In a message dated 11/27/2002, Marko writes:
> So what did you decide? Is a fuse blowing acceptable? > I'm sure others would be interested as well. > Hi Marko: So you want me to go on the record so I can get flamed? OK, here goes: The revised text in Issue 3 of GR-1089 (kindly posted to the group by Alain Servais) explicitly states that fuses are acceptable. The text is not 100% clear on whether the fuse has to be located on the module where the short was induced or whether it can be anywhere in the system, but it appears that the fuse could be anywhere. The only remaining gray area is whether the failure of a component or circuit trace is acceptable *provided* that it is located on the same circuit card where the short was induced, *and* that no safety hazard resulted. My interpretation is that this would be acceptable. This conclusion is based on the following considerations: 1) Issue 3 makes it very clear that failure of something other than a fuse is NOT permitted if it located on a module other than the one where the short was applied. 2) Issue 3 also says that the module where the short was applied does not have to work after the test is over. 3) It seems unreasonable to require a circuit card or module to continue to function after a short was applied on that module. The short itself simulates a fault condition that would require replacement of the module. If so, what's the point of requiring that nothing else on the module is damaged? It doesn't serve any of the reliability goals that are part of the implied intent of the requirement. For the module itself, it would seem that a sufficient criterion would be that no safety hazard occurs. I suspect that others may disagree with this interpretation, and I am open to discussion about it. The fact is that I would be inclined to use fuses at that module level on any new design unless I was VERY sure that the failure mechanism would not create a "fire, fragmentation, or electrical safety hazard" as stated in GR-1089. In the case that prompted me to post this question in the first place, the failure mode was an open circuit trace, on an inner layer of the circuit card where the short was applied, in an existing product. Under the circumstances, I am not inclined to insist on redesign of that product to retrofit a fuse. However, I would recommend the inclusion of a fuse in any similar product designed in the future. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 http://www.randolph-telecom.com
