On 01/07/2020 16:44, John Rumm wrote:
On 01/07/2020 13:42, wrote:

Well, that was much easier than I thought. I had both a suitable nylon
gland and the heatshrink in "stock" - job now done but there's a
supplementary question about MCB ratings.

The 2.5mm2 buried SWA sub-main is currently protected by a B16 MCB at
the head end but I'd like to increase it to a B25 because I have a
couple of B16 RCBOs at the remote end. 25A is inside the rating of the
cable so I don't see a problem. Is there any reason not to do this?

Chances are it will be fine, although it depends on the length of the
submain.

Factors that might come into play are maximum circuit impedance, and
possibly voltage drop (if you assume a higher design current)

If you add up the total round trip resistance presented at the end of
the submain in the event of a phase to neutral short, it needs to be low
enough to achieve a prospective fault current equal or higher than the
160A required to trip the B32 "instantly" on the magnetic part of its
response.

You can use the tabulated cMin value of Zs of 1.37 Ohms (down from the
2.73 Ohms for the B16 MCB):

http://wiki.diyfaq.org.uk/index.php/...2C_Cmin_factor


One probably ought to allow for the submain operating at capacity (i.e.
conductor temp of 70 deg C) at the time of the fault, which will push
the resistance up to 17.78 mOhms/m (L + N). So your length limit is
1.37/ 17.78 x10^-3 = c. 77m
It's going to be a B25 (not B32) at the head end so that gives me a max
length of around 98m - the actual length is 75-85m, split as 70m'ish of
buried SWA and 15m'ish of T&E running through the house from where the
SWA terminates to the CU.

Voltage drop you probably care less about, but if running lighting
circuits from the submain, then you are limited to 3% (6.9V) total drop.
If you took the design current as 25A, then then your drop would be 18
mV/A/m, so 6.9 / 25 / 0.018 = c. 15m (Vs c.22m for the current setup)

(voltage drop allowed for socket circuits is 5% or 11.5v)
Voltage drop isn't an issue. This supply runs a Well pump (for
irrigation) and now also supplies a couple of IP66 sockets for general
use at that end of the garden. In the future I may need to provide a
further extension from this sub-main to provide electrics at a gate, but
that's a long way down the job list!

BTW, everyone says put in a larger cable than you need when burying
cables - I wish I'd done that!

You are not alone!

Thanks for the prompts and sanity check.

I had a momentary brain-off wobble when a socket tester objected to the
ELI at the new sockets, but then remembered that the N-E link was a long
way away. Measuring from the TT earth to the TN-C-S earth gives about
120 ohms but I'll double-check the actual quality of the TT earth later
with an intermediate probe.