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Sparx please help

Postby ericmark » Sun May 25, 2008 8:17 am

You may have read:-
I am sure:-
“A lower breaking capacity is permitted if another protective
device or devices having the necessary breaking capacity is
installed on the supply side. In this situation, the
characteristics of the devices shall be coordinated so that
the energy let-through of these devices does not exceed that
which can be withstood, without damage, by the device or
devices on the load side.â€


Simply Build It

Postby sparx » Sun May 25, 2008 8:04 pm

Hi, Ericmark,
I did read this one and first thought was " the regs book says exactly that" but blowed if I can find it in latest edition!
I too have the excellent Hagar book but it is not that clear, my own thinking is that the test if carried out at incoming side of consumer unit main sw. may be 6.6kA [very high for a domestic supply, would suggest a Ze of around 0.03ohms, & implies house has sub-station in the back garden!].
The BS1361 cut out fuse could break such a fault current easily which would not be a problem for a consumer unit main switch, so assuming that is the set up and not an RCD incommer then can only think the testers are concerned about the MCB's 6kA rating. If so I would suggest the test be done on downstream side of the MCB's ie the outgoing cable term. to see what pscc they actually face with the added busbar ind their internal resistances taken into account.
I do believe I have only once come across the situation of main incomming MCB/main switch not being high enough rating for fault current and answer given by maker was "fit fuse upstream!" This was in a commercial set up where the new sub-station was put 10Metres away!
I absolutely agree with your reading of page 60 book 3 also the note which says for domestic installations the fault current is unlikely to exceed 6kA, also as long as no mcb is larger than 50A their fault current rating may be taken as 16kA! if supplied via a BS1362 type 2 of no larger than 100A, I don't think their are any consumer units fitted that don't fit BS numbers qouted except old BS 3036 types,
Sorry to go on but I think our testers need to read their guidance notes,
as you & I do,
kind regards SPARX
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Postby ericmark » Sun May 25, 2008 8:56 pm

Thanks I was wondering if I had missed something reg is 434.5.1 in new book page 76. Being honest I could have made the same mistake myself I didn't go to site with a 17th Edition under my arms. Easy sitting at PC with them in bookcase at arms reach.

73 Eric

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