Hi Brian:
First, you should measure the current in the ground wire with an oscilloscope and a current probe or measure the voltage across a low-value resistance such as 10 ohms in series with the ground wire. Second, you should measure the current in the ground wire with a RMS ammeter. These should confirm that the difference current is tripping the GFCI. If the high frequency is what is causing the GFCI to trip, you might want to consider re-designing the EMC filter to lower the high-frequency currents in the ground wire. An ideal EMC filter should return the currents generated in the power supply to the power supply, not via the mains circuits. The worst-case fix is to use a 1:1 isolation transformer between the mains and your equipment. Make the measurements so that you know what is causing the GFCI to trip. Best regards, Rich > -----Original Message----- > From: Kunde, Brian [mailto:brian_ku...@lecotc.com] > Sent: Monday, August 20, 2007 6:50 AM > To: ri...@ieee.org; emc-p...@ieee.org > Subject: RE: Ground Fault Circuit Interrupter tripping > > > Thanks for the great information. Let me see if I'm > comprehending it correctly. > > - A product can pass the leakage current test and not pose a > hazard to the user but still trip a GFCI. > > In such cases, is there an alternative to the GFCI? If the > NEC requires a GFCI in a location but a device used in that > location falsely trips the GFCI, can you wire the instrument > without the GFCI and still meet the NEC requirements? (are > their exclusions?). Is there something you can do, such as > provide an addition safety earth ground connection, instead > of the GFCI? > > If I'm correctly understanding the reason our product is > tripping the GFCI, there is nothing we can do about it. So, > our only choice is to find an exclusion or an alternate to > the GFCI that would satify everyone. Any suggestions would > be appreciated. > > The Other Brian > > > -----Original Message----- > From: emc-p...@ieee.org [mailto:emc-p...@ieee.org] On Behalf > Of rn...@san.rr.com > Sent: Friday, August 17, 2007 2:17 PM > To: emc-p...@ieee.org > Subject: RE: Ground Fault Circuit Interrupter tripping > > Hello John: > > > > I've never measured the 'frequency response' (trip current vs. > > frequency) of a GFCI (they don't occur in UK, we have RCCBs) but I > > suspect that the trip current is higher than the design value at > > higher frequencies. > > As far as I know, there is no frequency compensation in the > GFCI. The frequency response is determined by the core > characteristics and the IC amplifier characteristics. > > Virtually all US/Canada GFCIs use the National Semiconductor > LM1851 IC. The data sheet includes some good information on > GFCI circuits and circuit performance. There are no > frequency response specs that I could find. Here is the URL > for the LM1851 and data sheet: > > http://www.national.com/mpf/LM/LM1851.html > > Here is another web site for GFCI information: > > http://www.codecheck.com/gfci_principal.htm > > The original question pointed out that the leakage current > was in spec, but the load nevertheless tripped the GFCI. The > only conclusion is that the leakage current meter and the > GFCI do not have the same measuring circuits -- which they > don't. We do know that the leakage current measuring network > accounts for the response of the body to frequency, while the > GFCI has no such metering circuit network. > > > Best regards, > Rich > > > Richard Nute > Product Safety Consultant > San Diego > > > - 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 emc-p...@ieee.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 emcp...@ptcnh.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald: emc-p...@daveheald.com All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________
