wrote:
On 19 Aug, 16:58, Andy Wade wrote:

Thanks John and Andy for your replies.

Zs needs to be watched here, given that the feeding distribution circuit
is 16 mm^2 - twin & earth, quite possibly, although no specific cable
type was mentioned - so possibly only a 6 mm^2 incoming CPC to the
board.

The cable is T&E.

So 6mm^2 it is then....

I'd want to take a loop reading at the board before going too
much further.

Note that the answer here is somewhat moot in the sense that you have an
RCD so the actual impedance is not critical. My comment about designing
as if it was not there is really just a belt and braces thing - if you
can show that there would be sufficient fault current to operate the MCB
quickly enough then you have the reassuring knowledge that your circuit
is protected even if the RCD should fail.

The last earth loop impedance reading was taken about 2 years ago and
the reading was 0.3 ohm. Is this OK or should I get it checked again?

Where was that reading taken? Is it the the supply earth impedance
measured at the main distribution board near where the supply enters the
property, or is it the complete earth loop measured at the auxiliary CU
position (i.e. including the 16mm^2 T&E run to the CU)?

The 5 to 0.1 sec opening time on a 40A type B MCB is 200A. You can
therefore afford a 1.15 ohms total ELFI to play with.

I have gone down the route of puchasing a separate RCD shower consumer
unit. However there is insufficient vertical space where I would like
to place the new unit and therefore can these consumer units be fixed
horizontally so that in effect it will be on its side. Will this
effect the operation of the MCB or the RCD?

You would have to consult the manufacturers data sheets here. My only
concern would be if adequate cooling of the MCB will be maintained in
the "wrong" orientation. (while it is rare to see domestic CUs arrange
the breakers horizontally, its quite common in industrial ones)

Failing that are the any regulations requiring the unit to be a
certain height from the floor?

It ought to be accessible without a ladder etc. Other than that, use
common sense.


--
Cheers,

John.

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