Fuse voltage rating

[nickjb]

I've bought a non working cnc machine which is missing some of the control system and I'm building it up. The motors operate from a 40v DC supply. It has automotive style blade fuses for each motor. These fuses are rated at 32v. I can find fuses rated at 58v but cannot find a supplier. So here's the question, why are standard car fuses only rated at 32v? The insulation is not an issue at that voltage (and it would be fine at 10x the voltage anyway) . Only thing I can think of is the risk of arcing if it blows but 40v could only arc a tiny fraction of a mm. I'll continue searching for 58v fuses but if I can't find them what actually could go wrong

 

[ericmark]

The atmosphere can ionise, even the gases in a standard light bulb can ionise hence the flash when they blow, so at some voltage fuses change from air gap to sand, I have never really studied at what point they change over, it also effects the speed at which it ruptures, for 40 volt I would not really worry and would fit any fuse of correct amp value and not worry about voltage. I note with the American blade fuse some have a wire and some have a S shaped strip, but since I first saw them in 1980 I have never seen any details about why they have different designs.

 

[bernardgreen]

With AC the arc through plasma will reduce each time the voltage goes through zero and hence extinguish after a few cycles

With DC there is no zero and the arc will continue until the metal at the ends of the arc has melted / eroded and the arc gap has increased far enough that the arc breaks.

With the wrong fuses a DC motor may continue to run after the fuse has blown, the current through the arc being enough for the motor to continue running.

Like wise with the control contactors. If you are replacing them then the contacts must separate far enough apart to ensure the arc created when the contacts open will break. Magnets are often used to bend the arc to extend its length beyond the length it can sustain.

 

[Tigercubrider]

Where are you buying the fuses?

I don't count myself as qualified to answer but if you buy automotive fuses from a car shop, it's hardly surprising that they are rated for 32v as cars are 12v

Maybe commercial automotive are higher? Or marine ones?

 

[securespark]

Can you get in touch with the manufacturer and ask them which fuses you need?

 

[bernardgreen]

Telephone exchange fuses are designed for 48 volt DC supplies.

 

[SUNRAY]

If you are replacing them then the contacts must separate far enough apart to ensure the arc created when the contacts open will break. Magnets are often used to bend the arc to extend its length beyond the length it can sustain.

I'd forgotten the magnet thing, also on big DC contactors a jet of compressed air is sometimes used.

 

[ban-all-sheds]