Hi Ron:
> I have come across a problem where a product is exceeding 3.5 ma of touch > current. The product in question has 2 line filters and 4 Switching Mode > Power Supplies. Because of the addition of the fourth power supply the > overall touch current can be as much as 4.0 ma. Ideally, we do not want to > make the product so it has to be hard-wired to the building mains. > > Because of the high touch current, does anyone know what the relationships > are between different touch current sources? I've heard that touch current > is not linear when you add up the different sources and phase shifts between > L-G and N-G might be a problem. However, I'm looking for some detailed > information on this issue so I can provide ways to investigate the problem > and hopefully reduce the overall touch current. The principal source of touch current is the mains voltage. Most of the touch current is via the Y capacitors in the line filters. Touch current IS linear. Gary Tornquist presented a paper at the August PSES Symposium that explains how touch currents from several sources add. (The resistance of the touch current meter must be accounted for when summing touch currents from several sources.) Phase shifts do not affect the touch current or the measurement. Several messages have suggested how to reduce the touch current. Basically, you have too much line-to-ground capacitance. You mention 2 line filters and 4 SMPS. Each SMPS should have its own line filter. Do you really need the other 2 line filters? If you can eliminate them, you'll probably meet the touch current limit. Another way is to buy one SMPS that provides all of the power from one SMPS. The touch current >from one large SMPS is much smaller than from four smaller SMPS. By the way, worst-case touch current is at max line frequency. You should test at 60 Hz, not 50 Hz. Best regards, Rich This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Ron Pickard: [email protected] Scott Douglas [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
