Correct Consumer Unit Set-Up For a TT Supply

[securespark]

Hi guys. There seem to be a few different trains of thought on how to cater for a TT supply so it complies with the 17th.

I think there's a real problem with the 0.2 s disconnection time, now that 61008/61009 devices are 0.3 s.

How would you do it?

 

[1john]

surely a dual 30mA board complies although if you want RCBO route and they are not double pole then a 100mA upfront as old school (which contreveins)

 

[Adam_151]

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 )

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 )

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

 

[Spark123]

If you start delving into the standards for BS(EN)61008 and BS(EN)61009 then you will find that when "significantly higher" (2x) than rated tripping currents are applied even type S RCDs should trip within the required 200ms.
The 2x was the value printed in the original version of BS7671:2008 was removed in the corrigendum - I'd imagine owing to a lot of upset people and manufacturers with incompatable testers.
The wording in BS7671 is now just "significantly higher".
However the 2x is still in those two EN documents.

 

[kai]

I am very surprised that single pole RCBO's are still recognised in the 17th, as the neutral earth problem still exists.

I thought that the use of double pole RCBO's would have been made compulsory, and the single pole RCBO's phased out, just like voltage operated ELCB's were some years ago.

 

[1john]

it's a shame that we dont use double poled MCB's alot as this would make isolation of problems much easier. In spain i noticed every domestic install had an RCD upfront but all d/p mcb's, where as i dont agree that any RCD upfront is a good idea, at least with these installs you can properly isolate a faulty circuit.