On Monday, 24 August 2020 23:06:44 UTC+1, John Rumm wrote:
On 18/08/2020 15:56, tabbypurr wrote:
On Tuesday, 18 August 2020 15:37:12 UTC+1, John Rumm wrote:
On 18/08/2020 11:41, tabbypurr wrote:
On Tuesday, 18 August 2020 00:33:28 UTC+1, John Rumm wrote:
On 17/08/2020 22:09, Scott wrote:
On Mon, 17 Aug 2020 21:57:17 +0100, John Rumm
wrote:
On 17/08/2020 16:59, Scott wrote:
On Mon, 17 Aug 2020 16:20:35 +0100, John Rumm
wrote:
On 17/08/2020 14:04, Cursitor Doom wrote:
On Mon, 17 Aug 2020 02:12:17 +0100, John Rumm
wrote:

Why do you think the system was introduced with 1, 2, 3,
5, 7, 10 and 13 amp fuses unless a lower rated fuse had
safety benefits?

For the limited cases where overload protection is useful.
They will all handle fault currents.

Whatever you call it, if there is only 2 amps of it instead
of 13 amps nothing will convince me that is not safer.

Define safer?


You're barking at nonexistent shadows there. Any appliance can
get a smouldering insulator fault that takes 3A but not 13A.
That's one way appliances catch fire.

And will that smouldering insulation result in an large enough rise
in current draw to blow a fuse?

Often yes, often not. Ultimately it either burns out without harm, blows a fuse or catches
fire.

Well yes, that covers most of the possibilities. Blows a fuse and
catching fire also being possible.

The problem is, its easy to generate more than enough heat to
start combustion with only a kW of power to play with, and a fuse
in a plug will happily deliver that.

Indeed. That will be solved some time in the future by electronic
load characterising fuse type circuits.

I admire your faith. To work you would need sophisticated monitoring
electronics integrated into every appliance, and they would need careful
tuning by the manufacturer to recognise what is an unusual current
profile for the appliance. Something that in many cases will not be
possible, and prohibitively expensive in cases where it is. So in the
real world economics will probably prevent that happening.

Load characterisation & electronic protection has already been done. It's too expensive for home use. But computing power is very cheap, and getting cheaper. It won't take too long before it's built into whatever the future replacement for RCBOs is. There's no requirement to put them inside appliances.


Solutions that work at the circuit level are typically very limited in
effectiveness. (see the tests on circuit level AFCIs).

of course. They do improve over time.


For now we use fuses & mcbs
which we know only catch a percentage of such events. A 3A fuse
catches more than a 13A fuse, it blows with less energy passed.

The "energy passed" or let-though only really takes on significance when
interrupting fault currents. During normal operation, the energy
delivery from any size of fuse will deliver to all intents and purposes
unlimited power to start a fire.

the reality is they frequently don't result in fire, things often burn to open circuit. So the let-through energy does matter.

I would contend that (particularly with modern appliances) plug fuses
rarely have a significant influence in reducing appliance fires in non
fault current scenarios.

History tells us otherwise. Imagine replacing them with bolts & tell us whether you're as safe.


If they did have significant influence, then you would expect the UK to
show a notable reduction in electrical fires compared to the rest of
Europe where plug fuses are not used, and the only protection outside of
the appliance will be the 16 or 20A MCB for the circuit.

which does a better job than a 13A fuse. Faster means less energy.


However we have more fires per person than say France, and a similar
number compared to Germany.

(ans as you know, most electrical fires stem from misuse of appliances,
followed some long way after by faulty appliances, with faulty fixed
wiring some distance behind those)

The number of electrical fires seems to be dominated by the number of
electrical appliances in use, so wealthy nations lead the way.


IMHO Smoke alarms, RCDs etc will make a far more meaningful
contribution to safety then obsessing about fuses in the vast
majority of cases.

Indeed. Correct fusing is still wise. I don't think anyone
recommended obsession though.

You are not recommending it, but you are going on about it - endlessly.

I'm going on about it no more than you. And it is not endless.


Especially when you are not really suggesting anything substantially
different from what I said in my first post.

someone didn't pay attention

We just pointed out that picking an
appropriate fuse is safer..

And while true in many cases, as a blanket statement its incorrect since
there are counter examples.

0.001% of counterexamples does not make it a bad idea.


In cases where its true, the amount of "safer" is very small and
difficult to qualify for *most* cases.

It is a challenge. I daresay looking into early electrical regs & insurance figures would answer that to some extent, but only as far as the the practices of the time, which were obviously a lot worse.


Trying to suggest that is obsession is
childish.

they are old enough, they may not include their own
protection, for

many modern ones don't either as your tablelamp example shows

Because it does not need it...

Generally true. Of course there are also appliances that do.

So you agree with what I say, yet still feel the need to argue?

no

those, yes its important they are fitted with the "right"
fuse. For

ah, we agree after all

I said that right from the outset.

others it matters less than many worry about.

[1] 100A should be a fusing time well under 0.1 secs on a 13A
fuse. So for a PVC flex we can work out the conductor size
required to cope with the I^2 . t let through energy with the
adiabatic equation:

s = sqrt( 100 ^ 2 x 0.1 ) / 115 = 0.27mm minimum CSA

(115 being the k factor for PVC insulated cable)

So even the smallest typical 0.5mm^2 CSA flex would be fine
with any fuse.

Yup. But the chinese flea bay special with copper coated steel
mains lead would catch fire.

Since it probably has the dodgy non fused plug to go with it, its a
moot point!

most don't have those. Picking a fuse to reduce fire risk is not a
moot point, it is the central point of this thread & good sensible
practice.

Its is rather missing the point. If you want to improve safety, don't
use sub standard mains leads in the first place,

You can tell that to end users till you're blue in the face, it's not gonna happen because most don't know and some don't care.


don't try to cure their
sins with a sprinkling of holy water after the fact.

I don't know anyone suggesting that


(and many of those "fake" flexes would not take 3A sustained load
anyway)

many do, some don't

You would be a negligent to use one in the first place.

no, just a normal member of the public that knows no different.


How can you
seriously argue the merits of different fuse ratings when using a lead
which poses orders of magnitude more risk?

that is precisely the sort of situation where good fusing practice makes most safety difference. The idea that all appliances out there in the UK are safe with a 13A fuse is simply well & truly wrong.


Proper flex that's partway broken also would not cope, and that
is not a rare failure mode.

Under fault conditions it may well blow at the weak part of the
flex as well. A fuse is not going to necessarily help even if it
blows at the same time. Under normal operating conditions the
appliance may stop and start or not work reliably - but again the
fuse is not going to help.

The flex may blow oc, the fuse may blow preventing a fire or the flex

Under normal operating conditions why would a fuse blow just because
there is a break in the flex? It does not blow when you turn the
appliance on and off with the switch, so why would it when you turn it
on and off with a break in the flex?

When flex etc shorts, either a fuse blows oc or the flex damage blows oc. Could be either.


may catch fire. A fuse that lets less energy through before blow
improves the odds. I'm not sure there's much serious debate left on
that point.

Your logic is flawed[1], however I agree, no further debate is worthwhile..

[1] You are assuming it will blow.

If you think that you really haven't followed what's being said.

The safety benefit of appropriate fusing was established a very long time ago.


NT