John, What you highlight is one of the differences between traditional ICT equipment and traditional radio equipment. ICT equipment tends to use switch mode power supplies where any inter-winding screen is connected to the neutral for EMC purposes. From what you say, radio equipment tends to use 50 Hz transformers where connection to PE has the benefits you outline. Please do not fall into the trap of thinking that Notes contain requirements, they should not and in this case do not. There is a general requirement that any Protective Earth path must 'out-live' the over-current device in the supply line, clause 2.6 of IEC 60950-1 has lots of requirements on various aspects of this. The note you are referring to in C.2 is just a reminder of these requirements. It particularly applies to foils that may be much thinner than the adjacent primary winding (otherwise the transformer would be difficult to make) since it is important that any short from winding to screen causes the up-stream fuse to blow rather than causing the screen to locally melt. There is another difficulty with foil screens in that the transformer manufacturers sometimes add insulating tape over the top and bottom of the screen (for manufacturing reasons) which then means that the creepage path is effectively longer from primary to PE than it is from primary to secondary. Presumably, the wire screens you refer to in radio equipment are simply bare wire (not enamelled?) I'll certainly take your word for what is common practice in the radio industry, but when it comes to safety I'll check it out for myself. Over and out, Richard Hughes Safety Answers Ltd.
From: j...@jmwa.demon.co.uk [mailto:j...@jmwa.demon.co.uk] Sent: Friday, June 27, 2003 5:21 PM To: emc-p...@majordomo.ieee.org Subject: Re: Dielectric withstand voltage for power supplies I read in !emc-pstc that richhug...@aol.com wrote (in <44E75E95.7A170092 0ba45...@aol.com>) about 'Dielectric withstand voltage for power supplies' on Thu, 26 Jun 2003: >[R_Hughes >] Technically incorrect: it is also possible to have a design >where the secondary circuit is floating from earth and where you have a >double-wound transformer with a screen connected to protective earth. It's >called 'Method 2' and the requirements are in IEC/EN 60950-1 clause 2.2.3.2. >In this case, breakdown from primary to secondary is rendered unlikely >because breakdown from primary to protective earth is made to be more >likely. It is also permissible to construct PCBs or other components this >way. Admittedly in practice this is seldom done because by the time you >have made the protective screen 'meaty' enough you may as well have provided >insulation. Actually, it's often done like that in professional audio equipment, to which IEC/EN 60065 applies. I am more familiar with that standard, in which there is no special mention of the technique, and I'd forgotten that EN60950 goes into the method more formally, in 2.2.3.2 of IEC/EN 60950-1:2001. The technique stems from traditional a.m. radio receivers, where the interwinding screen provides greatly increased immunity to conducted disturbances entering from the mains supply. The implications of it for safety were realised when IEC/EN 60065 began to be applied in earnest to professional audio equipment, around 30 years ago. It allows the signal circuits of individual pieces of equipment to be optionally connected to the PEC or not, which is particularly useful for outside broadcast equipment, to prevent earth currents flowing in the screens of signal cables. For the EMC purpose, a single layer of winding, which the winding machines can put on automatically, has been found effective, but the current-carrying capacity of such a layer of thin wire might have insufficient current-carrying capacity for the safety purpose, and this is dealt with in Annex C of IEC/EN 60950-1. However, in the NOTE to C.2, there is, despite a disclaimer, an implication that a foil screen is necessary. The same note says that the requirement is that an overload device operates in the event of a fault before the screen is damaged, and this may be satisfied by a screen of quite thin wire if the operating current of the overload device is less than 1 A or so. But don't take my word for it! -- Regards, John Woodgate, OOO - Own Opinions Only. 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
