levels of rcd protection on TT system - long post - prepare cuppa first!

[suffolkspark]

Hi all, need some opinions on the following please>

Origin is a three phase 60A TN-C-S supply.it goes into a changeover switch (small PTO driven generator across the yard) and then into a wylex three phase board. the incomer is 100A 4pole 100mA type S to provide discrimination to other downstream RCBO's. there are several different outgoing circuits from this board so please bear with me:

2x tp+n supplies go to chiller control panels in the same room on 32A MCB's which cool the adjoined cold store. PVC double insulated tails ref method B
1x sp inside lighting circuit on 6A 30mA RCBO PVC singles ref method B
1x sp outside lighting circuit on 6A 30mA RCBO pvc singles ref method B
1x sp single socket on 16A 30mA RCBO pvc singles ref method B
1x tp+n sub main to a control panel in an irrigation shed 20m away containing bore hole pump and various other pumps. - 32A tp mcb SWA ref metho D + additional 16mm earth
1x single phase sub main to site office container - the cpc is separated for this and linked to an earth electrode - 20A 30mA RCBO - T/E ref method C
2x single phase sub mains to camp sites - 30mA rcd protected boards on the end of these and again separated from the TN-C-S as with the site office and linked onto earth electrode. (there are also earth electrodes at the camp site end) 40A mcb SWA ref method D + additional 6mm earth
1x tp+n sub main to a dis board in an adjacent sorting/packaging warehouse (steel construction + asbestos cladding - structural steelwork is bonded) - within this is another wylex board with 30ma protection via rcbos to everything except a three phase 16A motor running a bin lift and 2x plugged in single phase conveyors linked into the same system though an emergency stop button/contactor in a clearly labelled socket. 32A mcb, 5 core YY cable

As it stands 30mA protection is installed on all single phase circuits (except the camp site sub mains). All sub mains + the irrigation panel, chiller panels, and conveyors only have the 100mA type S to back them up but all Zs values are well within what is allowed, bonding to structural steelwork is good, as is the earth electrode.

Now the spanner in the works is this - its an apple farm, obviously unless you count bees there's no livestock though. BS7671 clearly prohibits the use of TN-C-S in agricultural and horticultural installations. The camp site and office have correctly been separated from the TN-C-S side and are well labelled, infect everything is well labelled. My issue is its probably classed as a farm as it farms apples? but the electricity supplies aren't typically farmy, ie a sorting and packing warehouse and a cold store. with only a forklift running between the 2 buildings. areas with tractors etc actually don't even have power to them.
Should i be recommending the installation be put on TT in its entirety because of reg 705.411.4?

I know the 100mA type S is acceptable for the distribution circuits and it also ensures continuity of service, and discrimination from downstream rcbo's... 705.422.7. but the final circuits, ie the 2x chiller panels, conveyor setup, and irrigation panel require at least 300mA protection for protection against electric shock, does the 100mA type S comply with this? 705.411.1 (iii) I cant find in the regs a yes or a no? The 16A tp+n circuit doing the conveyors does contain a double socket outlet but it is clearly labelled it is just for use of the single phase conveyors, these are wired in SY. an adjacent 30mA protected double socket is labelled general use.

Ra was 17ohms for the earth electrode 0.2 Ze on the TN-C-S side

What are your thoughts please chaps

 

[westward10]

A TN-C system uses a neutral which acts as a neutral and cpc so you only have a line and neutral, at the point of connection say to a socket you link the earth terminal to the neutral. I have personally never come across this. GN7 gives further advice for TN-C-S systems in certain areas of an agricultural installation.

 

[Ian1981]

The TNC part is the DNO supply cable on a tn-c-s system as the neutral is also the means of earthing.
TNC in a consumers installation is prohibited.

 

[westward10]

It isn't unheard of I believe it was quite common in Southern Ireland.

 

[Ian1981]

I've never come across one and I doubt I ever will in this country.

 

[Wilko]

The TNC part is the DNO supply cable on a tn-c-s system as the neutral is also the means of earthing.
TNC in a consumers installation is prohibited.

This is my thought too - so why do they single it out in the special locations to say don't use it ? Are they really saying all agric supplies from DNO must be TT or TNS ?

 

[Ian1981]

This is my thought too - so why do they single it out in the special locations to say don't use it ? Are they really saying all agric supplies from DNO must be TT or TNS ?

They are principally concerned with a broken pen conductor resulting in bonded and exposed conductive parts rising to mains potential
Bad news if your a horse.

 

[Ian1981]

I had an installation the other week on a warehouse unit and the earth into the unit supplied by the DNO was non existent.
DNO came out dug up the ground and found that when the new plastic service water pipe had been installed they had cut through the separate earth to the building.
Never informed anyone and just taped the ends of the cable and left it.
DNO repaired but refused a tn-c-s installation into the unit as it had exposed structural steel all over the place.
Installation was made into a tn-s.
Reasons where if the pen was broken and raised the steel to mains potential

 

[westward10]

This is my thought too - so why do they single it out in the special locations to say don't use it ? Are they really saying all agric supplies from DNO must be TT or TNS ?

It could be a historic reason. TN-C-S is not prohibited but can be refused on the grounds of the difficulty of bonding all extraneous-conductive-parts it has nothing to do with a lost supply neutral, the DNO would not consider this. Each site is unique and the risks would have to be assessed especially the presence of livestock where I would suggest a TT system is definitely employed.

 

[Ian1981]

It could be a historic reason. TN-C-S is not prohibited but can be refused on the grounds of the difficulty of bonding all extraneous-conductive-parts it has nothing to do with a lost supply neutral, the DNO would not consider this. Each site is unique and the risks would have to be assessed especially the presence of livestock where I would suggest a TT system is definitely employed.

Just one example Caravan installations are prohibited from being tn-cs for especially the reason of a broken pen conductor.
Also the reason I was refused a tn-cs in the warehouse

 

[westward10]

Yes you are quite correct but in agricultural it is the difficulties of bonding extraneous-conductive-parts.

 

[Wilko]

Thanks for the feedback Guys and to OP for tolerating my hijack.
There are animals on site, so I will crack on with a TT installation

 

[suffolkspark]

There is exposed structural steel in the packing shed, doors are metal too, can easily get a 10mm earth to it if its acceptable to be on pme this is all that is required accross the site, or do i TT the lot. If the 100ms upfront does satisfy that reg 705.411.1 (iii) and discon times arent relevant then thats doable too. Why is this bloody regs book so open to interpretation!!!

 

[Ian1981]

Yes you are quite correct but in agricultural it is the difficulties of bonding extraneous-conductive-parts.

There is exposed structural steel in the packing shed, doors are metal too, can easily get a 10mm earth to it if its acceptable to be on pme this is all that is required accross the site, or do i TT the lot. If the 100ms upfront does satisfy that reg 705.411.1 (iii) and discon times arent relevant then thats doable too. Why is this bloody regs book so open to interpretation!!!

i find the guidance notes are the most helpful when trying to decipher bs 7671.
The guidance notes do explain in more detail as bs 7671 is just a set of regulations and is not meant to go into details I feel

 

[westward10]

There is exposed structural steel in the packing shed, doors are metal too, can easily get a 10mm earth to it if its acceptable to be on pme this is all that is required accross the site, or do i TT the lot. If the 100ms upfront does satisfy that reg 705.411.1 (iii) and discon times arent relevant then thats doable too. Why is this bloody regs book so open to interpretation!!!

If you can get a 10.0 cpc across that would be my choice.

 

[suffolkspark]

I can, i can infact more easily fit an inline rcd to protect the conveyors and bin lift, isolate the cpc at the end of the YY cable, and use the structural steel of the warehouse as my earth thus having that as TT, would save crawling through a long loft space!