i am working on an old alamo electra tube amp. on inspection before installing a 3 prong i noticed that the fuse is rated at 250 V. is it safe to just switch this out for 120? wouldn't that change the ampere rating also? any tips on finding the correct and safe fuse would be appreciated.
chris
chris
Fuses are specified in terms of current for your unit. The fuse voltage has no effect on that as sofa points out. What you don;t want to do is put 32v fuses in 120v circuits. 250v fuse in a 120v circuit is perfectly fine.
Remember, when a fuse is intact, there is almost zero volts ACROSS the fuse. The only time there is any voltage from one end to the other is when the fuse is open. There will be a tiny voltage drop across a good fuse due to the tiny resistance of the fuse, but we can ignore that. SO the fuse voltage rating is about how many volts can safely be across a blown fuse. If a fuse that can handle 120v would be OK, then certainly a fuse that can handle twice that is OK too.
Remember, when a fuse is intact, there is almost zero volts ACROSS the fuse. The only time there is any voltage from one end to the other is when the fuse is open. There will be a tiny voltage drop across a good fuse due to the tiny resistance of the fuse, but we can ignore that. SO the fuse voltage rating is about how many volts can safely be across a blown fuse. If a fuse that can handle 120v would be OK, then certainly a fuse that can handle twice that is OK too.
Just for interest,
When a fuse blows the wire " Element" melts, This causes a small gap and an arc forms that conducts until the gap in the melting fuse wire gets big enough to stop the arc. This is very fast however max fault current will flow for this time and do damage. Some fuses have sand inside so that the gap gets filled and the arc is stopped "quenched" faster. There are Time lag, to take into account magnetisation current of Tx's, quick blow to protect electronics and diodes.
The voltage rating is as already posted to ensure that the arc is quenched during a fault "element gap after current is exceded".
Fun link:
Electrical fuse burning with increasing current - YouTube
Regards
M. Gregg
When a fuse blows the wire " Element" melts, This causes a small gap and an arc forms that conducts until the gap in the melting fuse wire gets big enough to stop the arc. This is very fast however max fault current will flow for this time and do damage. Some fuses have sand inside so that the gap gets filled and the arc is stopped "quenched" faster. There are Time lag, to take into account magnetisation current of Tx's, quick blow to protect electronics and diodes.
The voltage rating is as already posted to ensure that the arc is quenched during a fault "element gap after current is exceded".
Fun link:
Electrical fuse burning with increasing current - YouTube
Regards
M. Gregg
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When you measure voltage across any resistance aren't you measuring the decrease in voltage? So when you measure a fuse in a 120 volt circuit you get 0 or close to because there is no decrease you have 120 volts both sides. However when that fuse blows you interrupt the circuit, you have no flow and your meter shows a decrease of 120 volts So when the fuse is intact you have 120 volts across it and it is only when the fuse blows that you have zero volts. Just a thought,I do agree that a 250 volt fuse is fine in replacing a 120 volt fuse.
Take care,
WILD1
Take care,
WILD1
The point realy is not resistance.
Think spark plug..When a fault occurs the current drawn will only be limited by the resistance/inductance of the mains supply. So only the resistance of the cables etc limit the current. Now how long it takes for the fuse element to go open circuit is dependant on the heating effect caused by the fault. So forget the 10A fuse what current flows until the element opens is full current. The fuse does not limit the fault current untill it opens.The speed of operation is very important for damage limitation.
When the fuse element opens an arc is drawn this can be greater depending on any back EMF produced by transformer windings etc the (higher the voltage the greater must be the gap to break the arc). The arc will continue to jump the gap in the fuse element until it burns away to a distance great enough to stop the arc. This has nothing to do with fuse element resistance. The speed of operation of the fuse is dependant on the current drawn. Looking at Auto disconnection of supply if the fault to earth does not draw enough current the fuse will heat slower and the damage will be greater. Thats why no resistance is allowed in a safety earth connection.
Regards
M. Gregg
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Wild1 had it right to begin with, then changed his answer at the end.
I just don't want anyone thinking an open fuse has 0 volts across it.
Yep
I think Volt drop is a bit confused in this context..Obviously there will be a volt drop across the fuse under fault conditions linked to resistance of the fuse element or it would not heat, however this has nothing to do with voltage rating and the gap required for fault clearance.Regards
M. Gregg
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Final comment,
Regards voltage being non relevant and current being the only issue for fuse operation. Volts X Amps =Watts = Heat.
So here is food for though..
would the fuse element heat faster with higher voltage or operate at the same time at the same current rating..would a higher voltage fuse used on lower voltage with the same current blow slower..
So why have different voltage rating fuses at all except for arc gap.
(Operation time)?
For our purposes as long as the fuse is above the working voltage of the circuit it will work. There is always more ..LOL
Regards
M. Gregg
Regards voltage being non relevant and current being the only issue for fuse operation. Volts X Amps =Watts = Heat.
So here is food for though..
would the fuse element heat faster with higher voltage or operate at the same time at the same current rating..would a higher voltage fuse used on lower voltage with the same current blow slower..
So why have different voltage rating fuses at all except for arc gap.
(Operation time)?
For our purposes as long as the fuse is above the working voltage of the circuit it will work. There is always more ..LOL
Regards
M. Gregg
No. The heating of the fuse depends on ITS resistance and the current through it. This develops the voltage across it - very small until it blows.
The voltage rating is of the package, not the fuse wire. Why bother making a 36V rating when 230V will do for everything from 0 to 230V? (Except the 36V one could be smaller)
You are assuming a resistive short..If the short is to Earth then the voltage across the fuse is supply. The current through the fuse is dependant on the supply combined with the fuse resistance which is low. You are saying that a resistor wattage is developed by its resistance this is not true its the volt drop across it the current will vary dependant on voltage drop.
I do not think the voltage across the fuse will be low with 240V across it. Its almost like saying that a lamp will not glow brighter with twice the voltage. The energy dissipated will be higher and so speed will be faster. Im not saying that the element is different just that the voltage will dissipate more power in the fuse element for the same current.
Regards
M. Gregg
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32V fuses are cheaper than 250VAC rated fuses in the 1/4x1 1/4 size. A lot of them are sold for cars, and many people use them for AC circuits when they shouldn't (like the factory parts room where I previously worked, until I complained). Furthermore, in the larger currents, 32V fuses can still be glass, or self indicating, whereas the 250 VAC fuses above 15A are in ceramic now from the primary vendors. This means you have to use a device to determine if the fuse is blown, which slows repair work.
A third rating is "interrupt current" which doesn't apply much to consumer products. The power source can deliver a certain current into a short circuit. Fuses of a smaller physical size can interrupt a smaller short circuit current. A one house transformer in the country can produce only maybe 1000 amps short circuit. But a large transformer in a factory can produce more than 10000 A short circuit current. In that case use of a larger fuse is indicated.
An interesting misapplication of the 3AG 250 VAC fuse was in 1968 when Canada regulators required fuses on the B+ side of the power transformer of Hammond H100 organs. These were 250 VAC fuses in a 450 VDC circuit. Hammond had been getting a way for years with no fuse whatsoever, since they made the whole consumer electronics thing up, and oddly these units didn't have a primary AC fuse either. The use of 0-3 Leslies or tone cabinets drawing power from the main organ confused the issue of the main fuse utterly. But IMHO they should have used 500 V rated fuses if B+ fusing was required. Copied protection devices of this below rating design are still being sold.
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That is not how I would look at it.
The load defines the current in the fuse. The effect of the fuse (impedance) is negligible UNTIL the heating by the load current causes the fuse dissipation to rise, it self heats and finally melts. Rupture.
It is actually quite complex as the fuse is highly non-linear. Which is why it can be difficult to get the right fuse rating for a toroid Xformer. Lots of short large pulses on switch-on (Inrush) cause erratic fuse failure.
AFAIR Littelfuse had some good app notes in their catalog with graphs of fuse failure modes v time etc
The load defines the current in the fuse. The effect of the fuse (impedance) is negligible UNTIL the heating by the load current causes the fuse dissipation to rise, it self heats and finally melts. Rupture.
It is actually quite complex as the fuse is highly non-linear. Which is why it can be difficult to get the right fuse rating for a toroid Xformer. Lots of short large pulses on switch-on (Inrush) cause erratic fuse failure.
AFAIR Littelfuse had some good app notes in their catalog with graphs of fuse failure modes v time etc
I agree,
However at instant short circuit there is no load to supply..
Regards
M. Gregg
No....... In my conclusion I said that when the fuse is intact you have 120 volts across it [no decrease] and your meter would read 0,when the fuse blows you have 0 volts across it [a decrease of 120] and your meter reads 120. The use of resistance in the meter for measuring voltage is the resistance I was referring to. However to say the resistance in the circuit is irrelevant is not right. I think it is very relevant. After all isn't it the resistance to current flow that causes the fuse to heat up and blow? I do agree that voltage has every thing to do with arc. That is why spark plugs use very high voltage at very low amperage.
Have a great day,
WILD1
Have a great day,
WILD1
Indiana Jo,
Yes I agree the fault currents are very high before fuse interupt. Only limited by the inductance of the Power Tx and supply cables. The package like in HRC fuses must contain the energy. I prefer silica and ceramic fuses..some have tell tails like Siemens "milk Bottle" can't remember the package number..
Fuses in B+ now there is an interesting subject.
Regards
M. Gregg
Yes I agree the fault currents are very high before fuse interupt. Only limited by the inductance of the Power Tx and supply cables. The package like in HRC fuses must contain the energy. I prefer silica and ceramic fuses..some have tell tails like Siemens "milk Bottle" can't remember the package number..
Fuses in B+ now there is an interesting subject.
Regards
M. Gregg
Measuring voltage?
I think this is the point you would have 120V across an open fuse (Side to side), however to other side of supply you have 0 volts at the load end of the fuse.
Regards
M. Gregg
I think this is the point you would have 120V across an open fuse (Side to side), however to other side of supply you have 0 volts at the load end of the fuse.
Regards
M. Gregg
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