On 16/03/2012 11:40, Martin Brown wrote:
On 15/03/2012 17:45, Dave Plowman (News) wrote:
In ,
Martin wrote:

Or in the case of our local church fete a pair of 3kW kettles on a
mostly rolled up mad hot extension lead plugged into the wall. They did
it last year as well completely melting the inside of the reel. *SCARY*

Two 3 Kw kettles on the same extension should have blown the plug fuse
and
quickly.

Not quickly enough! About 10 minutes continuous use I would guess and
kettles tend to get used with only a small amount of overlap apart from
at the beginning when they boil both to get started. I can assure you
the 13A plug fuse did not blow in the time the water took to boil.

If there is only a small overlap in use then you would not really expect
the fuse to blow. That is diversity in action for you ;-)

The fuse also recovers from thermal stress much faster than the bulky
reel of wound up and increasingly hotter cable.

Its reaction is also as fast as its recovery though...

However the cable heating aspect is not really something you can expect
a fuse to do anything about, since it needs to be sized to provide
adequate fault protection for the cable in the first place, and overload
protection for the *maximum* rating of the cable in the second place.
However with a fully or partially wound lead you will not get anywhere
close to the maximum load before it overheats. You ideally need an
additional thermal protection mechanism in the drum of the lead itself
to cope with this.

I suspect part of the problem *IS* that people think the fuse will
always protect them if they are doing something wrong.

Possibly... many don't understand that its there primarily for fault
protection any not overload. The first its very good at, the second its
"ok" at in severe cases, but a bit patchy in the moderate cases.

--
Cheers,

John.

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