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?

define waste of time

So if your life support system were to fail with a blown fuse when there
is a mains spike because some well meaning but uninformed person
replaced the manufacture fitted 13A fuse with a 2A one "because its
safer", that would be better?

With respect 99.999% of appliances are not life support. That different safety issues apply to that miniscule percentage is not news.


Think it though logically, what are you trying to achieve? Say you have

more safety

a table lamp that takes a single B22 fitting lamp. What is the highest
power lamp that it will take in that format? 250W or about an amp? Is
there any way a lamp could draw say 2kW for a sustained period to put
its flex at risk of dangerous overheating? The answer is not really (at
least not without determined and conscious action by the user to be a
complete numpty).

Now you say, ah but, its a lamp, 3A fuse is better... in a sense you are
right, there is no downsize other than needing to replace the one fitted
by the manufacturer. But what have you actually achieved? The thing that
does matter (i.e. power gets swiftly disconnected when you stick your
chair leg down on the flex and cause a short circuit), will happen
regardless of fuse rating, since you have say 100A of fault current to
play with[1].

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.


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

any insulator can fail smouldering. All appliances include insulation.


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

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


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

ah, we agree after all


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


NT