Are 3A plug fuses really necessary? Why not always 13A?

The fact the fuse can't provide overload protection in the common circumstance of the lead being used without being unreeled is probably why they often have heat operated overload cutouts in the reel. Realistically the fuse is not going to provide overload protection, just fault protection for the flex for modern small electronic appliances. I thought that was what you said earlier in the thread?

Reply to
Roger Hayter
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So why did you mention it?

So why did you mention it?

Reply to
Dave Plowman (News)

where a 13A fails to. It will open when that chinese 0.1A mains lead is sh orted, and when the 2 slice toaster element droops so far that suddenly it' s drawing too much current and about to set fire to the bread.

does the job, that's it. Debating how well it protects what appliances in w hat ways doesn't achieve anything.

What the fuse achieves varies in each case. Sometimes it provides needed fa ult protection, more often not. Sometimes it provides overload protection, more often not. But it need not be analysed case by case, just put the righ t fuse in and you get whatever the best protection is a plug fuse can provi de each time.

NT

Reply to
tabbypurr

For extension leads on a reel, yes.

There is a second class of lead which are the multiway sockets on a (typically shorter) lead. There the 13A fuse also has to provide overload protection since its possible for the user to abuse them. (and this is also a cases where I would have some sympathy with fittings say a 10A fuse to some of the cheaper examples of those since tests have shown a number to not really be capable of sustained 13A load).

Indeed it is. I was just trying to get the bottom of what Nige was actually attempting to achieve with his lowest possible fuse that's adequate scenario - I understand the adequate bit - it must offer fault protection, but assuming we have got past the "it gives adequate fault protection" stage, there seem to be diminishing returns.

Depending on the appliance, it could end up being an exercise in blowing lots of small fuses working your way up to the optimal one.

Most appliances are not going to need specific overload protection and many that do may include a 750mA (or whatever) 20mm glass cartridge fuse for the purpose. Those alone though won't provide fault protection (inadequate breaking capacity usually), so sny BS1362 fuse in the plug will do in many of those cases.

Reply to
John Rumm

But a small significant percentage of appliances have neither short nor overload protection on a 13A fuse. Why you don't get that I really don't get.

NT

Reply to
tabbypurr

I "get" that crusty old crap, and 5A extension leads need specific fusing.

I also get that if one understands these things there is no harm in selecting a lower rated fuse.

However we live in a world where appliance designers accept that many users won't be able to make a judgement call on this, and hence designing them to remain safe on a 13A fuse is the only prudent option.

I also get that any electrical goods for sale now will have to be CE marked, and hence suitable for sale anywhere in the EU. How do you think that these devices which you believe are "neither short nor overload protection on a 13A fuse", are going to fare in countries other than the UK, where 16A circuit breakers are commonly the only fault or overload protection between CU and appliance?

Lets do a sum...

Lets assume we have a circuit with a B32 MCB and its at its maximum length permitted by the 1.44 Ohms Zs.

Lets now take 1.8m of skimpy 0.5mm^2 flex and add on its ~156 mOhm, giving a total of ~ 1.6 Ohms.

So we get a PFC of 230/1.6 = 162A which is just enough to open the MCB on the magnetic part of its response.

So lets check the conductor size required:

s = sqrt( 162^2 x 0.1 ) / 115 = 0.44mm^2

So even then, the flex lives to fight another day.

So if I were going to boil this down to advice for non technical users, I would say leave whatever fuse it came with in it. If you need to replace it, use the same one. In general this is not something to worry about.

If you are skilled, and have a particular scenario in mind where you know that you will be running a small flex appliance outside of its design parameters (like running it on the end of 100m of extension cable plugged into an already long circuit), by all means fuse down. For normal every day stuff (aka the general case), you are more likely to hurt yourself tripping over the lead.

Reply to
John Rumm

overload protection on a 13A fuse. Why you don't get that I really don't g et.

and new crap that is often CE mismarked.

Some are mismarked, sometimes they don't bother CE marking them. CE marking doesn't mean too much in practice.

obviously they are neither short nor overload protected, and cause fires, s hocks and death. What about that is not completely obvious?

So those chinese goods with barely 0.1mm2 are uprotected on a 13A fuse. And that 1960s hifi with 0.1mm dia mains wiring inside ditto.

So you don't recommend goods be protected against short and overload. OK, b ut there's no point defending that.

But if you compare death rate, failure to fuse properly is the bigger risk. If you don't see an issue with people having at risk appliances, what is t here left to say. You seem to live in a naive world where every appliance i s new and compliant. Real life is just not like that.

NT

Reply to
tabbypurr

I ask again. Why are you buying such things that you know to be rubbish? If you're not, no point in telling the world they should fit an appropriate fuse, as they're not listening. And wouldn't understand anyway.

Must have been real rubbish if it didn't have an equipment fuse.

Reply to
Dave Plowman (News)

Lots of people buy cheap shit in the hope it will do a simialar thing to a full priced version of the same thing. Some just because it looks the same and others might think they own a better product than they actually do, or it's an image thing where they fool themselves too. if people didn't want such things we wouldn't be getting fake things coming into the UK or anywhere else.

It can also cost more to adhere to any regualtions that are imposed by an a uthority.

I handed out a 'classic' 3.15 AS 20mm fuse 2 weeks ago a long time since I' ve handed one of them out.

Reply to
whisky-dave

I am not sure what your fascination with dangerous and substandard goods is in particular.

Yes there is loads of crap that is electrically unsafe and being sold illegally.

Don't lul yourself into a false sense of security that sticking a 3A fuse in it its going to suddenly make it safe - it will likely still be dangerous!

The lack of fusing or inadequate mains cable is usually the tip of the iceberg. Look closely and you will often see metalwork coupled directly to mains, inadequate isolation between mains and LV side of circuits. Low quality components (like transformers with inadequate insulation between primary and secondary windings), lack of thermal protection, unsafe plastics and smoke / fume hazards and so on.

e.g. one of many:

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I you want to keep safe from cheap crap goods, then don't buy them in the first place.

So your advice to someone using one of these appliances in Germany for example would be?

They are unsafe when connected to the mains.

See above: 'I "get" that crusty old crap, and 5A extension leads need specific fusing.'

(although to be fair to 60's hifi, much of it was a better quality construction than some of the modern crap).

You are mistaken.

To paraphrase that nice Mr Pauli, you are not even wrong.

Link to stats demonstrating an increased death rate from using the supplied fuse please.

You seem to be engaging in an exercise in reductio ad absurdum.

Now, allowing for the caveats that we have already highlighted like thin extension leads and old kit etc (remember we are talking in general here, the majority of cases, not absolute black or white; hell I want an

*argument* with someone on the internet!)

I am struggling to think of any of the hundreds of electrical appliances around me where this is an issue... I can't bring to mind many. An oldish set of Christmas lights perhaps? I don't recall buying anything with a mains lead of less than 0.5mm^2 in decades. I have had equipment supplied with non conforming plugs on (inadequate overhang round pins, no internal fuse) - they *may* also have had thin wire as well, I don't know I did not look, but I don't really think the bin cared much though.

I do have on odd one here though. A new Cambridge Audio HiFi amp. The English section of the user guide makes a big point about including a section for the "UK only" and includes instructions on fitting a plug with the requirement that it must contain a 3A fuse, and if you remove the plug then to provide fusing elsewhere. Its a curious section since if you look at the lead its of adequate cross section to be protected on a 13A fuse (or for that matter a 16A MCB), and a quick glance though the ventilation slots shows internal overload fusing with a 2A time delayed

20mm fuse, and warnings silk screened around it to replace only with a like fuse. The multilingual manual has blank sections in the same place in all the other languages. Not sure if it was an exercise in "because we could", or just tin knickers.
Reply to
John Rumm

Interestingly, I just had a counter-example where a 3A fuse might have prevented equipment damage due to a fault. A Grundfos pump (mention the name because it is the second one to do the same) failed short-circuit and melted a trace on the controller PCB, without actually blowing the 13A plug fuse or tripping the MCB. A 3A fuse might have prevented this. However, I haven't checked whether the oil pump inrush current is too high for a 3A fuse yet.

Reply to
Roger Hayter

There's a very big difference between a fake and something merely sold as cheaply as possible. The former is an attempt to deceive. So everything else goes out the window. The latter to capture a sector of the market.

A low cost product may or may not be value for money. The obvious one being power tools. A Lidl cordless drill at 40 quid almost certainly isn't as good as a 100 quid Makita. But may perform DIY tasks more than adequately.

Anything on sale in the UK should conform to UK regs. No point in having them if not enforced.

I've got a nice big selection box of 20mm fuses. Hope they don't have a shelf life. ;-)

Reply to
Dave Plowman (News)

Did that pump come with a fitted 13 amp plug? Must admit to never having seen one that did.

A well engineered heating system would include specific fuses for each of the outboard components. Although I do realise few will.

My Viessmann boiler has individual fuse protection for everything outside of the main PCB. Using through hole PCB mount fuses. Which are not listed as being replaceable. Working this out saved me the cost of a new PCB when the gas valve failed due to getting soaked in water, and took that fuse.

Reply to
Dave Plowman (News)

Except that is not a counter example - since pumps are not domestic appliances supplied with pre-fitted lead and plug. They intended to be installed as a component of a larger system by competent people who would understand the system fusing requirements.

The circulator on its own would probably be OK on 3A (its an induction motor and has quite high inrush - but its a small motor), however 5A would likely be more appropriate for the whole CH system.

Alas PCB traces are quite often the unintentional weak spot in many systems. I had a cooker hood the blew a couple of traces on its main switching PCB when one of its 40W incandescent lamps failed short at switch on. It was 3A fused and the fuse was fine!

Reply to
John Rumm

I don't think anyone has suggested that anyone should now design an appliance to depend on the presence of a plug fuse less than 13 amp for safety. However I do not think that for devices that only require small currents (including all your surges etc.) there is any harm done in fitting a lower value fuse. Whether anything might be achieved by that depends on the specifics of the attached appliance in terms of its on board safety features, quality of materials and manufacturing standard and adherence to current regulations.

Reply to
bert

Yes I am expressing myself badly, aren't I? I am talking about an oii-fired combi boiler which contains two circulating pumps. It has no internal fuses at all. It is controlled from a standard CH controller which has no fuses on inputs or outputs and is connected to the mains by a standard three pin plug. This choice is no doubt affected by the adjacent placement of a socket the previous owner intended to run from a generator.[1] I am not sure I am willing to stock more than two kinds of plug fuse, and 13A and 3A seem to be the preferred ones.

I was thinking of the oil pump and fan, which have a common motor with a significant torque but I have no details except that it has a starting capacitor and no obvious brushes so I suppose it is some kind of induction motor.

[1] The socket is to be connected to a generator which is intended to be earthed to a rather weedy earth stake outside. In conjunction with the house PME system this is a whole other can of worms.
Reply to
Roger Hayter

I've just given a packet of 5A fuses to my neighbour, they definitely exist. In my toolbox I have packets of 3A, 5A and 13A.

jgh

Reply to
jgh

I think you can even get 1.5A in that size. Used to see 10A but not for years now.

Reply to
The Natural Philosopher

For me, this is where the fuse rating lies. Estimate the working current of the device and limit the current input. Any mishaps handling the circuitry will have the same current limit. Could be a life saver?

...Ray.

...Ray.

Reply to
RayL12

and the middle ground were you get 'fake' chennel no.5 a friend of mine us ed to be a lookout down oxford street while somneone sold such things out o f a suitcase.

But they are rarely if ever faked.

But what if it's branded Makita and cost £40.

I guess you've never watched Rip of Britain.

me too 100s of them also 5/8th and 1 1/4 inches from 50ma to 15 amps. The shelf I keep them on is metal so I think it will last a while I might be worried if it was wooden, but then the lastest from steven harkwins is t hat he thinks the human race (as we know it) only has 1000 years left.

Reply to
whisky-dave

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