With the mentioning of ratings, what do voltage ratings mean? Why does radio shack sell 250v fuses? The darn thing would blow WAY before that anyway, its the current that matters...
It is not only magnitude of current that matters. It is current and time. I recall a quote I read once: "Anyone who thinks that a 1 Amp fuse will blow at 1 Amp has never been involved in the fuse selection process."
And it is true.. Fuse selection is not a trivial process. Depending on the particular fuse characteristics, that 1 Amp fuse may finally blow after it had 1 Amp flowing for 10 minutes! Each fuse is different - check the manufacturers data very carefully. Some are quick some are slow. Typically, if you need your fuse to blow immediately at current X, then you will likely pick a fuse that has a fast trip time at something less than X.
If the current is sufficiently limited, then the fuse won't blow. The ratings have alot to do with how the fuses are tested because they are expected to work at the voltages they are rated for. So, if I manufactured a fuse and found that it could work reliably at up to 500V why would I test and rate it for say 150V? I would lose all those customers who needed a 500V fuse. Of course, it could be installed in lower voltage environments as well.
These voltage ratings are generally the highest possible voltage that the fuse can work reliably (no arcing, reduced life etc..). And they also have much to do with the fuse material.
Now on something like a transistor I could understand it, you might not want 250v across the thing.... and maybe in a switch you could have arcing.
But seriously, why rate some things with a voltage when it seems that it should be rated as something else?
In many cases, somethings have ratings where it does not matter so much what they are specifically as long as you know what they are and what everything else is measured against (Example, opamps with data pertaining to various supply voltage ratings)
Other times, the ratings come from limitations of the material and assembly of the devices (Example, power mosfets with voltage ratings high enough to avoid avalanche breakdown of the semiconductor junctions)
Sometimes it has to do with convenience / practicality (Example, some ceramic capacitors rated for 1000V may actually be just as reliable at 1426.2V but who will test them at that voltage? Answer: No one. Test and hence, rate them at 1000V, call the extra 426.2V a safety margin and call it a day.)
Sometimes it has to do with all the above as well as a little bit of marketing. Example, hair dryers - voltage rating = 125VAC. That happens to be what we buy from the power company. Well, the motor and heating element can be run just fine on 140VAC but since everyone is buying this appliance to be plugged into the wall, they rate at 125VAC (not counting high line conditions which is already built into the rating)
To really understand why different things have seemingly unrelated ratings, you really need to study the manufacturers data. But you can rest assured, there are very good reasons the ratings are what they are.
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