Farm Mains Upgrade

[flyingsparks]

Got to price for a mains upgrade on a farm.
Currently there is a PME earth connection at the origin which is sited in a small stone building by the road. In here is a switch fuse to supply the house, one to supply the dairy, one for the farm buildings and one for the workshop.

Single Phase supply.
My plan is to fit a new dist board, with DP Main switch. Then a 300mA TD RCD for the farm buildings supply via a 63A MCB, another 300mA TD RCD for the dairy via another 80A MCB, a 30mA RCD and 50A MCB to the workshop and a 100mA TD RCD and 63A MCB for the house.

May seem overkill on RCD's especially considering the price, but thought that way nuiscence tripping would be kept to a minimum.

Was thinking to divorce the PME earth immediatly at the origin, and TT it here. Some of the steel frame buildings are already TT earth at the minute.

Is DP MCB's required considering it will be TT even though there is a DP RCD up front? Thoughts on this setup appreciated.
Thanks!

 

[JohnW2]

Is DP MCB's required considering it will be TT even though there is a DP RCD up front? Thoughts on this setup appreciated.

Are you referring to the 'distribution' MCBs mentioned in your post? If so, I can personally see no reason, or advantage, in their being DP. As for downstream MCBs protecting final circuits, I suppose you might want a means of DP isolating a final circuit which developed an N-E fault, so that you could use the other final circuits without taking out the upstream TD RCD - and you might even feel that it would be nice to have a means of DP isolation of individual final circuits reasonably close to those circuits. However, unless there are regs specifically relating to farms of which I'm unaware, I don't think there is a requirement for any OPDs to be DP (let's face it, if they were fuses, they wouldn't be allowed to be 'DP'), even though it's TT. Were you thinking of some other reason for needing DP ones?

Talking of fuses (and as an aid to my education!), I'm slightly surprised that you are contemplating MCBs, rather than fuses, for these distribution circuits - wouldn't fuses be more likely to give you discrimination from whatever was downstream?

Kind Regards, John

 

[ban-all-sheds]

What's the purpose of all those 100 & 300mA TD RCDs?

 

[JohnW2]

What's the purpose of all those 100 & 300mA TD RCDs?

I would imagine that one purpose would be to provide adequate fault protection (ADS) to the distribution cables (I have the same problem within my house, which is why I have to have up-front TD RCDs, even though all final circuits are RCD protected). Similarly (if there are any such circuits) to provide fault protection to any final circuits that do not have other RCD protection.

Kind Regards, John

 

[flyingsparks]

What's the purpose of all those 100 & 300mA TD RCDs?

705.411.4 TNCS can't be used so will be TT.

And

705.411.1 (iii)

 

[RF Lighting]

Personally I would not TT the entire supply. The only place which really needs to be TT'd (along with some other earthing requirements) is the dairy.

You could do the outbuildings if you like, but as long as they are just barns rather than cattle sheds they'll be fine with a PME earth, and the workshop and house would be best off with the suppliers earth and good old fashioned large fault currents. You may still need local 30mA RCD(s) for shock protection though.

If you use the PME to supply the submains to the dairy, but isolate and insulate it before it terminates onto the dairy installation, then you can keep the RCD protection local to the dairy, and even divide the dairy up to avoid a single fault wiping out the whole installation.

 

[JohnW2]

Personally I would not TT the entire supply. The only place which really needs to be TT'd (along with some other earthing requirements) is the dairy. ... You could do the outbuildings if you like, but as long as they are just barns rather than cattle sheds they'll be fine with a PME earth, and the workshop and house would be best off with the suppliers earth and good old fashioned large fault currents.

I'm still in learning mode (not a farm person!)! Taken at face value, 705.411.4 appears to fairly generally prohibit TN-C-S in all 'agricultural and horticultural premises' (the definition of which in Part 2 would appear to encompass barns and all sorts of things) - so how do you reach the conclusion above?

Kind Regards, John

 

[RF Lighting]

I suppose it boils down to how you interpret the regs, and what you class as the agricultural premisis! I work on the premis that the agricultural installation is those directly involved in farming, and where livestock will or may be present.

The prohibition on TN-C-S earhting is there to protect livestock from the risks associated with the possible PD between suppliers earth and true earth.

As long as the barns are just barns (for the storage of hay, machinery etc) and not cattle sheds, as I mentioned earlier, then there is no need, or logic in treating them differently from the shed in your back garden, which cattle will not have access to either!

Certainly the house is a house regardless of whether it's on the farm land or not, and no special precautions are required just because it's on a farm.

 

[flyingsparks]

Certainly the house is a house regardless of whether it's on the farm land or not, and no special precautions are required just because it's on a farm.

Sorry have to disagree there, according to the regs..
Read 705.411.4


Although I do agree with you, don't see why the house should be any different to any other house.

 

[RF Lighting]

That is for TN-C installations, not TN-C-S

 

[westie101]

Strictly speaking it is possible to technically use T-N-C-S in and around farm animals, but can only be done at the building stage by incorporating steel mesh around the area and then bonding it.

Practically it's pretty much a no, no!

 

[JohnW2]

I suppose it boils down to how you interpret the regs, and what you class as the agricultural premisis! I work on the premis that the agricultural installation is those directly involved in farming, and where livestock will or may be present. ... The prohibition on TN-C-S earhting is there to protect livestock from the risks associated with the possible PD between suppliers earth and true earth. .... As long as the barns are just barns (for the storage of hay, machinery etc) and not cattle sheds, as I mentioned earlier, then there is no need, or logic in treating them differently from the shed in your back garden, which cattle will not have access to either!

That is logic/common sense, and I totally agree with that, but I was talking about the regulations. The reg prohibiting TN-C-S, without qualification, is in a chapter, the scope of which (705.1) is said to be "agricultural and horticultural premises", which, in turn, is defined in Part 2 as:

Rooms, locations or areas where:
- livestock are kept, OR
- feed, fertilizers and animal products are produced, stored, prepared or processed, OR
- plants are grown, such as greenhouses

The second two of those categories obvioulsy encompasss many of the things which you feel (on the basis of common sense) would be OK with TN-C-S, since no livestock are (usually!) involved.

Certainly the house is a house regardless of whether it's on the farm land or not, and no special precautions are required just because it's on a farm.

701.1 certainly excludes "Rooms, locations and areas for household applications and similar" from the whole of 705. However, 705.441.1 specifically states that the TN-C-S prohibition applies to "residences and other locations belonging to agricultural or horticultural premises according to the definition of 'Residences and other .... premises' in Part 2", and that definition relates to the connection of electrical installations of those places via CPCs or extraneous-c-ps to the 'agricultural and horticultural premises'. I don't fully understand how/when that could apply, since it would not be possible to have a TN-C-S supply to, say, a farmhouse, which was connected via a CPC to some agricultural premises, without that agricultural premises itself also have a (seemingly forbidden) TN-C-S supply!

As so often, I suppose it may come down to a balance between common sense and regulations! I suppose that the thinking of the regs might be that, even if designed and intended as, say, a barn for storing hay, if there is livestock around, it's hard to guarantee that none will wander into the barn - so there might be some common sense in the apparent breadth of their prohibition of TN-C-S.

Kind Regards, John

 

[JohnW2]

That is for TN-C installations, not TN-C-S

I forgot to mention that. Yes, it says "TN-C", but I presume it actually means TN-C-S, since an installation with a PEN/CNE throughout would not be permitted, and presumably, in practice, does not exist. One (unless 'one' was a DNO) presumably would not even be allowed to run TN-C (i.e. a PEN) in the distribution circuit to the outbuilding.

KInd Regards, John

 

[JohnW2]

Certainly the house is a house regardless of whether it's on the farm land or not, and no special precautions are required just because it's on a farm.

Sorry have to disagree there, according to the regs.. Read 705.411.4 ... Although I do agree with you, don't see why the house should be any different to any other house.

Yes, but as I've just written, if you read the definitions that only seems to apply if the 'house' in connected to an agricultural premises via a CPC or extranous-c-p. ... and then there is the last sentence/paragraph of 750.1, which appears to exclude 'areas for household applications and similar' from the entirety of Chapter 750!

... so, as so often, it's more-or-less as clear as mud

Kind Regards, John

 

[Spark123]

The BGB says TN-C, although the regs seem to allow a TN-C system in other cases the ESQCR doesn't allow it without permission from the secretary of state.
That isn't to say another country that uses BS7671 can't use it, that just depends on their local laws.