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Old 9th Aug 2019, 11:50 pm   #121
turretslug
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Default Re: 13 amp fuse for 1.5mm mains flex?

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Originally Posted by gramophone1 View Post

Placing the fuse embedded within the transformer windings is something I have never heard of. Is this a cheaper option than using changeable fuses ?

For a machine of this quality, it seems a strange choice. Once the transformer goes, it will be cheaper to buy a replacement deck second hand.
It is awkward, but not that unusual. Manufacturers are more worried about being sued for consequential loss (burned-down houses, deceased relatives etc.) than whether a few consumers will be to repair their kit a little more cheaply. Transformers aren't the wax-and-cardboard-filled lurking incendiary devices that they once were, but they can still heat up quickly and destructively before an external fuse blows. The internal fuse is often a type with a link made of low-melting point alloy and being buried amidst the windings ensures that it breaks the circuit in good time. Being thoroughly embedded also makes it difficult to work around by bodging short of a complete new transformer.
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Old 10th Aug 2019, 12:11 am   #122
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Default Re: 13 amp fuse for 1.5mm mains flex?

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Thanks for the information.

Model is Teac V -3000 cassette deck. It is a three head model. A very good machine.

Placing the fuse embedded within the transformer windings is something I have never heard of. Is this a cheaper option than using changeable fuses ?

For a machine of this quality, it seems a strange choice. Once the transformer goes, it will be cheaper to buy a replacement deck second hand.
I'm not an expert in transformer protection, I'm just aware that sometimes the protection is internal to the transformer. Perhaps this also allows it to trip if the transformer overheats ? I imagine it would be cheaper than using a replaceable fuse, although recently the tendency has been to fit these in the now almost ubiquitous IEC mains inlet connector where the additional cost should be very small.

The V-3000 service manual is available from here https://www.hifiengine.com/manual_li...c/v-3000.shtml (you have to register, but it's free). I can't see any indication of an external fuse in the mains circuitry.

The relative strangeness of the choice probably depends on the point of view. From a customer's point of view having an accessible fuse might seem like a good idea. But from the manufacturer's point of view it takes up additional cost and space and is one more thing to go wrong. Your deck has lasted 15-20 years I guess. Perhaps they fail so infrequently that this lifespan is common and by now the manufacturer reckons that in the unlikely event of an unfixable failure, well, you've had your money's worth ?

EDIT: Post crossed with turretslug's.

Cheers,

CJ
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Old 10th Aug 2019, 12:30 am   #123
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Default Re: 13 amp fuse for 1.5mm mains flex?

So, is this where we have got to –

"You should fit a fuse in the plug that matches the rating of the mains cable. UNLESS the equipment being supplied has no internal fuse on the mains input – and such a fuse may be hidden away out of sight within the windings of the mains transformer, or buried out of sight in the innards of your soldering iron, table lamp, hair drier, amplifier, etc, etc – and good luck with finding that out, so, just make a guess or toss a coin on that one!" ?

Have I got that right?

OR, of course, you could just always fit a plug fuse that matches as closely as possible the normal power consumption of the equipment, which will always give as good or better overall protection than a fuse that just matches the rating of the cable....

All my life I had thought that the fuse rating matching the consumption of the appliance (not just the rating of the cable) as closely as possible was the principle on which the BS 1363 fused plug and ring main was based....

Mike

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Old 10th Aug 2019, 1:40 am   #124
Lucien Nunes
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Default Re: 13 amp fuse for 1.5mm mains flex?

Fitting small electronic goods with a low-rated plug fuse may not add much by way of safety . One amp can cause a major burnout if the unit is only intended to consume 100mA, and even a perfect fuse can't tell the difference between one amp being used legitimately by a machine at work, and one amp of overload toasting the transformer of a machine in standby. That is the reason for embedding a thermal fuse in the windings of the transformer. It can detect whether the power is being dissipated in the wrong place even when the input current is within normal bounds.

Integrally protected transformers are not a major liability if, as usual with electronics, the secondaries are protected too. A short on a DC power rail, for example, will rapidly blow the secondary fuse long before the transformer heats up. The only time the embedded fuse is likely to act is when the windings are already shorted and the transformer useless anyway. Note that the secondary fuses may not be conventional cartridges. They may be fusible resistors, ICPs or PTCs in modern equipment.

The idea of matching the plug fuse to the load is somewhat peculiar to the UK. Anything made for sale in Europe must be safe when protected by a 16A fuse.or MCB. The idea of a fused plug allowed the use of 30 / 32A circuits for 13A sockets rather than one 13A circuit each. But this innovation dates back 72 years to when the UK was accustomed to having circuits matched to the loads. BS 1363 carried on this practice with different plug fuses for different applications; useful at the time but increasingly redundant.
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Old 10th Aug 2019, 9:04 am   #125
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Default Re: 13 amp fuse for 1.5mm mains flex?

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... OR, of course, you could just always fit a plug fuse that matches as closely as possible the normal power consumption of the equipment, which will always give as good or better overall protection than a fuse that just matches the rating of the cable ...
But if you do that then you'll guarantee that the BS1362 fuse runs hotter than it needs to, so you'll have reduced one risk while increasing another (many of us have seen cooked BS1363 plugs - their overheating is not a myth). All we can try and do is to pick the best balance point.

Cheers,

GJ
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Old 10th Aug 2019, 10:45 am   #126
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Default Re: 13 amp fuse for 1.5mm mains flex?

That would explain my findings, and why I can't locate the fuse. It must be embedded within the transformer

This forum is a fountain of knowledge. I have learnt something new, and that is always a good thing.
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Old 10th Aug 2019, 5:27 pm   #127
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Default Re: 13 amp fuse for 1.5mm mains flex?

Many times there is a label stating, "Internally Fused". If the transformer is tape wrapped just use a razor knife and cut the tape where the connecting leads emerge. Many times the fuse is located right behind the tape. Replace or jumper it, your choice. If it's cast plastic, good luck!
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Old 10th Aug 2019, 5:28 pm   #128
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Default Re: 13 amp fuse for 1.5mm mains flex?

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That would explain my findings, and why I can't locate the fuse. It must be embedded within the transformer
But isn't that a rather risky assumption? AFAIK there's no way of knowing if a transformer has a thermal fuse built in (without ripping it to bits). I suspect that many appliances (e.g. those from 1960s/70s/80s) won't have this, and then the only protection on the primary side of the transformer will be the plug fuse - which will be then protecting the whole appliance, not just the cable.

This is a very useful thread, and I've learnt a great deal about the use and rating - and limitations - of fuses. It does all seem to boil down to an uncomfortable compromise between frequent nuisance fuse-blowing (and overheating plugs) with too small a fuse, and greater risk of overcurrent with a larger one. My old Antex 25W (that's 110mA) soldering iron has a 6mm ext. diameter cable that's good for around 6A. I'd be uneasy with a 5A fuse in the plug - even a 3A one seems risky, since I'm pretty sure there won't be any internal protection. OK, the only likely cause of problems is an o/c element, but even so....

I've found Lucien's and GJ's comments particularly interesting and informative - so thanks to them!

Mike

PS - my post crossed with usradcoll1
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Old 10th Aug 2019, 6:09 pm   #129
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Default Re: 13 amp fuse for 1.5mm mains flex?

I used to work for a company which made plugs and sockets and nuclear power stations. We were only allowed to take 8A from a 13A plug. Even now, at home, I stick to 2kW if possible.

I have tried to replace the thermal fuse in transformers and have always failed. Soldering sets them off and, if they are bridged, the transformer overheats indicating a shorted turn. Therefore they seem to do their job well.

I use a 1A plug fuse for radios, clocks, soldering irons etc. and have never had one to blow. I see no need to overfuse and cannot see how this weak fuse can overheat the plug intended for 13A.
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Old 10th Aug 2019, 6:10 pm   #130
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Default Re: 13 amp fuse for 1.5mm mains flex?

I fear that there's a degree of over-analysing and cable/fuse-rating-paranoia going on here.

In a 13A plug (or a 2-pin 'europlug' fitted with a clip-on 13A adapter, or a Schuko-type plug with its adapter - as seems to be more and more common with consumer-gear these days) any fuse is there to protect the flex, not the device-being-powered.

Some - in my experience a massive minority - appliances like hairdryers, coffee-machines, toasters, radios, hair-straighteners, cassette-players, clock-radios, PVRs, set-top-boxes - may have internal fuses and/or a thermal fuse in the transformer.

3A or 13A fuses in 1363-plugs are the default - I'd say that if you open up such a plug in 95% of the cases you'll find the fuse is 3 or 13A.

Despite what some apparently see as reckless "over-fusing" and "under-cabling" the world is not burning down.

Don't over-analyse; background, persistent worry and anxiety are more likely to kill you than an over-fusing-related 'event'!

Fuse as the manufacturer recommends, use flexes that are rated to handle the expected load, then forget about it and enjoy your life, just as 99.999% of the rest of the population does.
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Old 10th Aug 2019, 6:27 pm   #131
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Default Re: 13 amp fuse for 1.5mm mains flex?

The soldering iron is a good example of the need to examine possible failure modes. What can go wrong here?

Any short across the incoming terminals will blow the plug fuse instantly. As the iron is hand-held and the cable subject to frequent flexing, deterioration is quite common. Whilst conductor fatigue usually just breaks the circuit, it is not unreasonable to anticipate a short, in which case we can take advantage of a 3A fuse to limit the energy dissipated in the arc.

Any fault that would dissipate excess heat without a dead short must involve bypassing some fraction of the element. However, the element wire is so fine and fragile that the heat or arcing at the miniscule point where the short occurs will tend to destroy it and leave the element open-circuit. In other words, it is not unlike a fuse itself, If somehow a robust connection were made say halfway along the element, that did not burn itself clear, the power dissipation would be doubled in half the length of element. This might survive for some time running more than twice as hot as normal, while drawing 220mA instead of 110.

The iron might get rather hot but at no point is the cable at risk of overload. To damage 0.75mm² (6A) cable takes maybe >15 amps for some minutes, but there is no way the tiny element would dissipate the corresponding >3kW for more than a fraction of a second before blowing like a fuse and disconnecting its ends from the terminals. Not even a 1A plug fuse will help here, the element is already a better fuse.
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Old 10th Aug 2019, 6:57 pm   #132
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Default Re: 13 amp fuse for 1.5mm mains flex?

My cassette deck is working fine, and is very clean inside. I am not going to start pulling the transformer apart. It must have some form of protection, and the thermal fuse seems the most likely. I think the deck is from 1990, so almost thirty years old, and it is still working fine. It could probably do with the azmuth realigned etc. Excellent sounding deck though.
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Old 10th Aug 2019, 9:18 pm   #133
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Default Re: 13 amp fuse for 1.5mm mains flex?

Quote:
Originally Posted by TrevorG3VLF View Post
I used to work for a company which made plugs and sockets and nuclear power stations. We were only allowed to take 8A from a 13A plug. Even now, at home, I stick to 2kW if possible.
Same with electric car chargers, 10A max for a BS1363 plug as that is supposedly its long term rating (taken from mikeselectricstuff's video on EVSE chargers).
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Old 10th Aug 2019, 10:47 pm   #134
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Default Re: 13 amp fuse for 1.5mm mains flex?

Quote:
Originally Posted by TrevorG3VLF View Post
... I use a 1A plug fuse for radios, clocks, soldering irons etc. and have never had one to blow. I see no need to overfuse and cannot see how this weak fuse can overheat the plug intended for 13A.
The plug is heated by thermal conduction from the fuse, not (except in certain fault conditions) by ohmic heating of its own conductors. A 1A fuse has a higher resistance than a 13A fuse. So if you pass 1A through each of them the 13A fuse will stay cool but the 1A fuse will get hot (heat power is I-squared R). This heat, the heat generated in the fuse, can damage the plug.

The current rating on the side of the fuse is not the fusing current, which is why a 13A fuse will pass 20A without blowing. The number on the side of the fuse is the current beyond which the fuse will get so hot that it risks damaging the plug. A 1A fuse passing 1A will be at its safe temperature limit.

Under normal circumstances your radios, clocks, soldering irons etc are probably drawing rather less than 1A, so even a 1A fuse won't risk damaging the plug. But I wouldn't use a 1A fuse in the plug for a device which drew 1A normally.

Cheers,

GJ
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Old 10th Aug 2019, 11:56 pm   #135
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Default Re: 13 amp fuse for 1.5mm mains flex?

Vintage radios and items with mains transformers are particularly difficult to protect with fuses.
The failure mode of mains transformers is usually shorted turns on the primary. This is unlikely to draw enough current to blow even a 1A fuse, but will cause severe overheating. A 1A fuse will pass 1.5A without blowing. That's 360W. The transformer will catch light without the least effect on the fuse!

I've had a transformer incinerate in a '50's woody: The smell from the noxious, dense smoke that filled the downstairs of the house lingered for days.

The only way to guard against this is with a thermal fuse in contact with the transformer. Of course we then open up that old can of worms of 'modified equipment' if we fit one, but that's a risk I'm prepared to take.

In my experience, I would say almost all mains transformers in domestic equipment from the '70's onward have an embedded thermal fuse. Whenever an item came in with an open circuit transformer, it was always a thermal fuse failure. The thermal fuse is only there to sense temperature and not to blow on excess current. It's the only way manufacturers can guard against transformer overheating and their use has to be a good thing.

Cheers
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Old 11th Aug 2019, 10:49 am   #136
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Default Re: 13 amp fuse for 1.5mm mains flex?

Quote:
Originally Posted by GrimJosef View Post
Quote:
Originally Posted by TrevorG3VLF View Post
... I use a 1A plug fuse for radios, clocks, soldering irons etc. and have never had one to blow. I see no need to overfuse and cannot see how this weak fuse can overheat the plug intended for 13A.
The plug is heated by thermal conduction from the fuse, not (except in certain fault conditions) by ohmic heating of its own conductors. A 1A fuse has a higher resistance than a 13A fuse. So if you pass 1A through each of them the 13A fuse will stay cool but the 1A fuse will get hot (heat power is I-squared R). This heat, the heat generated in the fuse, can damage the plug.

.......

Cheers,

GJ
Correct. However will a 1A (or 3A) fuse running at its rating get hotter than a 13A fuse ditto?

I doubt it..... I would expect the lighter gauge wire in the lower current fuse to need less power to raise it to its fusing temperature.

TBH, this (100+ posts) thread has all the hallmarks of the "Bicycle Shed Effect".
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Old 11th Aug 2019, 11:07 am   #137
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Default Re: 13 amp fuse for 1.5mm mains flex?

Under BS1362 the fuse rating is the current at which the fuse is dissipating 1W https://www.pat-testing-training.net...cteristics.php. I'd be reasonably sure that the ceramic body, end caps and sand inside are the same for all BS1362 fuses. But maybe the 13A wire is better at conducting the heat into the end caps rather than the sand ? To a pretty good approximation though I'd expect them all to be equally hot when running at their current rating. Avoiding plug damage is one of the points of the current rating, after all.

Quote:
Originally Posted by Herald1360 View Post
... TBH, this (100+ posts) thread has all the hallmarks of the "Bicycle Shed Effect".
Careful, or I'll start a rant about why fast-blow fuses are all pretty much the same but time-delayed ones vary over a huge range of blow characteristics (don't worry mods, no I won't, at least not here ). If this thread stops even one person starting an electrical fire though then it'll have been worth it. Even if it's just a passing visitor.

Cheers,

GJ
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Old 11th Aug 2019, 11:33 am   #138
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Default Re: 13 amp fuse for 1.5mm mains flex?

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TBH, this (100+ posts) thread has all the hallmarks of the "Bicycle Shed Effect".
I disagree, for me it has been somewhat enlightening, for instance I didn't know about the 1W dissipation at rated current thing which is interesting.

D
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Old 11th Aug 2019, 11:45 am   #139
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Default Re: 13 amp fuse for 1.5mm mains flex?

I wouldn't argue about that aspect of the thread- notably the 1W info- but I do think the original question was thoroughly dealt with about 100 posts back.....
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