---------- From: treg-approval To: treg Cc: gary_schlussler Subject: Re: Telecom safety design List-Post: [email protected] Date: Wednesday, June 26, 1996 11:52PM
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. In message dated 96-06-27, Joe Randolph writes: >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. ----------- This is a problem that I will also be looking at in the near future. I have had some experience in testing misc. configuration for telco safety/fcc performance. Indeed you can find wirewound resistor/MOV solutions that will survive the FCC surges and provide sufficient protection during the UL overvoltage testing. However, there are some problems with this type of solution. First is that, considering a PCMCIA modem, wirewound resistors of sufficient wattage are quite litterally .... huge. Second is that there is no formal "control" method when manufacturing such a part to confirm that they will protect the device in a consistent manner, as the performance criteria that is of value in this testing concerns the fusing capacity of the resistor ... and its inductive component (which may or may not be controlled). However, you should be able to get this type of configuration through a safety lab, I don't believe they would through up a flag for this type of protection. On the other hand ... concerning surface mount fuses. I believe that most safety labs will have a problem using this component in an analog interface. the fundamental problem with surface mount fuses is that the very dimensions of their construction prevent them from complying with the working voltage ratings of a telco interface. My experience in this is a couple of years old ... so things may have changed. Most surface mount fuses, I believe, have voltage rating in the 50 Vdc area. If a product safety engineer sees this rating, they will not .... or shall I say ... should not... allow you to use this part in this application. Well I'll stop babbling at this point .... but would be very interested in others problems and solutions in this area of telco compliance (I'm interested if anyone has tested with inductors). Jim Taylor [email protected]
