Electronic – Why does a fuse voltage rating only signify the dielectric strength

IMPORTANT:

• It is far far better that a fuse sometimes blows when there was no need for it to, than for it to sometimes fail to blow when a fault condition exists.

Blowing a fuse. The term "blow" will be used here for the fusing of a fuse - the act of melting the fuse wire and breaking the electrical circuit. Terms such as "it blew a fuse" and "why did the fuse blow?" are common here. The term "blow" in this context may be less common in some countries. Using "fuse" which is correct, as in "the fuse fused", is liable to be too confusing :-).

Why do they blow?
Should they?
The purpose of a fuse is to protect equipment and wiring against the damaging effects of electrical faults which cause excess currents, and to disable equipment which is faulty. The fuse "blows" when the current carried exceeds the rated value for an excessive time. The higher the overload the shorter the period before the fuse blows. So, equipment which is meant to "draw" 10 amps but which has a short from phase to ground, so it draws, say, 100 amps, will blow its fuse in milliseconds. But, a piece of equipment which draws say double the fuse's rated value, may take many seconds to melt the fusewire and to blow the fuse. The ratio between trip times(time to blow) and "overload to rated current ratio" vary with fuse design and can to some extent be controlled by the manufacturer. This is a complete subject in its own right, but assume that a fuse will blow "after a while" at 2 x + overload and will blow almost immediately with say 10 x + overload.

A piece of wire can only be so smart ...
Because a somewhat complex task is being carried out by a deceptively simple piece of equipment (ie a piece of wire) and because the fuse is not always optimally dimensioned for the equipment used, the fuse sometimes "blows" when there is no significant or long term fault condition present.

To blow or not to blow ? - that is the question.
Dimensioning & surges.
Assume that a fuse will blow "after a while" at 2 x its rated value then we can expect it to run indefinitely at its rated value.

If we have a household circuit rate at 20 amps and a number of outlets rated at say 10A then it is possible to connect more load that the rated fuse value. If we connect say a 10A fan heater, a 5 amp one bar radiator (maybe in the next room), a 400 Watt plasma TV (about 2A), and some plug in mood lighting at say 1 A or less then all SHOULD be well. 10+5+2+1 = 18A. If somebody then turns on an electric jug rated at say 8A current rises to 26A. More than the 20A nominal value but less than the 2 x 20A = 40A we have said it will blow at. But if the plasma TV is off and is turned on suddenly the power supply input filters amy present a nearly pure capacitive load to the mains. The mains will be at random phase at TV turnon and usually a current spike will cause no problems. But on some random lucky (or unlucky) day the mains may be at the very peak of the mains cycle at turn on. The capacitor may have stored charge of opposite polarity from last turnoff leading to an even greater current spike. Add a possibly high mains voltage (as happens) and some heavy switching spikes from a nearby factory, or even domestic equipment (treadmill, welder, drill, sander, router, planer ...) Then load + capacitor spike + high mains + switching transient may lead to a very high short term load. And the fuse may decide enough is enough and melt. Or may not.

*Unlikely?*Is all the above likely to happen at once?
No. But as reported, the nuisance blowing happens only a few times a year. Ij the order of what is expected.

We could make the fuse rating higher (more amps)!
Yes. That is one solution. But the ability to react to moderate overloads is lost. Along with lack of protection may go loss of insurance, if the insurance loss assessors find a still intact 2 x 20A wire fuse in the smouldering ruins of your workshop.

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.