Submain Cable to house extension

[JohnW2]

yep, sure John I understand that. But in this case, I think we can treat it like a submain to a garage/outbuilding, and protect it by a 30ma RCD with MCB's protecting the individual circuits downline. Reason being, I can run the lights in the stairway (with the door to the garage) from the existing house circuit so you're not plunged into darkness. Also, as it is just a bedroom, bathroom and garage, there is no normal instance where an earth fault would occur at night.

I personally would say that's a very reasonable common sense approach, and probably even compliant withthe regs, given that you have given due consideration to the possibility that a single fault could give rise to an unacceoptable level of 'danger or inconvenience'. However, as I'm sure you understand, some people will automatically regard having all circuits covered by a single RCD as 'very poor design' (and probably non-compliant with regs) regardless of any considerations of the actual circumstances of the installation concered (and quite possibly without even giving any thought to such considerations).

Sod's law also means that the electrician has put the lights on the same side of the split-load board as the plug sockets, so it defeats the object of split-loading the board for separate RCD protected circuits in the first place. Strange there isn't any requirement to separate circuit types according to use on a split load board. hmmm.

I suppose one can argue that, given the vagueness of the regs, there is an implied requirement that not only should the circuits be divided but also that they be divided in such a fashion as to minimise 'danger and inconvenience'. Mind you, as I implied in an earlier post, even if things are arranged so that the sockets circuit remains live in the location of a lighting circuits failure, many (perhaps even most) houses won't have any lighting plugged in to any sockets, let alone turned on, at a time when the lighting circuit fails!

Kind Regards, John

 

[tew1]

Also, technically, 30ma RCD protection is only required for cables "buried less than 50mm in walls". There is no mention of ceilings (other than bathrooms). I guess it's because people don't hang pictures on ceilings. So, you could fit a 100ma RCD at the meter, with seperate 30ma RCD's in the extension's CU which would prevent discrimination.

 

[JohnW2]

Deleted (quoted instead of edited, in error!)

 

[JohnW2]

So, you could fit a 100ma RCD at the meter, with seperate 30ma RCD's in the extension's CU which would prevent discrimination.

No, you will very often not get discrimination between RCDs just because one has an IΔn of 100Ma and the other 30mA. The 100mA one would need to be (and could be) a time-delayed ('Type S') one to achieve such discrimination.

Kind Regards, John

 

[EFLImpudence]

But if it has to have an RCD it will have to be 30mA.

 

[JohnW2]

But if it has to have an RCD it will have to be 30mA.

If there is no other (30mA) RCD in CU, yes.

Kind Regards, John

 

[flameport]

I don't get why this is a problem. There is no issue with all the circuits being on one RCD.

Doesn't comply with BS7671, regardless of what you might think.

 

[flameport]

Are you saying that you don't feel that there is any way to install a ≥60A submain in a TN-S system which does not represent 'very poor design'? - even a Type B MCBs would be no good if the Ze is anywhere near the maximum possible with TN-S.

Yes - other than the actual Ze being lower etc.
However in reality, there are practically no circumstances where a 60A or larger submain would be required in a domestic property anyway.

This mess being discussed certainly does not need one, given that:

it is just a bedroom, bathroom and garage

Even if this had to be connected to a single circuit for some reason, a 20A circuit would probably be ample.

 

[JohnW2]

Are you saying that you don't feel that there is any way to install a ≥60A submain in a TN-S system which does not represent 'very poor design'? - even a Type B MCBs would be no good if the Ze is anywhere near the maximum possible with TN-S.

Yes - other than the actual Ze being lower etc.

That's an interesting view. Given that a designer can do absolutely nothing about the Ze of a TN system, it would seem a bit rotten to accuse them of 'very poor design' if they had done the best that could possibly be done with the installation they were dealing with.

However in reality, there are practically no circumstances where a 60A or larger submain would be required in a domestic property anyway.

I realise that my house is a bit atypical, but I have several. I agree that this is unusual but, as has been asked, what about just 'long meter tails', which is a much more common situation - with people all the time being advised that a switchfuse (presumably at least 60A) is required.

This mess being discussed certainly does not need one ...

I've already agreed with that.

Kind Regards, John