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Switches and Fuses...

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No both are maximum. If a switch is rated at 10A that is the maximum current the contacts will handle, regardless of voltage.

Same with a fuse. If it i rated at 10A that is the current it will blow at.
 
Gobbledok said:
Same with a fuse. If it i rated at 10A that is the current it will blow at.

Not quite. The fuse rated current is the max. cuurent that the fuse can carry indefinitely, without fusing.

To fuse it in a reasonable time, i.e. seconds, minutes, the current has to be at least 1.5 times to three times the rated current.
 
Yep understood. That was just a simplistic explanation to anwer the question.
 
Another thing to keep in mind about fuses is the voltage rating. The voltage rating means the minimum voltage that the fuse is going to interupt the flow of current. If you are using a 125V fuse on 12V it may not open the circuit when it's current rating are exceeded.
 
k7elp60 said:
Another thing to keep in mind about fuses is the voltage rating. The voltage rating means the minimum voltage that the fuse is going to interupt the flow of current. If you are using a 125V fuse on 12V it may not open the circuit when it's current rating are exceeded.

Sorry, you're the wrong way round - it's the MAXIMUM voltage it's guaranteed to be safe with after the fuse has blown, fuses don't really have voltage ratings, and anything under the rating is the same current regardless.
 
Fuses also have a maximum current that they can interrupt.

On high power mains circuits the short circuit current can be very large and the current can arc accross where the fuse wire used to be. Glass fuses can also have the glass explode.

The sand-filled fuses are like that as the sand can cool and quench the arc.

The overcurrent fuses in Fluke multimeters cost a lot because they are rated to interrupt something like 100,000 amps. If you don't have good fuses, and you leave it on amps and put the leads onto the mains, the current can get so large the current then jumps where the fuse used to be.

Then the user pulls the test leads off the cables, and arcs are formed from each test lead to the cables. What can then happen is that an arc jumps straight from live to neutral of the mains, and the fault current remains.

In that situation, if you are close to the transformer, the current can get very large and dangerous.
 
Nigel Goodwin said:
Sorry, you're the wrong way round - it's the MAXIMUM voltage it's guaranteed to be safe with after the fuse has blown, fuses don't really have voltage ratings, and anything under the rating is the same current regardless.

This is taken from the Littlefuse Electronic Designers Guide.
VOLTAGE RATING: The voltage rating, as marked on a fuse, indicates that the fuse can be relied upon to safely interrupt its rated short circuit current an a circuit where the voltage is equal to, or less than, its rated voltage. This system of voltage rating is covered by N.E.C. regulations and is a requirement of Underwriter Laboratories as protection against fire risk. The standard voltage ratings used by fuse manufactures for most small-dimension and miget fuses are 32,125,250 and 600.
In electronic equipment with relatively low output power supplies, with circuit impedance limiting short circuit currents to values of less than ten times the current rating of the fuse, it common practice to specify fuses with 125 or 250 volt ratings for secondary circuit protection of 500 volts or higher.
As mentioned previously (See DERATING), fuse are sensitive to changes in current, not voltage, maintaining their "status quo" at any voltage from zero to the maximim rating of a fuse. It is not until the fuse wire melts and arcing occurs that the circuit voltage and available power become an issue. The safe interruption of the circuit, as it relates to circuit voltage and available power, is discussed in the section on INTERRUPTING RATING.
To summarize, a fuse may be use at any voltage that is less than is voltage rating without detriment to its fusing characteristics. It may also be used at voltages higher than its certified voltage rating if the maximum power level available at the fuse under a "dead short" condition can only produce a low-energy non-destructive arc.

Based on the above I still maintain one should be aware of the fuse voltage rating.
 
k7elp60 said:
This is taken from the Littlefuse Electronic Designers Guide.
VOLTAGE RATING: The voltage rating, as marked on a fuse, indicates that the fuse can be relied upon to safely interrupt its rated short circuit current an a circuit where the voltage is equal to, or less than, its rated voltage. This system of voltage rating is covered by N.E.C. regulations and is a requirement of Underwriter Laboratories as protection against fire risk. The standard voltage ratings used by fuse manufactures for most small-dimension and miget fuses are 32,125,250 and 600.
In electronic equipment with relatively low output power supplies, with circuit impedance limiting short circuit currents to values of less than ten times the current rating of the fuse, it common practice to specify fuses with 125 or 250 volt ratings for secondary circuit protection of 500 volts or higher.
As mentioned previously (See DERATING), fuse are sensitive to changes in current, not voltage, maintaining their "status quo" at any voltage from zero to the maximim rating of a fuse. It is not until the fuse wire melts and arcing occurs that the circuit voltage and available power become an issue. The safe interruption of the circuit, as it relates to circuit voltage and available power, is discussed in the section on INTERRUPTING RATING.
To summarize, a fuse may be use at any voltage that is less than is voltage rating without detriment to its fusing characteristics. It may also be used at voltages higher than its certified voltage rating if the maximum power level available at the fuse under a "dead short" condition can only produce a low-energy non-destructive arc.

Based on the above I still maintain one should be aware of the fuse voltage rating.

But the article you quoted, agreed with what I said - not what you originally posted?.
 
Gobbledok said:
No both are maximum. If a switch is rated at 10A that is the maximum current the contacts will handle, regardless of voltage.

Same with a fuse. If it i rated at 10A that is the current it will blow at.


hmm..

By why then do relays say like 30VDC @ 1amp .. 125VAC @ .3amp

I mean relays are just switches too.. if you forget about the coil used to activate it..

are you sure the same doesnt apply for regular switches..

I saw one that said 5 amp @ 90VDC and 3 amp @ 90VAC ... :confused:

is 90 VAC really more than 90volts... peak to peak is 90VAC really 180 volts??

so confusing.
 
Peter_wadley said:
hmm..
By why then do relays say like 30VDC @ 1amp .. 125VAC @ .3amp
Yes, there will be a different current ratings for AC vs DC and for different voltages.
I saw one that said 5 amp @ 90VDC and 3 amp @ 90VAC ... :confused:
is 90 VAC really more than 90volts... peak to peak is 90VAC really 180 volts??
so confusing.
Usually, a switch can handle a higher AC voltage/current than a DC one. This is because the AC drops to zero as it reverses polarity. Thus when the switch arcs when switching off, the arc is stopped when the AC voltage crosses zero. With DC, the arc will be longer.
So you are on the the right track. There are different ratings for AC and DC switching but they are not necessarily always published.
 
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Peter_wadley said:
hmm..

By why then do relays say like 30VDC @ 1amp .. 125VAC @ .3amp

I mean relays are just switches too.. if you forget about the coil used to activate it..

are you sure the same doesnt apply for regular switches..

I saw one that said 5 amp @ 90VDC and 3 amp @ 90VAC ... :confused:

is 90 VAC really more than 90volts... peak to peak is 90VAC really 180 volts??

so confusing.

A switch (or a relay) must be able to open (and that means quenching the arc that appears when it opens) at the rated current - but it will also open at less than the rated current.

For use at a lower voltage, or in AC, it may be rated at a higher current as in your examples.

A fuse, on the other side, has only one nominal current, which is independent of the voltage.

At the nominal current (or below it) it should never open, al higher currents it will open if the overcurrent lasts enough time (the time it needs to open with a given overcurrent is the difference between "slow" and "fast" fuses)

But fuses also have a maximum current that they can open safely, and a maximum voltage that they can have when they do open. If you exeed those ratings it may not be able to quench the arc, the sustained arc lets current pass throu it (as if it were a welder :eek: ), and lots of nasty things happen - even explosions.

In low voltage / low power circuits the current that appears in a short circuit is not very high, and the fuse should be rated only for its nominal current, voltage and speed.

In high voltage or high power circuits the designer should determine the "short circuit current" and use fuses rated accordingly - that is fuses that won't fail when opening such current.
 
ecerfoglio said:
In high voltage or high power circuits the designer should determine the "short circuit current"QUOTE]

Thanks for the replies ecerfoglio and KC,

how can you determine the short circuit current?

If the power rails are shorted together..

and I = V/R...

R being an infidecimal amount .. while Voltage remaining constant (theoretically)

Current would be infinite..
 
Current would be infinite..
In an "ideal" circuit that would be correct. In reality, the wiring has resistance, a battery has an internal resistance, the windings on a power transformer have resistance, etc..... You'd have to take all these factors into consideration to figure out the short circuit current.
 
In practice fuses have a limited breaking capacity. Useing a fuse with a breaking capacity of 2kA on a circuit with a maximum short circuit current of 10kA could lead to disaster as an arc could form across the fuse and the whole thing will melt down.

To deturmine the short circuit current you need to know the impedance of the wiring and use Ohm's law and use a fuse with the correct breaking capacity.
 
Not that this was Asked by anyone, but some more useful info.

The Higher the voltage rating of the fuse, the higher its internal resistance.

This is important to know:
If you use a 250 volt rated fuse in a low voltage application, such as a 12 volt circuit. It will result in a much greater voltage drop across the fuse and less voltage to your load.

For low voltage devices buy "Auto fuses", which are usually rated at 32 volts.

And Avoid some Import fuses as Some of them are Not at all, what they say they are.

Take care...Gary
 
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