Why do 3.15 amp fuses exist?

[Circlotron]

I was snuffling through some junk at work the other day and I came across a packet of 3.15 amp fuses. Then I thought, why do they make such an odd value of fuse???

http://au.element14.com/webapp/wcs/stores/servlet/Search?catalogId=15001&langId=43&storeId=10184&categoryId=800000004790&st=fuse&pageSize=25&showResults=true&pf=110153352

[Galenbo]

http://www.vintage-radio.net/forum/showthread.php?t=13060

[mariush]

So basically an 1877 a French military engineer, Col. Charles Renard is to blame for 3.15A fuses, or more exactly ISO: http://sizes.com/numbers/preferred_numbers.htm

[AF6LJ]

I was going to say because the 3.14159 A fuses were too expensive.
But I thought better of myself. 

[SeanB]

Those Littelfuse tables bring back memories, especially of the PSU protected by 3 1A solder in Littelfuses, where the most common cause of a
PSU failing was a shorted diode ( one of 42), and if 2 shorted out in the same rail it would cook the transformer. The fuses would only blow ( 2 out of 3) after the transformer had well and truly cooked itself into a ruin. I used to check the power rails on every unit, irrespective of why it came in, with a scope looking for the odd shaped ripple that indicated a shorted or open circuit diode. I got a few open circuit diodes that way, and a single open or shorted diode would have very little observable difference on the rails aside from the missing cycle in every 6. Of course the original MC44 diode was obsolete........

[HighVoltage]

They are common in Germany as well and I never got a good answer to why they exist.
The large German retailer conrad (conrad.de) does carry them and as I was told a while back, they are selling lots of them.

[kedr_vostok]

So basically an 1877 a French military engineer, Col. Charles Renard is to blame for 3.15A fuses, or more exactly ISO: http://sizes.com/numbers/preferred_numbers.htm

Wow. This has answered a question that i've never thought to ask. Here's a write up from ABB about it.

http://www02.abb.com/global/ilabb/ilabb012.nsf/0/8d9c6e730f0dd3f6c1257935002aaac2/$file/The+strange+numbers+of+circuit-breaker+rated+current.pdf

[Marc M.]

Don't forget the little brother .315A GMA used in many Radio Scrap DMM's from at least the early 80's.  Those DMM's would pop that fuse any time you screwed up and dribbled any voltage down it's throat in OHM's mode  .  I probably could have bought a brand new Fluke for what I paid in .315 fuses over the years (did a lot of consumer electronics repair in the late 70's thru the 90's).

[JuKu]

I was going to say because the 3.14159 A fuses were too expensive.
But I thought better of myself. 

No, it is so that you can safely pass 3A through it. 
(No, you can't, you'd overheat the fuse. It is because starting from 1, going up by 25% and rounding gives the ISO R10 series. Why that one isn't rounded to 3.2 is the true odd question.)

[Circlotron]

I was going to say because the 3.14159 A fuses were too expensive.
But I thought better of myself. 

My favourite answer. 

[Chris C]

No, it is so that you can safely pass 3A through it. 

That's what I'd always assumed.  Fuses are not precision devices.  An editor on a well-known hobbyist site wrote a fuse tutorial, claiming a fuse would blow "if even one more electron than its rating passes through it".  Hah!  Then got offended when I and others took him to task over misinforming his readers.

It's interesting to learn the true story behind this rating.

[calexanian]

We actually use 3.15 amp fuses in one of our products. We arrived at them just because they worked well in our application, which happens to be driving motorized 24VAC valves for that product. Good old design by empirical evidence.  O0

[Zero999]

So basically an 1877 a French military engineer, Col. Charles Renard is to blame for 3.15A fuses, or more exactly ISO: http://sizes.com/numbers/preferred_numbers.htm

Same with resistors, capacitors & zener diodes.

[Delta]

I think the E series reisistor values were based around the tolerances available.

[Psi]

3.15 amp fuses are required to protect pi circuits.

[VK3DRB]

Damn good question and good answer. We take a lot for granted, but I never thought of asking the question why we have a 3.15A fuse.

Another thing very few engineers ever ask: Why is current depicted with the letter 'I', for example, I=5A. Why 'I' specifically? I know the answer to this one. Its historic. I'll let you do the research to find out why .

[GNU_Ninja]

No idea; But I just checked my desk drawer and found two unopened packets of 6.3 Amp 20mm Anti-Surge fuses (Farnell 151-526) ... 

[HighVoltage]

Another thing very few engineers ever ask: Why is current depicted with the letter 'I', for example, I=5A. Why 'I' specifically? I know the answer to this one. Its historic. I'll let you do the research to find out why .

This was taught in Germany in the University:

It goes back to André-Marie Ampère in France and since most SI units have French roots, so does this one.
"I" because of "intensity", at a time when this was synonym to what we call today current.