Electronic – Fuse rating and safety

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.

The voltage issue with fuses is limiting the energy available to the arc as the fuse opens, it is not uncommon to see inappropriately chosen glass fuses explode when overloaded from a sufficiently butch source. Fuses have limits on safe breaking capacity, and for small glass examples these can be as low as a hundred amps or so, which is also due to arc energy limits.

If seen in these terms the voltage rating of a series chain is clearly that of the lowest voltage fuse.

An important thing to note is that fuse voltage ratings for DC service are often very much lower then the rated voltage for AC service as the DC arc will not self quench.

Where series connected fuses can be useful is when you have a very stiff supply, like say a telecomms battery bank, if you want to fuse something close to the batteries at say 0.1A, you have a problem, because the prospective short circuit current can far exceed the rating of the biggest 0.1A fuse you can find. Lets say you have a PSC of 50kA... What you do is place a much higher current fuse before the little one, coordinated such that the big fuse will catch nearly direct short circuits and limit the energy delivered to the arc in the much smaller fuse.

Fuses are NOT simple devices, read the datasheets carefully.