No.

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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.

Or by decreasing the load.

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.

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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.

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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.

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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?

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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.

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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.

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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?

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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.

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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.

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?

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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.

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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.

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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.

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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.

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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.

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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.

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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?

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

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.

That still has nothing to do with the thread.

Just clearing your blowhole, dup?

Yes. Now stop watching me.

Mommmmmmm, dup's doing stuff. Mmmmooooooommmmmmmmm!

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

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.

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?

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, 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.

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.