Rich, Thanks for the Email in which you mention many of the points already discussed in IEC TC74. So what are we going to do with the new hazard based standard? Are we going to permit constructions that have been in IEC 60065 and IEC 60950 for years or are we going to introduce a test that is rather more realistic of the actual failure condition than simply running an earth bond test? You mention a few times the 25 Amp test. The designs I have seen that try and use this approach (and I don't ever recall seeing a design I was happy with) were all switch mode power supplies where there was an input fuse of about 2A, meaning that the test would be performed at 3A in the past (and will be performed at 4A in the future, if the CDs go through un-changed). You mention that current density caused by a small point of contact is the reason why a short from the winding to the earthed foil will cause the formation of a hole in the foil. Although I agree that it is part of the problem I am not convinced that it is the whole story. For me, another chapter in this story is the part played by the very high surge current that flows for a very short period of time. It is here where we could usefully apply the surge test proposed in WG8 by Taylor and Bahra, but rejected as being not sufficiently well thought through.
So there you have it, some of us Brits just like having a good discussion (what you call an argument). Unfortunately there were no Americans around to argue with in our morning so we just had to make do with arguing with one another! Glad to see you've joined the fray! Richard From: ri...@sdd.hp.com [mailto:ri...@sdd.hp.com] Sent: Saturday, June 28, 2003 5:43 AM To: j...@jmwa.demon.co.uk; richhug...@aol.com Cc: emc-p...@majordomo.ieee.org Subject: The transformer screen/shield conundrum Hi John and Richard: The transformer screen/shield between primary and secondary windings is intended to carry the fault current in the event of a failure of basic insulation between the primary and the screen/shield. The screen/shield must be capable of carrying the full fault current and is often tested using the 25-amp test. The object of the test is to connect to the screen/shield, and then pass the 25 amps through the shield to its transformer terminal and then to the PE terminal. The difficult part is how to connect the tester to the screen/shield. When the screen/ shield is a copper foil, a special transformer must be wound with a wire connected (soldered) to the foil screen/shield and brought out for connection to the 25-amp source. If this cannot be done, then a wire is soldered to the edge of the foil screen/shield by forcefully separating the sheet insulation that extends beyond the edge of the shield, inserting the soldering iron tip, and soldering the wire. (A constructional problem is that the screen/ shield must extend BEYOND the primary winding so that no part of the primary winding is "exposed" to the secondary winding.) (Another constructional problem is that the ends of the foil must be overlapped, but with insulation inserted between the overlaps; otherwise, the screen/shield would constitute a shorted turn and cause all kinds of electrical, magnetic, and thermal problems.) The connection of the tester to the foil screen/shield typically has a very large contact area. Any thickness of foil screen/ shield passes the test. The conundrum is that the failure of basic insulation could result in a point contact between the primary winding and the shield. The point-contact, because of its small area, has very high current density, and will blow a hole in thin foil (that otherwise passes the 25-amp test). With the hole in the screen/sheild, the fault to the foil opens, and the fault no longer exists -- even though there is a failure of basic insulation. The connection of the foil to the PE remains intact, and there is no shock hazard. (Of course, the heat of the point-contact fault may very well cause failure of the foil screen/shield-to-secondary functional insulation, which might very well result in a shock hazard from the secondary SELV circuit.) The point is that the test does not necessarily test what can occur within the transformer, yet will pass the transformer screen/shield construction. Nevertheless, when the basic insulation fault occurs, the hole in the screen/shield may very well end the event safely. 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: emc_p...@symbol.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
