Part of the reason why two levels of protection are required is because of what happens if one level fails. If the protective earth bonding connection fails, and the exposed metal is no longer earthed, the product will typically continue to operate normally. There is no indication to the user that a failure has occurred. If a second failure occurs that bridges the insulation gap between hazardous voltage and that exposed metal, the chassis will become energized. The user may still have no obvious indication of a failure until they contact that energized chassis.
Different standards committees have different ways of addressing this issue. TC 108 generally requires that the protective earth bonding be designed and tested in such a way to prove that it is robust enough. In many ITE products, the user accessible circuits are earth ground references, so providing basic insulation between SELV and earth ground is not possible. Other standards committees may have chosen other methods of protection based on the products they cover. TC 13 may have decided that basic insulation between SELV and earthed parts may be the best way to provide protection in their products. In this case, if the earth bonding fails, the basic insulation may be need to reduce the risk or arcing between hazardous voltage and the exposed metal. Ted Eckert Microsoft Corporation The opinions expressed are my own and do not necessarily reflect those of my employer, TC 13 or TC 108. From: Scott Aldous <00000220f70c299a-dmarc-requ...@ieee.org<mailto:00000220f70c299a-dmarc-requ...@ieee.org>> Sent: Tuesday, March 13, 2018 8:57 AM To: EMC-PSTC@LISTSERV.IEEE.ORG<mailto:EMC-PSTC@LISTSERV.IEEE.ORG> Subject: Re: [PSES] insulation between SELV and protective earthed accessible part- IEC 62052-31 Hi Vyas, I'm not familiar with this standard, but the general principle is that 2 levels of protection are required to protect operators from hazards. In the case of the "protective earthed accessible part", I imagine the standard assumes that a single fault could compromise the protective earthing, so Basic or Supplementary Insulation is also required between such a circuit and SELV. In the same vein, the table requires Double or Reinforced Insulation between an "unearthed accessible part" and SELV. On Tue, Mar 13, 2018 at 5:56 AM, Balmukund Vyas <balmukund.v...@ymllabs.com<mailto:balmukund.v...@ymllabs.com>> wrote: Dear All, IEC 62052-31 is standard for product safety requirements for electricity metering equipment. It has a table 20 (given below) which details out insulation requirements between various types of circuits. My question is, why Basic insulation is required between SELV circuit and protective earthed accessible parts? Isn’t a functional insulation is sufficient for this? Thanks B M Vyas - ---------------------------------------------------------------- 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://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 <sdoug...@ieee.org> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
