Hi Joe:
Sorry for the delay in my reply to your questions. SPDs are used on mains circuits, both between the poles and poles to earth. In this latter application, the SPD is in parallel with basic insulation. SPDs are also used on low-voltage external circuits that are subject to transient over-voltages such as antenna circuits and telephone circuits. The SPDs are between the external circuit and earth, or between the external circuit and the mains circuit. In this latter case, for the purposes of a transient over-voltage originating in the external circuit, the mains circuit is a connection to earth. Usually, within the equipment, these external circuits are isolated from earth and from equipment secondary circuits. Indeed, the standards require such isolation (to protect personnel touching such circuits downstream from the equipment). But, because these circuits are low voltage, the isolation system is not a safety isolation, i.e, is not a basic insulation. Nevertheless, because the external circuits are subject to transient over-voltages, the isolation system must withstand such over-voltages. Therefore, the isolation system is subject to an electric strength test. And, the isolation system can be bridged by an SPD at the discretion of the designer. The principle I described in my previous message on this subject applies here. In the case of an open- circuit failure of the SPD, the external circuit isolation system must withstand the expected transient over-voltages. Hence, a separate voltage- withstand test of the isolation system without the SPD. While the equipment is not required to have reliable earth, many of the requirements are excluded if the equipment does have reliable earth. The standard does not address the short-circuit or low-resistance failure of an SPD that is connected between the external circuit and the mains. However, external circuits are normally isolated from earth, so no transient current can flow in the reverse direction, i.e., from mains to the external circuit. I suppose the SPD can bridge the isolation, i.e., from external circuit to secondary circuit. If the secondary circuit is connected to earth, then the SPD simply conducts to earth. If the secondary circuit is not earthed, then the SPD conducts through the primary-secondary capacitance to the mains. Further questions or objections? Best regards, Rich - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <emcp...@radiusnorth.net> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
