Could the grounding safety issues we are confronted by have been avoided if the incoming AC supply had remained 'balanced' that is, not referenced to local ground anywhere?
Or to put it another way. Some where along the way it was decided to ground the neutral at the premises entrance. Why? What is the advantage? and to whom? I feel I should already know the answer, but I don't. Any offers? (which reminds me of the fact that it was the catalog of an American parts supplier which taught me why we have fused domestic plugs in the UK and not elsewhere; because the UK domestic wiring uses ring distribution through the house. The ring has a single thirty amp rated fuse and so can supply over twice the current rating of an individual outlet. Primary protection takes place inside the appliance outlet plug which has 13A fuse rating. The resettable magnetic breakers that are so familiar to everyone in 120V territory are unknown in the UK.) Newbie here. Brit working in US. Best Regards Ted Rook Crest Audio 201 909 8700 ext 213 >>> <[email protected]> 05-Nov-01 1:46:15 PM >>> Several years ago I had the need to explain this same issue. Similarly most of the inquiries came from the U.K. Here are some exerpts from a "white paper" done to address this issue: *************** This document addresses the nature and safety of two-wire ITE products. The international safety standard for ITE is IEC 60950. The referenced sections of IEC 60950 below are generally the same in unique country standards. SHOCK PROTECTION A major aspect of product safety for ITE is protection against electric shock. There are three equally acceptable methods of achieving this protection (section 1.2.4). Class I equipment employs only "basic" insulation, but ties all accessible conductive parts to a "ground" pin to protect the user in the event of a failure of the basic insulation. Such equipment requires a three-wire line cord and a reliable path to earth ground. Class II equipment uses "double" or "reinforced" insulation between primary voltage and accessible conductive parts to protect against electric shock. Such equipment uses a two-wire line cord, is not dependent on the integrity of the building's grounding system, and typically displays the "square within a square" symbol denoting double insulation. ITE products which contain no hazardous voltages (e.g. less than 42.4Vpk/60Vdc) are approved as Class III devices. NATURE OF CLASS II There is one aspect of Class II equipment that can confuse end users. Because no earth ground path is required or available, accessible conductive parts will "float" to some voltage less than the applied mains voltage. Typically this may be one-half of the mains voltage. This voltage is not considered a hazard under the standards as the available current cannot exceed the 250uA specified in Table 17 of section 5.2.2. Some individuals may be able to feel a slight "tingle" or "shock" at this low current level, although no electric shock injury should result. FINAL COMMENTS If a user is aware of a voltage on an accessible part, and suspects a hazardous condition, some simple tests can be performed to eliminate this concern. George Alspaugh ------------------------------------------- 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.
