TNS and TNC-S In same building

[Manator]

Hi guys, I did a site survey today in order to carry out periodic testing on a multiple storey building. There are two supplies presented with different earthing arrangements. One is TNS the other TNC-S, initial tests prove that due to the buildings bonding, both earthing arrangements are now joined.

I have never come across this before, so thought I would post up here to see what you all think.

I have not contacted the DNO,I would welcome any constructive idea's.

Background info.

This is a 5 storey building, the TNS supplies three distribution boards, the TNC-S supplies two distribution boards. These boards are situated on each floor via a dry riser system. All supply cables are SWA.

Each floor is cross bonded to steelwork and water. The gas only enters the building on the ground floor and is housed within a purpose built plant room with full auto controls.

Bonding measurements across each earthing system is 0.003 ohms, so they are definitely connected.

Both supplies are three phase 100amp per phase. Main earthing conductors are 25mm copper and connected to an earth bar. All bonding is 16mm copper. For those keen eyed amongst you the earth bar was separated, so that each supply was tested independently.

Any thoughts?

 

[Andy™]

cant see any problem providing the earths from both are linked

 

[husky2013]

in agreement with above  providing  the earth from the TNS and the TNCS are linked   and both supplies are from the same transformer  cant see a big issue

 

[Rob_the_rich]

Are the two supplies in the same room? And is it completely obvious that the earths are linked? I am thinking a sign should be there saying they are linked and to isolate both supplies before disconnecting.

If the PEN conductor to the TNC-S supply broke, some of the neutral current would be flowing down the supply sheath of the TNS system. A tester might isolate the TNS supply and disconnect the earth to do a Ze test. Could be dangerous.

 

[Manator]

Scoobed you for that Rob, your thinking along the lines that I was when I saw it.

The earthing is only joined because of the main bonding and supplemental bonding. I presume that both supplies are from the same supply, but would expect that the TNS would have been changed when the new supply was installed. I am due back there next week to begin testing and will look  deeper into the implications. In the meantime keep your ideas coming, like I say I may learn something here because I have no idea having not seen this before.

Sorry Rob, yes the two supplies are in the same room, in fact they are probably 200mm apart.

 

[husky2013]

maybe give the DNO a Call and ask them is there a specific reason as to why 2 types of earthing arrangements have been installed maybe they can shed some further light on the matter.

i do agree with rob,  some further investigation is definately warranted on site though,  do they have records of the installation or the designs, etc,

 

[leesparkykent]

If you linked both earthing  arrangements together then I cant see a problem.. I would then class it as TNC-S and the metallic sheath of the incoming TNS cable as an extraneous conductive part that requires bonding :Salute  

 

[Rob_the_rich]

Only other thing I can think of is that if an earth fault occurred on one system, it might trip an RCD on the other

 

[Essex1]

How much time are you planning on spending on this?   £80 and the DNO will change your TN-S to a TN-C-S. 

 

[Andy™]

Only other thing I can think of is that if an earth fault occurred on one system, it might trip an RCD on the other

how exactly is that going to work? its a bit like saying an earth fault in my house will trip a neighbours RCD

 

[Rob_the_rich]

how exactly is that going to work? its a bit like saying an earth fault in my house will trip a neighbours RCD

just thinking of imbalances, only needs a little bit, if there are already high protective conductor currents (lots of PCs) on one supply, then an earth fault on the other, might raise the potential on the earth enough to trip an RCD.

 

[Andy™]

but an RCD does not monitor earth, it only measures imbalance between live conductors. same as an RCD in a dual split should not trip on an earth fault on the other side

 

[Tony S]

in agreement with above  providing  the earth from the TNS and the TNCS are linked   and both supplies are from the same transformer  cant see a big issue

That is the bit that would worry me.

 

[Rob_the_rich]

but an RCD does not monitor earth, it only measures imbalance between live conductors. same as an RCD in a dual split should not trip on an earth fault on the other side

Of course it does. Don't know what I was thinking. Another brain fart. Sorry all.

 

[ProDave]

Are the two supplies in the same room? And is it completely obvious that the earths are linked? I am thinking a sign should be there saying they are linked and to isolate both supplies before disconnecting.

If the PEN conductor to the TNC-S supply broke, some of the neutral current would be flowing down the supply sheath of the TNS system. A tester might isolate the TNS supply and disconnect the earth to do a Ze test. Could be dangerous.

Surely that SAME situation would also apply if there were two TNC-S supplies to the same building with the earths linked as well? but I suspect nobody would be asking the quesion if they were both TNC-S.

If the PEN failed on one, you wouldn't even know about it in all probability, until someone came to do some work and started disconnecting stuff.

 

[Manator]

There has been some good input thus far, but everyone is missing a vital point.

The initial tests that I carried out would suggest that the DNO were just lazy and installed an upgrade without doing any extra work. The TNS supply (original) and the TN-C-S supply (new) is linked. Not as presented but I would bet my whole life savings that they are from the same transformer.

The problem I have is how do I describe the intake? I can not say its TN-C-S for the TNS because it is not presented as such.

It truly is a strange one, and one in the last 30 odd years that I have never seen.

 

[SLIPSHOD & SLAPDASH]

If there is 2 supplies then there is 2 installations ?

 

[husky2013]

perhaps  description can read  as   TNS Origin  and TNCS Secondary  no harm in making the observation of the secondary supply in that part of the cert

 

[NozSpark]

2 sources of supply....

however I don't like the idea of 2 different earthing types supplying the same building

 

[husky2013]

That is the bit that would worry me.

if both supplies are from the same transformer and are under control  then there is not an issue with both supplies being fed from same!

Manator,  hope u get to the bottom of it,  its not something i have come across but having said that  you have been in the game alot longer then myself, or at least if the client agrees to have the TNS upgraded to being a TNCS.  but please keep us updated be interesting to see the outcome of this