This question about cartridge fuses came up in our shop:
This "Tron" (Bussman FNQ) fuse is blown, and we don't have the right value in stock. Can I substitute this "Dual-Element" (Bussman FNM) in its place?

The fuse was protecting a pair of condenser fan motors and a control-voltage transformer primary. I said "Yes." Then, just to confuse myself, I looked the two styles up on the Bussman web site to see how they differ.

From the Bussman www site:

The FNQ (Tron) fuses:
A fuse with a built in delay that allows temporary and harmless inrush currents to pass without opening, but is so designed to open on sustained overloads and short circuits.

The FNM (Dual-Element) fuses:
Fuse with a special design that will utilize two individual elements in series inside the fuse tube. One element, the spring actuated trigger assembly, operates on overloads up to 5-6 times the fuse current rating. The other element, the short circuit section operates on short circuits up to their interrupting rating.

Ummm ... Okay ...
Even after getting my tweezers out and picking the language over, I get the idea that these two fuses do pretty much the same thing. Does anyone know anything about the real physical or functional differences between them? TIA ... !