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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

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Old 20th Jun 2019, 8:24 pm   #1
slidertogrid
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Default 3.15 amp fuses.

A lot of CRT TV sets had a mains fuse rated at 3.15 Amps. Is there a reason for this particular rating? Why not 3 Amp or 3.5 amp? It just seems a odd value. I don't suppose it has anything to do with Pi..?
It's not something I thought about or questioned during my time in the TV trade, but the other day while talking to a friend about the various, and sometimes nasty power supply circuits from Thyristor to switch mode I thought about it. Everything from the Philips G8 onwards seemed to use 3.15 amp fuses...
Any thoughts?

Rich.
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Old 20th Jun 2019, 8:43 pm   #2
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Default Re: 3.15 amp fuses.

I can not help on the reason but years ago we had light dimmers with this value fuse mounted in the lower edge of the dimmer. I would expect the dimmer would have containd a Thyristor.




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Old 20th Jun 2019, 8:50 pm   #3
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Default Re: 3.15 amp fuses.

It's been queried before:

https://www.vintage-radio.net/forum/...ad.php?t=13060

Lawrence.
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Old 20th Jun 2019, 8:55 pm   #4
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Default Re: 3.15 amp fuses.

Yes, it's the middle value of a geometric series based around the tenth root of 10 (all rounded to nearest sensible decimal).


1 1.26
2 1.58
3 2.00
4 2.51
5 3.16
6 3.98
7 5.01
8 6.31
9 7.94
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Old 20th Jun 2019, 8:59 pm   #5
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Default Re: 3.15 amp fuses.

It's part of an exponential series, where the ratio between successive terms is the same; the same idea as the resistance and capacitance series, but with a different number of values in each decade. The values in the "fuse" series (I suppose you could call it "E10") go 1 - 1.25 - 1.6 - 2 - 2.5 - 3.15 - 4 - 5 - 6.3 - 8 - 10.

The reason 3.15 crops up, is that 3.15 * 3.15 = 10, as near as dit is to swearing -- and certainly to a better approximation than you could practically manufacture a fuse within!

EDIT: Great minds obviously think alike. Either that, or fools lack the mental capacity to differ .....
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Old 20th Jun 2019, 11:34 pm   #6
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Default Re: 3.15 amp fuses.

DNO substation fuses are often 315 amps rather than 300 amps for similar reasons, and of course domestic power circuits are often 32 amps rather than the traditional 30 amps.
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Old 21st Jun 2019, 9:57 am   #7
slidertogrid
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Default Re: 3.15 amp fuses.

Many thanks for the replies. I didn't realise it had been covered before.
Rich.
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Old 21st Jun 2019, 3:41 pm   #8
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Default Re: 3.15 amp fuses.

When we moved offices circa 2000, each desk was provided with a bank of four 13A sockets. Each socket had an integral 20mm fuse holder fitted with a glass anti-surge 3.15A fuse. Plugging in things like fan heaters had the expected consequences! We didn't have on-site maintenance so to avoid delays I got a couple of packs of replacement fuses from Farnell of RS. Still have them somewhere.
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Old 21st Jun 2019, 4:46 pm   #9
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Default Re: 3.15 amp fuses.

We always fitted 3 amp mains plug fuses for small colour sets and 5 amp for large screen,never 3.15 amp.
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Old 21st Jun 2019, 4:52 pm   #10
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Default Re: 3.15 amp fuses.

I don't think that 3.15 amp fuses are available to fit 13 amp plugs, and certainly they are not readily available.
They are however widely used within equipment.
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Old 21st Jun 2019, 5:09 pm   #11
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Default Re: 3.15 amp fuses.

Looking at fusing time against current curve in the data sheet for a Cooper Bussmann brand 3A fuse to fit a 13A plug (on the RS web site), I doubt you could tell if such a fuse was supposed to be 3A or 3.15A
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