Standard Fuse Ratings

It's all based upon what the fuse manufacturers can sell. The odd-ball sizes generally reflect applications that represent a high demand / volume for the manufacturers to produce. For example, a 6.3A fuse probably correlates to the proper protection for a common AC motor size, or some ther commonly fused appliance or device.

Hi JRaef,

Sorry, I don't agree with that explanation on the basis that there would be more "odd" sizes than just these two. Also this doesn't account for the multiples of these values from mA through to hundreds of amps.

Regards

Chas

There are indeed many more "odd sizes" than just those two. One place they come into play is sizing fuses for control transformers, many of which compute to small but odd values. I doubt that JRaef's answer is the whole story, but it is a GA as far as it goes.

Here are the fuse ratings of Littlefuse and Ferraz. They are not necessarily standardized as '3.15 Amps and 6.3 Amps and their multiples'.

Littlefuse:

http://www.littelfuse.com/searchresults.html?No=0&Ne=16&RT=Part&NN=0%3aTechnology%3a44223

Ferraz:

http://us.ferrazshawmut.com/oem/media/pdf/A2DA6D.pdf

- MS

look here, there will be something that says it all.

http://www.google.ca/#hl=en&safe=off&ei=gpcUS-2hMpL7nAeGvejiBg&sa=X&oi=spell&resnum=0&ct=result&cd=1&ved=0CAYQBSgA&q=fuse+%2Bnomenclature&spell=1&fp=768bb7f9acd45c0b

Some fuses incorporate blown fuse indicators. Particularly HRC; if fuse rating is 32Amps 30 amps is the main fuse link and 2 Amps is the spring loaded blown fuse indicator. Fuses are designed in such a way that in actual use most current flows through main fuse wire and when main fuse wire fails then flows through spring loaded indicator fuse wire. This is purely my assumption. Fuse manufacturers may be able to explain better.

Interesting GA! I hadn't heard that rationale before, but it makes good sense. The US NEC has some different standard ratings (1, 3, 6, 10,...) Some specs allow you to calculate the fuse and select the next larger standard size; other apps need a closer range of selections. Is this tenth-root-ten a European standard?

GA The frequencies of fuses from the littelfuse link provided by msamad confirm this answer:-

Only 3 and 6 amps seem to stand out as exceptions.

Do you know what these frequencies are based on? I suspect that the higher 3A and 6A numbers result from the US NEC standards to which I referred, but I don't know--just a guess.

I haven't been involved much lately in UL-508 industrial control panel design, so I am remembering (or forgetting) some of the common selections that tend to fit control transformers that accommodate a number of solenoid valves and motor starter/relay coils that would be fed from such panels.

That tenth-root-ten system sure looks sensible, though....

Do you know what these frequencies are based on?

No.

1,2,3,6 and 10 are the values you get if you "round" an "E4" series to the nearest whole number:-

E10, with a little rounding, is also the basis for the sequence of metric screw thread diameters.

http://www.littelfuse.com/searchresults.html?NN=0%3aTechnology%3a44223

In the olden days of yore. Manufacturers had less control of their tolerances and only manufactured fuses according to standard gauge sizes they could draw using a standard fuse alloy. So they achieved the fuse rating in the center of the tolerance range thus the odd rating at lower amp allowances.

Coming up next vacuum tube technology.

This is not rocket science, the reason for intermediate ratings, specifically on Time Delay fuses is to accomodate protection for motors. For instance if you compare Fast Acting J fuses to Time Delay J fuses you will see the answer.

Regards;

Lew Silecky C.E.T.

Manager

Technical Sales & Marketing

Ferraz Shawmut Canada inc.