagnosed and fixed. The voltage differences between two buildings are generally very low, on the order of a couple volts, but due to fractional ohm impedances are capable of very high currents. Building maintenance procedures should routinely be checking for ground currents since they indicate faults in the power system. Note that in accordance with IEC 62102, 10base2 is an SELV circuit and should not be used for exposed plant (between building) wiring. Ethernet distance specs and application recommendations also go against exposed plant wiring. Ethernet 10base5 is considered a TNV-1 circuit and suitable for exposed plant use, is insulated and requires single point grounding. Optical fiber is a better recent development and the best answer for exposed plant lines these days. Note that IEC 62102 provides an extensive list of interconnect circuit types and what category they fit into with regard to shock level (SELV, TNV, hazardous, etc). It's an important reference for any engineer designing interconnect circuits in determining isolation needs. Bob Johnson ITE Safety From: owner-emc-p...@majordomo.ieee.org [mailto:owner-emc-p...@majordomo.ieee.org] On Behalf Of Knighten, Jim L Sent: Wednesday, March 05, 2003 7:51 PM To: Rich Nute Cc: ken.ja...@emccompliance.com; emc-p...@majordomo.ieee.org Subject: RE: Ethernet coax connection 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 jim.knigh...@ncr.com -----Original Message----- From: Rich Nute [mailto:ri...@sdd.hp.com] Sent: Wednesday, March 05, 2003 4:31 PM To: Knighten, Jim L Cc: ken.ja...@emccompliance.com; emc-p...@majordomo.ieee.org 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org 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
