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.

(note the word "insulation" not well known for passing high currents)

On the contrary, it's how electrical fires mostly start


Now there are some appliances that do have overload failure modes.
If

any insulator can fail smouldering. All appliances include
insulation.

See above.

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. 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.


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


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.


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

many do, some don't


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 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.


NT