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

 

[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 ...

 

[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!

 

[suffolkspark]

bottom of page 24........ pme not recommended unless a metal grid is in the floor! ffs I think I'm going to price to TT the bitch and be done with it.
Can you chaps just answer me the question of whether I still need to meet my 0.2 sec discon times for the 32A tp panels? as its not in section 705 - just says rcd ratings.
installing 30mA protection on each one would at least reduce the likelihood of the incoming type S tripping - if ever, as it stands now a decent fault on any of the pumps has the ability to trip the entire site.

ps thanks everybody for their input, including the hijacker! all aids the cause

 

[westward10]

Ìt isn't a cattle shed I would leave it as TN-C-S.

 

[Ian1981]

I

bottom of page 24........ pme not recommended unless a metal grid is in the floor! ffs I think I'm going to price to TT the bitch and be done with it.
Can you chaps just answer me the question of whether I still need to meet my 0.2 sec discon times for the 32A tp panels? as its not in section 705 - just says rcd ratings.
installing 30mA protection on each one would at least reduce the likelihood of the incoming type S tripping - if ever, as it stands now a decent fault on any of the pumps has the ability to trip the entire site.

ps thanks everybody for their input, including the hijacker! all aids the cause

It may not be in section 705 but disconnection times are disconnection times.
For TN you have 0.4/5 seconds for 230/400 v
For TT it's 0.2/1 seconds.
You can use the TN disconnect times for TT if you can get your zs values that low but I've never seen it done.

 

[suffolkspark]

might be a herd of funny looking creatures working in there though
Looks like misreading the TN-C not being permitted bit in the regs has got me in a mucking fuddle over almost nothing then! undersized earth to the warehouse will be getting C2, and I'm still going to C3 the fact that a reasonable fault on any of these 3phase items could cause the upfront RCD to trip loosing the entire site which would be a pain in the arse, especially if the cold stores were full!

 

[westward10]

Have you asked about nuisance tripping.