no mixing of earths?

[PeteC]

Hi, any ideas please?

A modern barn structure, sudivided, has a common internal sub-supply from a TNS head.
At the end an adjoining switch room for external plant, with a TNCS sub-supply from a TNCS supply head.

The building has steel frame (set into concrete) steel purlins and roof. Metal clad, metal everywhere.!

An on-roof mounted array (framework earthed through direct contact to metal parts~ TNS side) SMA 17000TL's connected to The TNCS sub-supply.
Only theory I can come up with is turn both TNCS and TNS sub-supplies into TTs, but the building structure would remain a common grounding?

 

[SolarCity]

You have TNS and TNCS on the same site? That seems strange.

It's a good question. Is there any connection between the TNS earth and the TNCS earth currently?

 

[PeteC]

No link currently. The TNCS supply is recent, installed to supply the external plant. Technical advice from accreditors advised if framework bonded to steelwork no bonding conductor needed to be brought off roof to TNCS MET (EMB) The inverters are mounted on lower concrete wall but within feet of exposed metalwork from both sides. Worse is the client has a liscence for another system he want's to connect to the TNS side (seperate buisiness/Mpan for fits) on the same roof?!

 

[SolarCity]

I would have thought TTing both incomers would be a good solution. The equipotential zone is relevant to both supplies so they will both need bonding together as far as I can see.

 

[PeteC]

Yes, my thoughts~ bonding the TNS and TNCS earthing together (probably from the same local grid) I would think could create one system raising earth potential on the other in fault conditions through differring Ze's and why mixing earths is so dangerous.

I'm hoping someone from the IEE will come back on this to explain more fully.

 

[moggy1968]

I would have thought sticking a spike in was the way to go for a simple solution

 

[jason121]

Are there two tranformers feeding in
one for TNCS
one TNS

 

[jason121]

I would have thought sticking a spike in was the way to go for a simple solution

Would you need a type B RCD for this

 

[moggy1968]

If your turning it into TT then yes you need an RCD but it depends on the characteristics of the inverters wether you need a B type. I believe G83 Sunnyboys don't but not sure about these ones.
It may be the DNO comes back that they will turn the TNS into a TNCS supply,, have to wait and see!

 

[jason121]

What about this
The final connection into a distribution board that contains one or more circuits protected by an RCD, shall be made in such a way that the outgoing circuit for the PV system is not connected into the outgoing side of an RCD installed for the protection of other circuits (ie in a split board – connected into the part not protected by the RCD) .

 

[moggy1968]

err, yes, and?
all that refers to is you can't share an RCD

 

[SolarCity]

Seperate RCDs for the PV system if you do go down the TT route.

 

[jason121]

Whats it matter if you two types of earthin system in one building?

 

[Gavin A]

alternative solution would be an isolating transformer inverter for the system that's run off the other supply, then there shouldn't be an issue as it's only the isolated DC circuit that's sharing the roof space )just remember not to bond the sheath of any SWA cable you might use for the DC side).

I think... but there may be a flaw to this I've not thought about.

 

[jason121]

What is the problem if you two types of earthin system in one building?

 

[topquark]

What is the problem if you two types of earthin system in one building?

Which one would you use for any bonding of the metal in the barn? You have two possibly differing potentials. The situation could be made even worse if the supplies are on differing phases as well.

Safest option has to be to TT the barn.

 

[Engineer54]

Which one would you use for any bonding of the metal in the barn? You have two possibly differing potentials. The situation could be made even worse if the supplies are on differing phases as well.

Safest option has to be to TT the barn.

Going by the OP's description of the Barn build, it's probably is a TT system in it's own right!! The problem being, is isolating the two differing earthing/bonding supply types. This is a DNO's total cock up, and it's they, that should be sorting this problem out, especially if these two supplies come from separately derived TX's....

 

[topquark]

Going by the OP's description of the Barn build, it's probably is a TT system in it's own right!! The problem being, is isolating the two differing earthing/bonding supply types. This is a DNO's total cock up, and it's they, that should be sorting this problem out, especially if these two supplies come from separately derived TX's....

Good point mate, hadn't thought about the frame itself effectively TT'ing it.

 

[jason121]

If there of the same trany shirly it wont matter go with the best eath path ie TNCS

 

[moggy1968]

it's the potential(?) for introducing different potentials in the same building.

I had this in my parents business when they expanded into a shop next door, one was TNS, one TCNS and the DNO put them both onto TNCS