Listed Voltages of Ampere Interrupting Capacity Ratings

[expozen]

Could someone help me understand why AIC fuse ratings are listed as amps of varying voltages? For instance, a class T fuse is rated for 20,000A @ 125V DC, and an MRBF is rated 10,000A @ 14V DC. If I wanted to compare the AIC of these two fuses wouldn't I want to compare their interrupt capacity at the same voltages? Is there a good way to convert the AIC to make this comparison? I would imagine that increasing the voltage will decrease the interrupt capacity? So the AIC of an MRBF at 125V DC is something like 800A, and the AIC of a class T fuse at 14V DC is something like 90,000A?

[john61ct]

AFAIK no; it's amps that burn not watts.

So at the very low end of their voltage range, tiny power watts will blow the fuse compared to at higher voltages.

My understanding is the voltage rating is a maximum, long as that circuit is below then it's just amps and time delay you need to worry about.

DC being much harder, a completely different scale from AC as well.

[expozen]

Oh... that makes more sense.

I had come to that conclusion because the MRBF fuse is was researching listed the AIC as:

10000A @ 14V DC
5000A @ 32V DC
2000A @ 58V DC

Which made me think AIC was a function of voltage.

So, if I wanted to protect a 12V CALB CA 180 AH LFP bank that has a cell internal resistance of 0.5 milliohms, the estimated theoretical short circuit current would be (4 Cells * 3.65 V) / ((4 Cells * 0.5 MilliOhms) + 0.5 MilliOhms external resistance) = 5,840 A

Which would mean I could use either a Class T or MRBF fuse type. And the only difference would be that the Class T would break in a 10th of the time as a MRBF would? Would that be a critical characteristic in this scenario?

[tanglewood]

The ratings decline with increased voltage because at higher voltages, the arc can jump a longer distance and potentially weld the circuit closed, thereby defeating the fuse. As a result, all fuses and breakers have both current and voltage ratings, and you need to stay below both limits.


Related to this, DC circuits are harder to break/fuse than AC because there is no zero-voltage crossing. A DC circuit will arc continuously, and as a result is much harder to extinguish. An AC circuit stops arcing every half cycle when the voltage goes to zero, the lights back up again. So any arc self-extinguishes at the zero crossing, making it easier to break the circuit and keep it broken. If you look at breaker ratings, the DC rating will pretty much always be lower than the AC rating, and the DC voltage typically much lower. High current, high voltage DC breakers can be very expensive as a result of this, and fuses are often used instead.