On 25/04/2018 00:55, Cursitor Doom wrote:
On Tue, 24 Apr 2018 21:57:46 +0100, Jimmy Wilkinson Knife wrote:

They are pretty **** actually. I had a kettle running at 2400W (10
amps) on a 5A plug fuse which lasted for 3 months of using it several
times a day before it blew. And a colleague had 2 tumble driers and 2
washing machines running on a 4 way power strip, which melted the strip
into a soggy mess and didn't blow the 13A fuse. Makes you wonder why we
have them at all. Surely they can design them to take triple load for 5
seconds (to allow for motors starting), then blow if it's over the
rating after that? Simply the temperature of the wire should reach a
certain point? But all we seem to have is "fast blow" which won't allow
for motors and "slow blow" which is in plug fuses and doesn't blow other
than a dead short.


I'm inclined to agree. Hardly anyone other than sparkies is aware that
any given mains fuse will pass a great deal more than its rated value for
a surprisingly long time

That bit is true, however...

during which much melting or even a fire could
possibly start.

That bit ought not be if the right fuse is used.

One of the absolute requirements for most fuses is that they offer
adequate fault protection - and that implies protection of the wiring
from melting and catching fire.

Such fuses are great for protecting against dead shorts,
but not much beyond that.

That's definitely not true. Its perfectly acceptable and normal to use a
fuse for overload protection. Its also acceptable (and in fact
desirable) that they will pass an overload current for a period of time,
since they should reflect the thermal performance of the circuit as a
whole. This is why the actual current level itself is less important
than the "bigger picture" combination of current combined with duration.

(There may be cases where you don't require overload protection at a
fuse however. Say for example at the origin of a dedicated circuit
feeding a hard wired bit of equipment. Then the fuse only need offer
fault current protection).


--
Cheers,

John.

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