Earthing Systems

[JohnW2]

There are some things which one thinks one understands, or takes for granted, until one (foolishly?!) starts thinking too much about them ...

... what does the second letter of an earthing system designation (e.g. TN, TT, IT) actually mean? I gather that this is defined in IEC 60364, to which I don't have access - but looking in books, and Googling, leads to a series of (generally similar) explanations which leave me confused.

I have no problem with TN-C-S or TT. In both those cases, the second letter ('N' or 'T' respectively) indicates the path from the installation (e.g. CPCs etc.) to earth, and hence the path that any L-E fault currents will follow (albeit that with TN-C-S, there is also somewhat of a parallel path to earth via the PME) - and that is consistent with what most of the explanations of 'the second letter' say.

However, what about TN-S? In this case, the installation's path to earth, and the path for any L-E fault currents, is via the Separate earth conductor - so what does the 'N' actually mean in this situation?

Kind Regards, John.

 

[ban-all-sheds]

http://www.tlc-direct.co.uk/Book/5.2.1.htm

 

[TheOriginalTonkaToy]

From wiki

International standard IEC 60364 distinguishes three families of earthing arrangements, using the two-letter codes TN, TT, and IT.

The first letter indicates the connection between earth and the power-supply equipment (generator or transformer):

T
Direct connection of a point with earth (Latin: terra);
I
No point is connected with earth (isolation), except perhaps via a high impedance.

The second letter indicates the connection between earth and the electrical device being supplied:

T
Direct connection of a point with earth
N
Direct connection to neutral at the origin of installation, which is connected to the earth

IIRC T is Terra, N is Neutral

 

[JohnW2]

http://www.tlc-direct.co.uk/Book/5.2.1.htm

Thanks. That's one of the many places I looked. As you know, it says:

The second letter indicates the earthing arrangement in the installation.
... N - all exposed conductive metalwork is connected directly to an earthed supply conductor provided by the Electricity Supply Company.

That is actually slightly different from most of the other definitions one finds. This one actually 'works' with TN-S, in that there is a DNO supplied earth conductor, but it still makes no sense of the fact that the letter is 'N' - "TS" would be a much more logical characterisation'

Most 'explanations' do not talk of a 'DNO supplied earth conductor' but, much more commonly, something like "the connection between earth and the electrical device being supplied" - which, again, makes it hard to see why the second letter should be 'N' in TN-S.

Kind Regards, John.

 

[JohnW2]

From wiki
The second letter indicates the connection between earth and the electrical device being supplied:
T Direct connection of a point with earth
N Direct connection to neutral at the origin of installation, which is connected to the earth

Exactly - so, as I asked, why the 'N' as the second letter of TN-S ?

Kind Regards, John.

 

[TheOriginalTonkaToy]

From wiki
The second letter indicates the connection between earth and the electrical device being supplied:
T Direct connection of a point with earth
N Direct connection to neutral at the origin of installation, which is connected to the earth

Exactly - so, as I asked, why the 'N' as the second letter of TN-S ?

Kind Regards, John.

Because earth and neutral are connected at the substation transformer and both are supplied to the consumer.

 

[JohnW2]

From wiki
The second letter indicates the connection between earth and the electrical device being supplied:
T Direct connection of a point with earth
N Direct connection to neutral at the origin of installation, which is connected to the earth

Exactly - so, as I asked, why the 'N' as the second letter of TN-S ?.

Because earth and neutral are connected at the substation transformer and both are supplied to the consumer.

But 'the original of the installation' is within the consumer's premises, not the substation, and the consumer does not (is not allowed to, and doesn't need to) 'earth' their electrical devices to the supplier's neutral with a TN-S system. Don't forget, what we are defining is an 'earthing system'.

In any event, "Because earth and neutral are connected at the substation transformer" is as true of a TT supply as a TN-S one, and in neither case is the neutral supply cable used for earthing the installation.

Kind Regards, John.

 

[bernardgreen]

Because earth and neutral are connected at the substation transformer and both are supplied to the consumer.

But supplying them both along a single wire is NOT the same as supplying both individual, each on its own wire.

Using one wire for both means the current in the neutral will produce a voltage between true ground at the sub-station and the false "earth" at the installations in all properties that use the "earth" that arrives along the neutral conductor.

 

[JohnW2]

Because earth and neutral are connected at the substation transformer and both are supplied to the consumer.

But supplying them both along a single wire is NOT the same as supplying both individual, each on its own wire.

Exactly. Do you have the same problem as me in understanding the 'N' in TN-S?

Kind Regards,John.

 

[bernardgreen]

Exactly. Do you have the same problem as me in understanding the 'N' in TN-S?

Having seen the present system evolve over the years has helped me to see the "logic" behind the lettering system.

It seems to be T as the starting point and then it goes via the N to the end user. Either as S = Separate conductors for the earth and the neutral or as C = Combined neutral and "earth" on one wire. ( also Cost cutting ).

At a transition point there is a - to indicate a change if there is a change from combined to separate.

TT is Terra and Terra with out any dash and no mention of Combined or Separate meaning the Earth is not carried along the network cables. By implicaton the earth at the installation has to be local.

I seem to recall that before earthing in domestic property was considered essential ( I go back a long way ) that the designation of T was a valid designation.

 

[JohnW2]

Having seen the present system evolve over the years has helped me to see the "logic" behind the lettering system.
It seems to be T as the starting point and then it goes via the N to the end user. Either as S = Separate conductors for the earth ....

Sorry, but you've lost me already as far as any 'logic' is concerned. What is it that you are suggesting 'goes via the N' in a TN-S system. It's certainly not the installation's CPC's path to earth - and it is, after all, the 'earthing system' which we are trying to define with these letters, isn't it?

Kind Regards,John.

 

[TheOriginalTonkaToy]

Because earth and neutral are connected at the substation transformer and both are supplied to the consumer.

But supplying them both along a single wire is NOT the same as supplying both individual, each on its own wire.

Using one wire for both means the current in the neutral will produce a voltage between true ground at the sub-station and the false "earth" at the installations in all properties that use the "earth" that arrives along the neutral conductor.

Eh? That's TN-C you're talking about. TN-S systems have separate conductors for neutral and earth.

 

[TheOriginalTonkaToy]

From wiki
The second letter indicates the connection between earth and the electrical device being supplied:
T Direct connection of a point with earth
N Direct connection to neutral at the origin of installation, which is connected to the earth

Exactly - so, as I asked, why the 'N' as the second letter of TN-S ?.

Because earth and neutral are connected at the substation transformer and both are supplied to the consumer.

But 'the original of the installation' is within the consumer's premises, not the substation, and the consumer does not (is not allowed to, and doesn't need to) 'earth' their electrical devices to the supplier's neutral with a TN-S system. Don't forget, what we are defining is an 'earthing system'.

In any event, "Because earth and neutral are connected at the substation transformer" is as true of a TT supply as a TN-S one, and in neither case is the neutral supply cable used for earthing the installation.

Kind Regards, John.

Erm, connected at the substation, which they are and both supplied to the consumer as in using separate conductors.

I suspect you're looking at it the wrong way. The IEC standard applies to the supplier, not the consumer - the earthing system is WRT to the supply.

 

[JohnW2]

Eh? That's TN-C you're talking about. TN-S systems have separate conductors for neutral and earth.

By my reading, that was precisely the point Bernard was making - that TN-C and TN-S are different, for the obvious reason.

That's why I'm perfectly happy with TN-C(-S) being called that, since earthing is being provided via the neutral (PNE) conductor. With TN-S, it's a totally separate conductor providing the earth, and the neutral has nothing to do with it - hence my question about the 'N'.

Kind Regards, John.

 

[JohnW2]

Erm, connected at the substation, which they are and both supplied to the consumer as in using separate conductors.

Indeed, and the earthing to the installation is provided via the separate earth conductor, not the neutral.

I suspect you're looking at it the wrong way. The IEC standard applies to the supplier, not the consumer - the earthing system is WRT to the supply.

I'm afraid that makes no sense to me. From the viewpoint of the supply (hence presumably the IEC standard), TN-C and TT are identical (until one adds PME to TN-C-S, but that's a separate and additional matter).

Kind Regards, John.