wrote:

Yup, ta for the confirmation... I was pretty sure it was only a
guidline, I just wanted to make sure it did not have some obscure regs
"heritage" that I had not spotted!


2 aspects come to mind:

Firstly, heritage is what it is.

15th ed IEE & (IIRC 14th & maybe earlier) had explicit rules for ring
circuits. These were moved to OSG after 16th ed published, so they are
now guidelines. The rule is now in OSG appendix 8 - Spurs page 149 in
my vintage 1998 yellow cover copy.

Yup, page 150 Annex 8 in the current one, with the specific bit on spurs
on page 153 now.

IIUIC you can't easily be fingered (part pee aside) if you follow OSG
to the letter, but you can depart if you are prepared to justify the
design.

Tis what I figured. I could not find any technical / moral reason for
not using the same layout as is used currently, but thought I would
check the collective memory banks to see if there was a reg I had
missed! ;-)

Secondly, the original post said the circuit dated from the 50s. If so
it would most likely have been cabled in imperial 7/029 (earth
conductor (CPC)is 3/029 IIRC) - that is approx 3mmsq with 1.25mmsq
earth. 7/029 has a rating of about 30A on the same basis that modern
2.5mmsq has 27A. The whole installation would also have run off a
conventional (probably wired) fuse box.

Yup certainly imperial sizes (and stranded). I did not actually measure
the cross section of the wire other than approximately by eye... did not
seem much point since it all needs to go really.

Now that RCDs are in wide use and we have precise instruments to
measure fault currents and we are very aware of things like earth loop
impedances and so on, there would be a major quibble with a 3/029 (ie
1.27mmsq) CPC. It is a tad too small - in the 'standard' fault

It would be a problem on a long spur, but as a short "finger" from a
ring it probably still copes.

condition this could leave a fault greater than the 50V maximum -
limiting the maximum cable run length. (Standard 2.5mmsq cable uses
1.5mmsq CPC & avoids the problem. Modern 4mmsq, with a 1.5mm CPC,
suffers from the same fault - which limits its popularity.

That and it is harder to work with anyway...


--
Cheers,

John.

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