On 08/02/2019 16:25, wrote:
On 08/02/2019 12:26, Andrew Gabriel wrote:
In article ,
Â*Â*Â*Â*John Rumm writes:
On 07/02/2019 18:38, wrote:
I haven't been able to find the rules for sub-main protection. Is it
permissible to run it from a 50A DP MCB in the garage CU, or does have
to be from a separate DP fused/switch from a Henley block?

No you can run from a MCB without any problem. Its commonly done.

One problem with using an MCB is you may get little if any
discrimination between that, and the downstream consumer unit
MCBs in the event of a fault. So if something shorts out on
a final circuit, you are quite likely to find the upstream
40/50A MCB trips, and you lose power on all the final circuits
on that submain. A BS1361 HRC cartridge fuse used to be a better
bet, but they have vanished (spare fuses still available, but
new fuse holders are not).


I was planning to use a 50A type C MCB to protect the sub-main and 16A
type B RCBOs in the new CU - this should give discrimination. The reason
for using 16A, rather than 32A, is that I have several radials and one
ring with spurs. All the high current loads are fed from the other CU,
in the garage.


That was the second point I meant to ask you about (I was tired
yesterday as I had had a weekend off work).

That's more about diversity than discrimination.

http://wiki.diyfaq.org.uk/index.php/Diversity

is worth a read if you have not already seen it.

--
Adam