A few comments in addition to the consideration of ground loops well addressed by David Sterner. These wires are subject to the electrical codes as well as the product standards. The risk of contact with hazardous voltages is considered to be unlikely unless run outdoors, in which case protectors are required at the building entrance. In the US, the building wires are considered NEC CL2 or CM wiring requiring 50mm or barrier separation from power circuits. Within the equipment spacings or insulation must meet the double protection requirements. Since many connections may be short, and are often between Class II equipment, grounds are often unavailable, undependable or impractical.
Bob -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of [email protected] Sent: Wednesday, November 07, 2001 8:37 AM To: [email protected]; [email protected] Subject: RE: IEEE isolation question. Jake Obviously you refer to 10/100 twisted-pair Ethernet with isolation transformers. ANSI/IEEE802.3 (ISO/IEC8802-3) requirements are for safety and EMC protection: Cat-5 cabling is self-shielding, impedance-matched transformer terminations provide ideal transmission lines. Each link is transformer terminated at both ends. Each link of a hub/switch is isolated from all other links. Consider the gigantic 'ground loops' if network wiring were earthed. 1) radiated emissions would be impossible to control 2) immunity would degrade because of reciprocity 3) DC currents from nodes at different ground potentials would pose safety issues Interconnecting earths helter-skelter throughout buildings is scary. Clock frequencies from PC's, hubs, switches, routers, modems, etc. could couple onto the ground lines causing untold interference. Any "hot" chassis among nodes/hubs/switches becomes a fire hazard. COAX Under 802.3, Ethernet Coax (10Base2) also is isolated from ground at each node. The cable interface is a transformer-isolated driver with level-shifting. The coax shield is supposed to be earthed at a single point between nodes (often ignored by installers). These are my opinions and not necessarily shared by Europeans. For an alternative viewpoint, read EN55022:1998. David Sterner ADEMCO Group -----Original Message----- From: Jake Jung [mailto:[email protected]] Sent: Wednesday, November 07, 2001 1:38 AM To: [email protected] Cc: [email protected] Subject: IEEE isolation question. Hello, I have start at new company this past week. We have ethernet products that are required to comply with the isolation requirements of IEEE802.3. We provide isolation of 2250vdc (1500vac) between the ethernet line and the secondary circuit and between the ethernet line and ground circuit. Does anyone know why these lines are not required to be grounded? If both sides of a line are connected to IEEE802.3 compliant products, I think that if there is a fault between the building power wire and a ethernet wire, the fault voltage will just float on the ethernet wire since there is not path to ground. If there are other cables attached to the device, these cables will also float the power voltage and be subject to contact if not connected on other end. Do anyone know the background on this requirement or have any thoughts on if this is correct thinking? Thank you for your help, Jake __________________________________________________ Do You Yahoo!? Find a job, post your resume. http://careers.yahoo.com ------------------------------------------- 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: Michael Garretson: [email protected] Dave Heald [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server. ------------------------------------------- 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: Michael Garretson: [email protected] Dave Heald [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server. ------------------------------------------- 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: Michael Garretson: [email protected] Dave Heald [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
