Is a 4 amp 120v fuse the same as a 40amp 12v?

[poulsbobill]

Is a 4 amp 120v fuse the same as a 40amp 12v? Can they be substituted as both are 48watt?

Thanks

Bill

[Miami Mike]

No way. Watts have nothing to do with it. (And even if they did, the figure you came up with, which is essentially meaningless in this context, would be 480 watts, not 48.)

The amp rating of a fuse is the amount of current that will cause it to blow, and the voltage rating is the amount of voltage the fuse can safely hold back without arcing inside after it has blown. You can safely use a fuse that's rated higher in voltage than the voltage of the circuit it protects, but if it's rated at a lower voltage, arcing inside may be possible in the event the fuse blows.

[MatC]

Thanks Mike, I always wondered what the voltage rating was all about :)

[AndyOne]

The voltage rating of a fuse is the safe voltage to use it at. Use a low voltage fuse in a high voltage circuit and if it blows it can explode.

Andy.

[poulsbobill]

You are right about the 480 watts...and thank you for the responses!!!!!!!!!

Bill

[Miami Mike]

...if it blows it can explode.Let's not get carried away now. :rolleyes:

[dleroi]

Let's not get carried away now. :rolleyes:

Actually, that is part of the equation. When a fuse blows, pressure is generated. The "blowing" will be more violent at higher voltages, partly because of the arcing you mentioned. If the fuse housing isn't able to withstand the pressure, it can rupture, or "explode". The fuse housing's ability to withstand the generated pressure is one component of the voltage rating.

[Mr DIY]

Let's not get carried away now.

Ok then ... how about this...

I built a 3 phase motor controller for use on a lathe motor. 220 Volt mains gets regulated to DC (about 380 Volts) before FETs do the switching. On a couple of occasions while testing the unit, a small 20mm glass fuse (in the 380V line) blew up spraying fragments of glass everywhere. It literally blew like a firecracker. Got quite a fright first time round.

Brian

[Miami Mike]

Yes, that's exactly what I meant by getting carried away. You're talking about 380 volt circuits now.

What's next, lightning strikes? ;)

[Acetronics]

The big problem ...

is that 120 or 220v can stop current flowing ( min distance between open electrodes ) ... but a blown one 12v probably WON'T stop current flowing under 120+ volts .

no use to put the fuse on the line ... then.

Alain

Alain

[pbunnstf]

"The amp rating of a fuse is the amount of current that will cause it to blow"

Actually - this is not correct - You need to look at the time/overcurrent graph for the fuse. Many 4 amp fuses will hold 4 amps for minutes/forever before blowing.. There are many time-overcurrent characteristics for different fuses - ie Slow blow and fast blow for small glass fuses.

Go to Bussman and look at the curves.

Voltage rating and short circuit interrupting capability are also very important characteristics that must be considered.

If you have a DC supply that can deliver 5 amps maximum - then any old fuse will work - but if you are fusing a line off a 12 volt bus that can deliver 10,000 amps it takes a much different type fuse.

Selecting a proper fuse is not as simple as it looks.

[Malc C]

Also I'm correct in thinking that it also matters if you are fusing an AC supply compared to DC one.

[Gary Warner]

Let's not get carried away now. :rolleyes:

They do explode. I've cleaned up many glass shards from fuses that exploded after a tv has been hit by lightning. Obviously the voltage applied at the time of damage was well in excess of the rated voltage value, but then again, so was the current. Never the less, glass fuses can and do explode.

[Miami Mike]

Gary, I was just kidding when I wrote "What's next, lightning strikes?" in post #9! :p

[Gary Warner]

Gary, I was just kidding when I wrote "What's next, lightning strikes?" in post #9! :p
I had a feeling you were. Thanks for clearing that up. :)

Oh, this reminds me.... Have you ever applied strait 120AC to a neon light? At the wonderful age of 7 I did. Picked glass out of my hands and face all night long. :eek:

[Zlatko]

Have you ever applied strait 120AC to a neon light?

I did this to 220VAC ... once ...

Just to be thorough regarding Malc C question.
"What is the relationship between AC and DC?"

I thought that AC fuses would be under-rated by a factor of 1.414 ( square root of 2 ) for 50Hz anyway but everywhere AC and DC fuses seem to be listed for the same capacity.

Can soemone please shed some light on this?

Cheers

[Miami Mike]

Any given RMS amount of alternating current ought to heat a fuse filament to the same temperature as the equivalent amount of direct current. I see no reason why a fuse current rating should be specified differently for DC or AC.

[Gary Warner]

Right. If the given voltage value is expressed in 'RMS' then its power equivilent to DC voltages that are the same value. Now if the AC is expressed in peak voltage, you can times it by .707 (for a sine wave) to get its RMS value. Household 120VAC is an RMS value.

[simingx]

No one really uses peak values for AC calculations... ;)

Anyway, the fuses I see at work are huge beasts... they're a foot long, 3 inches in diameter and weigh something like 10 pounds. That's what you get when you have to interrupt currents in the order of 50kA at 6.6kV... ;)

[pilotpete2]

Any given RMS amount of alternating current ought to heat a fuse filament to the same temperature as the equivalent amount of direct current. I see no reason why a fuse current rating should be specified differently for DC or AC.
Very true Mike :)
I'm pretty sure that the fact that RMS values are used for AC power systems could be used to counter the arguments that repeatedly spring up on these forums, that with the PWM of DC our ESCs use to control motor speed at part throttle, that the motor, battery and ESC "see" the full voltage or current, all the time, so in effect your "wattmeter" is lying to you, as it is only the average current that is being shown by the meter.
I would expect that with a PWM DC current that the fuse would fail when the average current, not the peak current procuced during the "on" time of the PWM duty cycle, reached the fuses rating.
Am I off base on my relating RMS averaging of AC with PWM of DC producing an average voltage, thereby an average current?
I would appreciate any feedback from all in this thread :cool:
Wishing all a Happy New year :)
Pete

[MatC]

RMS current should work out as you say, Pete: Power = I * V works out just the same whether it is DC or RMS AC voltage or current, provided that it is a purely resistive load.
Of course for any inductive or capacitive load you have higher currents and voltages than the power alone indicates.

I could believe that the voltage rating of fuses applies to peak voltage though, because it is the ability to withstand arcing that is being specified.