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.