RE: Armour cabling/domain registering

[Nigel Orr <Nigel.Orr@xxxxxxx>]

At 09:52 23/06/00 +0100, you wrote:
>It is there for physical protection of the cable NOT electrical
protection.
>The armour MUST be earthed but there must also be a dedicated core in
the
>cable that is the primary earth for whatever is at the other end.
>
>At least....thats how I understand it to be based on observing the
>practices employed by the site electrical contractors at work.

As usual, it's a little more complicated than that... this was a
news:uk.d-i-y posting from recently- if any of
it doesn't make sense, feel
free to ask!  I don't have a copy of the Regs (aka 'The 16th Edition' aka
'The IEE Wiring Regs' aka 'BS7671') to hand, but I can look up the
referenced table if you can't find them, or a guide to them, in your local
library.

>Re: Meter Tails
>Author:Andy Wade <ajwade@xxxxxxx>
>Date:2000/01/15
>Forum: uk.d-i-y
>
>Rick Hughes wrote ...
>
> > Did [can] you use the steel armour as the earth ?
>
>It is perfectly in order to use the steel armour wires as a circuit
>protective (earth) conductor (CPC), HOWEVER, you must verify that the
>cross sectional area (CSA) is adequate.  BS 7671 (the wiring
regulations)
>gives you two methods of determining 'adequate'.  You can either
calculate
>the minimum CSA using the adiabatic equation (see regulation 543-01-03)
or
>select a size using Table 54G.
>
>The calculation method requires knowledge of the earth fault loop
>impedance of the incoming supply, that of the distribution circuit in
>question, and also the operating characteristics of the relevant fuse.
>Using Table 54G avoids the need for calculation, but is a more
>conservative method and usually requires a larger minimum size than you
>would determine by calculation.
>
>For live conductors in the range 16 to 35mm^2 Table 54G gives the
minimum
>CPC size as 16mm^2 if copper, or 16*k1/k2 mm^2 for other materials,
where
>k1 is the adiabatic k-value for the phase conductor (from Table 43A)
and
>k2 is that for the CPC.  Assuming that the 'armoured cable' in question
is
>2-core 25mm^2 shaped-conductor, XLPE insulated type to BS 5467, we have
>k1=143 and k2=46.  The minimum CPC size then works out to
16*(143/46)mm^2,
>i.e.49.7mm^2.  Unfortunately the armour CSA for that cable is listed as
>43mm^2, so this will not comply with Table 54G.
>
>So (if still wishing to avoid detailed calculation) you could either
(a)
>run a separate 16mm^2 copper CPC, (b) use a three-core armoured cable,
or
>(c) increase the two-core SWA cable size to 35mm^2 -- this has 62mm^2
>armour CSA and so _will_ comply with Table 54G (problem solved).
>Personally I would go for option (c).  It will be cheaper than (b) and
>tidier than (a).  It will also reduce the voltage drop and power loss
on
>the circuit, which I think was fairly long (15m?).

Hope that helps- it's always worth a DejaNews search on uk.d-i-y for this
sort of query, all things electrical (and mechanical, structural and
plumbingical!) are discussed there in depth...

Nigel


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