On 19/02/2013 00:44, Alexander Lamaison wrote:
John Rumm writes:

On 18/02/2013 18:18, Stephen H wrote:
On 16/02/2013 20:00, John Rumm wrote:

(In reality a 20A plate switch and a single 2.5mm T&E would have been
adequate (i.e. in effect creating a switched unfused spur)).

I did actually sit down and think this through from a good design point
of view...

As stated, there is a 2.5mm2 ring main with a 32A RCBO.

snip

Ah, right I see the line of thought you are following. In fact you are
over worrying since this exact scenario has already been thought about
in the design of unfused spurs...

snip

There is nothing dangerous about what you propose, although its over
egging things a tad. Lots of design effort over the years has gone
into make sure that these things work well in reality, and while there
are theoretical weaknesses in the designs of unfused spurs, they prove
time and again in practice to be very robust and reliable.

I'm pinning this reply in my newsreader as I've never seen it explained
so clearly before. Perhaps this Q&A is also worth sticking on the Wiki
somewhere?

Well to be fair this has been discussed a few times in the past, so I
have had practice ;-))

I think most of it ought to be in the wiki... although if you find a bit
missing feel free to either highlight what, or for that matter add it
yourself (drop me an email if you want a wiki account) ;-)

Fault and overload currents are discussed here (and in other places):

http://wiki.diyfaq.org.uk/index.php?...g_A_Cable_Size

The effects on different circuit types along with that of other common
fault conditions is discussed he

http://wiki.diyfaq.org.uk/index.php?...Circuit_Faults

The responses of fuses and MCBs he

http://wiki.diyfaq.org.uk/index.php?title=Fuse
http://wiki.diyfaq.org.uk/index.php?title=MCB



--
Cheers,

John.

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