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]

Disconnection times have been replaced by rcd requirements in Section 705 for ADS.

 

[suffolkspark]

thanks, I did touch on that in my first loooooooong post
with reference to reg 705.411.1
(i) all circuits supplying socket outlets are indeed covered by 30mA RCD.
(iii) - which requires all other circuits are protected by a 300mA RCD, well they are covered by the 100mA type S, but is this acceptable? I genuinely do not know the answer to that question. - reg 705.422.7 says S type can be used but it does not say if that s also acceptable for ADS

 

[Ian1981]

Your 100mA time delay tripping times are 500ms max and 130ms min.
If it's protecting circuits with a 0.2 second disconnect time this maybe problematic depending on what the s type trips at.
with a 300mA rcd the max tripping time is 300ms and when testing the rcd needs to trip really below 200ms for a 0.2 disconnect time for circuits upto 32 amp.

 

[westward10]

The 100ma s type will provide a better level of fault protection. Have you decided if it is agricultural.

 

[suffolkspark]

its, as you say agricultural by definition, and that does include processing facilities according to the definitions in the BYB.

the type S went out in 176ms but this is with it having 0.2 ze as it is at present, I cannot confirm how quickly it will operate if it is on TT.

The thing getting me in a tangle now is the discon times, in the agricultural section there is no mention of the 0.2/1sec times only the level of rcd protection, as post #16

 

[Ian1981]

its, as you say agricultural by definition, and that does include processing facilities according to the definitions in the BYB.

the type S went out in 176ms but this is with it having 0.2 ze as it is at present, I cannot confirm how quickly it will operate if it is on TT.

The thing getting me in a tangle now is the discon times, in the agricultural section there is no mention of the 0.2/1sec times only the level of rcd protection, as post #16

If it's a TT installation then the 0.2/1 second disconnect times will apply for the final circuits

 

[westward10]

its, as you say agricultural by definition, and that does include processing facilities according to the definitions in the BYB.

the type S went out in 176ms but this is with it having 0.2 ze as it is at present, I cannot confirm how quickly it will operate if it is on TT.

The thing getting me in a tangle now is the discon times, in the agricultural section there is no mention of the 0.2/1sec times only the level of rcd protection, as post #16

Disconnection times were removed from Section 705. You are concerning yourself with a situation that does not exist because it is TN-C-S. Concentrate on what you have now for the EICR.

 

[suffolkspark]

its gained an unsatisfactory report because its in breech of reg. 705.411.4
so to remedy its going to have to go onto TT to start with.
are you saying 0.2/1sec don't apply to farms when on RCD ? that feels wrong?

 

[suffolkspark]

although the way I'm reading it that's what 705.411.1 is implying? and if so the 100mA is more sensitive than a 300mA and there is no mention of time, so it should comply as is if put on TT?

 

[westward10]

its gained an unsatisfactory report because its in breech of reg. 705.411.4
so to remedy its going to have to go onto TT to start with.
are you saying 0.2/1sec don't apply to farms when on RCD ? that feels wrong?

705.411.4 states a TN-C system shall not be used it does not say TN-C-S.

 

[suffolkspark]

oh bugger me ive been reading it as tncs all this time! I should put the regs book down after 10pm!

with reference however to an earlier reply of yours, the adjacent asbestos cladded steel building - the structural steel here is bonded to its local board in 10mm, but this only has 6mm YY going back to the DB at origin.

 

[suffolkspark]

the irrigation shed with bore hole however does take a 16mm earth with its SWA,

 

[suffolkspark]

but thoughts on pme out in a standalone shed with bore hole pumps at true earth potential

 

[westward10]

oh bugger me ive been reading it as tncs all this time! I should put the regs book down after 10pm!

with reference however to an earlier reply of yours, the adjacent asbestos cladded steel building - the structural steel here is bonded to its local board in 10mm, but this only has 6mm YY going back to the DB at origin.

Easy done your brain recognises shapes and makes you think TN-C-S because you rarely encounter TN-C. So the 6.0 cpc of the cable to the steel building is inadequate for PME bonding purposes and I would personally Code 2 this but that is up to you. I personally wouldn't see a problem with with the bore hole pumps.

 

[Wilko]

Easy done your brain recognises shapes and makes you think TN-C-S because you rarely encounter TN-C...

I had assumed that since TNC was out, that adding an S to the end didn't matter one way or tother (?).
So what does this reg 705.411.4 mean then? We would always have a separate CPC on circuits and submains within an installation and so it can never be TNC anyway (?).
I'd appreciate any discussion here cause as luck would have it I'm about to do an agricultural barn next week that would be readily PME by DNO (I think ) but I was about to not ask them and do one of my TT specials. Thanks ...