18th Edition changes regarding sub-mains

phatboy · 9 Sep 2019

Hi all,

A friend has had some pricing for installing some new heating on an off-peak tariff, which will involve a new sub-main from the (new) meter to the new heating CU position.

They are being advised that as part of this work the existing installation (Sockets, lighting etc. only) needs to be provided with an RCD at the meter box position. This existing sub-main is only being touched due to needing to provide isolation to both sub-mains (whole house) at the same time, and because of the change of meter.

I questioned why there was additional work than quoted 2 years ago, and they advised it was because this new RCD requirement for the sub-main was part of the 18th edition regulations.

Is it correct? I've not seen such reference when searching for changes in the 18th.

It bothers me a little, as I though installing an RCD at the start of a sub-main was a poor choice, as it results in all circuits becoming isolated in the case of a fault. In this case the existing CU is all RCBO.

Any guidance appreciated!

flameport · 9 Sep 2019

There were some changes to RCD requirements such as lighting requiring RCDs regardless of how the cables were installed, but nothing for submains specifically.
A consumer unit with all RCBOs would already cover those changes.

Submain might need an RCD depending on how it's installed - such as T&E concealed in walls etc. However that was the same under the 17th, no change to that.
If SWA, in steel conduit, surface fixed or similar then no RCD required.

JohnW2 · 9 Sep 2019

phatboy said:
.... which will involve a new sub-main from the (new) meter to the new heating CU position. ... They are being advised that as part of this work the existing installation (Sockets, lighting etc. only) needs to be provided with an RCD at the meter box position. This existing sub-main is only being touched due to needing to provide isolation to both sub-mains (whole house) at the same time, and because of the change of meter. ... It bothers me a little, as I though installing an RCD at the start of a sub-main was a poor choice, as it results in all circuits becoming isolated in the case of a fault.

What sort of earthing does the installation have (TN-S, TN-C-S or TT)?

Kind Regards, John

phatboy · 10 Sep 2019

JohnW2 said:
What sort of earthing does the installation have (TN-S, TN-C-S or TT)?

Kind Regards, John

TN-C-S

Thanks!

phatboy · 10 Sep 2019

flameport said:
There were some changes to RCD requirements such as lighting requiring RCDs regardless of how the cables were installed, but nothing for submains specifically.
A consumer unit with all RCBOs would already cover those changes.

Submain might need an RCD depending on how it's installed - such as T&E concealed in walls etc. However that was the same under the 17th, no change to that.
If SWA, in steel conduit, surface fixed or similar then no RCD required.

Thanks, so what was acceptable 2 years ago, is still equally as acceptable now? I don't want to get so deep into the debate with the contractor as to tell them SWA would be fine, but have advised said friend to get alternative quotes based on what I've heard so far!

flameport · 10 Sep 2019

phatboy said:
Thanks, so what was acceptable 2 years ago, is still equally as acceptable now?

Mostly yes - the main change was that previously lighting circuits did not need RCDs if the cables were in steel conduit or similar. However most actually did get RCDs due to the cables being T&E in plaster and similar.
Now all domestic lighting circuits require RCDs regardless of how the cables are installed.

JohnW2 · 10 Sep 2019

phatboy said:
TN-C-S

Thanks.

In case you wondered why I asked .... if it had been TT then, like me, you would probably need RCD protection for the distribution circuits ('sub-mains'). The way one avoids the problem you mentioned (of one fault on a final circuit taking out the entire board) is by having each of the those final circuits protected by a standard 30mA RCD or RCBO, and then use a time-delayed ('Type S') RCD to protect the sub-main. In that way, a fault on any final circuit will cause that circuit's RCD/RCBO to trip, and clear the fault, before the time-delayed one trips.

Kind Regards, John

SimonH2 · 24 Sep 2019

The obvious question is to ask the person saying this RCD needs to be fitted to explain specifically which regulation requires it. It could be that it is required due to cable type and routing, or it could be that it isn't required and the person claiming otherwise is just talking out of their backside. As said, there's been no recent changes that would mean such a submain has to be RCD protected now that weren't also in place "some years" ago.

Risteard · 25 Sep 2019

JohnW2 said:
Thanks.

In case you wondered why I asked .... if it had been TT then, like me, you would probably need RCD protection for the distribution circuits ('sub-mains'). The way one avoids the problem you mentioned (of one fault on a final circuit taking out the entire board) is by having each of the those final circuits protected by a standard 30mA RCD or RCBO, and then use a time-delayed ('Type S') RCD to protect the sub-main. In that way, a fault on any final circuit will cause that circuit's RCD/RCBO to trip, and clear the fault, before the time-delayed one trips.

Kind Regards, John

Not the case with single pole RCBOs and a neutral/Earth fault. Just one of the reasons why single pole RCBOs are crap.

JohnW2 · 26 Sep 2019

Risteard said:
Not the case with single pole RCBOs and a neutral/Earth fault. Just one of the reasons why single pole RCBOs are crap.

That is arguably true in the case of a TT installation which requires an up-front TD RCD, but that is a relatively rare situation, which only arises these days if some of the final circuits don't have RCD/RCBO protection and/or (as in my case) there are 'pre-CU' distribution circuits which require RCD protection. However, even in those pretty uncommon circumstances, N-E faults are so relatively uncommon that many/most people would probably go for a lifetime without ever experiencing the inconvenience to which you refer.

In my case, virtually all of my final circuits are protected by (DP) RCDs, so the issue does not arise.

Kind Regards, John

Risteard · 26 Sep 2019

JohnW2 said:
That is arguably true in the case of a TT installation which requires an up-front TD RCD, but that is a relatively rare situation, which only arises these days if some of the final circuits don't have RCD/RCBO protection and/or (as in my case) there are 'pre-CU' distribution circuits which require RCD protection. However, even in those pretty uncommon circumstances, N-E faults are so relatively uncommon that many/most people would probably go for a lifetime without ever experiencing the inconvenience to which you refer.

In my case, virtually all of my final circuits are protected by (DP) RCDs, so the issue does not arise.

Kind Regards, John

I would not consider 30mA RCD protection on all final circuits to be a good reason not to have an upfront S-type RCD, not least to provide a level of defence against tails making all earthed metalwork in the installation live. This is why I don't agree with fitting it in the DB either - but would always fit it upstream of it.

JohnW2 · 26 Sep 2019

Risteard said:
I would not consider 30mA RCD protection on all final circuits to be a good reason not to have an upfront S-type RCD, not least to provide a level of defence against tails making all earthed metalwork in the installation live.

In the common situation of very short tails between a meter and CU which are in close proximity, that would seem to be an extraordinarily unlikely happening. In any event ....

Risteard said:
This is why I don't agree with fitting it in the DB either - but would always fit it upstream of it.

... it would seem that, if one has your concerns, one can't really win. If the concern is that a fault in the supply conductors feeding the CU might result in "all earthed metalwork in the installation [becoming] live", then having the S-type RCD physically separate from the CU would merely transfer those concerns from the conductors between meter and CU to the conductors between the external RCD and the CU.

In my case I have very long 'tails' (distribution circuits) and therefore do need the Type S RCDs close to the origin of those distribution circuits. However, as I said, since virtually all of my final circuits are protected by (DP) RCDs, a fault on a final circuit will not result in the upstream Type S RCD operating (and taking out many other circuits).

Kind Regards, John

Risteard · 26 Sep 2019

JohnW2 said:
having the S-type RCD physically separate from the CU would merely transfer those concerns from the conductors between meter and CU to the conductors between the external RCD and the CU.

No it wouldn't because I would always house it in an insulated enclosure. After all it's not a similar switchgear assembly to a type-tested distribution board for use by ordinary persons. It's merely an RCCB in an enclosure.

AndyPRK · 26 Sep 2019

Nicc say use a tail stuffing glad into a metal cu to cover this on TT.

They also say keep tails as short as possible. Though sparks take this too litterly.

Risteard · 26 Sep 2019

AndyPRK said:
Nicc say use a tail stuffing glad into a metal cu to cover this on TT.

They also say keep tails as short as possible. Though sparks take this too litterly.

Even with a tails gland (which frankly I use regardless of Earthing arrangements) I would like to see an S-type RCCB in an insulated enclosure upstream. It's a better way of doing it.

diynot

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

18th Edition changes regarding sub-mains

Similar threads