Cut-Off Current

It opens really fast, just a tiny percentage of the half cycle that it would have to conduct, plus it's filled with silica that fuses and snuffs out the arc. All of those effects combining to never let the fuse conduct long enough to allow the current to rise to those high levels.

Yes, if the fuse opens while current is << prospective current there is much less energy to handle and peak current << prospective current. Assuming a good design, of course, so you only rely on a type-tested to standard product.

You may find the SIBA catalog at www.sibafuses.com of interest. It has cut -off current curves for 5 x 20 mm fuses with 300 kA breaking capacity at 250V AC in 100 mA to 12.5 amp rating. Types 70 001 40 and 70 007 40 refer.

Having unintentionally connected a DMM on 10 Amp range parallel to a 240V 4.6kA S/C supply without damage to meter or self, I can vouch for the energy limiting effectiveness of the SIBA 38 mm, 10 amp 1 kV, 30 kA prospective current version!

67model

I do not understand at all where you get the idea that a 2A can handle a 100 KA current. First none of the lower graphs include any fuse with a 100 KA prospective current. The lower graph plots the safe interrupt currents (Y axis) that each fuse rating selected can handle based of the prospective current (X axis). The prospective current is related to the arc flash potential of the circuitry feeding the line.

To clarify I'll take the 2A fuse attached to a grid capable of 1 KA and 20 KA of prospective current. (First, the 2A rating of the fuse means that 2A of current running through this fuse should never interrupt current. At 3A it might take several minutes for this fuse to open but it will eventually interrupt.) Now if the prospective current is only 1 KA an actual fault current of less than 150 A will produce an arc flash condition inside the fuse that will interrupt. Above 150 A with only a 1000A prospective current available the arc flash of the fuse elements vaporizing into a plasma ball may continue to permit current to flow long enough to not protect anything up or down stream. With 20 KA prospective current a 300 A fault in the same design 2A fuse will be violent enough inside the fuse to not maintain a conductive plasma ball. The fuse will safely protect. However, a fault current between 400 and 20,000 A may not successfully interrupt without grid damage happening.