Re: EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests)
Dear Friends, Thanks very much to the input regarding CB Tripping During Fault Testing. There were many thoughts on the subject and I am sure that next time the wall CB trips during fault tests at your third part certification laboratory, you have something to talk about - make sure they do not charge you for the discussion time! Peter _ Do you Yahoo!? Yahoo! Mail Plus http://rd.yahoo.com/mail/mailsig/*http://mailplus.yahoo.com - Powerful. Affordable. Sign up now h tp://rd.yahoo.com/mail/mailsig/*http://mailplus.yahoo.com
Re: EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests)
Hi Peter: My comments were based on the proposed requirement to test the PE path with the circuit prospective current transient, e.g. 200 amps from a 10,000-amp source for the period of time required to operate the overcurrent device -- say less than a second or so. (The 200 amps is a function of the contact resistances and the wire resistances, independent of the fault; the duration is a function of the overcurrent device. 200 amps is a reasonable number for plug-and-socket cord-connected products.) In order to get this maximum current, the fault must be near zero ohms for the duration of the current transient. To achieve near-zero ohms, the fault must be a large- area fault. A small-area fault is likely to fuse open due to the current density and resistance at the contact. (I had the unfortunate experience that such a test by a cert house used a small-area contact at a point where no basic insulation fault could occur; the PWB PE path was destroyed. We repeated the test at a large-area contact where basic insulation could fault, and the PWB PE path passed.) What if the over current device operates, the earthing path is compromised by the fault, but not destroyed? I believe this is the objective of the proposal -- to test the PE path with the circuit prospective current. I would expect the compliance criterion to be no damage to the PE path. What if the fault is of nonnear-zero impedance, the earthing path is damaged, but not opened, and resetting of the breaker does occur, but at some point the breaker holds due to the relatively high impedance? This scenario moves from withstanding the circuit prospective current to withstanding the steady-state current just below the operating point of the over- current device. I suggest that this is the objective of the existing requirement to test at twice the overcurrent device rating or 25 amps, whichever is less. a relatively complex earthing path, I have prepared a separate e-mail that includes some construction details and empirical data for a product in my lab. To be sent soon. I appreciate you sharing this data. Best regards, Rich 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
RE: EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests)
This thread has been largely theoretical. Let's look at some empirical test results for a product I just completed testing. The product has a redundant power configuration and nearly identical current paths for each of two power supplies, though one has about 2 in. longer traces on one side of one of the boards involved. There is no supplementary overcurrent protection between the appliance inlets and the input connections of the power supplies. The earthing path involves the following: filtered appliance inlet -- quick-disconnect on filter -- ~1.5 in. No. 18 AWG terminated in a ring lug -- earthing stackup on a PEM stud of ring lug (from filter), KEPS nut, ring lug for downstream earthing, KEPS nut -- ~15 in. No. 18 AWG to a header style, soldered through-hole interconnect -- traces -- soldered through-hole interconnect (for hot swappable power supply) -- soldered through-hole interconnect -- traces -- soldered through-hole to a header style, interconnect -- ~9 in. No. 18 AWG -- soldered through-hole to a header style, interconnect on the power supply -- internal power supply magic -- large, open-frame heatsink on power supply This testing was first performed in situ and as intended in normal use. I believe this test configuration should be used for the purposes of safety certification. In each of the following cases, the earthing impedance test current was maintained for 2 minutes. These tests were performed precompliance. I first tested the shortest path. before faulting test current: 20.0 A after faulting test current: 20.4 A before faulting: 0.008 Ohm (a 0.016 V drop across the path) after faulting: 0.006 Ohm (a 0.012 V drop across the path) Surprising to have a lower impedance final result. So much so, I assumed I must have done something incorrectly, reflowed a bad solder joint, initiated metal migration ... something, either during the fault test, the earthing impedance test or both. Based on the product's construction, I knew that some incidental current paths contributed to the very low earthing impedance. I then removed the assemblies of interest from the main chassis and retested on the other of the two circuits, so that only the current path of specific interest was involved. I left the main protective earthing connection intact on the chassis. Testing the longest path, before faulting test current: 20.5 A after faulting test current: 20.4 A before faulting: 0.038 Ohm (a 0.77 V drop across the path) after faulting: 0.037 Ohm (a 0.75 V drop across the path) Still compliant at a ~20 A current value and still an apparent *reduction* in the impedance of the earthing path. This is not coincidence and double checking my test methods along the way told me there were no errors. I performed a third test on the same sample, longest path, still outside the enclosure. before faulting test current: 20.4 A after faulting test current: 30.2 A before faulting: 0.036 Ohm (a 0.74 V drop across the path) after faulting: 0.041 Ohm (a 1.24 V drop across the path) The above testing was repeated in situ on a new test sample. The earthing impedance test, before and after, was set to 40 A. The results were very similar to those for the first in situ test, with almost identical calculated impedances, and the earthing path withstood the 40 A current very nicely. It should be noted that I performed the fault on a 120V, 20 A branch circuit. The product will be rated for 240 V and CSA 22.2 No. 0.4 requires the test be performed on a circuit with the voltage at the highest rating marked on the product, but I only have 20 A circuits on 120V circuits; my 208 V circuits, which I can boost to 240V, are all 30 A. Regards, Peter L. Tarver, PE Product Safety Manager Sanmina-SCI Homologation Services San Jose, CA peter.tar...@sanmina-sci.com 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
RE: EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests)
From: Rich Nute Sent: Wednesday, February 05, 2003 12:20 PM Hi Peter: Hi, Rich. This test implies a near 0-ohm fault to the PE, where the PE circuit includes a PE trace on the PWB. That's a reasonable assumption and is convenient for the purposes of testing. It is unlikely to be the only fault case, but that's irrelevant to compliance with the standard and should be considered internally, to the level of pain tolerable by any particular company. If there is a zero-ohm fault, an over-current device, somewhere, will operate. (Indeed, this is the function of the PE circuit!) A zero-ohm fault implies a large-area contact with a fair amount of contact pressure for at least the period of time to operate the overcurrent device. Consequently, the product must be removed from service and repaired before being returned to service. What if the over current device operates, the earthing path is compromised by the fault, but not destroyed? Let us not forget that there are many who will reset a circuit breaker ad infinitum, to failure, reimposing a fault repeatedly. (I spoke this afternoon to a coworker who is also landlord. One tenant consistently overloaded a branch circuit and reset the circuit breaker repeatedly, until it failed to close.) Each resetting of the circuit imposes a similar fault, with a progressively weaker earthing circuit. Let us assume that at some point short of circuit breaker failure, the earthing path becomes compromised enough that the branch circuit does not open the circuit. What if the fault is of nonnear-zero impedance, the earthing path is damaged, but not opened, and resetting of the breaker does occur, but at some point the breaker holds due to the relatively high impedance? We can let our imaginations wander from there and each believe as we will that thus and such will or will not, could or could not happen and debate the probabilities until the ruminants return hither. Bad stuff happens: dead-front switchboards explode, fires are started by minor appliances with safety certification house marks or questionable wiring practices, trains jump the tracks... If the 0-ohm fault is on the PWB, then the PWB will need to be replaced. It is difficult to imagine a fault of 0-ohm proportions that could be repaired without replacing the PWB assembly. Indeed, if the PWB PE circuit carries the high transient current, it may very well be that the supply conductors on the PWB may be blown off the PWB. So, I question whether the compliance criteria need be applied. If. The fault might occur anywhere in the earthing path. To give an idea of how a relatively simple idea can lead to a relatively complex earthing path, I have prepared a separate e-mail that includes some construction details and empirical data for a product in my lab. To be sent soon. There is also the much more variable solder in the earthing path. While manufacturing techniques have come a long way in terms of consistency, the amount of solder in a joint and the quality of the joint itself can play a significant role. It should be expected that a lower melting point solder will perform less well than a higher melting point solder. Appropriate process controls will have a positive effect. An ideal solder joint involves an amalgam at the joint with the conductors. The properties of the amalgam are typically greater than the property of either material alone. As in copper plumbing joints, an idea joint has very little solder between the two components being joined. And yet, mass production of electrical and electronic products, while generally yielding consistent-quality products when produced in a conscientious environment, can still have variability and initially undetectable problems that even HALT testing can't predict and HASS testing can't weed out. There will be very few companies with zero field returns where cracks develop in a laminate, solder joints fail or are imperfect to the point of eventually some flaw eventually rears its head. The goal is to at least offer the impression that a construction will not yield an insidious hazard at some point in the future. My recent experience has led me to believe that, aside from a few head scratching results, the test is *very* simple to perform and requires almost *no* additional test equipment, over and above an earthing impedance test setup and a modicum of ingenuity inherent in any engineer. My guess would be that the current path will be that of least resistance, which will minimize the current through the solder around the joint. So, I would doubt that the solder (of a good joint) would be much affected by the current pulse. Best regards, Rich There's no question that incidental currents can have a positive effect, even if not considered reliable. There's no denying that it is possible to comply with the test, even for a seemingly complex earthing path. There is every
Re: EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests)
Hi Peter: Not quite. I^2·t will tell you the let through current of the copper trace, but will not necessarily tell you if the construction will be compliant. The compliance criteria for this test include: * no damage to the trace (no lifting, probably no discoloration) * no damage to the PWB (no delamination, burning; I don't know if this includes burning off of solder mask) * before and after earthing impedance must comply with the 0.1 Ohm maximum impedance * no change in earthing impedance greater than 10% of the before and after earthing impedance results This test implies a near 0-ohm fault to the PE, where the PE circuit includes a PE trace on the PWB. If there is a zero-ohm fault, an over-current device, somewhere, will operate. (Indeed, this is the function of the PE circuit!) A zero-ohm fault implies a large-area contact with a fair amount of contact pressure for at least the period of time to operate the overcurrent device. (A point-contact fault would blow a hole in the copper trace due to very high current density at the point of contact.) Consequently, the product must be removed from service and repaired before being returned to service. If the 0-ohm fault is on the PWB, then the PWB will need to be replaced. It is difficult to imagine a fault of 0-ohm proportions that could be repaired without replacing the PWB assembly. Indeed, if the PWB PE circuit carries the high transient current, it may very well be that the supply conductors on the PWB may be blown off the PWB. So, I question whether the compliance criteria need be applied. There is also the much more variable solder in the earthing path. While manufacturing techniques have come a long way in terms of consistency, the amount of solder in a joint and the quality of the joint itself can play a significant role. It should be expected that a lower melting point solder will perform less well than a higher melting point solder. Appropriate process controls will have a positive effect. An ideal solder joint involves an amalgam at the joint with the conductors. The properties of the amalgam are typically greater than the property of either material alone. As in copper plumbing joints, an idea joint has very little solder between the two components being joined. My guess would be that the current path will be that of least resistance, which will minimize the current through the solder around the joint. So, I would doubt that the solder (of a good joint) would be much affected by the current pulse. Best regards, Rich 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
Re: EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests)
Hi Chris: 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? The 18 AWG readily passes the circuit prospective current test. This is because the current is transient, and is cut off before the wire in the cord can reach fusing temperature. 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). Fusing currents for wires are published in: Reference Data for Radio Engineers International Telephone and Telegraph Corporation 67 Broad Street New York 4, New York This reference says Courtesy of Automatic Electric Company, Chicago, Illinois. The approximate fusing current for 18 AWG copper is 82.9 amperes. The approximate fusing current of wires can be calculated from: I = (K) * (d**3/2) where d is the diameter of the wire, in inches K is a constant that depends on the metal Here are some values for K: copper:10,244 aluminum: 7,585 silver: 5,230 iron: 3,148 tin:1,642 The Standard Handbook for Electrical Engineers by Fink and Beatty has some additional data, including curves of current and time for each AWG. A couple of points for 18 AWG: 0.1 second:~720 amps 1.0 second:~220 amps 10.0 second:~ 82 amps 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 problem with mechanical connections to PWBs by means of screws is that the PWB base material is a plastic and is subject to cold-flow under compressive conditions. In the long-term, the connection can loosen. Not everyone pays attention to this, and, in practice, it is rarely a problem. One way around this is to use a wire from the board to the chassis. Best regards, Rich 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
RE: EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests)
Lou, I'm not proposing anything mind you, but you could save some space if you had a PWB mounted appliance inlet and you would still have to get the PEC to the chassis. Gary From: Lou Aiken [mailto:ai...@gulftel.com] Sent: Tuesday, February 04, 2003 8:36 AM To: Peter L. Tarver; emc-p...@majordomo.ieee.org Subject: Re: EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests) Why not provide a fuse to prevent deterioration of the PE trace on a PCB? Joking of course, but now that I have your attention, I would like to see this thread move away from the physics and discuss what practical reasons there are for using PC traces to provide earth fault circuits. 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: Peter L. Tarver peter.tar...@sanmina-sci.com To: emc-p...@majordomo.ieee.org Sent: Tuesday, February 04, 2003 9:53 AM Subject: RE: EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests) Not quite. I^2·t will tell you the let through current of the copper trace, but will not necessarily tell you if the construction will be compliant. The compliance criteria for this test include: * no damage to the trace (no lifting, probably no discoloration) * no damage to the PWB (no delamination, burning; I don't know if this includes burning off of solder mask) * before and after earthing impedance must comply with the 0.1 Ohm maximum impedance * no change in earthing impedance greater than 10% of the before and after earthing impedance results There is also the much more variable solder in the earthing path. While manufacturing techniques have come a long way in terms of consistency, the amount of solder in a joint and the quality of the joint itself can play a significant role. It should be expected that a lower melting point solder will perform less well than a higher melting point solder. Appropriate process controls will have a positive effect. These are some of the reasons some form of safety agency factory auditing of this type of construction is normal. Regards, Peter L. Tarver, PE Product Safety Manager Sanmina-SCI Homologation Services San Jose, CA peter.tar...@sanmina-sci.com -Original Message- From: Chris Maxwell Sent: Tuesday, February 04, 2003 5:32 AM Exactly! Chris Maxwell -Original Message- From: drcuthbert [SMTP:drcuthb...@micron.com] Sent: Monday, February 03, 2003 7:50 PM What is needed is the I squared t rating of the breaker. Then the (I^2)(t) rating of the PCB. Then you know if the PCB can take it. Dave Cuthbert 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
RE: EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests)
PC traces are easier to assemble and the assembly can be done in a tighter space. I think (just an opinion) that proper design could make this type of system more reliable as well with less chances of wires coming loose... -Original Message- From: Lou Aiken [SMTP:ai...@gulftel.com] Sent: Tuesday, February 04, 2003 11:36 AM To: Peter L. Tarver; emc-p...@majordomo.ieee.org Subject: Re: EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests) Why not provide a fuse to prevent deterioration of the PE trace on a PCB? Joking of course, but now that I have your attention, I would like to see this thread move away from the physics and discuss what practical reasons there are for using PC traces to provide earth fault circuits. 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 - Original Message - From: Peter L. Tarver peter.tar...@sanmina-sci.com To: emc-p...@majordomo.ieee.org Sent: Tuesday, February 04, 2003 9:53 AM Subject: RE: EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests) Not quite. I^2·t will tell you the let through current of the copper trace, but will not necessarily tell you if the construction will be compliant. The compliance criteria for this test include: * no damage to the trace (no lifting, probably no discoloration) * no damage to the PWB (no delamination, burning; I don't know if this includes burning off of solder mask) * before and after earthing impedance must comply with the 0.1 Ohm maximum impedance * no change in earthing impedance greater than 10% of the before and after earthing impedance results There is also the much more variable solder in the earthing path. While manufacturing techniques have come a long way in terms of consistency, the amount of solder in a joint and the quality of the joint itself can play a significant role. It should be expected that a lower melting point solder will perform less well than a higher melting point solder. Appropriate process controls will have a positive effect. These are some of the reasons some form of safety agency factory auditing of this type of construction is normal. Regards, Peter L. Tarver, PE Product Safety Manager Sanmina-SCI Homologation Services San Jose, CA peter.tar...@sanmina-sci.com -Original Message- From: Chris Maxwell Sent: Tuesday, February 04, 2003 5:32 AM Exactly! Chris Maxwell -Original Message- From: drcuthbert [SMTP:drcuthb...@micron.com] Sent: Monday, February 03, 2003 7:50 PM What is needed is the I squared t rating of the breaker. Then the (I^2)(t) rating of the PCB. Then you know if the PCB can take it. Dave Cuthbert --- 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
Re: EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests)
Chris, Douglas Brooks wrote an article about Preese's and Onderdonk's equations for fusing currents of wires, which was published in Printed Circuit Magazine. It can be downloaded from UltraCAD's web site at http://www.ultracad.com/fusing.pdf Appendix F of the book that I am writing for Kluwer, Robust Electronic Design Reference, will cover the ampacity (current-carrying capacity) of wires, printed circuit board traces, busbars, etc. The manuscript is due August 1st, so I had better get back to my writing... John Barnes KS4GL, PE, NCE, ESDC Eng, SM IEEE dBi Corporation http://www.dbicorporation.com/ 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
Re: EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests)
I read in !emc-pstc that Chris Maxwell chris.maxw...@nettest.com wrote (in 83d652574e7af740873674f9fc12dbaaf7e...@utexh1w2.gnnettest.com) about 'EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests)' on Tue, 4 Feb 2003: This would make heat dissipation different; and I would assume that it would make the fusing characteristics (I^2)(t) slightly different as well. Or even a lot different. The reason why I personally would not use a printed board trace as a PEC is that boards can develop cracks and thin copper patches, so I couldn't guarantee that every board would stand the test that the test sample passed. In this case, I don't think potentially destructive sample testing is adequate, either. The PEC needs to be 'four nines' reliable. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! 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
Re: EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests)
Why not provide a fuse to prevent deterioration of the PE trace on a PCB? Joking of course, but now that I have your attention, I would like to see this thread move away from the physics and discuss what practical reasons there are for using PC traces to provide earth fault circuits. 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: Peter L. Tarver peter.tar...@sanmina-sci.com To: emc-p...@majordomo.ieee.org Sent: Tuesday, February 04, 2003 9:53 AM Subject: RE: EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests) Not quite. I^2·t will tell you the let through current of the copper trace, but will not necessarily tell you if the construction will be compliant. The compliance criteria for this test include: * no damage to the trace (no lifting, probably no discoloration) * no damage to the PWB (no delamination, burning; I don't know if this includes burning off of solder mask) * before and after earthing impedance must comply with the 0.1 Ohm maximum impedance * no change in earthing impedance greater than 10% of the before and after earthing impedance results There is also the much more variable solder in the earthing path. While manufacturing techniques have come a long way in terms of consistency, the amount of solder in a joint and the quality of the joint itself can play a significant role. It should be expected that a lower melting point solder will perform less well than a higher melting point solder. Appropriate process controls will have a positive effect. These are some of the reasons some form of safety agency factory auditing of this type of construction is normal. Regards, Peter L. Tarver, PE Product Safety Manager Sanmina-SCI Homologation Services San Jose, CA peter.tar...@sanmina-sci.com -Original Message- From: Chris Maxwell Sent: Tuesday, February 04, 2003 5:32 AM Exactly! Chris Maxwell -Original Message- From: drcuthbert [SMTP:drcuthb...@micron.com] Sent: Monday, February 03, 2003 7:50 PM What is needed is the I squared t rating of the breaker. Then the (I^2)(t) rating of the PCB. Then you know if the PCB can take it. Dave Cuthbert 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
RE: EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests)
Not quite. I^2·t will tell you the let through current of the copper trace, but will not necessarily tell you if the construction will be compliant. The compliance criteria for this test include: * no damage to the trace (no lifting, probably no discoloration) * no damage to the PWB (no delamination, burning; I don't know if this includes burning off of solder mask) * before and after earthing impedance must comply with the 0.1 Ohm maximum impedance * no change in earthing impedance greater than 10% of the before and after earthing impedance results There is also the much more variable solder in the earthing path. While manufacturing techniques have come a long way in terms of consistency, the amount of solder in a joint and the quality of the joint itself can play a significant role. It should be expected that a lower melting point solder will perform less well than a higher melting point solder. Appropriate process controls will have a positive effect. These are some of the reasons some form of safety agency factory auditing of this type of construction is normal. Regards, Peter L. Tarver, PE Product Safety Manager Sanmina-SCI Homologation Services San Jose, CA peter.tar...@sanmina-sci.com -Original Message- From: Chris Maxwell Sent: Tuesday, February 04, 2003 5:32 AM Exactly! Chris Maxwell -Original Message- From: drcuthbert [SMTP:drcuthb...@micron.com] Sent: Monday, February 03, 2003 7:50 PM What is needed is the I squared t rating of the breaker. Then the (I^2)(t) rating of the PCB. Then you know if the PCB can take it. Dave Cuthbert 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
RE: EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests)
Exactly! There is lots of data and tables available on the web for steady state current; but I haven't found any sources that would give the (I^2)(t) values for wires or PCB traces. Such tables would take a great deal of mystery out of this subject. Right now, the best guess is to go by steady state current rating; but there must be faults in this. A PCB trace that can handle 10 Amps of steady state current has a totally different geometry than a wire that can handle 10 Amps of steady state current. This would make heat dissipation different; and I would assume that it would make the fusing characteristics (I^2)(t) slightly different as well. 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 | -Original Message- From: drcuthbert [SMTP:drcuthb...@micron.com] Sent: Monday, February 03, 2003 7:50 PM To: 'John Woodgate'; emc-p...@majordomo.ieee.org Subject: RE: EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests) What is needed is the I squared t rating of the breaker. Then the (I^2)(t) rating of the PCB. Then you know if the PCB can take it. Dave Cuthbert 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
EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests)
I read in !emc-pstc that cnew...@xycom.com wrote (in 85256CC2.005F2DA4. 0...@notes.fw.xycom.com) about 'EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests)' on Mon, 3 Feb 2003: 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. Until it trips, your CB lets through the **whole 200 A**. The trip current is practically irrelevant in this test; what matters is the trip TIME. The board trace may stand 200 A for 50 ms but not for 100 ms. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! 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
RE: EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests)
I've had extensive discussion with UL regarding the performance of this test. Below are my comments, taken from these discussions. -Original Message- From: Carl Newton Sent: Monday, February 03, 2003 9:20 AM 1. Three samples are tested; Intended to demonstrate repeatability of the test results. 2. Trace resistance is measured before and after test. Resistance cannot exceed 0.1 ohms, and cannot change more than 10% after test; The test datasheets I have from UL state the impedance before and after applying the fault is measured using an ohmmeter. I intend to use a lower current version of the earthing impedance test for this purpose (say, 20A, rather than 40A). 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. Some of this information is for the UL lab technician, in order to increase the reproducibility of the results and protect their equipment. For instance: *no power factor is specified in CSA 22.2 No. 0.4 *bus bars are what exist in UL's lab and are not a requirement to perform this test *UL's power panels this test is derived from will likely be capable of very large fault currents, so they will add resistance to limit the current *a service entrance circuit breaker is not necessary; this was chosen for it's larger interrupting rating, so as to not degrade the breaker too quickly under repeated fault conditions; you can use a plain old branch circuit breaker. 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. It's anyone's guess why a No. 16 5. Test continues until ultimate results occur; e.g. CB trips, trace opens, etc. Carl The preliminary testing I've performed in my lab indicates that the fault portion of the test can be over very quickly (probably ms, but I haven't tried to measure it), even with No. 18 AWG conductors and four connectorized interfaces involved. Regards, Peter L. Tarver, PE Product Safety Manager Sanmina-SCI Homologation Services San Jose, CA peter.tar...@sanmina-sci.com 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
Re: EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests)
Lou, It's my impression that the typical switching power supply, such as that used for ITE type equipment, will apply basic insulation between the primary circuits and earthed conductive parts on the primary side. They do this in order to minimize the creepage/clearance requirements and subsequently reduce the size of the supply. I'm referring specifically to Tables 2G and 2F of 60950. Thanks, Carl From: Lou Aiken ai...@gulftel.com on 02/03/2003 01:46 PM To: Carl Newton/XYCOM@XYCOM, emc-p...@majordomo.ieee.org cc: Subject: Re: EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests) 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
Re: EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests)
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
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
Re: EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests)
I read in !emc-pstc that Nick Williams nick.willi...@conformance.co.uk wrote (in p05200f03ba60957364e4@[192.168.1.28]) about 'EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests)' on Fri, 31 Jan 2003: At 12:22 + 31/1/03, John Woodgate wrote: There is a proposed amendment to IEC/EN 60950-1 requiring a test of the protective conductor network at *prospective short-circuit current* for the time it takes for the mains circuit protective device to operate. The details are controversial at present, because the test currents appear not to have taken into account the differences between prospective short-circuit currents in different wiring systems and supply voltages. Given that reservation, the lowest test current is 200 A. The amendment is aimed at protective conductors which are surface or internal traces of multi-layer printed boards. It is said that such traces have failed in the field under high-current fault conditions. -- Is the proposal to replace the existing test in the standard or to add an additional test only for certain special circumstances? It's additional. Is there any evidence that this test would actually result in a significant number of poorly designed products which currently pass the requirements of the standard being rejected? This is the claimed justification for the introduction. Field problems have occurred where printed board conductors have failed in high-current short-circuit conditions. The printed-board mounting versions of the IEC 60320 appliance connector encourage the use of board traces to carry the PEC; something that I would not be happy about, in principle. The existing test has its faults but it is easy to do with some very cheap apparatus. It strikes me that the cost of doing a test at 200+A is potentially very substantial. I don't think 200 A is too much of a problem, but testing at higher currents is proposed for some equipment. I don't want to be too explicit, because the figures in the draft are highly suspect (of applying to 120 V supplies!). If the result of an amendment to the standard is that significant numbers of self-certified products which have not been properly tested in this aspect of their design reach the market, then the net result will actually be a significant reduction in the safety of end users. I don't understand that. You mean that if people cheat, safety will be compromised? That's always the case. But in fact, the presence of the test may well concentrate attention on the need to make such traces substantial, whether they are tested or not. A cynic's view might also be that an amendment of this nature would suit the test labs and larger manufacturers fine, since they will be able to justify the cost of the apparatus required, whereas smaller manufacturers (and yes, small consultancy companies like mine) will not. Remember you don't necessarily need 200 A at 230 V. I can get 200 A at a bit over 1 V from a single turn on a big toroidal transformer. OK, I admit I'm putting two and two together and getting about seven but I believe one should get one's retaliation in first in these circumstances! Any amendment along the lines suggested should be prepared to sacrifice a fair degree of technical accuracy against the need for the test to be cheap, quick and easy to perform. It doesn't call for technical accuracy. You zap the equipment with the 200 A current for the operating time of the protective device and the PEC either remains intact or doesn't. Nowadays, standards writing should not just about getting accuracy and repeatability in testing but should also take into account the need to ensure that the requirements (and hence the tests) are actually possible to apply in the real world, and not just by people at specialist test houses. I quite agree, but as you indicate above, there isn't too much *active* support for that view. When I talk in the committees about low-cost testing, people tend to remain silent. In any case, at present it's difficult enough coping with the problems of the costly test equipment not measuring correctly or not being feasible (low-distortion, high- current mains supplies for IEC 61000-3-12, as a case in point). If you want a copy of the draft, to make comments to the BSI committee, please e-mail. Note that this offer can only be made to people in UK. Others should approach their national standards body. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! 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
EN60950 protective conductor test (was Re: Circuit Breaker Tripping Dring Fault Tests)
At 12:22 + 31/1/03, John Woodgate wrote: There is a proposed amendment to IEC/EN 60950-1 requiring a test of the protective conductor network at *prospective short-circuit current* for the time it takes for the mains circuit protective device to operate. The details are controversial at present, because the test currents appear not to have taken into account the differences between prospective short-circuit currents in different wiring systems and supply voltages. Given that reservation, the lowest test current is 200 A. The amendment is aimed at protective conductors which are surface or internal traces of multi-layer printed boards. It is said that such traces have failed in the field under high-current fault conditions. -- Is the proposal to replace the existing test in the standard or to add an additional test only for certain special circumstances? Is there any evidence that this test would actually result in a significant number of poorly designed products which currently pass the requirements of the standard being rejected? The existing test has its faults but it is easy to do with some very cheap apparatus. It strikes me that the cost of doing a test at 200+A is potentially very substantial. If the result of an amendment to the standard is that significant numbers of self-certified products which have not been properly tested in this aspect of their design reach the market, then the net result will actually be a significant reduction in the safety of end users. A cynic's view might also be that an amendment of this nature would suit the test labs and larger manufacturers fine, since they will be able to justify the cost of the apparatus required, whereas smaller manufacturers (and yes, small consultancy companies like mine) will not. OK, I admit I'm putting two and two together and getting about seven but I believe one should get one's retaliation in first in these circumstances! Any amendment along the lines suggested should be prepared to sacrifice a fair degree of technical accuracy against the need for the test to be cheap, quick and easy to perform. Nowadays, standards writing should not just about getting accuracy and repeatability in testing but should also take into account the need to ensure that the requirements (and hence the tests) are actually possible to apply in the real world, and not just by people at specialist test houses. Regards Nick. 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
