What Earthing system do I have?

[kingkevbo]

Hi,

I live in the Czech Republic and I have an old consumer unit that needs updating, and on inspection, it has an unusual earthing system (possibly not unusual for the Czech Republic). Here is a basic diagram:

Untitled

• kingkevbo
• 17 Oct 2011
• 1

It's a 3-phase system with a earth wire heading towards the ground ( I am assuming that its a ground rod, but the area around it has been paved so I cant actually see the rod) so it looks a bit like a TT but the local earth is commoned with the neutral in the distribution board. Is this normal?

Is there any disadvantage (or danger) in joining the earth rod with the incoming neutral on a TT system

If the local earth was not there, then I suppose it would look like a TN-C-S system, but the separation of neutral and earth happens after the meter, in the distribution board, rather than before

In the new consumer unit that I would like to fit a 3-phase RCD. Would this earthing system cause a problem with this?

Thanks.

 

[bernardgreen]

Is there any disadvantage (or danger) in joining the earth rod with the incoming neutral on a TT system

The answer is totally dependent on knowing how the three phase supply is referenced to earth at the substation. Without that knowledge it is best to say that connecting the neutral to the earth rod could create a hazard.

If the reference to earth is the star point of the three phase transformer and the loading on the network circuit is reasonably well balanced across the three phase then earthing the neutral is unlikely to be hazardous but in some circumstances high currents could flow in the connecting wire ( assuming a low impedance earth rod )

If the the reference to earth is not the star point of the three phase transformer then earthing the neutral WILL create a hazard

 

[kingkevbo]

Thanks for your reply.

I assume that, as this is how the system is installed I should not disconnect the link between neutral and earth until the earth rod is tested and verified as able to provide a satisfactory earth.

My concern is, with neutral connected to earth, am I effectively providing an earth for the entire network via my earth rod?

 

[Paul_C]

I cannot say for certain what the standard arrangement is now in the Czech Republic, or what was the norm when the supply might have been installed when it was Czechoslovakia.

However, I do know that many of the Eastern Bloc countries used to use TN-C arrangements at one time, where the earth at each socket was simply strapped to the neutral. Having a local earth rod connected to the neutral bar at the distribution panel would certainly have made sense with that system.

It may well be that they've moved to TN-C-S now with neutral & earth separated within the installation, but still with a common neutral/earth busbar at the main distribution panel with a local earth rod connected.

 

[kingkevbo]

Thanks for your reply

I have just opened the cabinet on the wall just after where the main supply comes in from the overhead cable and the Neutral is labeled PEN.

There is also a diagram on the inside of the door which suggests than the PEN conductor be earthed on the consumer side, presumably by an earthing rod.

I think that I am satisfied that the system is wired correctly now.

I suppose the system cannot really be described as a TN-C-S or PME system as known in the UK, as the PEN from the DNO cannot be gauranteed with overhead cables due to the possibility of a tree falling on the cable or someone stealing the cable or something.

 

[JohnW2]

I have just opened the cabinet on the wall just after where the main supply comes in from the overhead cable and the Neutral is labeled PEN. There is also a diagram on the inside of the door which suggests than the PEN conductor be earthed on the consumer side, presumably by an earthing rod. I think that I am satisfied that the system is wired correctly now.

Thanks for clarifying.

I suppose the system cannot really be described as a TN-C-S or PME system as known in the UK, as the PEN from the DNO cannot be gauranteed with overhead cables due to the possibility of a tree falling on the cable or someone stealing the cable or something.

It sounds as if what you are describing is, by definition, TN-C-S - since the N and E functions are combined ('-C') in the supply PEN, but then separate ('-S') within your property. Although I realsie that the separation happens after your meter, I don't think that stops it being TN-C-S. However, as you say, whether or not it is also PME is a different question. In the UK (and in most other places), TN-C-S without PME is not allowed, since it can become dangerous if the supply PEN is broken somewhere. It is certainly possible to have PME with overhead supplies (the PEN being connected to earth rods at poles etc.). Presumably your electrical supplier could tell you whether or not your supply is PME.

One of the consequences of having the PEN split after the meter is, of course, that (potentially very high) earth fault currents would have to flow through the meter. However, since the same current will be flowing through the L circuit in the meter, I presume that the meters are designed to be able to cope with that.

I can't really think of any problem of having your earth electrode (if that is what it is) as well. Apart from the meter being 'in the way', it is no different from what would happen with a supplier's PME.

Kind Regards,
John

 

[Paul_C]

I concur with John on all points. It's still TN-C-S, just with the point at which the neutral & earth separate being after the meter instead of before it as in Britain. The same arrangement is found in American homes where all the supplies are TN-C-S and the neutral/earth busbars are bonded together at the main distribution panel and a local earth electrode connected as well.

And we certainly have many homes wired for TN-C-S with PME on overhead supplies out here in the sticks.

 

[ericmark]

Think your questions already answered seems it's TN-C-S as to rod in the ground it's called an "extraneous-conductive-part" not "earth electrode" with a TN-C-S system it is of course like any other metal that is earthed and in the ground for example the water pipes.

I have heard it said that you can't have an earth rod with TN-C-S but although technically true it is only because it not called an earth electrode it's only the name you can't have not the item. It has confused many an apprentice.

So I see nothing wrong with what you have.

 

[JohnW2]

Think your questions already answered seems it's TN-C-S as to rod in the ground it's called an "extraneous-conductive-part" not "earth electrode" with a TN-C-S system it is of course like any other metal that is earthed and in the ground for example the water pipes.

This is all very semantic (and, I would say unimportant) but I think it's true to say that even a TT system's earth electrode fulfills the definition of an 'extraneous conductive part', but we then call it an 'earth electrode'!

I have heard it said that you can't have an earth rod with TN-C-S but although technically true it is only because it not called an earth electrode it's only the name you can't have not the item.

Yes, it's often said, but I don't think it is even 'technically true'. As Paul is often pointing out, in many countries a local earth electrode is actually compulsory with a TN-C-S/PME system, and I see no reason why is should be 'outlawed' in the UK (or Czech Republic). As you point out, it's no different from, say, a water supply pipe - or, indeed, a supplier's PME earth electrode. Indeed, in the case of one of my daughter's house, I happen to know that the closest PME electrode is only a few feet from her house - so how does that differ from a consumer-supplied electrode?

Kind Regards, John.

 

[Paul_C]

as to rod in the ground it's called an "extraneous-conductive-part" not "earth electrode" with a TN-C-S system

It's a metallic rod driven into the ground for the specific purpose of making a connection to the general mass of the earth. That makes it an earth electrode.

 

[securespark]

Not being familiar with this country's ways with regard to electricity supplies, I cannot be certain.

However, could the rod be something that the DNO (or equivalent thereof) connected up to the neutral?

Our DNO's would do this on a PME system.