BruceB wrote:
"John Rumm" wrote in message
o.uk...
Is there a nuance we have missed in the wiki here?

at the end of the section on:

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

We say "Also note that if one is exporting the equipotential zone, that
probably means the CPC of the submain is also being used as a main bonding
conductor, and so it will will have to meet the minimum CSA size
requirements for a main equipotential bonding conductor. Since this is
often 10mm² copper equivalent, this can preclude the use of some of the
smaller sizes of SWA since the copper equivalent of the armour may be too
small. In these cases a separate bonding conductor can also be used."

Rather than an 8x factor we have: "Note that for ease of use of the above
figures one needs to convert the actual CSA of the steel armour wires to a
"copper equivalent". To do this divide the quoted figure by 2.255. So for
example: a 2 core 2.5mm² cable with 70° C PVC cladding has a copper
equivalent armour area of 17 / 2.255 or 7.54mm²"



Yes there is a nuance missed I think

There are 2 separate calculations or issues.

Fault currents and protective conductor sizing. Adiabatic regime applies
which allows you to use ratio of 'k' values to work out minimum protective
conductor size. Table 54.7 gives an example.

Indeed - I think we have covered this angle well enough...

Protective bonding conductors. Reg 544.1 covers protective bonding
conductors. 544.1.1 introduces the phrase 'affording equivalent conductance
in other metals' which is then emphasised in the note at the bottom of Table
54.8. This is much more demanding and as was said in another post by Andy
Wade means you need pretty large (impractically large for domestic) sizes of
swa to comply if you want armour as a bonding conductor.

Yup, I think we need to create a new section on exporting equipotential
bonding to cover this requirement.

Not sure how we missed that first time thought - since the same words
are on the end of Table 54H in the 16th edition.

I think it might be wise to modify the tables we have that show the
armour CSA to also include the resistance per m based on table H.2 in
BS5467:1997

--
Cheers,

John.

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