In message <[email protected]>, dated Wed, 15 Dec 2010, Joe Randolph <[email protected]> writes:
>Rich's question about what the potential current path would be has >bothered me as well. I'm not sure what sort of scenarios to test for, >but there may be some sort of scenario involving the USB port and/or >headset port that could create a path through the body. That is crucial. At first sight, I would say that you cannot get an electric shock from a battery-operated hand-held device, even if it produces 15 kV (ESD simulator!) > I think I will need to show that under any single-fault condition, >the voltages that appear on the USB and headset ports remain within >SELV limits. Voltages with respect to what? A measurement between ground and the SELV circuit will return zero, apart from a very small voltage due to the 300 Hz current through the capacitance of the user's hand, the user's body and their conducting boots. This is in normal operating conditions. See below for a single-fault condition. You wrote: * A non-isolated DC/DC converter powered from the 3.7V cell phone battery generates a 150 V DC driver supply that will deliver about 5 mA into a 2K ohm load. * The 300 Hz AC output of the driver will deliver about 4 mA RMS into a 2K ohm load. * Both the 150 V DC supply and the 300 Hz AC output share the same circuit reference node with the rest of the phone circuits. * This circuit reference node is normally floating with respect to earth ground, but it can become grounded through the USB port. If the 'hot' end of either the DC or the AC supply (it's NON-isolated) shorts to the SELV circuits, you have a potential burn-up but no electric shock to the user. It is irrelevant whether the product is earthed via its USB connector or not. The only way a shock could occur, it seems to me, is a 2-fault condition where the high-voltage shorts to an isolated headphone output AND the insulation of the headphones or their connector fails as well. -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK Plural: data, criteria. Singular: datum (different meaning: use 'data element' for a single item), criterion. 'Effect' is a noun, 'affect' is a verb (except in psychiatry). - ---------------------------------------------------------------- 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://product-compliance.oc.ieee.org/ Graphics (in well-used formats), large files, etc. can be posted to that URL. 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 <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
