Detached garage and regs

[Jimmy Boy]

I hope this is in the right section.

I am hoping someone can refer to the correct section in 7671.

Incoming TNCS/PME

Brother in laws house 80's, set in CU with type 1 breakers no RCD protection at all.
( No funds for new board )

Detached garage T&E comes out of board to outside in to an adaptable box and continues into garage in SWA. 2 Way rewire-able fuses Looks like SWA is only 2 core but difficult to see

As per the old regs ( I think ) they have run no CPC from the dwelling instead earthing is provided by rod. IE a TT system

Additional protection is provided by a 30 mA oldd rccb, RCD tested trips on both 0/180 well within limits.

Measured Ze in garage is high at 68 Ohms but I believe this falls within the parameters for TT.

I want to submit this a board change for Part P assessment, I think and this is where I need the help guys that I should be looking at running a 16mm csa main earth back to the board or a met block in the outside meter box ( this would be easier as the main Earth from the PME disappears into the cavity ) as I am paying for the project I was planning on a 4 way board + MS with an RCBO for lights/sockets/pond/spare.

I do have all the readings ie Ze/Zs and pfc's for both buildings if this in anyway makes any difference to the answer given

As always thankful for your advice guys !

ATB Steve

 

[Adam W]

Running a 16mm earth from where to the MET?
The garage has it's own earthing supply - the TT you've converted it to.

 

[Sintra]

I hope this is in the right section.

I am hoping someone can refer to the correct section in 7671.

Incoming TNCS/PME

Brother in laws house 80's, set in CU with type 1 breakers no RCD protection at all.
( No funds for new board )

Detached garage T&E comes out of board to outside in to an adaptable box and continues into garage in SWA. 2 Way rewire-able fuses Looks like SWA is only 2 core but difficult to see

As per the old regs ( I think ) they have run no CPC from the dwelling instead earthing is provided by rod. IE a TT system

Not as the old regs perfectly acceptable to have outbuildings on their own TT.

Additional protection is provided by a 30 mA oldd rccb, RCD tested trips on both 0/180 well within limits.

Measured Ze in garage is high at 68 Ohms but I believe this falls within the parameters for TT.

Well within the parameters of 200 ohms.

I want to submit this a board change for Part P assessment, I think and this is where I need the help guys that I should be looking at running a 16mm csa main earth back to the board or a met block in the outside meter box ( this would be easier as the main Earth from the PME disappears into the cavity )

No need to run cpc back to house. Use main earth from rod and install a MET beside garage cu.

as I am paying for the project I was planning on a 4 way board + MS with an RCBO for lights/sockets/pond/spare.

Sounds fine as long as its a non metallic cu and all circuits are protected with RCBO's


I do have all the readings ie Ze/Zs and pfc's for both buildings if this in anyway makes any difference to the answer given

As always thankful for your advice guys !

ATB Steve

Hope this helps.

 

[calv68]

why dont you spend the money on a new db for his house and make that safe first, it will be cheaper than what your going to do in his shed and be better for your assesment.

 

[Jimmy Boy]

NovusSparkus I never converted owt this as was.

I am sure that practise had changed and CPC from dwelling was now considered good practice to meet disconnection times.

thanks for the great reply Sintra

 

[Jimmy Boy]

why dont you spend the money on a new db for his house and make that safe first, it will be cheaper than what your going to do in his shed and be better for your assesment.

Read my OP he can't afford it.

But I will try to talk him round

 

[Guest123]

Nowt wrong with the outbuilding being converted to a TT system. A lot of people will do this on a TN-C-S system as there may be metallic parts in the garage that would otherwise require additional protective bonding back to the main CU in the house(depending on sub-main size), water service, gas service steel framework etc.

As your results are within limits then I can see no reason for the suggested additional earth as long as the sub-main is correctly protected as the origin it's fine to leave as is.

 

[Jimmy Boy]

Great thanks guys if any one knows where this is detailed in 7671 tho I would be grateful.

ATB Steve

 

[Sintra]

Where a number of installations have seperate earthing arrangements any protective conductors common to any of these installations shall either be capable of carrying the maximum fault current likely to flow or be earthed within one installation only and insulated from the earthing arrangements of any other installation.
In the second instance if the protective conductor forms part of a cable the protective conductor shall only be earthed at the supply end. Reg 542.1.8 p 126.

 

[ezzzekiel]

damn hes good

 

[Jimmy Boy]

Yep he's good !

Thanks for input Sintra but I am having trouble getting this in context to a house/detached garage senario can you try to explain for me ? That reg says ' a number of installations etc..' it doesn't seem to say yo must do xyz on a detached building fed from a common supply etc, perhaps it's an interpretation rather than Black and white answer..on the other hand I could just be having a Blonde day..

ATB Steve

 

[ezzzekiel]

most of it is a interpretation and the rest a contradiction

 

[Des 56]

You have a seperate installation in the garage
It is supplied via a 2-core swa,the armouring of the swa requires connection at the supply end

It is then terminated into an insulated board at the garage end
The seperate installation at the garage end requires an MET to bond any services and the rod earth

Both systems are then seperate installations,although the swa feed to the garage is part of the house installation and its protective conductor connected at that end only

The only issue you then need to address., is the protection and load carrying capacity of the swa feeding the seperate garage installation

 

[Sintra]

The reason for detatching the earthing system from the house is as Lenny has posted to do with other services entering the detatched building. If you have another service eg water then this is an extraneous conductive part and needs a main equipotential bond back to the MET.
If the supply is TNC-S and you feed the detatched building with a 3 core 4mm cable then you need to install a seperate 10mm bonding conductor all the way back to the MET at the supply.
If you detatch the earthing system to the garage and install an earth electrode making it its own installation so to speak then your main bonding conductor only has to be taken back to the MET for this installation.

 

[Jimmy Boy]

I get the jist guys thanks very grateful for advice.

but given the existing earth rod, and given the close proximity of the garage, around 2 Mtrs to PME incomer, and given the better Ze/Zs for a swifter disconnect would I be better putting a 10 or 16 mm back to the main board, I reckon this would take me an hour,no other services in garage