Electronic – combine two identical fuses in parallel

The current rating of a fuse represents the minimum sustained current the fuse will blow at ... eventually. A 1A fuse will take 1A for a very long time without blowing, and if the fuse can dump some heat into the PCB or has airflow across it, may never blow at 1A.

The critical parameter is the \$I^2 \cdot t\$ rating, which gives you an idea of the energy (power and time) needed to blow it. (Remember that fuses are really meant to protect circuits when there are catastrophic failures.)

It's crucially important to match \$I^2 \cdot t\$ ratings, since if you replace a fast acting fuse with a slow-blow type, even thought they both say 1A, it's going to take radically different energy levels to actually blow them.

When the fuse is intact, you only have an \$I \cdot R\$ voltage drop across it. This drop is going to be nowhere close to the voltage rating of the fuse (else it acts like a big resistor and limits the energy available to your circuit.) Once the fuse blows, the voltage rating comes into play, which represents how much voltage potential the open fuse can withstand without flashing over and re-energizing the compromised load circuit.

Fuses are safety devices and they are specified and characterized to be used "standalone".

Although in theory you could characterize a fuse so that you could determine what happens when you put two in parallel, no vendor will do that, because it is something no sane designer would want to do.

Therefore, putting two fuses in parallel will go against the very purpose of a fuse, i.e. having predictable behavior under the specified conditions.

This means that, by doing that, you put at risk the equipment (and possibly its user) the fuse is intended to protect.

Bottom line: DON'T DO THAT!

BTW: a fuse is not simply characterized by its current rating, but by lots of other parameters, which vary among manufacturers. Think only of the "reaction speed". What do you expect if putting a 5A(T) (slow) fuse in parallel with a 40A(FF) (ultrafast) fuse? Which will blow first? Very messy and unreliable behavior is ensured!