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Why do 3.15A fuses exist?

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Any idea of the tolerance on that '3.15' figure? It sounds amazingly accurate, but presumably ambient temperature etc would alter the actual fusing current.
 
Any idea of the tolerance on that '3.15' figure? It sounds amazingly accurate, but presumably ambient temperature etc would alter the actual fusing current.

It's pretty wide anyway, as is the time it takes to blow - the majority of 3.15A fuses are anti-surge ones as well.
 
Yes, I think I've seen 315mA fuses too. But using this series, there are a lot of values missing in terms of what mortals like us can buy - so it's still strange.
I wonder how important tolerance actually is, for a fuse? (Thinks.... Hmmm, quite important, really...)
But the Renard series does explain capacitor voltages too, so that's another mystery cleared up!
 
Yes, I think I've seen 315mA fuses too. But using this series, there are a lot of values missing in terms of what mortals like us can buy - so it's still strange.

Standard values we keep at work are 50mA, 63mA, 125mA, 250mA, 315mA, 500mA, 630mA, 1A, 1.25A, 1.6A, 2A, 2.5A, 3.15A

This covers pretty well all TV's, Radios etc. :D

I wonder how important tolerance actually is, for a fuse? (Thinks.... Hmmm, quite important, really...)

No, it's really pretty vague - no need for it to be particularly accurate, a fuse is a VERY crude device.

If you put 4A through a 3.15A fuse it 'may' blow, but it will do so very slowly - fuses are to provide 'emergency' cover in catastrophic circumstances. For example if it was in the feed to the rectifier from a transformer, it's there to protect the transformer if one of the rectifiers goes S/C - in which case it will blow and protect the transformer - in such a case it will blow quickly, because the current will be MANY times that which the fuse is rated for. Assuming there's a more minor fault in the unit that causes it to draw 10% more than the fuse rating, it's quite likely it would never blow - that's not what a fuse is for (and it's a very unlikely fault anyway).

There's been plenty of discussion here over the years about microwave oven transformers - and a number of microwave ovens have a fuse (a very expensive HV fuse!) in the secondary of the transformer, to protect the transformer if the rectifier goes S/C - unfortunately, the strange design of the PSU means it has a capacitor in series with the transformer, and this means it doesn't always blow the fuse, so kills the transformer. The solution was adding a large (and expensive) over-volts diode across the capacitor, so if the rectifier goes S/C, so does the diode across the capacitor - which then blows the fuse. Unfortunately this means three fairly expensive parts need replacing, then they blow again immediately, and you find the magnetron is duff as well :D Even worse, the over-volts diodes commonly go S/C for no apparent reason :p
 
Lol - educational as ever. I was thinking, the + tolerance and - tolerance might be different - like a mfr might allow 10% under, so your 3.15A will carry only 2.84A, or 20% over, so it might carry 3.78A. Maybe it's really -0% +10%, though I would think wire gauge tolerance is much closer than that. Can't be bothered to work out what the change in carrying current would be for percent change in diameter :grumpy:
 
Always been a mystery. Anyone know?

Because we have ten fingers.

On a logarithmic scale (and a lot of electronic concepts depend on logarithmic scales), 3.16 is half way between 1 and 10. Another way to look at it - into the same resistor or other load, 3.16 A (rounded down to 3.15 A) is 10 times the power of 1 A (and you humans love things in 5's and 10's).

ak
 
I think that is in essence what the link I provided said. It all depends on the base used for the geometric series.

John
 
Also known as half-octave increments where an octave is double the previous value for a log scale.
 
315 Amps is a common size distribution fuse. Although never seen a 31.5 Amps fuse which are normally 32 Amps.
 

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So according to John's link, the 315 amp fuse belongs to the R10 series, but the 32 amp fuse belongs to the R'10 series.
The series make perfect sense, but the actual implementation seems rather inconsistent!
 
There are actually 31.5 Amps fuses in service too. I found them in an oil switch 11 kV HRC fuese rated at 31.5 Amps.
 
This data sheet from Littelfuse:-
https://www.littelfuse.com/~/media/...cle/blade-fuses/littelfuse_atof_datasheet.pdf
shows the temperature derating and the typical curves for one range of automotive fuses.
The curves for other types of fuse will vary, but those curves show how long those particular fuses will take with various overloads. There is a wide tolerance, as Littelfuse quote between 0.25 seconds and 50 seconds for a fuse to blow at 160% of rated current.
 
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