TNCS Outbuilding earthing arrangement

[swifters]

I have a large shed (affectionately known as the warehouse) that is fed by a 16amp rcbo. The main earthing arrangement at the distribution boards is tncs and now a more experienced electrician than myself has stated that the earth in the 3 core 2.5mm armoured cable should be connected to a separate earth rod and make its own tt earthing arrangement rather than be fed back to the main supply, stating parallel paths are the reason. Is he right? Could anyone point to it the new regs book?

many thanks, Jon

 

[Moses]

I have a large shed (affectionately known as the warehouse) that is fed by a 16amp rcbo. The main earthing arrangement at the distribution boards is tncs and now a more experienced electrician than myself has stated that the earth in the 3 core 2.5mm armoured cable should be connected to a separate earth rod and make its own tt earthing arrangement rather than be fed back to the main supply, stating parallel paths are the reason. Is he right? Could anyone point to it the new regs book?

many thanks, Jon

Its nothing to do with parallel paths!!!!

The reason is that as Main Earth on a TN-C-S is derived from the Main N at the supply head, if you loose the the N at the head for any reason, all earthed equipment could become live and your RCD's, RCBO's have no neutral/earth to work correctly, leaving a very dangerous situation.

Thats why a TT is recommended, I don't think you will find any reg nos.

 

[telectrix]

if you have extraneous parts in the shed, these would require a 10mm bonding conductor back to the house MET. also, no harm in adding a rod at the shed and keeping the TNC-S.

 

[Deleted member 9648]

Its nothing to do with parallel paths!!!!

The reason is that as Main Earth on a TN-C-S is derived from the Main N at the supply head, if you loose the the N at the head for any reason, all earthed equipment could become live and your RCD's, RCBO's have no neutral/earth to work correctly, leaving a very dangerous situation.

Thats why a TT is recommended, I don't think you will find any reg nos.

Funny how they always pick on outbuildings though....never outside lights/sockets etc etc.

 

[ample current]

there is no reason why you cant connect the out building to the MET in the house, as long as as your Zs meets requirements and as Tel says, if there is extraneous parts in the shed then these would need to be connected to the MET also with a reading below 0.05 ohms, only then would you have to TT and divorce the earth from the house.

 

[swifters]

Thats what I thought. The trouble is my multi tester trips the rcbo everytime I do a Zs test.

 

[telectrix]

so measure R1+ R2 and Ze , then calculate Zs.

 

[swifters]

Hadn't thought of that. Will do.

 

[JUD]

Thats what I thought. The trouble is my multi tester trips the rcbo everytime I do a Zs test.

Which tester is it. Does it not have a No-Trip setting??

 

[swifters]

Its a Di Log 9083p I think. It is meant to be non tripping but always does.

 

[Guest55]

............Is he right?

Not by a million miles lol.

 

[Engineer54]

I have a large shed (affectionately known as the warehouse) that is fed by a 16amp rcbo. The main earthing arrangement at the distribution boards is tncs and now a more experienced electrician than myself has stated that the earth in the 3 core 2.5mm armoured cable should be connected to a separate earth rod and make its own tt earthing arrangement rather than be fed back to the main supply, stating parallel paths are the reason. Is he right? Could anyone point to it the new regs book?

many thanks, Jon

Obviously not as experienced as you first thought, ...is he!! lol!!

 

[Engineer54]

Thats why a TT is recommended, I don't think you will find any reg nos.

That's why a TT is Recommended by Who?? ...Scaremongers??
It's no more dangerous than the PME system you have in the main house.

 

[Sav]

That's why a TT is Recommended by Who?? ...Scaremongers??
It's no more dangerous than the PME system you have in the main house.

I agree with Engineer54,
I wired a small office in a rear garden (approx 30 meters from main house ), 16mm armoured from isolator inside the house, then to office. I just extended the equipotential zone.
No need for TT

Sav

 

[swifters]

Many thanks everyone, I did think he may be a bit off the mark but I will calculate Zs just to be sure.

Right, got a reading of 0.5ohms for R1+R2 and 0.09 for Ze so even with adjustment for temperature table 41.3 states 1.44ohms or 1.15ohms adjusted for a 16amp RCBO so all good right?

 

[JUD]

Many thanks everyone, I did think he may be a bit off the mark but I will calculate Zs just to be sure.

Right, got a reading of 0.5ohms for R1+R2 and 0.09 for Ze so even with adjustment for temperature table 41.3 states 1.44ohms or 1.15ohms adjusted for a 16amp RCBO so all good right?

Is the RCBO a type B?

If so that's for a 32A. For 16A it's 2.87Ω (2.3Ω adjusted) so looks like you're well under.

 

[telectrix]

probably got a type c rcbo. in which case it is 1.44 (1.15 corrected)

 

[swifters]

I'm not sure if it's a type b or c. It's not as apparent as the MCB's. It's nsbs16 - something... They are Wylex RCBO's.

 

[JUD]

I'm not sure if it's a type b or c. It's not as apparent as the MCB's. It's nsbs16 - something... They are Wylex RCBO's.

Have a look at the pic at the bit in the circle...

This is a 16A type B. If it's type C it will say C16.

The bit underneath says NHXBS1B16 so that should give you a clue also

 

[swifters]

Yes, it's a B type. I was being blind.

 

[pencilpusher]

I have exported the equipotential zone to outbuildings before with no problems and mostly without having to bond any extraneous parts, because there have been none. Nice low ZS readings. No problem. (unless "Rodman" sees this! He thinks it is a big no no for some reason)

 

[Moses]

I have exported the equipotential zone to outbuildings before with no problems and mostly without having to bond any extraneous parts, because there have been none. Nice low ZS readings. No problem. (unless "Rodman" sees this! He thinks it is a big no no for some reason)

read post#2

 

[pencilpusher]

I have read all the posts! What about the house TNCS then? The world is all if's and but's!

 

[Moses]

I have read all the posts! What about the house TNCS then? The world is all if's and but's!

run a 2 core SWA to the hut, an RCd cu in the hut with TT supply from the SWA, and an earth rod at the hut end. SWA earthed at the supply end but the SWA earth insulated at the load end.

 

[pencilpusher]

^^^^^^^ What about the TNCS in a dwelling with no outside hut?

 

[rich.250]

Where do all these rumours come from.........?
Its all written in black and white (with green covers)
Exporting TNCS or PME is fine!! along as any extraneous conductive part is connected to the MET.
How many times have i heard the you can't do this rumours only to look in the regs to prove them wrong haha

 

[pencilpusher]

I have seen this on here many times. There seems to be an obsession about exporting the equipotential zone in outbuildings. Nothing wrong with it and its fine according to the Regs. Period.

 

[rich.250]

I can see there point about loosing the neutral.... as I've seen it a few times!!
But if it was that much of a concern TNCS supplies would be banned and it shouldn't occur with PME

 

[pencilpusher]

Here is the main thread about this very subject previously. Same old rumours.

http://www.electriciansforums.net/e...me-outbuildings.html?highlight=exporting+tncs

 

[Moses]

I have seen this on here many times. There seems to be an obsession about exporting the equipotential zone in outbuildings. Nothing wrong with it and its fine according to the Regs. Period.

In that case you need to ensure that you Zdb at the hut complies with the max for the earthing system that you are exporting.

are you going to calculate the distribution cable R1+R2, and hence calculate the Zdb, as well as the cable size for the expected load? not many do.

or as most people, just design for the expected load and then install the cable, and then take the Zdb measurement afterwards? and hope that it is within the regs?