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?