Can earth Rod be retained after TT to TN-C-S conversion?

[NeilMcC]

I'm looking at fitting a new Consumer Unit and the earthing arrangements seems to be TN-C-S (PME) but there is also a Earth Rod connected to the MET.

The house is fed by overhead wire. It turns out that it was converted from TT to TN-C-S but the earth rod connection has been retained. I know I'll need to use the distributor's earth to get a Ze value, but should the earth rod be disconnected or could it be retained going forward?

Is there a case for saying it should be retained as if there was an overhead line fault that only affected the CPC, the house would still have protection via the earth rod?

 

[Chr!s]

Hi Neil

The house is fed by overhead wire. It turns out that it was converted from TT to TN-C-S but the earth rod connection has been retained. I know I'll need to use the distributor's earth to get a Ze value, but should the earth rod be disconnected or could it be retained going forward?

Its not a problem leave it connected

Is there a case for saying it should be retained as if there was an overhead line fault that only affected the CPC, the house would still have protection via the earth rod?

Well the cpc in the tncs is also the neutral, so diverted currents will still appear on exposed conductive parts and extraneous conductive parts connected to the met.

Dependent upon the resistance of the electrode will decide as to how much protection the earth rod will offer.

 

[Larnacaman]

Neil McC,

Why not test the earth rod, then you'll know if it's providing a useful protection to the installation. But whatever, ....Leave the rod connected.

You can always chemically treat the earth rod, there are compounds that will help to lower the installed rod's resistance without undue corrosive side effects as was once the case, with chemical treatments....

 

[gselectrical]

Does an earth rod not have to be below a certain value to be connected to the DNO's earthing system? <20ohm's? Im probably wrong though!

GS

 

[PC Electrics]

The 200 ohm figure you're thinking of is that which is generally considered to be the upper limit to obtain a stable earth. It is a recommendation, but not a regulation.

You can leave the rod connected (in some places it's a requirement - Ireland I think?), but test it and record the Ra.

 

[NeilMcC]

Well the cpc in the tncs is also the neutral, so diverted currents will still appear on exposed conductive parts and extraneous conductive parts connected to the met.

.

Hi, thanks for your reply, I didnt understand the bit above, though. Wont all the current go to earth provided the earth is ok?

 

[Chr!s]

Hi Neil

Hi, thanks for your reply, I didnt understand the bit above, though. Wont all the current go to earth provided the earth is ok?

Well the current will divide, where and how much will depend upon the resistance of those paths.

You may have a electrode say at 150 ohms and an incoming metallic water supply which may have a resistance of 50 ohms, so current will divide between the two paths.

Now if you touch an exposed conductive part or an extraneous conductive part and say your outside of the equipotential zone some current will flow through you, how much will depend on the resistances of the other paths.

To maintain a touch voltage of 50 volts where a 7 kw load is connected your electrode Ra would need to be 2.1 ohms.

 

[Robin Spark]

Hi NeilWell the current will divide, where and how much will depend upon the resistance of those paths.

You may have a electrode say at 150 ohms and an incoming metallic water supply which may have a resistance of 50 ohms, so current will divide between the two paths.

Now if you touch an exposed conductive part or an extraneous conductive part and say your outside of the equipotential zone some current will flow through you, how much will depend on the resistances of the other paths.

To maintain a touch voltage of 50 volts where a 7 kw load is connected your electrode Ra would need to be 2.1 ohms.

You have confused me how you got to this 2.1 ohms ?

I thought touch voltage for say domestic purposes was calculated by

Ra * x I delta n <

50 volts.

Therefore your maximum resistance of earth rod is governed by protective device and not by the load.

 

[Chr!s]

Hi Robin

You have confused me how you got to this 2.1 ?I thought touch voltage for say domestic purposes was calculated by

Ra * x I delta n < 50 volts.

Therefore your maximum resistance of earth rod is governed by protective device and not by the load.

This formula is used Where Rcd protection is required.

When the pen/cne breaks the current will still flow via earthed paths so your rcd will not disconnect the load.

For touch voltage with diverted neutral current you use a different formula.

 

[Robin Spark]

Hi Plumber,

I was assuming 2 things

1. As it was a converted supply from TT to TNCS then the original CU would have at minimum 100ma main switch or RCD before it.

2. When the new consumer unit is fitted then normally all circuits will be protected by 30ma rcds/rcbo's unless pyro, swa etc used.

Erm...it was you who came up with Ra needs to be 2.1 ohms to disconnect before touch voltage got above 50volts.

Can you let me know what formula you used, thanks.

 

[Chr!s]

Erm...it was you who came up with Ra needs to be 2.1 to disconnect before touch voltage got above 50volts.

Lol. sorry my bad quoting skills again.

---------- AUTO MERGE Post added at 14:58 ---------- Previous post was at 14:48 ----------

Hi Robin

1. As it was a converted supply from TT to TNCS then the original CU would have at minimum 100ma main switch or RCD before it

2. When the new consumer unit is fitted then normally all circuits will be protected by 30ma rcds/rcbo's unless pyro, swa etc used.

This may be the case, but in the case of a broken pen/cne the protective devices will not disconnect.

So in our formula we need to remove disconnective element.

The Formula is Ra = .....RL x Vp

.................................. ----------

.....................................(Vs-Vp)

RL = load resistance (Vs2/wattage)

Vp = the touch voltage

Vs = supply voltage

 

[Robin Spark]

Thanks for that formula,

just wondered is it in the regs book/ guidance notes anywhere or is it from a course you did?

Dont ever rememeber seeing that one???

 

[Sidewinder]

Here's one to think about, I had a comment on my last NIC assessment for NOT fitting an earth rod at the board on a TN-C-S supply, can anyone think why?

It took my AE about 2 hours to find the reg number mind!!!

NO cheating, as I think I may have put it on the forum previously...

If I could ban searches temporarily I might!!!

 

[sellers]

Becuase your anual assessor was steptoe!!

 

[Sidewinder]

Sorry Sellers, he actually quoted a reg no.!

As much as I agree with Steps I ain't seen a reg. no. yet.

Sorry Steps!

 

[roys]

This thread has got me thinking, with some interesting comments being made especially the one where Sidewinder was asked about an earth rod on a TN-C-S, so if we are going down this road and bearing in mind the pole apart opinions that people have (sorry Steps and Larnacaman ), who have both brought up valid and strong points regarding TN-C-S earthing. If a property was TN-C-S and we exported the earth (I know, no groans, please wait) to an outbuilding why don't we earth rod (the way Steps likes) and export the earth as well. Apart from the fact the DNO might not like it would it not cover both of best worlds?

I did think twice about posting this as I don't want people falling out over this subject again, but have we ever mentioned combing earthing methods for an outbuilding?

Just a thought.

Steve

 

[Larnacaman]

Roy's,

We don't fall out as such, i value Steptoes opinions in many areas....i may not always agree with him, but that's what constructive argument's are all about. ..lol!!!!

I won't say anything here for the time being Roy, let's see what others have to say first, (including the new members) before i make any comment.

 

[roys]

Sorry Larnacaman, didn't mean to give the impression you fell out, I thought you both brought very good points to the party, in fact it has made very good reading, as you both put forward your points in a logical and thought out intelligent manner. It is a good forum for that.

Anyway as you say lets see where this goes

Cheers Steve

 

[NeilMcC]

Hi,

Thanks for all the interesting comments.

Robin Spark was right about the setup of the TT system previously - it was protected by a 100ma Main Switch/RCD. The reason for the change to TN-C-S was nuisance tripping during thunderstorms. After the change to TN-C-S the RCD Main Switch was taken out and just an isolator and Fuse Panel left - hence the interest in a new Consumer Unit.

I think I understand now why the Earth Rod was retained, thanks to Plumber's comments. The property is feed via overhead distribution, so I guess the chances of a PEN failure are higher than in a underground cable feed. In a PEN fault scenario, if current is being drawn by the Line it will still be connected to the house Earth causing potentially dangerous voltages on extraneous conductive parts. As Plumber's calculation showed, the Earth Rod would need to be 2.1 ohm or lower to keep the touch voltage 50v or less when drawing a 7kW load.

I did some measurements today and the Ze value for the distributors earth is 0.13 ohm and the Earth Rod was 132 ohm. So I guess I should try to improve the resistance of the Earth Rod! Could this be considered dangerous or is the chances of a PEN only fault, drawing a high load and touching an extraneous part at the same time too remote?

As to the questions about where this is mentioned in the Regs, I could only find under Fig 2 (Definitions, TN-C-S) on p33 "an earth electrode may be necessary at or near a consumer's location"; also 411.4.2 and 542.1.5 may be relevant. I wasn't aware of past discussion on this so sorry if I am stirring things up again.

Regards,

Neil

 

[Sidewinder]

Neil,

You are NOT stirring things up AT ALL! We like educated structured debate on this forum.

Don't have brb to hand.

I'll check your references over the weekend and see what they are, however, I KNOW by the section numbers (BIG CLUE) that they were NOT the ones my AE picked me up on wrt my earlier post.

As long as the 132 ohm is stable and the install is RCD'd as per TT requirements then the potential PEN failure should be no concern.

Else you really need the Re below 20 ohms, unless you are going to rely on the distributors earth.

Another clue...

To get this down, you can just add multiple rods, but in that case I would add a BEMT as per my other posts.

Remember resistors in parallel the total is less than the lowest.

HTH