I'm not so sure about that. The note to Table 41.1 acknowledges that, when disconnection is reliant on an RCD, achieving the disconnection times indicated in the table will require a fault current in excess of the IΔn of the RCD. A Type S RCD has a maximum trip trip of 0.15 secs at 5*IΔn, which is less than the required disconnection time of 0.2 secs for a TT installation. Do you not regard that as regulation-compliant?
Unlike what we most commonly see suggested here, the OP was not proposing to feed the outhouse from 'the house CU'. Rather, he was proposing to 'split the tails' (after the up-front RCD, which may or may not be time-delayed) and feeding the outhouse from one of those sets of split tails via a mini CU (equivalent to your switch fuse).
Exactly, and since the OP was proposing to have an RCD in his mini CU, that could be regarded as a 'simpler ('one box') solution than your 2-box solution of a switch fuse + RCD, couldn't it?
Yes, that latter is a 100A, 100mA time-delayed RCD. That RCD does little to provide protection to people against the effect of electric shocks - that's not what it's for (you need the 30mA non-delayed ones for that).
OK. Provided that you have downstream (i.e. in CUs) RCD protection for all circuits, then you could leave that up-front Hager as a 'main switch', since, being time-delayed, it ought not to trip if one of the other RCDs does (alternatively, once every circuit was RCD-protected, you could have it replaced with just a 100A simple switch). As far as I can make out, if you do ....
...and
.. then all circuits will, indeed, be protected by RCDs in CUs. The two RCBOs would not be 'affected', other than in the sense that the power to them (hence the circuits supplied) would be lost if the new RCD in the CU tripped (unless your electrician can 'split' that CU, so that the two RCBOs are not protected by the RCD). If the RCBOs remain protected by the (new) RCD, if one of them tripped, there would be a fair chance of the RCD in the CU also tripping - but that is no worse than if they were just MCBs (with which they could be replaced,if you wished).
Fair enough.
Can't you post them here?
Only if I look at the right column in Table 41.1. If I look at the wrong column it has a disconnection time of 70ms, and then I don't regard it as compliant.
It could. Most elegant, I guess, would be a small enclosure with an RCD and a fuse.
The instructions for putting photos in your post start here:
Ah - I thought you had maybe taken heed of winston, and thought it was right to look at the >230V columnOnly if I look at the right column in Table 41.1. If I look at the wrong column it has a disconnection time of 70ms, and then I don't regard it as compliant.
Yes, I suppose so, if you regarded a fuse as being preferable to an MCB - really just a variant on the OP's'mini CU'.
I did think about making a joke about having borrowed his glasses....Ah - I thought you had maybe taken heed of winston, and thought it was right to look at the >230V column
Not necessarily - their traditional use for them did not see them providing disconnection for circuits used for portable appliances, only those where a 5s time was allowed.
Don't know. My oldest copy of BS 7671 is from 2004, and U0 has been 230V since 1995. (Winston's dissatisfaction with the regulations is nearly 20 years old.)
Unless a very large cable on a very large MCB is being used, a fuse is the only way you'll get discrimination against MCBs in outbuilding CUs.
In a word, no
BAS was have a slightly bad moment and, as a result, introduced a red herring which probably confused things for you.
If you want other people to see them, and don't think it is right to expect other people to do the work of posting them for you then you should post them yourself.
How long ago was this 'traditional use'. The current Table 41.1 appears to relate to all final circuits ≤32A (i.e. most domestic circuits).
True - although I do think that, particularly in relation to domestic installations, people probably get unnecessarily excited about discrimination, and particularly when (as with the OP) provision has been made for emergency lighting. How often do these people expect a protective device to operate in their installations?!
It's horrible.
It's not the execution which I think is bad - it's the basic design.
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