In a message dated 96-06-26, Gary Schlussler writes: > I am researching a method of implementing UL 1950 3rd edition safety > circuitry into an international modem. My goal is functional survival > of FCC 68 lightening surges and nonfunctional (pass) after UL 1950 > power-line cross. This is generally a simple solution (1 amp slo-blow > 250V in series with T/R). Our product is a PC card modem though, and > does not have the necessary room for a fuse of this type.
Gary: Your comments and questions are right on target. I have been considering this same issue recently, and I do not have a proven solution. However, I have a few thoughts that I can share: 1) I believe that power cross, as simulated in the UL 1950 tests, is extremely rare. I do not believe there is any value in being able to survive the UL 1950 tests. The goal here should simply be a fail-safe response, as called for in the UL 1950 tests. 2) Lightning surges, on the other hand, are extremely common, especially in the southeastern USA. The goal here should be to survive "reasonable" lightning surges. 3) The definition of a "reasonable" lightning surge can be debated endlessly, but significant field experience has shown that surviving the FCC surges results in a very low return rate (typically less than 1%). My own experience suggests that the 100 amp peak current of the FCC metallic surge may be a bit severe (surviving 50 amps is probably OK). On the other hand, the 1500 volt peak voltage of the FCC longitudinal surge appears to be a bit on the low side. Longitudinal surge failures account for most of the field returns that I have seen in equipment that was designed to survive both of the FCC surges. 4) Recently, I have been studying the flow chart in figure 18d of UL 1950 (third edition). There appear to be several more options for compliance than there were under UL 1459. In UL 1459, you simply had to have a fuse to pass the M-1 test. In UL 1950, it appears that you can avoid installing a fuse under certain circumstances. These circumstances include specifying the use of a 26 AWG line cord, and/or providing a suitable fire enclosure. In each case, additional requirements apply. As yet, I have not quite worked my way to the bottom of this. 5) My preference would be to include a fuse if possible, so my first choice would be to find a fusible element that fits inside the PCMCIA form factor, survives the FCC surges, and operates properly for the UL 1950 tests. As yet, I have not identified a suitable device. Most picofuses will not survive lightning, and most will not stand off 600 volts after they open. Perhaps a carefully chosen wirewound resistor or inductor would do the trick. Unfortunately, the only way to find out would be to do some testing. Well, there's my two cents on this subject. I would be very interested to hear from other tregger's who may have a solution to this problem, or some ideas for a solution. The year 2000 is not that far away. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 617-721-2848
