Fuse question for electrical engineers

Question

I often work on 120V circuits that have 3amp glass fuses protecting them. When you look at the fuse it says 3A 240V on the fuse itself. My question is if the fuse allows 3A at 240V, by ohms law if it is installed in a 120V circuit it should allow 6A, correct? Is 240V a standard that all glass fuses are rated on? I don't know if a fuse would blow at 3A regardless of voltage. If not I may have to alter my allowable amperage when testing 120V circuits with the (factory installed) 3Amp 240volt fuses. Basically my question boils down to will the fuse blow at any voltage if it's over 3 amps?

Answer 1

The fuse will blow at 3Amps, the voltage rating is how far it will blow, for example taking your 240 circuit, if i put a 3A 120V fast blow and apply 240 at 4A it will blow, however it will not blow open enough to protect the ciruit since the voltage will be suffienct enought to arc over the small open in the fuse, and damage will be done. That is why it is recomended to always follow the circuit requirements when selecting fuses to protect your circuits.

Answer 2

Let's start with some basics.  Voltage is the force behind the electric charge.  Amp is the amount of current running with the charge.  120V circuit wired with one hot wire carrying 1.5 amps can be wired to carry 240V if two hot wires are connected to the appliance or whatever device with a resistance.  Each of the hot wires is 120V, 1.5 amps.  Together, the circuit is 240V, 3 amps.  Twice as much current is delived using a 240V circuit.  The fuse will blow if more than 3 amps is running through it regardless of voltage.  Most fuses are rated at least 240 volts.  

Answer 3

No, my Sansui Amp required changing 120v fuses to 230v even though I rewired the transformer for UK current.  I always mix up which is pressure and which is speed of current, if you check my early questions I got some good answers on this subject.

Answer 4

Fuses are very consistent in their current ratings, regardless of voltage.  Ohms law does not apply here.

A fuse rated for any particular voltage will break when its current rating is exceeded for a time.  The time is dependent on the amount of overcurrent.  This is clearly specified in the fuse manufacturers data.

Fuses are rated in the speed at which they will blow.  If say you were protecting a motor, you probably use a slow-blow fuse.  This way it wouldn't fail during start up surges or momentary overloads.

However, if you were protecting transistors, you might use a semiconductor fuse, They have an ultra fast reaction time.  There are other speeds in between as well.

So you can use a 32, 120, or a 240 fuse to protect a circuit up to 32 volts.  As long as its current rating is what the circuit requires.  

However, you can't use a lower voltage fuse to protect a higher voltage circuit.  Because these higher voltage fuses are designed to extinguish the arc created when the fuse blows.

Which means a 240 volt fuse in your 120 volt circuit will be just fine.  

You can go to Bussman or Littlefuse fuses they will have all the specs you could ever need.  .