This thread has been interesting. I am, at this moment, considering a design where I am almost forced to use a PC (printed circuit) trace for Earth ground.
It seems funny to me that most equipment has been historically made with 18AWG protective ground pigtail wires; and 25A ground fault tests have been used for years. Now that PC traces are being used for protective ground; we want to test with 200A or greater impulse currents? I'm curious about what would happen to your typical 18AWG line cord during this test. I'm wondering if the line cord would fuse open? There are a couple of handy charts on the web. One is at www.kepcopower.com/nomovax2.htm this is a nomograph of maximum operating current, AWG and IR drop in the conductor. The point "A" is generally considered the point of maximum IR drop. If you draw a line from point "A", through a wire gauge size; you'll get a max current. Of course this is steady state current; and the nomograph assumes a single wire. Wire bundles would be a worse case. It's too bad that this chart doesn't contain the "fuse" values for the wires as well (the I squared * T values). Another is at www.circuitboards.com/capacity.php3. This is a chart of max current for PC traces. Remember that this is for TRACES and planes only; it doesn't say anything about vias and other potential problems. At first pass, it seems that a trace size to handle twice the power cord's max current, (from the nomograph) with a 10degC trace temperature rise (from the PC trace chart), would be a good rule of thumb for the trace size. If I have room, I'll just make it bigger. Once we pay for the PC board fabrication, the copper is free! Even with an adequately sized trace; I can think of a few potential problems with the trace to chassis connection: 1. Many layout people open up PC traces or planes around vias so that only four little 20 mil wide bridges carry the current to the via. This is great for soldering heat relief; but BAD for current carrying capacity. These little bridges can fuse open in high current conditions. I am considering solving this by not putting any thermal reliefs around your Earth ground vias and using multiple vias. 2. Another problem with these traces is using plated through vias with screws through them. It has been found that plated through vias can crack when they are put under pressure from screws. Some power supply manufacturers solve this by bringing the Earth ground trace to the surface with vias near the chassis connection point; then route this to a solid plated pad on the surface layer for chassis connection. I am considering this same solution as well. 3. The third problem is mechanical. Once Earth ground brought to a pad on the circuitboard; then there is still the issue of getting a good mechanical mate to the chassis with a wide surface area. If the connection is made through a couple of teeth on a star washer; then there is a potential for localized heating. I'm just going to maximize surface contact area for this one. I'm also considering using multiple board to chassis connection locations. Every screw that connects the board to chassis is a potential Earth ground connection. The last "fuse" in any power system is the cord connected to the product. It seems to me, (just an opinion now) that a Earth ground system made to handle the worst case current of your worst case power input cable (along with some design margin) would stand a good chance of passing any regulatory test. Can any of the gurus see a problem with this? Chris Maxwell | Design Engineer - Optical Division email chris.maxw...@nettest.com | dir +1 315 266 5128 | fax +1 315 797 8024 NetTest | 6 Rhoads Drive, Utica, NY 13502 | USA web www.nettest.com | tel +1 315 797 4449 | This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
