RCD Main Switch?

[RF Lighting]

I Am working on a new build house located on a farm. The REC supply is I beleive to be a TT supply converted to TN-C-S. (the supply is a pole fed supply with an earth spike at the base of the pole but a PME terminal is provided at the cutout) The supply to the new house will be provided by cutting the tails and putting them into a 100A D.P. Henly block, then taking new tails to 1way metal clad consumer unit (with 100A D.P. Main Switch and a 63A MCB). This will supply a 35mm2 3 core SWA cable. The SWA is then connected to the new Consumer unit in the house. Does the main switch at the intake position need to be an RCD main switch? or can I just fit an RCD main switch in the new CCU in the house to save a trek to the intake position? or can I just fit a standard split load CCU? The existing installation for the farm is protected by an RCD main switch but I don't know if this is purely because it is a farm supply or if it is because supply type. Unfortunatly I don't know what the external Earth Fault Loop Impedence value is.

Here is a picture of the cutout:

(the earth is only 10mm2 but I will be upgrading to 16mm2 when I make the alterations)

 

[Spark123]

On a TN-CS sytem an RCD is required to protect all sockets reasonably expected to supply equipment outdoors and is also advisory for the shower. There are also additional requirements for agricultural premises (special locations), although I don't believe these to apply to the actual farmhouses but I may be wrong.
Is there no way of getting the DNO to run a new supply to the other building as 60A doesn't seem very big to be running two buildings, or am I reading this wrong?

 

[RF Lighting]

I think the cost of an extra supply might be an issue, also I am not sure how big the pole mounted transformer is. The 60A supply is not massive but the farm supply is only running a couple of fluorescents and a couple of sockets. Looking at the incommer it looks to me like a 16mm2 consentric cable so the REC may be able to upgrade the cutout without the need to replace the supply cable (I will make enquiries about this). I am unsure as to wether the house is classed as part of a special location installation with it being connected to the same supply or wether it is just a domestic dwelling fed from a special location.

 

[Spark123]

Just checked the regs, a dwelling solely for human habitation is excempt from section 605.

 

[RF Lighting]

Is an RCD main switch only required if the external ELFI is above a certain value or should it be fitted to all TT systems regardless of ELFI value? also is my earthing arrangement classed as TN-C-S even though there is an earth spike at the other end of the supply cable?

 

[Spark123]

Is an RCD main switch only required if the external ELFI is above a certain value or should it be fitted to all TT systems regardless of ELFI value? also is my earthing arrangement classed as TN-C-S even though there is an earth spike at the other end of the supply cable?

If your supply is TN-CS the max efli at the incomer should be 0.35ohms. The fact the DNO has put an earth spike in is part of the distribution PME system isn't anything to be woried about.
The only place and RCD "must" be used in a household TT system is to protect all socket outlets to the 50v rule, and be 30mA if expected to supply equipment outdoors.
For a TT with a high external efli (as opposed to TN) which it will be in most instances, an RCD is used to protect the installation to the 50v rule where a fuse can't be relied upon to disconnect quickly enough. The 50v rule becomes 25v rule in some special locations.

 

[RF Lighting]

Thanks for that.
Split load it is then

 

[Spark123]

On TT systems it is advisable to use a split load RCD board with a 100mA (time delayed) main,
which would be used for lights etc and a 30mA for sockets etc, but since it looks like you have
a TN system a normal Isolator and 30mA split load will be ok.

 

[RF Lighting]

Thanks again. I have not really had much experience on this sort
of installation so it's helpful to get some pointers.

(this is what happens when you do a favour for a friend)