Hello All: While we are on the subject of TNV-to-earth insulation, I would like to pose a related question regarding clause 6.1.2.1 in EN 60950-1 and the deviations in Annex ZB for Finland, Sweden, and Norway. This question relates to the requirement that "surge suppressors that bridge the insulation shall have a minimum breakover voltage of 1.6 times the rated voltage.....".
Suppose I have a piece of office equipment that is powered from the AC mains and has a rated voltage of 250 VRMS. The equipment also has a connection to a TNV-3 phone line. It uses a 3-wire AC mains plug that includes an earth ground. Inside the equipment, the surge protection on the TNV-3 phone line includes two 300 volt surge protectors, connected from tip to ground and >from ring to ground (there is a third surge protector connected across tip and ring, but that is incidental to the question at hand). I have seen this type of construction many times, and I believe it is widely used in the industry. Now, if I evaluate this construction for compliance with clause 6.1.2.1, I quickly conclude that the two surge protectors connected tip-to-ground and ring-to-ground must have a minimum breakover voltage of no less than (1.6)x(250) = 400 volts. Okay, this looks like a problem, but the exclusions in clause 6.1.2.2 leave some wiggle room. This particular piece of office equipment is not permanently connected, nor is it installed by a service person. However, it is possible to equip it with provisions for a permanent earthing conductor and then provide instructions for the installation of that conductor. Never mind how many users will actually read the instructions.... Now I go to Annex ZB and look at the deviations to clause 6.1.2.2 for Finland, Norway, and Sweden. The exclusions described here are much more restrictive, limited to permanently connected equipment or pluggable equipment type B, used in restricted access locations where equipotential bonding has been applied. In other words, a telephone central office or *maybe* the main telephone closet in an office building. Certainly, no stretch of the imagination would have this clause include the typical office environment. So, does this equipment fail to comply with EN 60950-1 for use in Finland, Norway, and Sweden? It appears to me that it fails to comply, but I can tell you there is a lot of product out there that meets this description. Think modems, fax machines, and small PBXs. I would like to hear what others in the group think. Am I missing something here? Thanks, Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) [email protected] http://www.randolph-telecom.com This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ieee-pses.org/ 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] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
