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:


If it were a BS1362 style fuse, the looking a bit to the left of the 3A
fuse curve would suggest that it might take 3A indefinitely:

http://wiki.diyfaq.org.uk/images/d/d...FusingTime.png


Yeah, here's a picture of the exact type he


https://images-na.ssl-images-amazon...._AC_SX355_.jpg

Probably fake - the end caps look too shiny to be Bussman

Here are some tell tails:

https://www.pat-testing-training.net...fake-fuses.php

(there is also more to a fuse than will it blow at an appropriate level
of overload. Actually having the capacity to interrupt the flow of
current, and not exploding in your face being some important ones!)

Seems there's even more leaway than I'd expected for extended
over-current tolerance. Perhaps we should never fit anything bigger
than a 10A fuse in a 13A plug? I think I'll toss out all my 13A fuses
to be on the safe side.

You could flip the argument, and say never fit anything other than a 13A
fuse. The purpose of the fuse (for any moderately recent appliance
anyway) is to provide *fault* protection to the flex - and a 13A fuse
will do that just fine.

The exceptions to this are some low quality multiway extension leads
where 10A would be more appropriate, or for historic appliances and
flexs, or cases where a manufacturer explicitly cites a smaller fuse.

I have seen this argument so many times before and have never
understood it at all.

ok.

I acquired a supply of 2 amp fuses and fitted
them to various low current appliances, without any difficulties. How
anyone can argue that limiting the fault current is anything other
than a safety enhancement confounds me.

Because you are confusing fault current with overload current. Fuses (or
any other circuit protection device have *no* ability to limit fault
currents. The only thing that limits a fault current the round trip
impedance of the circuit with the fault. So if that is 0.23 ohms, then
your fault current is 1,000 A regardless of the fuse fitted.

That will open any BS 1362 fuse pretty sharpish - the nominal current
rating is not important.

What matters is that the fuse has the breaking capacity to clear the
fault before something else (like the appliance flex) is destroyed /
catches fire.

This is a *totally different* discussion from overload protection - foe
example fitting a 13A fuse to a spur feeding multiple sockets on a ring
circuit.

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?

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?

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

Now there are some appliances that do have overload failure modes. If
they are old enough, they may not include their own protection, for
those, yes its important they are fitted with the "right" fuse. For
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.





--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/