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

[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.