John Rumm coughed up some electrons that declared:
Tim S wrote:
snip
A fuse is also probably better here in that it will discriminate with
just about any MCB you care to use.
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...
Not really - I like to have all potential questions answered as well ;-)
One presumes the distance to the shed lights remains the same either
way, so house circuit fed shed lighting is unlikely to dim less than a
submain based installation (more in all probability!). This sounds like
an example of complying with the letter rather than the spirit of the
rule ;-)
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 might use DP RCBOs for the outside circuits, or at least the sockets,
which are 2 module wide, so at the extreme the above occupies 14 ways
from a 20 way board.
I usually have a policy of making sure that anything outside does not
share a RCD with something inside. Since the outside circuits are a more
likely source of nuisance trips etc.
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.
Here I have installed two split load CUs in the house[1] (preceded by
one main switch in a separate two module enclosure)). Both TT, the
second smaller CU is reserved for outside stuff. The 30mA RCD carries
outside sockets and lights, the 100mA type S RCD directly feeding
things like the garage and my workshop (via BS 88 cartridge fuse
carries). The garage/workshop has another split load CU with inside
lights on the main switch part, and sockets and outside lights on the
30mA RCD section.
Nice
[1] When we got here there was one CU with everything sharing a single
30mA trip RCD. Needless to say a horse farting would plunge the place
into darkness.
Ow!
Cheers
Tim