John Rumm coughed up some electrons that declared:

Tim S wrote:

One thing I'm lost on, is why have an RCD (RCCB?) if we are satisfied
that the circuit will disconnect in less than 5 seconds (by my
calculations, it will disconnect in 3 seconds at worst) and if we are no
longer concerned about touch voltages.

I can't see the need for a RCD at the switchfuse position.

Me neither, but BigWallop's in that field so I won't discount anything just
yet, with no disrespect to anyone else.

To be fair, this comes up on the IET forum from time to time and the answers
are different every time, with the only common factor being the "3m rule".

When (if) I get a reply from EDF Technical, I'll post a summary here.

Ignoring them for the minute, I can see no need for anything beyond a DP
isolator.

Their 100A BS1361 fuse is quite capable of disconnecting my submain in 5s -
though it would not if the max permitted Ze of 0.8 Ohms was apparant (need
630A to meet 5s).

I used a science principle in my calcs - my measured Ze was 0.19 +/-5%
+/-0.07 Ohm

That's the uncertainty in a Megger 1552 plus not knowing which pair of leads
from 2 sets the LI test was calibrated with (0.04 Ohms between them).

So I'm working to the max Ze with the worst case uncertainty applied, which
is 0.34. Even with this figure the disconnect times are fine. That gives a
bit of latitude for their supply varying a bit over time.

So I don't know why the DNOs have this rule.

Unless they merely want an isolator at that position... Or the right to prat
around with their cutout fuse whever the mood takes them, safe in the
knowledge it won't make the installation unsafe...

When you get articles written by the ECA saying "stick an MCB in" you tend
to assume there's a reason. Even the NICEIC Snags and Solutions books don't
cover a simple supply extension.

What I suspect happens in reality is noone gives a hoot and just does
something reasonable. Me, being me, wants to be able to justify stuff with
confidence. Silly rabbit, me...



Example: You want to install a separate CU for external lighting at a
later
date. A box will allow you to take other tails off without disrupting
everything. But that's my opinion.


It's a good idea, though I'm personally not keen on multiple adjacent
CUs. My leaning is to leave enough spare ways that it will never be
aproblem

Can be handy if you decide you want a totally separate one for
outside/garden circuits for example.



Just for context, here's my final circuit list so far (I'm pretty happy with
this):

2 x 32A ring circuits covering kitchen and utility area
1 x 32A ring for rest of ground floor
1 x 32A ring for first floor
1 x 10A lighting for left side of ground floor (inc kitchen/dining room)
1 x 10A lighting for right side plus upstairs[1]
1 x 40A cooker circuit (we use gas, but best have a circuit in place)
1 x 45A distribution circuit for boiler + backup thermal store heating (3 x
3kW)[2]
1 x 16A radial feeding two external 16A weatherproof commando sockets[3]
1 x 10A external lighting, including direct feed to shed lights[4]
1 x 32A (or 25A TBD) radial to shed sockets.

[1] This arrangement gives a good split of load.

[2] It's more practical to feed one big cable round to the heating cupboard
and split with MCBs (as part of the control panel maybe) there. Only need
one RCBO at the head end, less ways in the main CU too.

[3] I like commando sockets for external use. I'll make up a couple of long
16A - 13A socket trailing leads for those long summers in the garden with a
laptop Naturally we'll have a convenient inside isolator.

[4] Why not submain to the shed? Well, my calculations show it's impossible
to meet volt drop requirements on the lighting component without using
excessively oversized cable. I have a need for a few outside lights so it
seemed sensible to maintain separate circuits back the the main CU and lump
all the lights together. Shed will have a couple of local DP isolators for
safety.

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.

So you see, I'm not too worried about expansion

Cheers

Tim