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.
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
Even worse, the over-volts diodes commonly go S/C for no apparent reason