Medium Voltage Power Fuse Interrupting Capability

[kartracer087]

Hello,

FOR ANSI: It is my understanding that the ultimate interrupting capacity of a medium voltage expulsion power fuse is its asymmetrical RMS 1/2 cycle ampere rating, is this true? Therefore, for any X/R ratio, that fuse must be capable of interrupting the asymmetrical RMS current and the symmetrical RMS interrupting is simply derived from the X/R circuit considering the maximum interrupting asymmetrical RMS interrupting rating of the fuse, correct? Therefore if a fuse with say 20kA asymmetrical is applied on a circuit that was tested to have an X/R ratio of say 20, that fuse would have a symmetrical interrupting of 12,500A RMS symmetrical. Now that same fuse would still have higher symmetrical interrupting ratings if the X/R ratio was less than 20, so long as the asymmetrical interrupting does not exceed the 20kA, is that also correct?

I know for circuit breakers its a bit different in that you cannot increase the symmetrical interrupting rating of the breaker when your X/R falls less than the tested X/R, if X/R is less than or equal to the tested X/R ratio, you use the symmetrical rating as the maximum the device can interrupt. If the applied circuit X/R ratio is greater than the tested X/R, you must derate the symmetrical interrupting which is listed on the breaker.

Thanks

 

[dpc]

The ANSI standard for expulsion fuses is based on asymmetrical interrupting at an X/R of 15, IIRC. S&C tests at X/R of 20 so that can be a little confusing. The symmetrical rating is adjusted based on the actual system X/R as you state. But that is simply a back-calculated number and not based on any additional testing, as far as I'm aware. The asymmetrical rating alone is the basis for the fuse short circuit capability.