Is TNCS worse than TT earthing?

[davetheglitz]

Just a few thoughts on the dreaded earthing systems - can anyone see any problems with this?

TNCS - generally hated as if the neutral is lost any load in the circuit routes the live back to the earth terminal making every metal object live. The RCD will have no effect on this.

TT - hated by some as if there is a N-E fault on the system the return current to the RCD is shunted by the earth rod impedance. Will not therefore trip correctly in fault conditions - but will trip when the imbalance is greater than 30ma - e.g. current taken 30A - 30mA shunted to earth via rod - hence trip.

Thought - add an earth rod to a TNCS system - see attached diagram.

If neutral is lost at prior to the RCD and a fault occurs after the RCD then RCD will trip as >30mA taken via the earth rod - removes objection to TNCS

If N-E fault occurs after the CU current is shunted across the RCD - approx half flowing through the RCD, half across it. Any small appliance in circuit (>60mA) would cause a trip- so hazard is identified before any fault can cause injury - removes objection to the TT system.

Arguably this is mixing earthing systems - but in reality I can't see what the difference is with this and bonding to a metal pipe which goes to ground.

Overall - I think that the TNCS is probably a better earthing system than TT provided it is all RCD protected and any bonding provides a route to ground.

Any thoughts?

 

[NozSpark]

My old house had PME which also had a rod, not a bad compromise

 

[gordy71]

This is what i suggested the other day but was told by steptoe that this is a big no no. Something about equipotential zones. My thinking is that any earthing is good earthing and as for differences in potential, well as long as everything is bonded to same point then there is no potential between anything. And the earth path is the one with the lowest impedence.

 

[steptoe]

This is what i suggested the other day but was told by steptoe that this is a big no no. Something about equipotential zones. My thinking is that any earthing is good earthing and as for differences in potential, well as long as everything is bonded to same point then there is no potential between anything. And the earth path is the one with the lowest impedence.

gordy71,

that is NOT what you suggested the other day,

the diagram shows a PROPER PME system,

the N and EARTH are split at the same point the rod is used,

that is what TNCS/PME should be like,

but its not always the case,

---------- Post Auto-Merged at 17:29 ---------- Previous post was made at 17:28 ----------

Just a few thoughts on the dreaded earthing systems - can anyone see any problems with this?TNCS - generally hated as if the neutral is lost any load in the circuit routes the live back to the earth terminal making every metal object live. The RCD will have no effect on this.

TT - hated by some as if there is a N-E fault on the system the return current to the RCD is shunted by the earth rod impedance. Will not therefore trip correctly in fault conditions - but will trip when the imbalance is greater than 30ma - e.g. current taken 30A - 30mA shunted to earth via rod - hence trip.

Thought - add an earth rod to a TNCS system - see attached diagram.

If neutral is lost at prior to the RCD and a fault occurs after the RCD then RCD will trip as >30mA taken via the earth rod - removes objection to TNCS

If N-E fault occurs after the CU current is shunted across the RCD - approx half flowing through the RCD, half across it. Any small appliance in circuit (>60mA) would cause a trip- so hazard is identified before any fault can cause injury - removes objection to the TT system.

Arguably this is mixing earthing systems - but in reality I can't see what the difference is with this and bonding to a metal pipe which goes to ground.

Overall - I think that the TNCS is probably a better earthing system than TT provided it is all RCD protected and any bonding provides a route to ground.

Any thoughts?

Dave,

that is a PME system

 

[davetheglitz]

Dave,that is a PME system

Agreed - but small difference is that the earth rod is added at the terminal rather than relying on the multiple earth paths which could be anywhere. Also this rod is not provided by the DNO - but by the householder.

The main thought is that lots of electricians dislike TNCS - but if the bonding goes to ground or an earth spike is added then it's better than using TT alone. Just trying to see if my thinking is correct or I've missed something.

 

[Sidewinder]

There are certain requirements in 7671 to actually DO this!

This is not one of the 7671 requirements per se, however, I also had an issue with a large complex TN-C-S / PME install, where by I had to export the PME earth to some steel cabins, I could not separate them off as only TT as they were adjacent & I mean adjacent to structural steelwork connected to the main install earth terminal.

This was already rodded close to the origin, directly from the main earth terminal.

I discussed this with my NICEIC AE for reassurance, his suggestion was add a rod at the remote DB

 

[ADS]

There are certain requirements in 7671 to actually DO this!This is not one of the 7671 requirements per se, however, I also had an issue with a large complex TN-C-S / PME install, where by I had to export the PME earth to some steel cabins, I could not separate them off as only TT as they were adjacent & I mean adjacent to structural steelwork connected to the main install earth terminal.

This was already rodded close to the origin, directly from the main earth terminal.

I discussed this with my NICEIC AE for reassurance, his suggestion was add a rod at the remote DB

 

[Sidewinder]

Sorry ADS,

I disagree, and I am not going to get into a debate over this tonight, I have some important work that I am finishing off.

My terminology may not be "exactly" correct, however, the principle is the same.

I am fully aware of the descriptions in GN8.

However, you cannot have mixed earthing systems in the same building or in other certain circumstances, thus it is essential to retain the TN-C-S/PME supplied earth and add additional earthing in the form of earthing rod(s).

 

[ADS]

I have a few 'terminology' questions for anyone who agrees that you can have a PME supplied TN-C-S system and connect a 'rod' to the same system.

1) In the above described installation, what do you call the conductor connecting the 'neutral link' at the cut-out to the MET?

2) In the above described installation, what do you call the conductor connecting the 'earth rod' to the MET?

3) What is the name of the 'earthing system' applied to the installation?

If your answer to 1) & 2) above is 'Earthing Conductor' I'm afraid that cannot be - a number of 'publications' tell us there is only one 'Earthing Conductor' in an installation.

Happy days:^O

 

[Andy™]

adding a rod is no different to bonding a metallic service pipe/sturctural steel....

 

[PC Electrics]

I know we're not supposed to link to other forums, but this is 5 years old and pre-dates this forum, so I'm hoping Admin will forgive this 'once in a lifetime' transgression (he used to be there as well after all):

The Ultimate PME/TNS Exportation! B-) : Screwfix Community Forum

 

[davetheglitz]

Ah Yes - I Remember It Well!

It's taken me 5 years to think I might understand it!!!

 

[steptoe]

I know we're not supposed to link to other forums, but this is 5 years old and pre-dates this forum, so I'm hoping Admin will forgive this 'once in a lifetime' transgression (he used to be there as well after all):The Ultimate PME/TNS Exportation! B-) : Screwfix Community Forum

WTF has TNS got to do with TNCS/PME.?

it is a totally different earthing structure,

BTW, I would assume sidewinder is referring to a structure that is within touch distance(as defined in BS7671) and therefore 'classed' as the same location. (at least I hope he is...)

---------- Post Auto-Merged at 20:54 ---------- Previous post was made at 20:51 ----------

Agreed - but small difference is that the earth rod is added at the terminal rather than relying on the multiple earth paths which could be anywhere. Also this rod is not provided by the DNO - but by the householder. The main thought is that lots of electricians dislike TNCS - but if the bonding goes to ground or an earth spike is added then it's better than using TT alone. Just trying to see if my thinking is correct or I've missed something.

not according to the drawing you provided Dave, sorry, but you show them split at the same point.

[quote name='Andy

 

[Andy™]

of course it is,

basics of earthing,

headbang

otherwise why do we bother adding a rod to old 'TT' systems that have never had one installed in the first place.?

it was aimed more at those why say its 'dangerous' / not allowed to install a rod on a TNCS supply

 

[ianmacd]

As mentioned above, the diagram is just a PROPER PME installation as delivered in other countries rather than this dodgy TN-C-S claimed PME that most people get in this country. This is what DNO's should be doing.

---------- Post Auto-Merged at 21:08 ---------- Previous post was made at 21:05 ----------

Agreed - but small difference is that the earth rod is added at the terminal rather than relying on the multiple earth paths which could be anywhere. Also this rod is not provided by the DNO - but by the householder.

So where is the earth rod connected to? If its to the PEN then its proper PME, if its not then you have 2 earthing systems in contravention to 542.1.1.

---------- Post Auto-Merged at 21:09 ---------- Previous post was made at 21:08 ----------

[quote name='Andy

 

[ianmacd]

Its completely acceptable to rod a pme supply at the main earthing terminal,theres plenty of guidance on this, and sometimes recommended.

Care you quote this 'guidance' that seems to disregard regs?

 

[steptoe]

Its completely acceptable to rod a pme supply at the main earthing terminal,theres plenty of guidance on this, and sometimes recommended.

there is a LOT more to it than simply rodding it(TNCS).

a lot more info needs to be known of the particular installation before you could even consider this path of action,

and this is why I strongly advise against it,

if someone is asking the question regarding this type of installation then it would seem to me they do not fully understand the concept well enough to even consider something of this nature.

it can end up an almost lethal installation if done incorrectly,

especially in domestic, when there should not be any reason to do this in the first instance, none that I have ever come across yet anyway.

 

[ADS]

Its completely acceptable to rod a pme supply at the main earthing terminal,theres plenty of guidance on this, and sometimes recommended.

Whether it's acceptable or not is open to debate (as we've seen)

The thing is, you are not 'rodding' a PME supply if you connect the 'rod' to the MET - you are 'rodding' the consumer's installation, and indeed, changing the characteristics of the installation earthing system.

To 'rod' the PME supply, you need to connect your 'rod' to the supply neutral (PEN) - which you are not allowed to do.

I know you are going to say that they are all connected together anyway - but them's the rules......the cut-out is the cut-off, so to speak.......one side is the supply, with Line and PEN, the other side is the installation, with Line, Neutral and Earthing Conductor.

 

[Chr!s]

there is a LOT more to it than simply rodding it(TNCS).a lot more info needs to be known of the particular installation before you could even consider this path of action,

and this is why I strongly advise against it,

if someone is asking the question regarding this type of installation then it would seem to me they do not fully understand the concept well enough to even consider something of this nature.

it can end up an almost lethal installation if done incorrectly,

especially in domestic, when there should not be any reason to do this in the first instance, none that I have ever come across yet anyway.

I disagree completely, people are showing a lack of understanding

 

[Andy™]

ADS - look at this a different way

if there was a metallic object sticking out of the ground (say an old disused metallic pipe). you would be required to bond it (since its almost guaranteed to be extraneous if its metallic). now are you 'changing the characteristics of the installation earthing system' by adding an earth to it? yet you magically are if you installed a rod, which has the exact same effect....

also, not one person has yet stated a reg which says a rod must not be installed on a TNCS system, yet there is a reg which states a rod must be must be installed in certain conditions