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Other Vintage Household Electrical or Electromechanical Items For discussions about other vintage (over 25 years old) electrical and electromechanical household items. See the sticky thread for details.

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Old 6th Jul 2019, 5:26 pm   #81
rambo1152
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Default Re: 13 amp fuse for 1.5mm mains flex?

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I once has a 13A to shaver adapter that was fused at 1A.
I can't remember the fuse length now though.

I have one here, it's 20mm or 3/4 in

I borrowed one from a Premier Inn reception once and the fuse was wrapped in tinfoil.
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Old 6th Jul 2019, 6:41 pm   #82
TrevorG3VLF
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Default Re: 13 amp fuse for 1.5mm mains flex?

I use 1A plug fuses to power valve radios with valve rectifiers. Never had one to blow but there is no turn on surge.
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Old 6th Jul 2019, 6:59 pm   #83
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Default Re: 13 amp fuse for 1.5mm mains flex?

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Originally Posted by rambo1152 View Post
I borrowed [a shaver adaptor] from a Premier Inn reception once and the fuse was wrapped in tinfoil.
Might it have been used by a previous guest to (attempt to) power a travel hairdryer, kettle or something fitted with a Schuko plug?
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Old 6th Jul 2019, 7:54 pm   #84
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Default Re: 13 amp fuse for 1.5mm mains flex?

If we are talking about switch on surge. Does each surge shorten or weaken the fuses longevity ?
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Old 6th Jul 2019, 8:17 pm   #85
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Default Re: 13 amp fuse for 1.5mm mains flex?

Switch-on surges weaken fuses. Tiny bits of the fuse wire can be vaporised, and the heating and cooling with consequent expansion and contraction work-hardens it. Eventually, the switch-on current proves too much and the fuse gives up.

Some appliances with large (or just marginal) transformers have an NTC thermistor in series with the primary, or more complex thermal or electronic delay devices having contacts that short out a series resistor after a time delay to allow some flux to build up in the core.
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Old 6th Jul 2019, 10:53 pm   #86
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Default Re: 13 amp fuse for 1.5mm mains flex?

The fuses normally specified for clock points, shaver adaptors and certain other mains power accessories are to BS646. These are 5 x 19mm and therefore technically distinct from the 5 x 20mm fuses popular within electronic goods. The breaking capacity of a BS646 fuse will have been certified as meeting the standard, whereas the breaking capacity of 5 x 20mm fuses varies.

I agree with the usage of low ratings of plug fuse for appliances that are not well protected internally, which usually means not made to modern standards. There are caveats though; even a 1A fuse will allow a consumer electronic device to dissipate over 300W for long enough to catch fire, so it is not a panacea for all possible overload scenarios. Also, I have found specimens of apparently reputable 1A fuses that were not silica filled, which probably had a higher let-through than filled types in the event of a short-circuit. (2A always seen to have been filled.)
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Old 7th Jul 2019, 10:02 am   #87
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Default Re: 13 amp fuse for 1.5mm mains flex?

My O level syllabus included being given 3A, 5A etc fuses (probably Bussmann or similar) for assessment. They routinely carried 1.5 times their rated current for many minutes before failing. (I assume the default format for plug fuses is 'slow blow')
I have an old looking glass 500mA fuse (without sand), not sure where it came from or what it's intended use was. Possibly for a plug on an electric blanket..
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Old 7th Jul 2019, 10:56 am   #88
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Default Re: 13 amp fuse for 1.5mm mains flex?

Quote:
They routinely carried 1.5 times their rated current for many minutes before failing. (I assume the default format for plug fuses is 'slow blow')
That does not make them 'slow blow', the BS1363 specification I believe places them somewhere between 'fast' and 'medium' using the modern categories.

An overload of 1.5xIn is a very low overload that is hard for a fuse to detect, and most fuses will not react for a long time if at all. The heat input that causes the temperature rise to melt the element follows the square of the current, so over an extended period when the fuse is in thermal equilibrium, i.e. as much heat is escaping as is being produced, a 1.5x overload only produces a rise of 2.25x normal working temperature. The maximum power dissipation of a plug fuse is typically 1W, so the additional 1.25W has got to take it from permanent endurance to melting, which is hard to achieve consistently with a copper wire fuse.

Where the distniction between fast and slow fuses comes in is with a surge of perhaps 5xIn, typical of PSU inrush, producing a momentary heat input of 25x normal. If that takes the element over its melting point, as it often will with a simple copper wire fuse, it blows before the heat has had time to dissipate. If instead the element has high thermal mass, typified by the traditional slow-blow fuse with a coiled spring and slug of solder, it can absorb that heat impulse without a corresponding rise to 25x normal working temperature and then dissipate it over a longer period. It will still react to a low overload of 2xIn over a longer period, as thermal equilbrium will be achieved and the melting part of the element (the solder) will catch up with the heat dissipating part (the wire).

Ultra-fast fuses are designed to have very low pre-arc values to protect semiconductors. They have specially shaped elements often of foil with one or more slots in. Under thermal equilbrium, the wider areas of foil work as heatsinks for the narrow areas, but in the event of a short-circuit the adiabatic heating at the isthmus very rapidly raises it beyond melting point.
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Old 7th Jul 2019, 11:01 am   #89
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Default Re: 13 amp fuse for 1.5mm mains flex?

The default format for plug fuses is not slow-blow, in that the fuse wire is not loaded with a thermal mass, or tensioned with a spring, or formed in a particular way to delay the blowing in the event of a current surge. The sad fact is that the transition between a wire's continuous current rating i.e. the current which it will carry without degradation for, say, thousands of hours, and its fusing current i.e. the current needed to blow it acceptably quickly is not sharp. A 3A fuse will carry 3A pretty much for ever but it will not blow quickly at 3.01A.

The fusing characteristics are indicated here https://www.pat-testing-training.net...cteristics.php.

We shouldn't put glass-bodied fuses without sand in plugs. Their breaking characteristics are often much poorer than proper BS1362 fuses, which means an arc may continue for long enough after the wire has melted to allow serious damage to occur downstream. And they can also shatter explosively, leaving lots of sharp glass fragments to hurt you in more obvious ways.

Cheers,

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Old 7th Jul 2019, 11:21 am   #90
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Default Re: 13 amp fuse for 1.5mm mains flex?

Thanks guys, i had a suspicion it wouldn't be quite that simple. The 500mA example is of a certain age, indicated by one of those applied labels with a 'gold-leaf' appearance.
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Old 7th Jul 2019, 2:36 pm   #91
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Default Re: 13 amp fuse for 1.5mm mains flex?

The 1" x 1/4" fuse was a standard pre-war UK glass fuse size, designated by Bulgin as "short type", available in ratings from 150mA to 3A: extracts from the 1936-7 Bulgin catalogue attached. They were said to be "Abolutely fireproof" and to provide "Absolute safety from danger of fire". Bulgin used to do double-pole fused 2-pin 5A plugs and fused chassis sockets that took a pair of them (supplied with 1A fuses unless specified otherwise). I suppose the 1" glass fuse was discontinued in the UK post-war so they wouldn't get used in 13A plugs.
Attached Files
File Type: pdf Bulgin Fuses 1936.pdf (318.8 KB, 18 views)
File Type: pdf Bulgin Fuse plugs1936.pdf (195.1 KB, 17 views)

Last edited by emeritus; 7th Jul 2019 at 2:59 pm.
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Old 7th Jul 2019, 5:25 pm   #92
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Default Re: 13 amp fuse for 1.5mm mains flex?

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Originally Posted by emeritus View Post
I suppose the 1" glass fuse was discontinued in the UK post-war so they wouldn't get used in 13A plugs.
Not so. They are still made (e.g. by Cooper Bussmann) and sold by RS (amongst other companies I guess). For example :

https://uk.rs-online.com/web/p/cartridge-fuses/8495529/

No, they are not suitable for use in 13A BS1363 plugs for (hopefully) obvious reasons.
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Old 7th Jul 2019, 7:32 pm   #93
gramophone1
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Default Re: 13 amp fuse for 1.5mm mains flex?

None of this is an exact science. It seems that fuses are flawed to a degree. For example a 13 amp fuse can work way above its specification, pulling 20 amps continually without failing. So when does a 13 amp fuse fail ? I can't find any information with exact tolerances. But this would suggest a safety issue, if your cable fails even when it is within the allowed parameters for the fuse value used.
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Old 7th Jul 2019, 10:14 pm   #94
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Default Re: 13 amp fuse for 1.5mm mains flex?

The time/current characteristics of 3A and 13A plug fuses are given in the link in GrimJosef's post above. A 13A fuse is not expected to operate below 21.6A (1.66xIn), above that, how long it takes depends on the current as shown by the curves.

I think you are overlooking the important point that cable ratings are calculated with reference to the fusing characteristics of normal fuses and circuit breakers. Cable rated at 13A will tolerate 50A for the few seconds it would take a 13A fuse to blow. The rule as I mentioned above is that the cable rating (once corrected for ambient temperature and any other derating factors if applicable) must be no less than the fuse rating, and the fuse rating must be no less than the continuous load of the appliance. If those conditions are satisfied, it is safe, because the behaviour of the fuse has already been taken into consideration when publishing the ratings.
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Old 8th Jul 2019, 4:37 pm   #95
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Default Re: 13 amp fuse for 1.5mm mains flex?

Quote:
Originally Posted by rambo1152 View Post
Quote:
Originally Posted by Refugee View Post
I once has a 13A to shaver adapter that was fused at 1A.
I can't remember the fuse length now though.

I have one here, it's 20mm or 3/4 in

I borrowed one from a Premier Inn reception once and the fuse was wrapped in tinfoil.
As I'm interested in this sort of thing, I picked up an adaptor to convert a BS1363 plug to a Shucko receptacle.
There was a neatly lettered notice on it, " Concierge Deposit 50P".
I'm sure, it was worth more than 50P.
Dave, USradcoll .
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Old 8th Jul 2019, 9:30 pm   #96
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Default Re: 13 amp fuse for 1.5mm mains flex?

They are the small ones - 5/8" we suppose.

I don't feel in the least guilty pinching up the spring claws in a 13A plug to fit one of the smaller fuses.

If it needs a 315mAT it can have it.
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Old 8th Jul 2019, 9:56 pm   #97
GrimJosef
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Default Re: 13 amp fuse for 1.5mm mains flex?

What sort of mains cable fitted to a 13A plug is so weedy that it needs a 315mAT fuse to protect it ?

Cheers,

GJ
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Old 8th Jul 2019, 11:01 pm   #98
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Default Re: 13 amp fuse for 1.5mm mains flex?

Quote:
Originally Posted by GrimJosef View Post
What sort of mains cable fitted to a 13A plug is so weedy that it needs a 315mAT fuse to protect it ?

Cheers,

GJ
True, the primary reason a fuse was included in the design of the BS1363 plug was to protect the flex, but that shouldn't preclude us from putting in a lower value to give added protection to the appliance.
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Old 8th Jul 2019, 11:47 pm   #99
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Default Re: 13 amp fuse for 1.5mm mains flex?

I have got an Antex X-25 that had the plug fuse clips pinched up and a much lower fuse fitted.
There were two reasons one being that the only other fuse I had was 13 amp and the other was that there was no internal fuse in the soldering iron.
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Old 9th Jul 2019, 8:47 am   #100
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Default Re: 13 amp fuse for 1.5mm mains flex?

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... that shouldn't preclude us from putting in a lower value to give added protection to the appliance.
Well there are those who argue that it should. The appliance is supposed to be fully protected in and of itself ('fully' in the sense of having no need at all for additional protection). Once we start using the plug fuse for appliance protection we run the risk that we will become sloppy about doing the job properly in the appliance itself.

An appliance fuseholder should be labelled with the correct rating and only a fool would fit a larger fuse there. But plugs are not labelled (or if they are it will be in accord with BS1363) and Joe Public is much more used to the fact that they should only properly be fitted with a BS1362 fuse. So when the dodgy tiddler botched into the plug fails he will 'do the right thing' and fit a proper one, unaware that the appliance might now not be protected.

BS1362/1363 is a safety system. I'd no more recommend non-compliance with it than I'd recommend that people make their own brake pads for their cars or try to refill their own fire extinguishers.

Cheers,

GJ
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