Chris, the theory behind the ground rods appears to be to maintain the floor plate at the local earth ground potential during an EFT or ESD test event. My first thought is why would the floor plate potential change relative to the local earth ground? With the equipment bonded to the floor plate the only path to local earth ground for the current is via displacement current. There will be some displacement current, but how much and what effect will it have on the test? The displacement current will occur when the DUT potential changes during a transient. The DUT then capacitive couples to all of the surroundings. Let's put some numbers to an example: Figure 1 pF per inch of DUT diameter. Say the DUT diameter is 25 inches, the EFT rise time is 5 ns, and the DUT potential moves to 1 kV relative to the floor plate. That's 5 amps of displacement current going to the floor plate, the earth beyond the floor plate, and upwards to who knows where. We can think of also as a capacitive divider. C from DUT to ground in series with C of ground to floor plate. The ground to floor plate C is a couple of nanofarads with the floor plate 12" above earth ground. Say all of the 25 pF of DUT C is to earth ground. That's an 80:1 capacitive divider; the DUT jumps up 800 volts and the floor jumps 10 volts relative to ground. With 10 volts, 2 nF, and 5 ns, there is a ground-to-floor current transient of 4 amps. But, there isn't really 25 pF >from DUT to earth ground. If it is 1 pF (I don't think it is even this high) the current is only 0.16 amp- not much in the presence of tens of amps of EFT current. I can think of ways to model this on a PC or with an electrolytic tank but I think a better way is to measure it. The floor-to-earth potential ,during an EFT, can be measured by connecting an oscilloscope between a ground rod and the edge of the floor plate. The floor-to-earth current can be measured with a suitable current probe inserted in a wire connecting the ground rod to the floor plate. Then what do you do? You have a number but no specification that says if the number is acceptable or not. On the other hand, what harm is there to the EFT test if there are ground rods? It probably can't hurt and probably doesn't help except to satisfy someone's empirical desire to ground everything as if ground has magical properties. Dave Cuthbert Micron Technology
_____ From: [email protected] mailto:[email protected]] On Behalf Of Chris Wells Sent: Wednesday, April 06, 2005 5:54 AM To: [email protected] Cc: [email protected] Subject: EMI ground plane reference versus NEC single point earth bonding In our prescreening lab we have a ~9M square metal floor plate that we tie direct to the Generator when doing IEC61000-4-4, -6 testing etc. The Generators and coupling devices are bonded high frequency wise to the plate and are also referenced to safety ground as per the NEC going back to the transformer earth grounding pad. In my view that is adequate for these high energy tests. the return path of the test signal is back to the plate, and all the gear is well within the permiter of the plate. However I am getting pressured to add localized ground rods next to the floor plate. I do not see how tying this plane to the earth has much EMC meaning unless we were doing emission uV type measurements. For that work we have a large GTEM which sits on the plate as well. I do see that a redundant local ground would violate NEC and that fault or surge currents would take the parallel path to my redundant ground rods causing risk to equipment and personel. We are on a hill and have been hit by lightening many times. We are also a power lab and are testing 3 phase equipment (480V P:P) As such I must be mindful of the safety topic perhaps more than others. If we were to instal an isolation transformer at the room that serviced all the gear and DUT, then we could establish a local ground that would satisfy both NEC and EMC type testing but the cost is an issue that would have to be rationalized and explained clearly to upper management. ( the transformer would have to be rather large and the building is leased) Right now I can not support the argument for either local ground rods or the isolation transformer. So please help me understand what the rational is for earth grounding locally the reference plane with ground rods. Is it like I thought, a uV issue with emission tests or does it actually have some bearing on high energy EFT, RF conducted type tests? If you can copy my work email that would be helpful. [email protected] IT won't let me get the list at work :-( Thanks in advance! Chris Wells Eaton Electrical ---------------------------------------------------------------- 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: Scott Douglas [email protected] Mike Cantwell [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 ---------------------------------------------------------------- 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: Scott Douglas [email protected] Mike Cantwell [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
