John Rumm coughed up some electrons that declared:
Agreed, they'll generally discriminate for faults upto around 2-3kA + even for the biggest final circuit MCB.
Hager do an interesting range of MCBs, the HMF series, type C in double pole upto 100A which will discriminate with most of the standard Hager CU MCBs (and presumably RCBOs, need to check, not in the data sheet).
Discrimination is close though, with the max fault current limited to 600A in one common case. That's quite close. Chances are that a fault any distance down a final circuit would be OK, but close to the CU might be borderline. OTOH, despite Hager failing to quote a let-though energy figure for the 100A device, I suspect it would, looking at data for the lower current devices in the range.
One of those design choices that's not obvious, but a balance of compromises...
I should perhaps clarify that the shed submain would originate at the CU too, so distribution characterstics upto the CU are common to both solutions.
What I found was that using separate circuits, one can take advantage of the permissible volt drop being 5% of nominal supply voltage for non lighting circuits (lighting being 3% of course). With lighting rolled in on a 38A combined circuit, it meant a jump to the next size of cable (10mm2 in this case, rather than 6mm2 which would be fine for a 32A socket circuit). I have chosen 5A to be the design load on the leg of the lighting circuit going to the shed - should be suitably overstated.
The "wastage" of running a second bit of 1.5mm2 is mitigated by the the fact that there'll be a bit of 1.5mm2 running in that general direction from the house for lighting on another tool shed and any other lighting requirments in the garden. It's a very specific set of conditions I have, but teh twin circuit solution does buy guaranteed discrimination of shed lights vs shed angle grinder on shed socket, whereas that's a very close call for a submain.
It's not the classic solution, but I don't feel it's weird beyond it's merits.
I should have said - it's an all RCBO solution. That's one of the reasons I settled on Hager - generic single pole RCBOs are around 20-something + VAT,
30-something for type C which isn't bad for a quality brand.Nice :)
Ow!
Cheers
Tim