RCD splitting question

[piggyitm]

Hey guys!!
Can anyone tell me if I'm wrong.
I've run a 16mm Twin from mains to a 2 storey extension where I'm putting in a new board.2 rings 2 lights and smokes
I have to put in an RCD at the mains end so I was gonna put the new board on a 100a Main SW.
Is that Ok or do I have to still put in a Dual RCD split load to cover me for division of circuits?
Cheers

 

[High Tower]

Wrong in many ways!! Why 16mm and why twin and earth?? Have you done your calcs taking max demand and length of conductors into account, more so, you cannot supply this kind of load from a existing rcd as when combined with other circuits the rcd will overloaded and in the event of a fault the rcd would trip at the origin causing neuceince tripping at the house and leaving the occupants of the house with issues trying to get it reset.

This needs the attentions of someone with a higher degree of competence

The way to do it is split the meter tails into henly blocks into a switch fuse, swa to the flat from there, suitably sized as per calcs. This then negates the need for RCD at origin. Terminated into a split load twin RCD. Board in the flat.

 

[Deleted member 9648]

Wrong in many ways!! Why 16mm and why twin and earth?? You cant dismiss this out of hand,it may well be adequate....more info on the loading,earthing system,and installation method is required Have you done your calcs taking max demand and length of conductors into account, more so, you cannot supply this kind of load from a existing rcd as when combined with other circuits the rcd will overloaded How do you know that?and in the event of a fault the rcd would trip at the origin causing neuceince tripping at the house and leaving the occupants of the house with issues trying to get it reset.

This needs the attentions of someone with a higher degree of competence

The way to do it is split the meter tails into henly blocks into a switch fuse, swa to the flat from there, suitably sized as per calcs. This then negates the need for RCD at origin. Terminated into a split load twin RCD. Board in the flat.

I agree that an RCD at the origin is not best practice,but more info is needed on the type of earthing system...is the reason for an RCd at the origin because the earthing is TT?....or are you thinking the T/E distribution circuit will be buried and need RCD protection?
Also the OP states the distribution circuit will be on a 100a main switch....will there be any overload/fault protection for the proposed 16mm?.....A lot more info is needed here to give an answer.

 

[High Tower]

I agree that an RCD at the origin is not best practice,but more info is needed on the type of earthing system...is the reason for an RCd at the origin because the earthing is TT?....or are you thinking the T/E distribution circuit will be buried and need RCD protection?
Also the OP states the distribution circuit will be on a 100a main switch....will there be any overload/fault protection for the proposed 16mm?.....A lot more info is needed here to give an answer.

To be fair even on a TT, a split load twin RCD should satisfy the regs and would render any at origin pointless, and I get the impression he was suppling it from another 30mA on a exsisting board, hence switchfuse and henleys.

I know I should have asked for more info, to be fair I normally do! But on this occasion I was trying to avoid the normal 20 post back and forth trying to clear up the basics.

 

[Engineer54]

Hey guys!!
Can anyone tell me if I'm wrong.
I've run a 16mm Twin from mains to a 2 storey extension where I'm putting in a new board.2 rings 2 lights and smokes
I have to put in an RCD at the mains end so I was gonna put the new board on a 100a Main SW.
Is that Ok or do I have to still put in a Dual RCD split load to cover me for division of circuits?
Cheers

If this is an ''extension'' to an existing property, Why are you installing a separate CU remote from the existing CU?? If you don't have enough ways in the existing CU you could either add another below /above
existing, or change the existing CU for a CU with enough ways to cover this new extension to the property.

Having 2 remotely separated CUs in a single dwelling property is a nonsense....

 

[Deleted member 9648]

If this is an ''extension'' to an existing property, Why are you installing a separate CU remote from the existing CU?? If you don't have enough ways in the existing CU you could either add another below /above
existing, or change the existing CU for a CU with enough ways to cover this new extension to the property.

Having 2 remotely separated CUs in a single dwelling property is a nonsense....

Why??.....running a single distribution circuit to a separate local DB may be a whole lot easier than running back a shed load of final circuits to the origin.

 

[Deleted member 9648]

To be fair even on a TT, a split load twin RCD should satisfy the regs and would render any at origin pointless, and I get the impression he was suppling it from another 30mA on a exsisting board, hence switchfuse and henleys.

I know I should have asked for more info, to be fair I normally do! But on this occasion I was trying to avoid the normal 20 post back and forth trying to clear up the basics.

Well no mate....if it's a TT an RCD with be necessary at the origin to provide fault protection to the distribution circuit...ideally a 100ma time delayed with a split rcd board in the extension.....if it's a TT and thats how the OP is going to do it

 

[Engineer54]

''Why??.....running a single distribution circuit to a separate local DB may be a whole lot easier than running back a shed load of final circuits to the origin. ''


Put it this way then, ....If it was my house and you suggested running in another CU, for what is, when all said and done just an extension of my house you'd be gone!!! Two remotely separated CUs in a single dwelling is not my idea of a correct electrical installation. ....It's just an easy option!!!

 

[Deleted member 9648]

''Why??.....running a single distribution circuit to a separate local DB may be a whole lot easier than running back a shed load of final circuits to the origin. ''


Put it this way then, ....If it was my house and you suggested running in another CU, for what is, when all said and done just an extension of my house you'd be gone!!! Two remotely separated CUs in a single dwelling is not my idea of a correct electrical installation. ....It's just an easy option!!!

It depends on the size of the house obviously....but the above statement as a generalisation is misleading and incorrect....for example the two storey extension may be a separate annexe...granny flat?....in which case a separate DB would make sense. If the house was a large one running final circuits back to the origin may result in excessive final circuit legnths....a second DB would solve those issues.

 

[piggyitm]

Ok.The existing board is 15th Edition and obsolete on a TNS.The idea was to henley into the tails with some kind of RCD config to protect the cable and deal with 17th regs about 50mm depth.Is this wrong Also?The question was do i still need to divide my circuits accross 2 rcds in my new board even if it's got an rcd at the main?

 

[Engineer54]

Ok.The existing board is 15th Edition and obsolete on a TNS.The idea was to henley into the tails with some kind of RCD config to protect the cable and deal with 17th regs about 50mm depth.Is this wrong Also?The question was do i still need to divide my circuits accross 2 rcds in my new board even if it's got an rcd at the main?

Well that's a bit daft isn't it, having an 30mA RCD protecting 2 further 30mA RCDs or RCBOs!!!

So what's the plan then, leave what your calling an obsolete CU for the existing installation, and provide a spanking new CU for the extension?? So the different parts of the house will be wired to differing wiring Regs also??

Sorry, i just can't get my head around having 2 completly seperate CUs in a SINGLE dwelling house.

 

[pushrod]

Ok.The existing board is 15th Edition and obsolete on a TNS.The idea was to henley into the tails with some kind of RCD config to protect the cable and deal with 17th regs about 50mm depth.Is this wrong Also?The question was do i still need to divide my circuits accross 2 rcds in my new board even if it's got an rcd at the main?

Don't put 2 ordinary rcds on the same circuit - No discrimination.

 

[pushrod]

Is the extension functioning as part of the main house? If it is i would be more tempted to go along the lines of extending the lighting circuits and sorting out the old CU. With extra circuits for the new sockets and smokes. Just my 2 pennyworth

 

[nickblake]

Im with pushrod here if you add further cu's then they would have to be connected to the original cu or if your lucky enought to have a main switch installed as you have to provide a single point of isolation , i have installed 2 cu's in a small bungalow , but this was due to the existing cu being above a ceiling height so i installed a small main board with a 3 pole isolator then subed out the main circuits to one board and eco 7 to another , and protected by 2 63 amp mcbs , customer was chuffed as she didnt have to climb on the work tops to reset a tripped mcb

 

[Engineer54]

It depends on the size of the house obviously....but the above statement as a generalisation is misleading and incorrect....for example the two storey extension may be a separate annexe...granny flat?....in which case a separate DB would make sense. If the house was a large one running final circuits back to the origin may result in excessive final circuit legnths....a second DB would solve those issues.

First off .... It wouldn't then be a ''Single Dwelling'' house if the extension was to be an annexe or separate flat, would it?? Then you would be correct...

If this was a large house to start with, it would be very doubtful the owner would be adding a two storey extension, ....not impossible, but doubtful!! I would suggest that the now present size of house is in all probability a 4 bed-roomed house converted from a 2 or maybe even a 3 bed-roomed house. ....So how many 4 bed-roomed houses have you seen with 2 completly separate CUs supplying it's electrical needs???

I stand firmly behind my statement, ...it's certainly not misleading and it's not incorrect either. If we were talking about a very substantial 3 or 4 storey house, or of the like you could possibly have a valid point.

 

[High Tower]

The more I read, the more i find out the more I think with 15th edition equipment then possibly the whole house could do with attention to bring it up to the same standard as the new addition and incorporate it into one installation

 

[piggyitm]

So i have to install a new board to create a new way rather than tap off the tails and put in an extra 17th CU that can feed the extension board because there is no supplied main isolator and because the house is only 4 bedrooms?Isn't this just like tapping off a busbar chamber with a new board and labeling it DB2 then making the extension DB2/1 at both ends?

 

[pushrod]

So i have to install a new board to create a new way rather than tap off the tails and put in an extra 17th CU that can feed the extension board because there is no supplied main isolator and because the house is only 4 bedrooms?Isn't this just like tapping off a busbar chamber with a new board and labeling it DB2 then making the extension DB2/1 at both ends?

Just replace present CU with a larger one. Arrange for supplier to fit an isolator.

 

[piggyitm]

Hey Hightower while your on...same extension....do i have to install mains interlink smokes in the existing house to connect in with my extension smoke install?
And do i need to change 25% of the lights in the existing to conform with L1A?Also there are some existing spots that aren't fire rated in the kitchen will these have to have hoods AND all the sockets are below 450mm

 

[High Tower]

As far as I know fire hoods and socket Heights arnt retrospective, so no for them but you will have to update smokes mate, and as for lighting do a quick search on here as there was a thread debating the lighting issue, not sure what the outcome was

 

[Deleted member 9648]

First off .... It wouldn't then be a ''Single Dwelling'' house if the extension was to be an annexe or separate flat, would it?? Then you would be correct...That info wasnt available in the OP,therefore your general statement about having 2 DB's in one propety was misleading

If this was a large house to start with, it would be very doubtful the owner would be adding a two storey extension, ....not impossible, but doubtful!! I would suggest that the now present size of house is in all probability a 4 bed-roomed house converted from a 2 or maybe even a 3 bed-roomed house. ....So how many 4 bed-roomed houses have you seen with 2 completly separate CUs supplying it's electrical needs??? That info was not in the OP either....you appear to have an uncanny ability to decipher imformation that is not provided

I stand firmly behind my statement, ...it's certainly not misleading and it's not incorrect either. If we were talking about a very substantial 3 or 4 storey house, or of the like you could possibly have a valid point.

.....If the property is as you describe I absolutely agree with you,final circuits should be run to a single DB...but as this info was not provided your all encompassing statement concerning additional DB's in a single property is misleading and incorrect.......you dont have to agree,but you are wrong.

 

[malcolmsanford]

Well no mate....if it's a TT an RCD with be necessary at the origin to provide fault protection to the distribution circuit...ideally a 100ma time delayed with a split rcd board in the extension.....if it's a TT and thats how the OP is going to do it

Though we now know it's a TN-S system the sole use of RCD protection for a SWA distribution cable is not totally correct. Reg 411.5.2 does state the prefered method of fault protection should be an RCD, but providing that the Zs allows disconnection time then another type of protection device can be utilised such as a BS 88-3 ( 1361) fuse.

I know that in design you may not be able to achieve the required Zs by calculation, but in reality with earth paths and bonding it may be achievable.

 

[Amp David]

Absolutely loath submains run in twin nad earth. Its the skin flint way IMO!

 

[High Tower]

Absolutely loath submains run in twin nad earth. Its the skin flint way IMO!

I did try and make that point, I diddnt say it was against regs just said it's a bad idea,

I still say that if it's not possible to amalgamate the new circuits with the existing then the best way in
My mind would be split the tails, switch fuse then swa to the remote board.

 

[Amp David]

DIdn't see your post High Yower, will teach me to read the full thread in future lol

 

[High Tower]

No your post was spot on, just this topic has caused quite the debate

 

[Deleted member 9648]

Though we now know it's a TN-S system the sole use of RCD protection for a SWA distribution cable is not totally correct. Reg 411.5.2 does state the prefered method of fault protection should be an RCD, but providing that the Zs allows disconnection time then another type of protection device can be utilised such as a BS 88-3 ( 1361) fuse.

I know that in design you may not be able to achieve the required Zs by calculation, but in reality with earth paths and bonding it may be achievable.

Technically correct.yes...but in practice the chances of getting a low enough Zs for a 60a bs 88 fuse with a rod are around zilch.

 

[Deleted member 9648]

Absolutely loath submains run in twin nad earth. Its the skin flint way IMO!

I'd do it in SWA as well.......

 

[High Tower]

Technically correct.yes...but in practice the chances of getting a low enough Zs for a 60a bs 88 fuse with a rod are around zilch.

theres no rod, its TN-S

 

[Deleted member 9648]

yeah I know...but those posts originated before the earthing system came to light.

 

[High Tower]

yeah I know...but those posts originated before the earthing system came to light.

sorry mate diddnt realise

 

[tuckermot]

upgrade to new board (more ways)

extend original lighting circuit ( 10a mcb instead of 6a mcb in new board ) allowing that original cable is not 1mm and using watts divided by volts to get amp usage of lights old and new )

wire smoke alarms of original / new circuit using wire less smokes

run new ring back to new board

job done !!!

 

[tonys]

tuckermo

How did you get on with the interview for that electrical/mechanical apprenticeship ?

 

[tuckermot]

hi tony

did not get there mate my normal job came 1st ( pays the bills ) still looking to go full time but just doing bits and bobs at mo !!!! .I have let part p run out as i can not compete with other sparks who just do work any way !!! and are not botherd about certs and building controle

 

[tonys]

Bad ain't it mate, from my point of view, if loads said stuff part p, there's nothing the authorities could do about it. They do not have the time or money to chase people.

Keep at it, there's a load of decent homes work up your way.

 

[tuckermot]

thanks tony

 

[Engineer54]

.....If the property is as you describe I absolutely agree with you,final circuits should be run to a single DB...but as this info was not provided your all encompassing statement concerning additional DB's in a single property is misleading and incorrect.......you dont have to agree,but you are wrong.

I suggest you read the original OPs post, No where in that post, did it talk about granny flats, annexes, or flats, You did that!! I read the post as it was described, and i related to it, and that was as ....'A House Extension'' .

Now unless we are talking about a rather large sprawling house, there will never be a need for having 2 CUs in remote separate locations. 2 CUs Yes maybe, ...one for off peak heating tariff's, or as a means of increasing the number of circuits. But again both located at a common position.


So let me say this again, albeit a little modified to suit yourself, ''The use of 2 remotely separated CUs in a standard sized single dwelling house is a nonsense!!!'' And that is not misleading, nor is it wrong!!!

 

[tonys]

Engineer54

Your not putting with nonsense then, I reckon your part p qualified lol

 

[Engineer54]

Nope, ....i don't even live in the UK these days ...haha!!

 

[tonys]

I know, you drop the hint how many times a day lol

 

[Engineer54]

Haha!!... I don't normally ''Hint'' about anything!!... Well so i've been told ...lol!!

 

[Deleted member 9648]

I suggest you read the original OPs post, No where in that post, did it talk about granny flats, annexes, or flats, You did that!! I read the post as it was described, and i related to it, and that was as ....'A House Extension'' .

Now unless we are talking about a rather large sprawling house, there will never be a need for having 2 CUs in remote separate locations. 2 CUs Yes maybe, ...one for off peak heating tariff's, or as a means of increasing the number of circuits. But again both located at a common position.


So let me say this again, albeit a little modified to suit yourself, ''The use of 2 remotely separated CUs in a standard sized single dwelling house is a nonsense!!!'' And that is not misleading, nor is it wrong!!!

Your original posts at the start of this thread(before there was any notion of the extent of the extended property,or it's intended use) inferred that a second remote DB in a single dwelling is never acceptable and even a cowboy practice....that is absolute rubbish and it is that I took issue with.There are instances where a separate DB would be a better installation method than running multiple final circuits a long distance to a main DB,some of those instances have already been acknowledged,others come to mind.
In the part of your quote highlighted you have suddenly inserted "standard sized"....where was this when your original posts were stating that a second CU is never acceptable?.....you were as unaware as I was of all the facts,that is why my first post asked for more info before answers could be given....

 

[Engineer54]

Your original posts at the start of this thread(before there was any notion of the extent of the extended property,or it's intended use) inferred that a second remote DB in a single dwelling is never acceptable and even a cowboy practice....that is absolute rubbish and it is that I took issue with.There are instances where a separate DB would be a better installation method than running multiple final circuits a long distance to a main DB,some of those instances have already been acknowledged,others come to mind.
In the part of your quote highlighted you have suddenly inserted "standard sized"....where was this when your original posts were stating that a second CU is never acceptable?.....you were as unaware as I was of all the facts,that is why my first post asked for more info before answers could be given....

It's pointless going on and on about this, i think it's been thrashed out enough now....
All i as trying to point out in my last post was that i always understood, that the OPs original post was an extension and nothing more, and that is what i commented on, in my original post.
I didn't suddenly insert ''Standard'', ... as i stated that was to satisfy your side of the argument.

 

[Deleted member 9648]

ok....

 

[piggyitm]

The 16mm in question is a 30m run straight through joists 400m apart .SWA would have been a bit of a chew.
I can't see the problem,if its labeled up,how the distribution layout should be a concern.
Anyway if I change the CU for a dual RCD board and come from a way that's 80a 30ms protected and feed my new board that has 5 circuits,how do I allow for division of circuits at my new board?
For those who are thinking lack of competence,would I be ***** to sit here on a saturday night fishing for the answer with a missus pestering for some romance............?

 

[High Tower]

The 16mm in question is a 30m run straight through joists 400m apart .SWA would have been a bit of a chew.
I can't see the problem,if its labeled up,how the distribution layout should be a concern.
Anyway if I change the CU for a dual RCD board and come from a way that's 80a 30ms protected and feed my new board that has 5 circuits,how do I allow for division of circuits at my new board?
For those who are thinking lack of competence,would I be ***** to sit here on a saturday night fishing for the answer with a missus pestering for some romance............?

Why are you here if you have the option of the jump??

U mad??

 

[pushrod]

Can i ask is this your own property or someone elses?

 

[piggyitm]

She's just made me watch the ****est film i have ever seen.....Valentines Day!
Obviously, it's now time for some punishment............