The 'main distribution board' needs to house CBs only to protect the cables to the CUs.  (CUs will include RCDs whether required or not)

As above, consider the type of cables for the submains - if ordinary insulated & sheathed and concealed in a wall it'll likely need 30mA RCD protection - hence with no discrimination with downstream RCDs. SWA avoids that problem, but isn't easy to run neatly or rear-entry to a CU in a domestic setup where aesthetics tend to be demanding.

My preference is for HBC fuses for submains - better for discrimination and with most designs you get terminals designed for larger conductors, rather than the usual MCB style shallow cage clamps which don't always do well with large stiff 7-strand.

  - Andy.

Thanks, Andy

I've been looking at sub-main cabling options. I think the best option is probably SWA.

Should be able to run this down from the 'main distribution board' to underfloor and then up to the CUs which will be in cupboards, so I don't mind SWA cables coming in the bottom.  Avoids the need for mechanical protection although I'll need to look at ratings where this will be in a wall behind plasterboard.

Also, I think if I use 3-conductor SWA then I don't need to run any additional separate earths to the CUs.  I do need to check that point though.

Discrimination is an issue, so don't want RCDs in the 'main distribution board' (except if I decide to take a garage feed from there).  Like the idea of the HRC fuses, but will need to check the loop impedance to work out whether there could be enough fault current for correct operation of these.

If you have one, I'd quite like a recommendation for which manufacturer's products to look at, particularly for the 'main distribution board'.

Thanks
John

Like the idea of the HRC fuses, but will need to check the loop impedance to work out whether there could be enough fault current for correct operation of these.

Rough rule of thumb (making all sorts of semi justifiable assumptions like N and PE have the same impedance etc) - if voltage drop is OK then a fuse (5s disconnection time) or a C-type MCB are likely to be OK for Zs. Certainly do the maths to be sure. but if generally if Zs is an issue, v.d. is going to be a bigger one.

If you have one, I'd quite like a recommendation for which manufacturer's products to look at, particularly for the 'main distribution board'.

Ah, now you're asking. The lack of an affordable mult-way fuse based DB for domestic use has been debated on here a few times. Single way units (e.g https://www.tlc-direct.co.uk/Products/WYDSF100M.html or even cheaper variants) are readily available but no-one seems to do multiway versions. (You can go for a industrial or Ryefield type unit, but rapidly get into silly money for a limited domestic setup). Some of us have resorted to DIN rail mounted 58x22 fuse holders, a switch disconnector, MCB sized DIN rail terminals for N and an assortment of bits of bus-bar in a suitable enclosure for our own home use ... but recent regulation tightening has made that a little more awkward to justify.  I guess the official answer is a few of the 1-way units, Henley blocks and an upstream switch disconnector (e.g. REC2) if the DNO hasn't already provided one. Simple enough and I suppose easy to maintain but not particularly neat.

   - Andy.

John R. said:

If you have one, I'd quite like a recommendation for which manufacturer's products to look at, particularly for the 'main distribution board'.

If you now want fuses, you only need a couple, but perhaps allow for a third to peel off to the garage or outbuildings.

A TPSN switch-fuse(-disconnector) could be used. Eaton Glasgows are built like a brick dunny - they pop up pretty regularly on eBay, if price is an issue.

Thanks Andy and Chris

I did actually see the Eaton boards on eBay having seen some silly prices elsewhere.

I was looking at DIN rail mounted fuse holders able to take up to 25sq.mm cables.

Ideally I need to find a board that will take SPDs too.

John R. said:

I was looking at DIN rail mounted fuse holders able to take up to 25sq.mm cables.

If it helps at all, this is the kind of thing I used: https://uk.rs-online.com/web/p/fuse-holders/1444544 (although mine had a different brand label). The way the holder opens (the front rotates down to open) means that a lot of DIN rail modular enclosures don't fit (and of course you'd be limited to steel enclosures these days for domestics). 58x22 is the size of the normal BS 88 (BS 1361 as was) "cut out" fuses - so will take 63A, 80A, 100A etc.

Ideally I need to find a board that will take SPDs too.

You can get "main switches" with a built in SPD (e.g. https://www.tlc-direct.co.uk/Products/WYREC2SPD.html ) - which might be useful if you were going down the Henley blocks and several separate 1-way switch fuse route.

  - Andy.

John R. said:

I was looking at DIN rail mounted fuse holders able to take up to 25sq.mm cables.

If it helps at all, this is the kind of thing I used: https://uk.rs-online.com/web/p/fuse-holders/1444544 (although mine had a different brand label). The way the holder opens (the front rotates down to open) means that a lot of DIN rail modular enclosures don't fit (and of course you'd be limited to steel enclosures these days for domestics). 58x22 is the size of the normal BS 88 (BS 1361 as was) "cut out" fuses - so will take 63A, 80A, 100A etc.

Ideally I need to find a board that will take SPDs too.

You can get "main switches" with a built in SPD (e.g. https://www.tlc-direct.co.uk/Products/WYREC2SPD.html ) - which might be useful if you were going down the Henley blocks and several separate 1-way switch fuse route.

  - Andy.