On 08/02/2019 12:26, Andrew Gabriel wrote:
In article ,
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).

Yup, I have noticed that CU MCB based fuse carries are much harder to
find these days.

If you can size your sub main MCB a couple of ratings higher than the
highest downstream one, or, fit a type C at the sub main head end, you
might be able to mitigate the discrimination problems.

Having said that there are no guarantees, ISTR Adam mentioning a case
where an appliance fault took out the main incomer fuse, leaving its
plug fuse un-blown and the circuit MCB un-tripped!


--
Cheers,

John.

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