On 16/03/2012 17:17, Dave Liquorice wrote:
On Fri, 16 Mar 2012 14:02:12 +0000, John Rumm wrote:

Two 3kW kettles probably would take out the plug fuse in under a minute.

Thats not quite a 100% overload. I think the average BSwhatever
plug top fuse would last longer than that(*). Wether the plug top
could take the heat generated is another matter...

(*) Wonkypeida says for a 13A fuse 1-400s @ 30A (6.9kW). My
experience of urns and kettles on the same 13A plugtop is that it
take quite a number of minutes for the fuse to blow and the plug top
melts as well.

I was going from the graph in BS1362:

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

However to be fair that is the lower limit of the range of possible
values, so it may well last longer depending on the fuse.

Also worth keeping in mind that with low resistance items like notional
3kW heaters, they are often quoted as 3kW at 240V (which suggests an
element resistance of 12.5 ohms) but that is only 2875W at 230V. So
small shifts in voltage make a noticeable difference. Factor in the
effect of the load on the actual voltage drop on the lead and you can
get quite substantial amount of load shedding.

Say its a 1.5mm^2 flex and assume 24mOhm/m round trip resistance. Try
and draw 20A through 20m of that, and you have just dropped another 600W
of load.


--
Cheers,

John.

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