"k" factors for Medium Voltage Fuses? 2

[rcw retired EE]

Is there a "k' factor for MV fuses to allow increasing the rated KAIC for operating voltages lower than the rated voltage rated?

My co-worker is checking a design of fused 4.16 kV motor starters. Short circuit levels under some unusual conditions are very close to the fuse interrupting rating. A sales engineer from the MCC supplier thought there was a "k" factor for fuses similar to the old ANSI MV circuit breaker rating factor.

For breakers, the interrupting rating could be raised by the ratio of the rated voltage to operating voltage within the range of <k. (Constant MVA interrupting rating). Example: a 5.0 kV, 50 kA breaker operating at 4.16 kv, with k=1.2, could have an interrupting rating raised by 5.0/4.16 = 1.20 for a rating of 60 kA. That standard is no longer applicable for modern breakers that are current limited and not MVA limited.


I haven't found a similar standard for fuses in the manufacturers' literature. Is there one?

Thank you

 

[magoo2]

I don't think so. A fuse element is responsive to current and reduced operating voltage doesn't change that.

 

[rcw retired EE]

I agree, Magoo, that the current rating or Time-Current charqacterisitc should not change. But could the maximum short circuit interrupting rating be increased for lower voltages within limits? If the fuse is designed to dissipate a fixed amount of kw-seconds as it clears, is it a constant MVA interrrupting device like the old oil and air blast breakers? At a 10% lower voltage could it safely interrupt 10% more current?

I think it would depend on the type of fuse and the mechanism used internally to interrupt the current and dissipate the resultant energy.

BTW, I am not suggesting trying this, I was just curious if it has been done.

 

[rcw retired EE]

No need to go further on this academic exercise. Even if the 50 kA fuse could be applied at 50 kA, the MCC can't. Intersting idea though.

Our solution is the usual: first check the calculations and data used in the short circuit study, add impedance if necessary to get a reasonable safety margin.

 

[bacon4life]

The interrupting rating of Bay-O-Net fuse 358C14 ranges from 600A at 23kV to 1500A at 8.3 kV. Not exactly a K factor, but does indicate voltage can impact interrupting ratings.