On 07/02/2019 19:17, wrote:
On 07/02/2019 18:50, Cynic wrote:
I've not got tables to hand but check your earth loop impedance
against disconnection times for the 40A mcb

The sub-main earth loop impedance (20m of 10mm2 and 20m of 16mm2) should
be about 60mOhm (37mOhm + 23mOhm) so about 0.4 ohms total (TN-C-S) -
about 600A fault current. A type C MCB needs 10x rated current, so 500A
for a 50A MCB. Unless I've boobed it should be OK.

You might have boobed it - twice, if I am not mistaken!


1. You are using 240V not 230V for your fault current. (Max Ze for a 50A
C type is 0.37ohm and remember that is when the cables are at 40 deg)


2. Your resistance calculations should be 16mm earth and 16mm live
(unless you wish to chop out the 6mm cpc of the 16mm T&E and not bother
using it)

Either way you are over the 0.37 ohms allowed.

It might be better to swap the head end with some thing like this

https://www.tlc-direct.co.uk/Products/CGMSF100.html

--
Adam