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

In article , John Rumm writes

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Reply to
Mike Tomlinson
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In article , John Rumm writes

And this is part of the reason why appliance flexes are so short nowadays: not only is the manufacturer saving a few pennies but they're ensuring the flex is short enough with low enough resistance for a fault current to blow the 13A fuse in the plug.

Reply to
Mike Tomlinson

Just to note that I've just replaced the 5A fuse in an extension lead for the second time in a week. Rated 4A coiled and 5A uncoiled. We use it for lights but passing builders tend to plug anything into anything. So yes, the fuse does protect the cable :-)

Reply to
David WE Roberts

In article , Frank Erskine writes

Pretty sure 5A is still commonly available.

The RS catalogue does BS1362 fuses in 1A, 2A, 3A, 5A, 7A, 10A and 13A.

In clock plugs, yes, 1A fuses are (were) common. Clocks used to be connected with what was essentially bell wire.

For myself, I fit 3, 5 or 13A depending on what is appropriate for the load. 3A for things like lamps, 5A for stuff like PCs and TVs, 13A for washing machine, fridge/freezer, dishwasher, etc.

I fitted a 1A fuse in the plug for something recently, buggered if I can remember what it was though. Something like christmas tree lights IIRC.

Reply to
Mike Tomlinson

Indeed. However the wiring regs separate these out into overload and fault since they need to be treated separately.

Except in this case we are not using it outside. The link the OP referred to was a very specific question about whether a small electronic device can be adequately "protected" by a 13A fuse. The answer to which is yes. The protection that the fuse is there to offer is to limit the effects of fault current - i.e. stopping the mains lead bursting into flames, vaporising and spewing hot metal and plastic about the place etc when someone sticks a metal chair leg through it. Once you can demonstrate that you have achieved that level of protection, then your job (and that of the fuse in the plug) is done.

This is a completely separate issue from minimising damage to the device itself because some internal motor got latched on when it should not have, and its now sucking more power than it ought. Yes that is a "fault" in the device, and that may result in overcurrent (wiring regs use of the term). If its a problem, then the designer can do something about it.

True, but not relevant to the point under discussion.

If you want to protect electronics you are designing now from damage resulting from overload, then you need a fuse (or thermal cutout etc). However you can't rely on the one in the plug, since outside the UK it won't be there.

Indeed. And for those, a lower rating fuse may be required in the plug.

Don't confuse the situation as it applies to fault current protection for the flex on a new device one is putting onto the market in the EU now, with old kit built to different requirements and standards.

Also don't assume that just because a 13A fuse will meet the fundamental requirement for fault protection on new kit that there is no benefit you using a lower value fuse in some cases. There might be.

As I said, in these cases if the maker said the appliance must be protected by a 3A fuse then so be it. If they designed it to rely on a

3A fuse in a plug, that is fine, use one and not a 13A. They would not be able to do it now (for a product to be marketed internationally) but it does not mean it did not happen in the past.

Fault current protection is a minimum requirement - something that must be taken into account and protection put in place for. None of that precludes doing more than the minimum.

Reply to
John Rumm

The irony that has been highlighted before, the one thing that what many people refer to as a "kettle lead" won't fit, is a kettle!

(i.e. an IEC cold condition connector won't go into an IEC hot condition appliance).

IIRC, all fuses that are not 3 or 13A are required to be black now...

They are fine on just a normal surge protected supply - but they are not usually put onto UPS supplies. (many UPSs will have a dedicated surge suppressed outlet that is not maintained during mains interruption for the purposes of powering stuff like laser printers)

Modern ones can be worse rather than better. Old ones tended to heat the fuser and keep it hot. They might have a long startup time, but then once hot, were ready to go. More energy efficient ones will use "instant on" fusees, but these may require higher short term currents to minimise the delay in printing the first page.

Reply to
John Rumm

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Na, just air...

Shows you want a fault current (wiring regs meaning) can do!

Reply to
John Rumm

I remember the ads now Har, Har, Harmony... :-)

Never noticed that on a flex - only CE and other markings.

Reply to
polygonum

In message , Mike Tomlinson writes

My approach too. They can argue about the semantics as much as they like but to me it make sense to have the minimum fuse necessary and also not to rely on the safety device as an inherent part of the design.

Reply to
bert

They're very different. It was common for old appliances to rely entirely f= or overload protection on the plug fuse, and often to require it to be 5A r= ather than 15A. It was also common to simply not consider the issue.

There often is, a 3A fuse ruptures much quicker than 13A in many fault/over= load scenarios, and that can prevent fire sometimes.

I think the normal recommendations were 5A, 2A, and sometimes 1A, and later= 3A. But tracking any such recommendations is unlikely to work out in many = cases, and many manufacturers simply ignored the issue. Some of the safety = issues were very basic, a lot didn't even have a cordgrip. I recall one rel= atively well regarded 1960s product that used a strip of ali foil for earth= ing, stapled down to connect it.

Exactly.

NT

Reply to
meow2222

Surely for fuses that should be bs1362.org.uk ?

Reply to
The Other Mike

Man at B&Q :

So, you're agreed. The jargon = a peculiar version of English.

Reply to
Mike Barnes

I remember having one computer which needed a 'kettle lead' with a notch cut out of it (which may indeed be a *real* 'kettle lead' connector, but I never did check). PSU was rated to 800W IIRC, and it was probably running at close to that.

Reply to
Jules Richardson

Fused plugs do exist here in the US, most often seen on Christmas tree lights (the ones that are made here, rather than the China-built death traps) - they have a little trapdoor in them for fuse access (US plugs are tiny in comparison the the UK's chunk-o-rama ones :-) and a pair of fuses that are around 10mm in length.

cheers

Jules

Reply to
Jules Richardson

I have a working mains clock on the desk here, with starter and tick/silent gismos. The clock connecter fuse is the same size as the ones on 5A Wylex plugs.

Reply to
Graham.

You are correct, but a little slower - it may go to 4A before the fuse breaks. If current is say 1A and the fuse 3A, then the current increases for whatever to 4A the fuse may not break so quick, but will eventually, so giving protection. The fuses are designed for fault current, which means, as one poster in the link stated a 3A and 13A will break as quick as each other - on a fault.

I only scanned the link, but one poster I think was saying have all fuses

13A. So if he had a lamp with 0.25mm flex and 13A fuses he thinks he is safe. OK on a lamp it is difficult to see how the current would be great enough to melt the cable as the bulb would blow way before any current is dangerous enough. They only time the current would be high enough to melt the flex would be if there is a fault, which would be momentarily and the fuse would then snap. I "think" that is what is he on about. He says the 13A fuse will break faster on a fault than a 3A. BTW, I recall as a kid my sister's cheap Italian (looked good though) table lamp switch melting. It obviously had a "fault" and" a 13A fuse. A 3A fuse would have snapped stopping the melting, which was a fire hazard. The switch could be classed as a part of the cable and the appliance the light bulb.

The secret? As fuses have dropped into 3A and 13A in common use (Only specialist outlets sell anything else these days) never install flex that is rated less than 3A and fit correct sized fuse to protect the cable. The cable should never be rated less than the fuse.

Some fit larger rated fuses because of surge. If there is surge then fit anti-surge fuses, not a large rated fuse.

Reply to
Doctor Drivel

I'm not convinced. In decades past I've seen loads of fuses repaired with b= its of fusewire, or worse just a thin strand of copper wire. The failure of= such fuses was entirely tame, the downside was just black deposits inside = the plug, requiring a new plug. I've seen plenty of glass fuses fail on mai= ns too, and again nothing dramatic.

How could they create that clip? I suspect by bypassing the protection of b= oth incomer fuse and CU entirely, and picking an unusually meaty (non)fuse,= and passing the full 6kA through it, perhaps at above mains voltage too. I= t sure looks like bs to me, but not 1362.

NT

Reply to
meow2222

Fusing on the neutral pole was banned here eons ago. A blown neutral fuse causes a faulty appliance to look dead, but be live.

A 10mm long fuse must have too low a breaking capacity to cope with more current than the breaker would act on. With the US fixed wiring system's lowish current breakers this shouldn't be a problem though.

NT

Reply to
meow2222

You're the only person I've ever heard describe pound shops as specialist outlets.

Nurse!

NT

Reply to
meow2222

True but how do you calculate the minimum fuse necessary, it's not always that easy.

Reply to
whisky-dave

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