On 18/11/16 22:49, wrote:
On Wednesday, November 7, 2012 at 6:40:34 PM UTC, David Robinson
wrote:
Not my question, or belief, but found here...

http://forums.digitalspy.co.uk/showthread.php?t=1755197


Is there a counter-argument?

Thanks, David.

To "the fuse is there to protect the cable" ? Hmmm, that's rather to
vague, and technically incorrect. I think you need to ask what are we
protecting really, and I beg to differ that the fuse is there to
protect the cable, here are some reasons why:

Really what we are protecting is ourselves and other property from
electrocution and fire, The simple fact is that if the cable is
overloaded then it will melt and cause a fire, it will then also
short out the supply causing further risk. To illustrate lets
consider the service fuse in your electricity supply, without it any
short or overload would probably take out at least part of your
street. It's easy to forget that the fuse is there to protect against
overload because in most cases it's blown due to a direct short, but
if you are running a 3KW heater on a 5A lighting circuit with a 13A
fuse, that wire is going to melt sooner or later, but even assuming
it doesn't, over time the excess heat will make the plastic brittle
and one day it will crack, perhaps when you pick up the cable! A 13A
fuse will certainly protect a direct short an a 3A cable as it won't
really have time to melt before the fuse blows, but if you want a
demonstration of what can happen try dropping a some 13A stranded
wire across a 12V car battery, make sure you aren't holding it and
there is nothing combustable around though as the whole length will
instantly burn all it's length before it vapourises- It's quite
spectacular, yo might want to wear suitable eye protection as it
could be quite bright.




Now try putting your 13A rated bare wire in series with a 13A fuse
across a car battery. I guarantee that the wire will be fine. Google
"Let through energy" for more detail.

The fuse is designed to heat up and melt before the cable heats up to
beyond its operating temperature (70C for PVC insulated).

Yes, a 13A fuse will not help a 3A cable, but you should have had a 3A
fuse there in the first place.

As for your 3kW heater on a 5A lighting circuit - yes, the breaker at
the source WILL protect the cable and will trip before the cable gets to
70C for an otherwise correctly designed and installed circuit.