Earthing System

[studentspark]

Is this a TN-S do you think.

 

[Notch7]

Im sure a pro will be along, but in the meantime: Im not certain the information is in the picture........

 

[ericmark]

It looks like TN however only the DNO can say if TN-S or TN-C-S as only they know is some where along the supply root is the earth and neutral have been combined or not.

A big clue as it if TN or TT is the ELI, although a TT supply can have a very low ELI in domestic unlikely to be under the 0.35Ω considered as limit for a TN-C-S or 0.8Ω considered limit for TN-S however with a three phase supply to commercial that does not really work. In one job I spent 6 months knocking in earth rods, non over 8Ω and main earth pit under 1Ω and the incoming earth connected through a large resistor to limit earth current.

So only real way to know is ask the DNO.

 

[JohnW2]

It looks like TN ...

It does, and if it were TN-C-S, one would expect to see a 'PME sticker'.

... however only the DNO can say if TN-S or TN-C-S as only they know is some where along the supply root is the earth and neutral have been combined or not.

If the installation's earth is derived from a separate conductor (i.e. the sheath of the incoming cable) then it is TN-S, regardless of what connections there are between neutral and earth outside of the installation (and, after all, there has to be at least one such connection, at the transformer). The installation is TN-C-S only if the installation's earth is derived from the neutral of the incoming supply cable.

Kind Regards, John

 

[ban-all-sheds]

It looks like TT. You can see that that g/y is taped to the sheath of the supply cable, and is heading off somewhere - to an earth rod?

 

[ericmark]

It does, and if it were TN-C-S, one would expect to see a 'PME sticker'.

If the installation's earth is derived from a separate conductor (i.e. the sheath of the incoming cable) then it is TN-S, regardless of what connections there are between neutral and earth outside of the installation (and, after all, there has to be at least one such connection, at the transformer). The installation is TN-C-S only if the installation's earth is derived from the neutral of the incoming supply cable.

Kind Regards, John

I don't have latest version of BS7671, I am still on 2008 version, however that version if anywhere between the step down transformer installation and the premises the earth and neutral are combined it is classed as TN-C-S as the risks are the same, i.e. if the combined earth neutral is lost the earth can rise in voltage, so use of TN-C-S is banned for petrol stations, caravan parks, boat marinas etc.

 

[chivers67]

It looks like TT. You can see that that g/y is taped to the sheath of the supply cable, and is heading off somewhere - to an earth rod?

There's also a Brass connector block sticking out to the right of the Neutral with an Earth connected going to the MET, Perhaps the DNO just wanted people to guess

I think TN-C-S .

Would like to see another photo taken back a bit more

 

[JohnW2]

I don't have latest version of BS7671, I am still on 2008 version, however that version if anywhere between the step down transformer installation and the premises the earth and neutral are combined it is classed as TN-C-S as the risks are the same, i.e. if the combined earth neutral is lost the earth can rise in voltage ...

I'll have to look to see what BS7671 says, but what you say above makes no sense to me. If the installation's earth is derived from the sheath of the cable (i.e. TN-S), rather than from the incoming neutral, how on earth can 'loss of the neutral' have any influence on the potential of the installation's earth (unless there is yet another simultaneous network fault)? What am I missing?

Kind Regards, John

 

[DetlefSchmitz]

I think Eric is saying that a section of cable under the street can get replaced with a section without an earth sheath, presumably because of the lack of availability of the other kind.

 

[ericmark]

I think Eric is saying that a section of cable under the street can get replaced with a section without an earth sheath, presumably because of the lack of availability of the other kind.

Exactly this does happen, I would guess with petrol stations, caravan parks and marinas the DNO must inform the customer if a TN-S is converted to TN-C-S however supply not covered with BS7671 so don't know the rules, but you can see from head if TT or TN and in some cases you can see when it's TN-C-S but you can never be sure it's TN-S unless the DNO tells you.

 

[JohnW2]

I think Eric is saying that a section of cable under the street can get replaced with a section without an earth sheath, presumably because of the lack of availability of the other kind.

If that is true, and the installation's earth is connected to the sheath of the incoming cable, then the installation won't have an earth at all (except, maybe, a sort-of TT one if there is some of the sheath exposed to the earth/soil) - and there still wouldn't be any reason why the potential of that 'earth' (or non-earth) should be influenced by a neutral fault, is there?

I wonder if eric is thinking of the situation with some types of cutout/arrangement which result in the fact that, without opening the cutout, one cannot be sure whether a G/Y emerging out of it and going to the MET is connected to the incoming sheath or the neutral?

Kind Regards, John

 

[JohnW2]

Exactly this does happen ...

yes, that happens. However, if it does, then the installation has to derive its earth from the neutral (not the cable sheath), and hence would be 'obviously TN-C-S.

The DNO could not leave a situation in which the sheath had become, or been made, discontinuous without making sure that all installations previously relying on that sheath as a TN-S earth were changed to deriving their earth from the neutral.

The supply entering my house is 'PMEd' - but that does not, in itself make my installation a TN-C-S one - it's actually TT, by virtue of what I have connected by MET to (and earth rod). Similarly, an installation with a TN-S will obviously only change to being a TN-C-S one if the earthing conductor is moved from the cable sheath to the neutral.

Kind Regards, John

 

[studentspark]

Bit more info
Its central London (Westminster)
Ze of 0.09
PEFC 2.5 kA PSCC 2.5 kA

So doubt a TT
Readings made me think TNCS but connection/separation must be under the pavement

Earthing conductor taped to the supply cable disappears in a hole in the basement wall, seemingly connected under the pavement...

 

[JohnW2]

Readings made me think TNCS but connection/separation must be under the pavement. Earthing conductor taped to the supply cable disappears in a hole in the basement wall, seemingly connected under the pavement...

That's a new one on me. Is it actually allowed to have a TN-C-S earth entering the building separate from the supply cable?

In fact, is it technically actually a TN-C-S installation if one does that, given that the split between PEN and N & E doesn't occur within the property?

Kind Regards, John

 

[DetlefSchmitz]

If that is true, and the installation's earth is connected to the sheath of the incoming cable, then the installation won't have an earth at all (except, maybe, a sort-of TT one if there is some of the sheath exposed to the earth/soil) - and there still wouldn't be any reason why the potential of that 'earth' (or non-earth) should be influenced by a neutral fault, is there?

Kind Regards, John

So you are suggesting that if the replacement cable happened, the DNO would leave the earth floating? I would have thought they would connect it to the Neutral. Anything else would be scandadous.