RCD Protection of tails to flat

[Badged01]

1st floor flat on PME supply. Service head on lower floor below G/F flat. Tails to 1/F flat some 10m away protected by switched fuse after meter. Cant see run of tails but 99.99% sure they are buried in the wall as no conduit/trunking evident and they also go through the communal staircase area.

Questions:
1) Should these tails to the flat be RCD protected from the switched fuse? and
2) Should this RCD be time delayed to discriminate with the RCD's in the flat consumer unit?

I believe they should be RCD protected and time delayed to meet regs but would be interested to know if there is an alternative rationale for why they shouldnt.

 

[tony mc]

Its the depth of the cable that you would have to look at.

If the tails were buried less than 50 mm into the wall then RCD protection would be required and it would need to be 30mA.

If you were to place a 30 mA RCD on the switched fuse you would have no discrimanation with any up stream devices fitted at the flats CU!

 

[ExArmy]

i've just come across this on an EICR, tails from meter cupboard to other side of house(15M) from a 63A SF. i'm thinking code 3 as i'm limited to know how they are ran, and if i were to install tails to a board on a new build lets say as they are all like this now with the meters outside then they would have to be in the cavity, otherwise mechanically protected and in general well out of the way. wouldn't want to rcd them as could cause nuisance tripping, couldn't garantee discrimination between the board RCD's and the ones outside

 

[Badged01]

Read somewhere that provided the run from the meter to the consumer is short and in the cavity (3m seems to be the magical distance) then unprotected tails is considered acceptable. Certainly cant find anything in the regs to make this correct but all the new build houses Ive seen (built to 17th edn) dont have RCD protection for the tails to consumer unit!

As for yours ExArmy 15m seems long but Im guessing its an 'old' build so a code 3 seems acceptable, perhaps with a recommendation for a bit of further investigation?

 

[Spazz]

Dont mean to steel the thread, but I got a similar situation

Block of flats
60A (BS3036) Isolation at meters
SWA from meters running 30m to a building behind (connected to meter building so no need for TT) (buried 700mm deep)
SWA terminates in the electric cupboard and joins to tails which feeds the flats.
The tails travel up the wall then into the ceiling and up a stud wall to the CU.
tails in stud wall are about 50mm deep (right on the edge of 50mm).

Where should I put the RCD protection?? at the meters or in the electric cupboard

Could I change the 60A isolation for RCDs? then how would I stand with 30mA at meters and then 30mA on the CU in each flat??

 

[malcolmsanford]

Tails are subject to the DNO discretion. As we are using their protection device to cover us for fault protection, the DNO have the final say on how long these tails are to be.. So this is why you will not find anything in the BS 7671-2008 concerning this,

A rule of thumb for us is the "3 metre" length. You may though find tails longer in say a purpose built estate or flats, this is because the designer had consultations with the DNO and it was agreed on by both parties, I know this because I have done this.

As for mechanical protection of the tails, ie if they are buried in the wall. Then yes if they are installed buried then they do need to be buried and protected as to the BS 7671-2008, they are deemed to be inside the installation and the ESQCR-2002 covers reference to our standards with regards to any system within the installation itself.

Often tails are indeed inside the cavity and dispensation within the building regulations allow for this.

Also in the original OP, there is no reference to how old this installation is. Regulation 522.6.101 was introduced into the 17th, so any installation designed prior to this would not require RCD protection to these tails

 

[Badged01]

Nick,

I’ll assume the 60A is correctly sized to protect the SWA then tails upto the flat CCU’s & also that the tails on the wall do not require RCD protection (e.g. mechanically protected).

Depending on your ‘view’ of which side of 50mm the stud wall tails are, if you decide they do need RCD protection then the requirement as I see it is only from the point at which the stud tails join the wall tails. Practically though I would guess that any protection fitted is best done in the electric cupboard (you also protect the wall tails by default at the same time). Most certainly this RCD protection would have to be time delayed to discriminate with the flat CCU RCD’s. I don’t recall anything in the regs stating that RCD protection for cables in walls can’t be time delayed?

I can’t see any reason why you couldn’t fit a time delayed RCD at the 60A isolator either (not instead of), other than the practicality of if it does trip all the flats loose power and the flat occupiers also have to come over to that building to reset! If Im reading it right and you do it in the electric cupboard then each flat has a separated delayed RCD protecting it.

 

[Richard Burns]

Nick,

Depending on your ‘view’ of which side of 50mm the stud wall tails are, if you decide they do need RCD protection then the requirement as I see it is only from the point at which the stud tails join the wall tails. Practically though I would guess that any protection fitted is best done in the electric cupboard (you also protect the wall tails by default at the same time). Most certainly this RCD protection would have to be time delayed to discriminate with the flat CCU RCD’s. I don’t recall anything in the regs stating that RCD protection for cables in walls can’t be time delayed?

I can’t see any reason why you couldn’t fit a time delayed RCD at the 60A isolator either (not instead of), other than the practicality of if it does trip all the flats loose power and the flat occupiers also have to come over to that building to reset! If Im reading it right and you do it in the electric cupboard then each flat has a separated delayed RCD protecting it.

An RCD that is providing protection to cables buried in walls must be a 30mA device that will disconnect in <40ms at 5*I delta n if you have a time delayed RCD then the minimum time is 150ms so it would not comply.

 

[malcolmsanford]

Badge I think that

1. Sourcing a 30mA S type RCD would be a challenge in itself
2. And by using it your going to be breaking the regulations with the tripping times especially in this case

 

[Badged01]

As the tails will still be protected by the 60A switched fuse was thinking of a 100A 30mA RCD but valid point on the 150ms for the time delayed RCD!

 

[tony mc]

I don't think a 30 mA RCD is manufactured .

If you are installing a RCD to protect the tails buried less than 50 mm into a wall then it would have to be a 30 mA RCD and not a time delayed device as that would be against the regs!

See post 2!

 

[Spazz]

Thanks everyone - I also contacted Napit about this and they said it doe's not matter where the RCD is installed.
The only issue is the fact that if the RCD trips it would cause a nuance to the occupier having to reset 2 RCDs instead of just the one.

Im going to install it in the electric cupboard in the same block as the flats so its easy to reset if needed.

I am trying to persuade the landlord (project manager) to put another stud on the back of the wall with the tails on but hes loath to do so as its going to cost him more money - if I have to install the RCD its included in the quote.

 

[tony mc]

Are you installing the tails ?

I take it when you install the RCD at the meter cupboard you are still retaining the OCPD for the tails?

Post 2 !! I did cover what Napit has told you!

Im going to install it in the electric cupboard in the same block as the flats so its easy to reset if needed!


I can see you getting further calls to this premises especially if the first one starts tripping!

 

[somersetsparks]

Badge - Did you actually install these tails? I would certainly not entertain the idea of rcd 'protecting' them, In my view another method has to be found, there is no discrimination between ccts in the fuseboard, one dodgy kettle etc. and grandads falling down the stairs trying to reset it, or phoning the electricity board to report a power cut.

Some people may shun me for this but if you can't see how they are installed, then could you not limitate that?

 

[Badged01]

No the tails were already there, probably been in some 20+ yrs!

- - - Updated - - -

No the tails were already there, probably been in some 20+ yrs!

 

[somersetsparks]

No the tails were already there, probably been in some 20+ yrs!

- - - Updated - - -

No the tails were already there, probably been in some 20+ yrs!

Well I would leave them be then, if you put a 30mA rcd on them you are IMO, worsening the installation

 

[Spazz]

I am installing these tails - the issue I have is that the landlord wont think about double layering the stud wall increasing the gap between the plasterboards to over 150mm. I remeasured it yesterday and the partitions are 75mm plus 7mm for each plasterboard (80mm (ish)) So only 30-40mm to the tails.

The other flats are easier because they are installed behind plasterboard and against a solid wall giving me over 50mm of protection.

The flat in question is on 1 level and one floor up - and covered by emergency lighting and on its own CU so that wont be effected if the RCD trips. I will have to put a note on the CU to check other CU in electric cupboard.

Thanks again.

 

[tony mc]

If you are responsible for the install then IMO I would be looking at ways of protecting the tails and getting away from a single RCD at the front end.

I would imagine a dual RCD 17th edition board is fitted in this flat and if so the RCD's are not going to be effectivein relation to division of installation.

It will be a lottery as to which RCD trips if a fault develops'

Even with emergency lighting in place IMO I would try and sort this issue out before the tails are installed as it could be a nightmare for a owner occupier of the flat in the future and it is a departure from the regs !

 

[Richard Burns]

I am installing these tails - the issue I have is that the landlord wont think about double layering the stud wall increasing the gap between the plasterboards to over 150mm. I remeasured it yesterday and the partitions are 75mm plus 7mm for each plasterboard (80mm (ish)) So only 30-40mm to the tails.

The other flats are easier because they are installed behind plasterboard and against a solid wall giving me over 50mm of protection.

The flat in question is on 1 level and one floor up - and covered by emergency lighting and on its own CU so that wont be effected if the RCD trips. I will have to put a note on the CU to check other CU in electric cupboard.

Thanks again.

As you are installing the tails then you need to use SWA to avoid the requirement for an RCD at the supply, then you can wire as normal thereafter. The cost for 3 core SWA versus 2 x tails plus earth is going to be minimal over one floor.