On 07/11/2012 18:40, 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?


Counter to what?

Frank1 is obviously missing a clue.

grahamthompson and Nigel Goodwin seem to have explained more than
adequately.

Its easy enough to check. From the BS7671:2008 table 41.3[1], the
maximum earth loop impedance for a circuit protected by a B32 MCB is
1.44 ohms. That is what gives a fault current on the longest path of the
circuit of around 160A - or enough to open a B32 MCB on the "instant"
part of its trip response.

So we could take that as an example of the minimum fault current the
flex will need to withstand when someone shorts it.

Lets say the appliance has a flex with 0.5mm^2 CSA conductors. For a
short flex (i.e. typically 1.8m) you can ignore the additional
resistance of the flex, so can just assume the prospective fault current
of 160A. Now that should open a 13A fuse in less than 0.1 secs (probably
significantly less). So all you need to know is, will the flex will
carry that fault current for long enough to allow the fuse to do its
stuff without either vaporising or bursting into flames.

So we can work out the minimum size (s) of conductor required for that
with the adiabatic equation[2] and the PVC cable type k value of 115.

s = sqrt( 160^2 x 0.1 ) / 115

s = 0.44mm^2

Which is less that the actual flex CSA of 0.5mm^2, so in this particular
example the flex would actually be adequately protected by the 32A MCB
at the origin of the circuit (although the fuse will probably open
before the MCB gets a chance to operate)

If you try that sum on a shorter circuit - or one with a lower Ze, then
you may get an ELI of say 0.5 ohms at the end of the flex. That gives a
fault current of 460A which would need a conductor size of aver 1mm^2 to
be protected by the main MCB, so here the fuse would be essential.

(if the circuit were protected by a BS3036 rewireable fuse, or the
circuit a tad out of spec, the flex longer or thinner etc then the fuse
in the plug would be essential).



[1]
http://wiki.diyfaq.org.uk/index.php?...Loop_Impedance

[2]
http://wiki.diyfaq.org.uk/index.php?...abatic _Check

--
Cheers,

John.

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