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
