This regards last month's discussion of fuse selection and transients, initiated by Fred Waechter and responded to most excellently by Rich Nute and Heber Farnsworth. My apologies for the tardiness of this minor point of clarification, but e-mail was a low priority for me in December.
Specifically, Heber said: >3. Interrupt ratings of 100,000A and higher are available, but >seldom needed, and devilishly hard to verify. The interrupt rating is >*NOT* the current through the fuse! Rather, it is the short-circuit >current available at a test source. A warning: some fuse specifications >add up to 4 feet of wire in series with the fuse, that is present when >testing the fuse, but not present when measuring the source >short-circuit current. This rather deceptive practice leads to listed >fuses which will NOT safely break their rated break current. In >addition, AC fuses specify specific current phase angles, and DC fuses >specify certain source inductance, which may or may not represent your >use situation. My experience is limited to some four + years of testing fuses for UL, so I can't address non UL standards. That said... The interrupting rating currents are based on what the power utilities say could be available at the service entrance to a structure. A typical service to a residence was supposed to get something around 10kA at 125Vac. The larger currents are based on industrial or large services to other facilities, but are certainly decades old. It has never been clear to me why, for small tube fuses (supplementary overcurrent protection fuses), the interrupting ratings are very low (as low as 35A) for 250Vac interrupting ratings, except that many fuses that pass at 125Vac, 10kA, break or explode when tested at 250Vac. It's almost as if the curve was adjusted to meet the (upper end of the) capabilities of the fuses, rather than what's available from the source (I would appreciate any further illumination <pun intended> on this conundrum). The rationale for using the additional wire between the source and the fuse is based on the practical premise wiring. The diameter of the wire is based on the current rating of the fuse and the US NEC (NFPA 70, National Electrical Code), Table 310-16; the length of the wire is based on an approximation of common wiring practices and the nearest anticipated outlet device, the closest piece of hard wired utilization equipment, switchgear, etc., to the service entrance. Even though the interrupting rating is large, it's unlikely that currents on that order will ever make it to the protected equipment (as both Heber and Rich pointed out). The testing and ratings are based on practical situations. An engineer designing the power system for any facility would have this in mind when selecting appropriate overcurrent protection devices and make certain of the capacity of the utility at the service entrance. Most readers of this forum won't need to deal directly with any of the above, but it doesn't hurt to know. Regards, Peter L. Tarver Nortel [email protected]
