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

 

[Leesparkykent]

The RCBO's are they single module?

 

[suffolkspark]

Yes

 

[Leesparkykent]

Yes

If they are single module they more than likely will be solid neutral so wont be no good for discrimination with the upfront RCD.

 

[suffolkspark]

In the event of single circuit faults the rcbo will still operate before it the type S will it not? there is propper discrimination for the camp site supplies anyway, what are your thoughts on dropping it to TT with regards to the rcd protection afforded to the items mentioned?

 

[westward10]

Your first post is far too long to take in can you post what you actually want to know.

 

[suffolkspark]

in a nutshell:
site is likely classed as agricultural?
should it be taken off TN-C-S?
if so is the 100mA type S adequate to protect control panels and conveyor motors?

sorry I do drool on sometimes just wanted to get all the information in there somewhere

 

[westward10]

An orchard would be deemed agricultural by definition. I am still totally confused, are the packing warehouse and cold store currently TT. Are these buildings at the orchard site or remote.

 

[suffolkspark]

Complete site with the exception of the camp site and office container is tncs, the orchards wrap around the buildings to be fair

 

[westward10]

Are you doing an EICR or carrying out works there.

 

[suffolkspark]

An EICR. i know the owner so im sure they would want it all putting right, its just thinking how i can go about it

 

[westward10]

From what you describe it sounds like the installation should be TT or perhaps was TT but this is no longer the case. The submains supplying other buildings which require main protective bonding would require a cpc adequate to support PME bonding and for a swa cable it would have to be a 50.0, 4 core minimum to support 10.0 bonding. As to whether the entire site should be TT and there is a suspicion it may originally have been is not possible to say from a distance. Do you have a copy of GN7 to refer to.

 

[suffolkspark]

It probably once was as the boards dont look very old so it has probably been faffed with, Would the 32A control panels be required to just have 1sec disconnection time on TT?as it would be simple enough to get 30mA protection to the conveyors circuit, but if not i guess id need to fit a30mA rcd within each control panel.

 

[westward10]

You need to complete the EICR and consider remedial ramifications after however I feel you are undertaking a task you may not be competent to do.

 

[suffolkspark]

forgot you cant ask questions on here without being deemed incompetent, ciao

 

[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