Carl, If the primary supply circuitry and components provide double or reinforced insulation, nothing can become live in the event of a single fault, the test becomes unnecessary, and I would argue that fact.
If the design does not provide double or reinforced insulation, the test sounds applicable from points that could become live in case of a basic insulation fault. Regards, Lou Aiken, LaMer LLC 27109 Palmetto Drive Orange Beach, AL 36561 USA tel ++ 1 251 981 6786 fax ++ 1 251 981 3054 Cell ++ 1 251 979 4648 From: <cnew...@xycom.com> To: <emc-p...@majordomo.ieee.org> Sent: Monday, February 03, 2003 11:19 AM Subject: RE: EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests) > > > > A slight divergence from the EN specifically, but I thought that the > following would be helpful to this thread: > > I am presently working this issue with a UL engineer in accordance with > UL 60950, 3rd Edition. I also have the UL 60950 3rd Ed. Test Data Sheets. > Their "Protective Earthing Trace Earth Fault Current Test", UL Doc. 190.eng, > per Section 2.6.3.3 requires the following in my case: > > 1. Three samples are tested; > > 2. Trace resistance is measured before and after test. Resistance cannot > exceed 0.1 ohms, and cannot change more than 10% after test; > > 3. AC source is 240 Vac, 200 amps (20A circuit breaker X 10), power factor > is 75 - 80% through shorted bus bars with a 20/30 A (20 in my case) > service > entrance type circuit breaker in series with the testing terminals. The > circuit > breaker is connected to the bus bars by 1.22 m (4 ft.) of #12 AWG wire. > > 4. The test circuit is connected to the DUT via the grounding lead of the > 1.82 m (6 ft) power supply cord. If cord is not provided, then #16 AWG > wire is used. > > 5. Test continues until ultimate results occur; e.g. CB trips, trace opens, > etc. > > My UL guy tells me that I should expect the typical service type CB to be rated > up to + 10%. So it appears that I need to concern myself with a burst of > current > up to approximately 22 amps for the 20 amp AC circuit that my product is being > evaluated for. > > Carl > > > > > > From: "Chris Maxwell" <chris.maxw...@nettest.com> on 02/03/2003 09:29 AM > > Please respond to "Chris Maxwell" <chris.maxw...@nettest.com> > > To: emc-p...@majordomo.ieee.org > cc: (bcc: Carl Newton/XYCOM) > > Subject: RE: EN60950 protective conductor test (was Re: Circuit Breaker > Tripping Dring Fault Tests) > > > > > 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 > > > > > > > > ------------------------------------------- > 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 > 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
