Hi Pete:
Over the years I have periodically been asked to troubleshoot problems with 60 Hz harmonics appearing in the audio signals of telecom equipment. The culprit is usually the type of EMC caps that you refer to. The easy-to-understand case is when two conventional Y-caps are connected from line to the the earth ground lead in the AC mains cable of Class 1 equipment, and the chassis of the equipment is also connected to this same ground lead. If for any reason the earth ground lead is not actually connected to earth, the two Y-caps form a voltage divider that modulates the equipment chassis, with respect to earth ground, at one half the mains voltage. Since Y-caps are typically limited to 4700 pF maximum, the touch current is not excessive. I personally have never felt a shock from such configurations, so maybe the mechanism is different for the cases you have described. In terms of the noise problems that I have investigated, the hard-to-understand case is with Class 2 equipment that has no ground lead. The switching converters used in most Class 2 power supplies typically contain a single capacitor, in the range of 2200 pF, that directly bridges the input and output windings of the isolation transformer. This causes the Class 2 output of the supply to ride on a common mode 60 Hz (plus harmonics) waveform, with respect to earth ground, equal to one half of the mains supply. One would expect this common mode voltage to have no effect on the equipment being powered, since the equipment is isolated from ground. However, experience has shown that parasitic impedances between the equipment and ground can cause the 60 Hz harmonics to appear in the audio of high-gain paths such as hands-free speakerphone microphones. I can eliminate the audio problem by removing the physical capacitor that bridges the isolation barrier, but it is my understanding is that for most switching power supplies, removing this capacitor causes the power supply to fail conducted emissions. Again, due to the limited value of the coupling capacitor, I have never personally sensed a shock from such configurations, so what I am describing may not be the cause of the electric shock problem you describe. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) <mailto:[email protected]> [email protected] <http://www.randolph-telecom.com> http://www.randolph-telecom.com From: Pete Perkins [mailto:[email protected]] Sent: Tuesday, January 14, 2020 6:14 PM To: [email protected] Subject: Re: [PSES] Class I vs Class II safety constructions Scott et al, You are correct in that there seems to be a split between electrical appliances and electronic equipment and that the latter is more Class II than the former, there is chatter about an issue that is growing among class II equipments. All of the electronic equipment which has been the primary user of SMPS for decades typically uses EMI filters to mitigate the noise generated by the switching action which get fed back into the line. These are used on Class I equipment where the noise is capacitively coupled to earth/ground to be dissipated and keep much of this noise from feeding back into the mains/line; likewise these filters are also used on class II equipment for the same purpose – however the class II equipment does not have any earth/ground sink in which to drain the unwanted signals. So, apparently, this electrical charge is fed to the equipment chassis and the chassis voltage builds up until it reaches some equilibrium value. The EMI folks discover this when they find that the equipment discharges to the probe before they are ready to induce a charge into the equipment. The voltage developed on the equipment seems to be a hi value but limited charge (due to the limited capacitance of the chassis to absorb it). Altho there are not yet specific numbers, this doesn’t seem to be a safety hazard at this point. However, it is also unknown as to how this affects the filtering of the mains noise which was the desired result. The EMC lab techs don’t like the equipment to zap them first rather than the other way around. Does anyone on this thread know of a paper on this which would contain some specific results? Or of some researchers who are chasing this? Or have personal experience with this. It would sure be nice to get some feedback on this. This is a great opportunity to expand our experience and provide a basic understanding as to the efficacy of this process in both equipment applications for both safety and EMC. :>) br, Pete Peter E Perkins, PE Principal Product Safety & Regulatory Affairs Consultant PO Box 1067 Albany, ORe 97321-0413 503/452-1201 IEEE Life Fellow <mailto:[email protected]> [email protected] From: Richard Nute <[email protected] <mailto:[email protected]> > Sent: Tuesday, January 14, 2020 12:58 PM To: [email protected] <mailto:[email protected]> Subject: Re: [PSES] Class I vs Class II safety constructions Hi Josh: Yes, you are correct. Both of these standards specify the equipment be Class I. 60335-2-38 applies to commercial-use griddles. 60335-2-75 applies to commercial food or drink dispensing equipment. Thanks, and best regards, Rich From: Wiseman, Joshua <[email protected] <mailto:[email protected]> > Sent: Tuesday, January 14, 2020 11:59 AM To: [email protected] <mailto:[email protected]> ; [email protected] <mailto:[email protected]> Subject: RE: [PSES] Class I vs Class II safety constructions It’s been a few years, but I seem to recall there was a particular IEC/EN 60335-2-xx standard that required tubular sheathed heaters to be grounded. This would force PE to be brought in. Additionally, IEC 60335-2-38 and IEC 60335-2-75 have requirements for Equipotential grounding. If I remember correctly IEC 60335-2-75 does not allow Class II products. Josh Joshua Wiseman Systems Engineering Staff Engineer, Product Safety/EMC Ortho Clinical Diagnostics From: Richard Nute <[email protected] <mailto:[email protected]> > Sent: Tuesday, January 14, 2020 2:38 PM To: [email protected] <mailto:[email protected]> Subject: Re: [PSES] Class I vs Class II safety constructions Hi Scott: I have seen no safety standards or codes that specify which products must be Class I and which products must be Class II, except in the USA washers and dryers must be Class I. As far as I know, the decision is that of the manufacturer. I have been associated with a manufacturer who has made the same product both ways. In my case, one of the factors in deciding Class I or Class II was cost (e.g., a 3-wire cord was more expensive than a 2-wire cord). I suspect a major factor is “momentum” of the manufacturer: we made it this way last time, and we know how to do it this way. A product with a grounding (3-wire) power cord is a Class I product regardless whether it has no accessible conductive parts. Unlike a Class II product, a Class I product does not bear a marking attesting that it is Class I. Note that a Class I construction necessarily includes Class II construction, e.g., appliance inlet which is all-insulated. We ignore the Class II construction portions of a Class I product. I checked our electric kettle (which has accessible metal) and electric coffee-maker (which has the heater plate accessible metal). Both are 2-wire. Neither has the double-insulated symbol. Both are UL-certified. Best regards from beautiful snowy Bend, Oregon, USA, Rich From: Scott Xe <[email protected] <mailto:[email protected]> > Sent: Tuesday, January 14, 2020 6:59 AM To: [email protected] <mailto:[email protected]> Subject: [PSES] Class I vs Class II safety constructions In terms of safety level, both constructions are given the equivalent protection against electric shock. In electrical appliances, Class I is used most whereas Class II is employed in most electronic products. Is there any background for such design route? In some cases such as induction cookers, the enclosure is plastic/glass - no any internal metal part exposes to the outside surfaces. The product is not marked with a double square symbol and comes with a 3-pin plug. Why is this type of product not classified as Class II rather than Class I with the plastic/glass enclosure? Thanks and regards, Scott - ---------------------------------------------------------------- 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 <[email protected] <mailto:[email protected]> > 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://www.ieee-pses.org/list.html (including how to unsubscribe) <http://www.ieee-pses.org/list.html> List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected] <mailto:[email protected]> > Mike Cantwell <[email protected] <mailto:[email protected]> > For policy questions, send mail to: Jim Bacher <[email protected] <mailto:[email protected]> > David Heald <[email protected] <mailto:[email protected]> > - ---------------------------------------------------------------- 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 <[email protected] <mailto:[email protected]> > 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://www.ieee-pses.org/list.html (including how to unsubscribe) <http://www.ieee-pses.org/list.html> List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected] <mailto:[email protected]> > Mike Cantwell <[email protected] <mailto:[email protected]> > For policy questions, send mail to: Jim Bacher <[email protected] <mailto:[email protected]> > David Heald <[email protected] <mailto:[email protected]> > - ---------------------------------------------------------------- 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 <[email protected]> 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://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
