Look at the loop: GND-PLANE -- AE CM-impedance -- Clamp -- EUT CM impedance ---- GND-PLANE
It's a simple loop consisting of 2 resistors of 150 Ohm and a 10V (low impedance) voltage source in between. If you look at the way the pre-set level for clamp testing is calibrated, you will notice that at the AE side a 150 ohm CM resistance to GND is used. This creates 33 mA in a 2 x 150 Ohm = 300 ohm loop impedance at 10 V EMF level setting. That is the theoretical intended stress level. If during a test the AE side is terminated with 0 Ohm CM impedance instead of 150 Ohm, then the EUT current will be entirely determined by the EUT impedance to ground. IF the EUT has an standard 150 Ohms to ground then (10V)/150Ohm = 66 mA If the AE-CM impedance is lower, then the current will be higher, ultimately Limited by the impedance of the ferrites at the AE side of the clamp. If the AE side has infinite impedance to ground (what happens if only the cable is put into the clamp without AE) then the current will be much lower, and any EUT will pass, making this test senseless. It's important therefore, when testing using a clamp, that the AE CM-impedance "closely" resembles 150 Ohm, using a CDN at the supply (or other) port of the AE. A clamp test can only be carried out if the AE side is impedance controlled, or the current to the EUT must be monitored all times. (as mentioned in other replies to this post) A clamp test cannot be carried out without an impedance at the AE side Be it a CDN (making the clamp redundant) or be it a real AE, positioned at 10 cm over ground plane AND at at least one cable terminated With another type of CDN (possibly simpler. While a CDN can be used without a connected AE side cable, the clamp CANNOT, because the current loop will be opened. If you don't not believe, draw the current Loops to prove it. Gert Gremmen Ce-test, qualified testing bv Van: [email protected] [mailto:[email protected]] Namens [email protected] Verzonden: maandag 17 september 2007 14:34 Aan: [email protected] Onderwerp: Conducted RF immunity When testing with EM clamp, the IEC/EN61000-4-6 gives a requirement to not exceed Imax (e.g Imax = 10V / 150 ohm = 66mA). We use a current clamp to measure on a 2-wire unscreened cable, and it is placed between the EUT and EM clamp. The current is measured to approximate 130mA. The test lab say it is due to the EUT impedance that the current is that high, I mean that the max current should not exceed 66mA whatever the EUT impedance is. How is this Imax requirement to understand? Best regards Amund Westin Oslo, Norway - 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/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] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc - 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/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] 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 ______________________________________________________________________
