> 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 possibility that some product might not make the grade. I am not stumping to add the requirements into the international community of safety standards. I am only trying to determine compliance with a standard's requirements that applies to products under my care and for which I am attempting to protect the interests of my customers. We must ask ourselves what level of pain we are willing to accept and when we prefer to receive it. Regards, Peter L. Tarver, PE Product Safety Manager Sanmina-SCI Homologation Services San Jose, CA [email protected] This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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