Farm Mains Upgrade

[westie101]

Just a thought, is the farm the only customer fed from a transformer, is there an earth rod connected to the Neutral at the cut-out.
It could be PNB which is a different animal (pun intended) to PME

 

[JohnW2]

I sense confusion. T-NS (Corrected to TN-S from TN-C after RF Lighting pointed out my error) is the type of earthing provided by the DNO where the Neutral and Earth are separate (sort of) all the way back to the substation and the N is only connected to earth art source (the substation) ... There are millions of customers (a lot of them farms) supplied by this way and there is no restriction on using it in any premises

Indeed, but neither us, not Chapter 75 of BGB are talking about that.

T-N-CS is where the DNO combines the neutral and the earth as one conductor and also earths this combined conductor at multiple points,....

You are surely describing PME. Although TN-C-S without PME is (AFAIAA) not allowed in UK, in concept TN-C-S can exist without PME, can't it?

Kind Regards, John

 

[westie101]

in concept TN-C-S can exist without PME, can't it?

In theory yes

 

[JohnW2]

Ah that one. It actually says: -

(4) A consumer shall not combine the neutral and protective functions in a single conductor in his consumer’s installation.

Well, yes, but since the consumer is not allowed to have those functions combined within a single conductor, if the supply were provided as a 'two wire' one (presumably with PME), the DNO would 'split off' an earth connection from the N at the origin of the installation, in which case it would be a TN-C-S supply, wouldn't it?

Kind Regards, John

 

[flyingsparks]

I sense confusion.

T-NC is the type of earthing provided by the DNO where the Neutral and Earth are separate (sort of) all the way back to the substation and the N is only connected to earth art source (the substation)
There are millions of customers (a lot of them farms) supplied by this way and there is no restriction on using it in any premises

I sense confusion too. That description sounds very much like TN-S to me

That had me confused too!! Looks like westie made a boob up!!

 

[JohnW2]

After all, if it just said "TN", you would presumably take that to encompass both TN-S and TN-C-S - so does not "TN-C" also encompass TN-C-S?

That was my immediate thinking too, but perhaps it is only TN C installs, which as you say John, would be a bit strange considering BS7671 doesn't allow TN C within the consumers setup.

Quite. I still think that the BGB's "TN-C" was meant to include (in fact, virtually always be) TN-C-S.

Kind FRegards, John

 

[westie101]

Well, yes, but since the consumer is not allowed to have those functions combined within a single conductor, if the supply were provided as a 'two wire' one (presumably with PME), the DNO would 'split off' an earth connection from the N at the origin of the installation, in which case it would be a TN-C-S supply, wouldn't it?

Correct and the customer cannot re-combine them. Generally it doesn't cause any issues until street lighting supplies get involved.
A few long stories there!

And I will not tell of a timeshare near here

 

[JohnW2]

Coming back to the RCD issue, I believe the regs still want an RCD on all circuits, including submains of at least 300mA. That is why I specified the 300mA TD RCD's at the origin. - 705.411.1.(iii) Do you agree?

As I said right at the start, if it's all going to be TT'd, I don't see that you have any alternative but to have 'up-front' RCD protection, to provide fault protection to the submains/distribution circuits - just as I have to do in my house.

Since it's single-phase, I suppose you could have just one 'up-front' TD RCD covering all the submains (assuming you could find one with adequate rating), but you probably wouldn't want one device to be able to take everything out.

In passing, you never responded to my initial comment/question about the use of MCBs, rather than fuses, to protect the submains.

Kind Regards, John

 

[JohnW2]

Well, yes, but since the consumer is not allowed to have those functions combined within a single conductor, if the supply were provided as a 'two wire' one (presumably with PME), the DNO would 'split off' an earth connection from the N at the origin of the installation, in which case it would be a TN-C-S supply, wouldn't it?

Correct and the customer cannot re-combine them.

Indeed, but I don't think that Chapter 75 of the BGB (agricultural etc.) is concerned about the possibilities of customers re-combining them (which they would do). Rather, I believe that the have the 'standard concern' about TN-C-S supplies - namely that a supply side fault can theoretically result in high pds between the installation's 'earthing terminal' (hence COCs and exposed-c-ps) and true earth.

Kind Regards, John

 

[JohnW2]

Yes and no! Again it depends where you feel the farm installation starts and stops. ... The 300mA RCD is there to prevent fire, particually due to rodent damage of wiring.

I don't think this has got anything specifically to do with farms - as I said, I have the same issue in my house. 411.3.2.4 requires, in a TT installation, a disconnection time not exceeding 1 second for a distribution circuit (submain). With TT you almost certainly won't get any disconnection at all in response to a fault, let alone within 1 second, from an OPD - so, AFAICS, one has no choice but to use an up-front RCD. Are you suggesting that there is a way around that regulation?

Kind Regards, John

 

[westie101]

'standard concern' about TN-C-S supplies - namely that a supply side fault can theoretically result in high pds between the installation's 'earthing terminal' (hence COCs and exposed-c-ps) and true earth.

Which can all be controlled by correctly thought out design which is why TN-C-S is not banned from agricultural buildings. But as I say very difficult to do where it is not a new build.

 

[flyingsparks]

Just a thought, is the farm the only customer fed from a transformer, is there an earth rod connected to the Neutral at the cut-out.
It could be PNB which is a different animal (pun intended) to PME

There is an earth rod connected to the MET, however I think the transformer also supplies another property next door. Not going there for a few days so cant check this.

 

[flyingsparks]

In passing, you never responded to my initial comment/question about the use of MCBs, rather than fuses, to protect the submains.

I know fuses would be better, but where all this is located it needs to be IP rated as the rain could possibly blow in. I was going to use an IP Dist board to house it all.

 

[flyingsparks]

Thanks for all your comments, a very interesting thread with lots of different interpretations of the regs! I think I will have another think about this one on the best way to do it, as like you say (RF I think!) it seems daft to TT the house too. Think I will keep that PME, do away with the 100mA TD and do it that way. Thanks again!

 

[westie101]

however I think the transformer also supplies another property next door.

If that is the case it can't be PNB