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Elk
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A technical fuse question

My question may be based on a misunderstanding of basic electricity so please correct me if I have the fundamentals wrong.

My understanding is that watts is a measurement of electrical power, the actual work that can be performed. We can obtain more electrical power by increasing either the current or the voltage.

Fuses are rated in amps. Most fuses for household items are 250V. We are told that we can use 250V fuses of the same amperage rating for our nominal 120V circuits.

Thus, if I need a 5 amp fuse for an amplifier I get a 250V 5 amp fuse.

What I don't fully understand is how the 250v fuse properly protects my 120v circuit.
250V * 5 amps = 1,250 watts
120V * 5 amps = 600 watts

Thus, it isn't the watts or electrical power that is blowing the fuse, it is the current. Otherwise the the 5 amp fuse wouldn't protect my 120V circuit.

So, is an amp is an amp, regardless of the voltage? That is, does an amp have the same affect on a fuse regardless of the voltage? Would the fuse blow at 160 watts in a 32V circuit (32V * 5 amps = 160 watts). At 5 watts in a 1V circuit?

Am I correct that the fuse blows any time that 5 amps passes through it because resistance increases as current increases? That is if voltage increases, current drops and more electrical power goes through the fuse. As voltage drops the current must rise to perform the same amount of work - the fuse blows once the current gets high enough, even though very few watts of work can be performed with the amount of power passing through the fuse because the voltage is low.

Bottom line: it isn't the amount of electrical power (watts) that blows fuses, it is the current. We think it is the electrical power that blows the fuse because voltage remains more or less constant in our homes. Thus, when we ask a circuit to perform to much work by demanding more watts that it can supply, it blows the fuse as the current must increase to do the work. Correct?

Jan Vigne
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Re: A technical fuse question

No.

Quote:
My understanding is that watts is a measurement of electrical power, the actual work that can be performed. We can obtain more electrical power by increasing either the current or the voltage.

Fuses are time constant. They don't care if you place them in a twenty watt amplifier or a two thousand watt amplifier. What blows a fuse is the amount of current it sustains over a specified amount of time. Fast blow and slow blow fuses maintain a constant amperage for different time periods. You should never replace a fast blow fuse with a slow blow fuse. And specified voltage does matter.

bjh
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Re: A technical fuse question

Quote:
And specified voltage does matter.

Just a minor clarification... It is my understanding that a fuse should not be employed in a setting that exceeds its max. voltage rating, e.g. a 125Vac rated fuse should not be used in 220Vac setting. The opposite apparently is ok, i.e. a 5A/220Vac fuse can be used where a 5A/125Vac fuse is called for (assuming approprate type, e.g. slow-blow).

I suspect you meant as much.

Elk
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Re: A technical fuse question

I think everyone here appreciates not to replace a fuse with the wrong type or with an inadequate voltage rating - at least I hope everyone does.

This isn't what I am after. I am trying to get a full intellectual appreciation of how a given fuse would react under various conditions. My example is a 250V 5 amp fuse.

So...will it blow attempting to carry 5 watts in a 1V circuit?

Do the 5 amps in the above example act upon the fuse - causing it to blow - the same as 5 amps in a 250V circuit (1,250 watts).

If not, why not?

Jan Vigne
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Re: A technical fuse question

Quote:
So...will it blow attempting to carry 5 watts in a 1V circuit?

Do the 5 amps in the above example act upon the fuse - causing it to blow - the same as 5 amps in a 250V circuit (1,250 watts).

I don't understand. The fuse doesn't give a crap about watts. If the current draw through the fuse excedes the fuse's amperage specification over an amount of time that excedes the fuse's time specification, the fuse will interrupt the circuit. The wattage output is inconsequential since it involves voltage and load with no time constant. If enough current is drawn through a fuse over a long enough time, the fuse blows. If an amplifier outputs "X" amount of voltage and "Y" amount of current into a "Z" load for a nanosecond, it has produced wattage. The rail fuse at the other end of the amplifier doesn't care about that.

I don't understand just what sort of "intellectual appreciation" you require to understand a blown fuse. Amps ain't watts.

Elk
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Re: A technical fuse question

We all know amps and watts are different; one is current and the other power.

We all also know that a fuse will blow if the current passing through it is greater than the fuse will carry.

The generalities are easy, but what about the specifics?

A fuse in a 1V circuit providing 5 watts of power is carrying 5 amps.

Will this cause a 250V 5 amp fuse to blow?

If you don't know, just say so.

Anyone else know?

Jan Vigne
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Re: A technical fuse question

Quote:
A fuse in a 1V circuit providing 5 watts of power is carrying 5 amps.

Will this cause a 250V 5 amp fuse to blow?

In the most basic answer, no.

First, a 5 amp fuse is meant to withstand a 5 amp current draw for an amount of time determined by the type of fuse, fast or slow blow. If the current draw excedes 5 amps (for an amount of time that excedes the fuse's limit), then the fuse will interrupt the circuit.

Secondly, operating at 1 Volt a 250 Volt fuse has more headroom. But you are still ignoring time as an issue with the fuse. If a 5 amp fuse were subjected to a higher than 5 amp draw for a sufficiently long period of time, then the fuse will still interrupt the circuit no matter the operating Voltage.

I do not know the tables to suggest how long the 250 Volt fuse would remain intact when operating at 1 Volt. Perhaps someone else can provide that information or you can find it through a search engine. That is, if you must know those values.

I would suggest it would be wiser to select a fuse with a voltage rating closer to the operating voltage of the circuit. You do not place 250 Volt fuses in a 6-12 Volt automobile circuit for just this reason. Picking the correct fuse for any given application is not like choosing a breakfast cereal. Different fuses for different circuits.

If you must use a 250 Volt fuse in a low voltage circuit, you would begin with a fast blow fuse at substantially lower amperage than the circuit requires and work your way upward until you find the fuse that stays intact as the circuit draws excessive current over the specified time period and then back off at least one step for the working fuse.

Is this all hypothetical or are you trying to determine what fuse to use in a 1 Volt circuit?

59mga
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Re: A technical fuse question

Quote:
The fuse doesn't give a crap about watts. If the current draw through the fuse excedes the fuse's amperage specification over an amount of time that excedes the fuse's time specification, the fuse will interrupt the circuit. The wattage output is inconsequential since it involves voltage and load with no time constant. If enough current is drawn through a fuse over a long enough time, the fuse blows. Amps ain't watts.

Jan is correct on all points.

A bucket with a 1 gallon capacity will hold 1 gallon of fluid...water, milk, beer it doesn't matter. A 10 pound bowling ball and a 10 pound cannon ball both fall to earth at the same rate of speed. A 15 amp circuit breaker will conduct 15 amps of current. A 5 amp fuse will conduct 5 amps of current.

Elk
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Re: A technical fuse question

This is all entirely hypothetical. If I needed a 1V 5amp fuse I would try to find one.

Quote:
A 5 amp fuse will conduct 5 amps of current.

1) So, in my example, a 250v 5 amp fuse carrying 5 watts of power in a 1V circuit would blow, correct? That is, any time a 5 amp fuse "sees" 5 amps it will blow, regardless of voltage?

........................................................

Jan earlier wrote: "Secondly, operating at 1 Volt a 250 Volt fuse has more headroom."

2) What does headroom mean in this context? How is it quantified?

3) This sure appears to say that an amp isn't just an amp. So is an amp at 1V different than an amp at 250V as far as the fuse is concerned? How is it different?

Jan also wrote: "If enough current is drawn through a fuse over a long enough time, the fuse blows."

True, time matters; for example, fast blow fuses are constructed differently than slow blow fuses. Slow blow fuses can handle brief transients greater than their rated capacity to allow for start-up surges.

4) Does voltage have an impact on the time it takes to blow a fuse?

Jan Vigne
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Re: A technical fuse question

Quote:
So, in my example, a 250v 5 amp fuse carrying 5 watts of power in a 1V circuit would blow, correct? That is, any time a 5 amp fuse "sees" 5 amps it will blow, regardless of voltage?

No, you've not paid attention. Go back and read my last post again. The amperage through the fusible material must exceed the fuse rating for a specific amount of time before the material melts. A 5 Amp fuse will pass 5 Amps for an indefinite amount of time.

Quote:
This sure appears to say that an amp isn't just an amp. So is an amp at 1V different than an amp at 250V as far as the fuse is concerned? How is it different?

Well, show me the amp. Then show me the voltage. Finally, show me the load and the work that is being accomplished. Until the point of work being done, neither of the other two exist. If voltage represents the potential for work, do you suppose more work can be accomplished at 1 Volt or at 250 Volts? That would depend on the amount of amps available. But into the same load there is the potential for more work at higher voltages than at lower. And therefore the potential for more heat (watts, the work done) at higher voltages at the same amperage rating.

However, it is the resistance to the work being accomplished which acts upon the fusible material by conversion to heat which eventually, over time, melts the material. If you consider there can be no resistance to the potential for work but only the actual work itself, voltage is a minimal factor in a fusible material. But cannot be divorced from the fuse rating.

1) Why do you suppose fuse manufacturers list the voltage rating of a fuse?

2) If voltage did not matter at all, why not just list the fuse type by family in order to indicate size, shape and suggested usage?

3) Wouldn't the inclusion of the voltage specification of a fuse indicate voltage plays a role in the ability of the fuse to protect a circuit?

Ergonaut
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Re: A technical fuse question

Quote:

No, you've not paid attention. Go back and read my last post again. The amperage through the fusible material must exceed the fuse rating for a specific amount of time before the material melts. A 5 Amp fuse will pass 5 Amps for an indefinite amount of time.

Hi Guys

In-line Protection technology is actually very complex in its design, and testing it even more so -- for designers it is a complex science -- for users it should be kept simple...and Jan's point about "time" is a critical design parameter of it.

I am merely a simpleton user and employ the technology.

When we are designing protection into our amplifiers - it is current over time we are most particular about.

We treat the Voltage rating of a fuse as the "up-to" rating
So if a 32volt - it works in a circuit "up-to" 32Volt ... no more - up-to 120volt -- up to 250volt and so on. I know the pedantic amongst us will see this is imperfection, when you read the manufacturers references to this figure about arcing at the break point etc. Lets not go there otherwise we get bogged down in unecessary science which is just going to send us over the horizon for just a millisecond or two of protection time here and there.

There's- Inrush current - peak current - constant running current - reference to ambient temperatures in Deg C etc etc etc.

(There is a manual available from Bussmann on all of this- and I cite that work - go check it out -)

Failure is equal to exposure over time.

So Jan's point about time is the most valuable part to a designer. I dont want the protection tecnology to come in prematurely, nor to be too late.

So under bench simulation conditions we will work out what is "good enough" - it is never precise. The math gets us to 30 clicks of where we need to be -- the rest is bench experiments until we're happy.

Simple fudge...

Inductive load (Motor, Transformer) any thing with "in-rush current" people should use Time delay fuses.

Semiconductor circuits - (no in-rush current) people should use Quick blow.

I think ELK's question is trying to make sense of it from workload (WATTS) - my advice is dont think of a fuse like a speaker -- you can have very little voltage a high inrush current and still blow your fuse as per the spec of that fuse. Stick with current over time.

59mga
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Re: A technical fuse question

Quote:

Quote:
A 5 amp fuse will conduct 5 amps of current.

1) So, in my example, a 250v 5 amp fuse carrying 5 watts of power in a 1V circuit would blow, correct? That is, any time a 5 amp fuse "sees" 5 amps it will blow, regardless of voltage?

A 5 amp fuse will blow when the current draw EXCEEDS 5 amps.

CECE
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Re: A technical fuse question

Not necessarily. JV is correct TIME matters, checkout the BUSSMAN or LITTELFUSE web sites, all kinds of infomation, not speculation. Ever wonder why there are thousands of different types of fuses? so many parameters involved in fusing ckts, current is one of the many itmes, though probably the most important issue, but how it see's that current is a design parthe websites of BussMan and LittelFuse and a few other mfgs...you will see there is no such thing as AUDIO GRADE fuse. Audio grade fuses came from the marketing, ad, BS labs of scammers. There is no such grade. What won't an audiophile not beleive?

Jan Vigne
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Re: A technical fuse question

Quote:
there is no such thing as AUDIO GRADE fuse. Audio grade fuses came from the marketing, ad, BS labs of scammers. There is no such grade. What won't an audiophile not beleive?

Just had to repeat yourself one more time, eh, dup? That idea has nothing to do with the thread. Stick to the thread and we'll all be better off, particularly since we've read your ideas about audio grade fuses a few hundred times before.

Elk
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Re: A technical fuse question

Thanks for the efforts, everyone.

The bottom line is that it is clear that the issue is as complex as I expected and that no one here, except perhaps Ergonaut, really understands how the various factors relate to each other.

Ergonaut's responses are very helpful: "In-line Protection technology is actually very complex in its design, and testing it even more so -- for designers it is a complex science" and "So under bench simulation conditions we will work out what is "good enough" - it is never precise. The math gets us to 30 clicks of where we need to be -- the rest is bench experiments until we're happy."

This explains why the majority of us have only a vague sense of what really is going on. We know that exceeding that amp rating of a fuse will cause it to blow. Thus, we know that we should by the same fuse rating when replacing a blown fuse but not much - if anything - more.

The Bussman materials I found address fuse failure as a function of two variables, current and time. I don't see references to the impact that differences in voltage has, although I may be missing this. This is what I am interested in learning.

I acknowledge that the following question was imprecise: "a 250v 5 amp fuse carrying 5 watts of power in a 1V circuit would blow, correct? That is, any time a 5 amp fuse "sees" 5 amps it will blow, regardless of voltage?"

I should have made this >5 amps of 1V current over whatever period of time one wants to subject the fuse to this current.

Does anyone know whether the fuse will blow under these conditions? If not, why not?

As fuse manufacturers list different fuses for different voltages, voltage obviously matters. How? How is an amp at 5V different than an amp at 120V? (It appears that an amp isn't just an amp, and there is more to fuses than simply current against time).

I'm still curious about the concept of "headroom" in fuses. Is this a real concept? If so, how is it quantified?

I can speculate with the best of you. I would prefer actually knowing however.

59mga
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Re: A technical fuse question

Quote:
Not necessarily. JV is correct TIME matters...

I stand corrected, DUP. I was just trying to make the point that a fuse will pass the current that it is rated to pass, not blow when the current flow reaches that rating. Yes, time does matter. A 5 amp fuse will, eventually, melt down if that current draw continues for too long. (That's why there are slo-blow fuses...to withstand short duration overloads.) And, yes, there are numerous parameters in fuse design: A 1/32 amp fuse is physically much smaller than, say, a 5 amp fuse. Fuses are rated for current-carrying ability and for maximum voltage of the circuit in which they will be used. High-current fuses have heavier fuse element and have a relatively large diameter. Low-current fuses may be smaller with a minimal fuse element.
A low-voltage circuit fuse can be physically short where as fuses for high-voltage circuit are long. This prevents any high voltage that is in the circuit from jumping across the fuse termination points once the fuse element is blown.

And I agree with you on the "audio grade" fuses. I've never seen a video grade fuse either.

Jan Vigne
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Re: A technical fuse question

Quote:
I should have made this >5 amps of 1V current over whatever period of time one wants to subject the fuse to this current.

Does anyone know whether the fuse will blow under these conditions?

The intention of a fuse is to interrupt the circuit when the current draw exceeds the amperage rating of the fuse. Time is not negotiable. I'm curious as to why you won't accept that answer.

Quote:
As fuse manufacturers list different fuses for different voltages, voltage obviously matters. How? How is an amp at 5V different than an amp at 120V? (It appears that an amp isn't just an amp, and there is more to fuses than simply current against time).

There is more potential for work at the higher voltage. How long would to take to make your toast if the toaster were limited to 1 Volt but could draw 5 Amps? Voltage ratings for a fuse are meant to give a working range which then has an effect on time. Time, however, is still not negotiable.

Elk
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Re: A technical fuse question

I have no difficulty with the concept that time is a critical variable. But this isn't my question.

My question is the degree to which differences in voltage makes a difference.

Once again, fuse manufacturers list different fuses for different voltages. Thus, voltage obviously matters. How does voltage come into play in the design of fuses? That is, how is an amp at 5V different than an amp at 120V as far as a fuse is concerned?

(We all know that higher voltage at a given amperage can do more work, but this isn't the question. The question is whether differences in voltage at a given amperage affect a fuse differently.)

Do you know? If so, please tell me.

Similarly, consider the hypothetical of >5 amps of 1V current passing through a 250V 5 amp fuse. Assume further that the fuse is subject to the current for a long time (read: I am taking time out of the equation to get an answer to my question).

Will the fuse will blow under these conditions? If not, why not?

Again, do you know?

I remain curious about the concept of "headroom" in fuses which Jan introduced. Is this a real concept? If so, how is it quantified?

Jan Vigne
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Re: A technical fuse question

Quote:
That is, how is an amp at 5V different than an amp at 120V as far as a fuse is concerned?

Are you looking for an answer that says the fuse will remain in circuit for this specific amount of time at 1 V and for this specific amount of time at 250V? The answer, I would assume, would be stated in hundreths of a second and very dependent upon the load at the end of the circuit. More than likely just long enough to do damage but not long enough for you to react to the situation.

Quote:

Similarly, consider the hypothetical of >5 amps of 1V current passing through a 250V 5 amp fuse. Assume further that the fuse is subject to the current for a long time (read: I am taking time out of the equation to get an answer to my question).

It would seem clear from the answers already provided that you cannot remove time form the issue. Current and time are the most important factors in when the fuse opens. The answer still stands that the fuse will open when its amperage rating is exceeded independent of voltage. The fuse companies do have tables which indicate the variables of fuses. Have you gone to these to find your answer?

I believe we've reached the point of discussing angels and pinpoints or dead is dead when you want to make voltage the only variable consideration. Are you only interested in how much time elapses between one fuse and another? That is how I read your question. Again.

Quote:
I remain curious about the concept of "headroom" in fuses which Jan introduced. Is this a real concept? If so, how is it quantified?

Can I ask why, after all of this, you don't call a fuse manufacturer and ask the people who should know?

Jan Vigne
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Re: A technical fuse question

I'm also curious how this question came to you while reading the current (no pun) issue of Stereophile.

CECE
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Re: A technical fuse question

Maybe cus' somethings blows in the current issue? And it ain't realted to a ckt overload? If the voltage rating on a fuse has one so befuddled, how bout voltage ratings on anything, why are switches made for different applications, and different voltages, even if they are rated for the same current? If teh fuses have you baffled, why not use ckt brakers...oh, they have the same ratings, short ckt current voltage ratings, and more specs, and all different time/current characteristcs, for all different applications. On motor ckts, ya use differet ckt breakers than reistive loads only. For refrigeration loads teh breakers need to be HACR rated...it goes on and on, and ..there are no service panels or ckt breakers audio rated......sorry. So when you add a line for soem high powered amps, ya gots to use those regular breakers, and just suffer from teh non audio rated service panel too.

Jan Vigne
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Re: A technical fuse question

That still has nothing to do with the thread.

CECE
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Re: A technical fuse question

Yes. Now stop watching me.

Jan Vigne
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Re: A technical fuse question

Mommmmmmm, dup's doing stuff. Mmmmooooooommmmmmmmm!

CECE
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Re: A technical fuse question

bertdw
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Re: A technical fuse question

The voltage rating of a fuse defines how much voltage it will withstand after it blows. Exceed that rating and it may arc (short).

Elk
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Re: A technical fuse question

Quote:
The voltage rating of a fuse defines how much voltage it will withstand after it blows. Exceed that rating and it may arc (short).

That's been my suspicion. That this is related to the interrupt rating as well.

Can you answer my curiosity question as to whether an amp in a 1V circuit is different than an amp in a 120V circuit? (as hypothetical examples).

Do you know if >5 amps of 1V current would blow a 5 amp 250V fuse?

I've dug through the fuse manufacturer literature and haven't found an answer there either.

I don't mean for this to be controversial, simply a question about the basic characteristics of electricity. So far no one appears to know, although there is lots of speculation and commentary.

Does anyone know? I admit that I don't.

bertdw
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Re: A technical fuse question

The fuse has no idea what voltages are present in the circuit until it blows and drops all the voltage across the now open circuit. Let's say the fuse has a resistance of a tenth of an ohm. The formula E=IR, where E is voltage, I is current, and R is resistance, tells us that 1 volt across the fuse will produce 10 amps, and blow a lower-rated fuse. The fuse doesn't know whether there's 1 volt on one side and zero on the other, or 500 volts on one side and 499 on the other. Let me know if I can clarify this further, but it won't be tonight.

CECE
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Re: A technical fuse question

Smart fuses DO know which side has what voltages...is that why AUDIO GRADE fuses have ARROWS? Oh, yeah. The Smart Fuze 2000, it knows, when it blows, and it blose when it knows, but do it knows it bloze before it blose? Fuses, the final frontier of audio nonsense...no wait, I'm sure there will be more. And what does it take to BLOW YOUR FUZE?

59mga
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Re: A technical fuse question

No one has mentioned "fuse fatigue"...the tendency for a fuse to fail after so many hours of passing current.

Yes, the voltage rating does indicate at what voltage an arc may occur - that's why some fuses are larger than others. The lower the voltage the (physically) smaller the fuse can be.

I've never seen a fuse with polarity. Is there a manufacturer that makes AC or DC fuse? How about positive or negative fuses?

DUP: Does a smart fuse know when it is becoming fatigued?

Elk
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Re: A technical fuse question

Thanks, bertdw. This was what I was getting at.

Fortuitously, I happened to have dinner Friday evening with an engineer who used to work as a designer with Wadia (first incarnation, when they were in Wisconsin). (I lead a high-speed run on some local twisty roads - he came along, having just acquired a new Porsche. Great to meet someone with two shared interests.)

He explained that an amp really, truly is an amp - regardless of voltage. Thus, once the amperage exceeds the fuse rating the fuse should blow. That is, 1 amp of 1V current will blow a 1 amp 250V fuse. This answered my initial question: is an amp an amp? Yippy! An answer!

However, things get complicated (as ergonaut previously explained) when one needs to pick the appropriate fuse for a given circuit as the fuse behaves as an active component. The details are actually quite fascinating and beyond what I could accurately recite hear.

Now for the annoying part: he opined that "audiophile" fuses actually make a difference. He has no idea why - even after having cut one open, although it does look different inside from a "regular" fuse. He indicates that the largest improvement is in the bass and that anyone with reasonably discerning ears will hear the difference on a good system. It bugs him that he and others can reliably hear the difference, and he can't explain the difference electrically. Fun!

Time to buy an audiophile fuse or two and experiment. If I can't hear a difference, it was cheap enough to try and I can pass the fuse on to someone else who would enjoy testing it out. If I can hear a difference...cheap tweak!

ecopen
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Re: A technical fuse question

@Elk, I really only read the first quarter of the posts in this thread, but for you or others coming later, I thought I could clarify something, since you were after some intellectual satisfaction.

You were calculating the power based on the amperage and the voltage "of the circuit". But remember the fuse is placed in series with your circuit (or should be). So the voltage drop of the circuit with the fuse in it is the voltage drop of the fuse plus the voltage drop across the rest of the circuit (Kirchoff's voltage law). So when you plug the 1 V from your example into P=IV, you've used the wrong voltage. The voltage dropped across the fuse is set by Ohm's Law, V=IR, where R is the resistance of the fuse and I is whatever current is being drawn. So the version of the wattage you're interested in should be the P=I^2*R. The R of the fuse is fixed, and the current, I, quoted in the fuse spec is the one that provides the wattage it is handling when it will blow. Since that R is fixed, that's why the fuse spec only needs to quote the amperage at which it will blow.

Turner
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Fuses aren't components

I'm glad you posted this Elk, because I had the same question. Thinking in terms of V = IR I was reading the 250V on the 5amp fuse I bought at the automotive store (and the 250v on the fuse I pulled out of my boat's 12V system), and I was wondering if I needed to compute a different level of amperage protection proportionate to the difference in voltages. V = IR is the basic DC formula we learn in school and it works when you apply it to an isolated resister.

Reflecting on these comments, I'm thinking that we may need to remember that, a fuse has essentially no resistance.
For instance, hooking the fuse up directly to the power supply would produce essentially infinite amps and would blow the fuse either way. In this case, the DEVICE is imposing a resistance and the fuse is just a conduit. The amperage level is set by the device. Apparently, from the comments, in that context amps are all that matter. if the device is letting 3amps pass our 5amp fuse will handle it. If the device shorts out and the amperage spikes the volume of electrons will heat up the wire and blow it. Since all electricity travels at essentially the same speed in a supply wire, regardless of voltage pushing on them it makes some sense that amps transmitted by the device would be all that matter in the case of a fuse.

Like you I had the same question, but thinking about it as a wire conducting fixed amperage that is set by an external device, I can begrudgingly accept that amps transmitted is all that matters. I'm no expert, and this still speculation on my part, but I had the same question and thinking about the comments in this post this is how I'm interpreting them and it's making sense to me.

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