These days dual RCD split seems to be the way to go, the issue of a fault between the tri-rated links free RCDs and a cpc in the unit was brought up, but the conclusion that if you exercise good workmanship then its not likely and the NICEIC recognise the use of such a CU on TT supplies, obviously its going to have to be plastic otherwise you'll need an up front 100ma slugged RCD
I wouldn't worry about the 0.3 disconnect time at 1x, because frankly if you are not hitting 5x on a 30ma device then you have bigger problems (like the electrode being in a plant pot or something
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Not sure if you have seen my post on the matter but been commisioning some industrial units this week... Zdb at the boards was 1.41 on the main stretch of units and with nothing bigger than a B16 in them... the circuits will still disconnect even if the upfront RCD should stick
Oh and if you have got side by side RCDs (ie. in a dual RCD cu, or multiple CUs off henleys) then when you do the touch voltage calc for max electrode resistance, add the RCDs
For example if I had a dual 30ma RCD CU for socket outlet circuits, then I had a a separate board with a 100ma RCD incommer for circuits I deemed to not need 30ma protection (say lights....either cos they are surface, in galv, or the installation is non domestic), then I had a board with a 300ma slugged device for running submains off... then
50/ (0.03+0.03+0.1+0.3) = 108 ohms
Other thing to wacth with the 17th is even if you have the lights surface, etc... you cant go for a '16th edition DB' with 100ma slugged and 30ma devices... as it falls fowl of the lights now falling under 0.2 sec disconnection time.. rather than the 1 sec exclusion for fixed equipment... but then you knew that already
As said above, avoid single pole RCBOs (unless up back it up with an upstream device and are prepared for neutral earth faults to take out the upfront device
... its your name on the design... not mine
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Sorry if I've rambled on a bit... its late and I'm now off to bed, I'll re-read this in the morning and fix any cock-ups