TN-C-S supply - worth adding an earth electrode?

[WellKnownSid]

We have a property we're renovating and is currently being re-wired. TN-C-S supply - but there is only a single connection to earth at the pole top TX itself which is 50 feet from the the house.

Given that the mains water service is plastic (despite this, it has been cunningly bonded to a 6mm CPC ), would it be sensible to add an earth electrode at the house to give some additional protection?

The single phase 11kV pole top TX only feeds our property, which is in an isolated rural location.

Thoughts?

 

[DaftPunk]

No you must not install an electrode. If you are unhappy with the current earthing arrangement then contact the dno.

 

[ericmark]

You can't fit an earth electrode simply as it will not be called an earth electrode but it will be an extraneous-conductive-part there is of course nothing stopping you from bonding extraneous-conductive-parts in fact you are required to bond items with that label.

I can't see the point in moving from TN-C-S to TT but if your the only house then not likely to be TN-C-S more likely to be TN-S.

With the exception of electric car charging outside, caravan and boat supplies your better off with TN-C-S than TT.

So you will find many telling you that you can't fit an earth electrode but that is only because a steel copper coated rod put into the ground would not be given that name.

 

[westie101]

electric car charging outside

In regard to this and TNCS supplies I found a report recently where an investigation was done into the added risks of this, the conclusion was it posed no significant added risks to the public due to shock so was acceptable.

 

[EFLImpudence]

Given that the mains water service is plastic (despite this, it has been cunningly bonded to a 6mm CPC

If you mean the bond is connected to internal copper piping, then that is a requirement unless you can determine absolutely that the pipe is NOT extraneous.

If you mean the bond is connected to the plastic itself then obviously this is pointless.

 

[JohnW2]

If you mean the bond is connected to internal copper piping, then that is a requirement unless you can determine absolutely that the pipe is NOT extraneous.

I've often wondered how you would apply your 'standard test' in this situation. Is it not true that, in the vast majority of primarily copper-plumbed properties, there would be a very low impedance/resistance from the pipework to the MET by virue of the CPCs of immersions, boilers, CH pumps/valves etc.? What would your test be, and how would you apply it?

Kind Regards, John

 

[JohnW2]

You can't fit an earth electrode simply as it will not be called an earth electrode but it will be an extraneous-conductive-part there is of course nothing stopping you from bonding extraneous-conductive-parts in fact you are required to bond items with that label. ... So you will find many telling you that you can't fit an earth electrode but that is only because a steel copper coated rod put into the ground would not be given that name.

Is this not really just a case of playing with words? IMO, an earth electrode doesn't really cease to be an earth electrode just because it represents an extraneous-c-p, any more than a water supply pipe ceases to be a water supply pipe because it represents and extraneous-c-p.

I would have said that the main issue is 'what is the point?'. Adding an earth electrode (whether one calls it that or a 'bonded extraneous-c-p') will obviously achieve nothing so long as there is an effective TN-C-S earth. In the very rare event of 'failure' of the TN-C-S neutral (hence also earth), it would leave one with a TT installation (and no neutral), but I'm not really sure to what avail.

...if your the only house then not likely to be TN-C-S more likely to be TN-S.

Even with the OP's overhead supply?

Kind Regards, John

 

[EFLImpudence]

I've often wondered how you would apply your 'standard test' in this situation.

It is impractical so to do.
That is why (I think) bonding is recommended unless...

Is it not true that, in the vast majority of primarily copper-plumbed properties, there would be a very low impedance/resistance from the pipework to the MET by virue of the CPCs of immersions, boilers, CH pumps/valves etc.?

It is and so, as you know, bonding , which may not actually be required and so is earthing, will not introduce a hazard.

What would your test be, and how would you apply it?

I would measure out of curiosity but it is more visual really.
If your copper pipes just go up the kitchen wall to the boiler and to the taps and can be seen to be NOT extraneous then it may not be bonded.
However, as in a lot of cases if it is not certain where the pipes go then bonding, just in case, would be better.

 

[EFLImpudence]

Is this not really just a case of playing with words? IMO, an earth electrode doesn't really cease to be an earth electrode just because it represents an extraneous-c-p,

But an earth electrode which is not effective as the earth is just an extraneous-c-p.

any more than a water supply pipe ceases to be a water supply pipe because it represents and extraneous-c-p.

No, but those two things are not related.
It is not the earth therefore is just an extraneous-c-p.

I would have said that the main issue is 'what is the point?'.

Precisely.

 

[JohnW2]

Is this not really just a case of playing with words? IMO, an earth electrode doesn't really cease to be an earth electrode just because it represents an extraneous-c-p,

But an earth electrode which is not effective as the earth is just an extraneous-c-p.

I don't think that argument really works. As I've often described, the water pipe entering my house is a far more effective earth than my earth electrode, but I still (I would say correctly) continue to call the pipe an extraneous-c-p and the rod the 'earth electrode', not vice versa. Would you call my water pipe 'the earth' because it represented easily the 'most effective earth' for my installation (I would think not)?

Kind Regards, John

 

[EFLImpudence]

Would you call my water pipe 'the earth' because it represented easily the 'most effective earth' for my installation (I would think not)?

No, because it is not allowed and your installation is configured for a TT.

Plus, it could disappear tomorrow.

 

[JohnW2]

I've often wondered how you would apply your 'standard test' in this situation.

It is impractical so to do. That is why (I think) bonding is recommended unless... ... I would measure out of curiosity but it is more visual really. If your copper pipes just go up the kitchen wall to the boiler and to the taps and can be seen to be NOT extraneous then it may not be bonded.

Fair enough.

However, as in a lot of cases if it is not certain where the pipes go then bonding, just in case, would be better.

I'm not sure I understanf this "lot of cases". There is surely only a need for bonding if any of the pipework (or things connected to it) 'exits' the property. Metal soil/waste pipes might sometimes present such an opportunity (in which case they should be main bonded, anyway) but, in the absence of them, is there every really any doubt as to whether the water pipes 'go' anywhere which would result in a requirement for them to be bonded? If there is a gas supply, then there will usually be continuity between that and the water pipework (e.g. at boiler), but provided the gas pipe is properly bonded, that would not, in itself, create a need to bond the water pipes, would it?

Kind Regards, John

 

[JohnW2]

Would you call my water pipe 'the earth' because it represented easily the 'most effective earth' for my installation (I would think not)?

No, because it is not allowed ... Plus, it could disappear tomorrow.

Indeed.

... and your installation is configured for a TT.

It would be TT either way, wouldn't it, the only issue being that, as you say, 'reliance' on the water pipe is not allowed (not the least because it could theoretically disappear tomorrow)? I'm not quite sure what "configured for a TT" really means. I have a MET which, amongst other things, has two G/Y cables connected to it - one goes to the earth electrode (~75&#937 and the other to the water pipe (~0.25Ω, so long as it remains present).

Kind Regards, John

 

[WellKnownSid]

If you mean the bond is connected to the plastic itself then obviously this is pointless.

Yes, wrapped around a plastic pipe and clearly pointless...

I would have said that the main issue is 'what is the point?'. Adding an earth electrode (whether one calls it that or a 'bonded extraneous-c-p') will obviously achieve nothing so long as there is an effective TN-C-S earth. In the very rare event of 'failure' of the TN-C-S neutral (hence also earth), it would leave one with a TT installation (and no neutral), but I'm not really sure to what avail.

What about a loss of the (single, unprotected) source earth?

 

[JohnW2]

I would have said that the main issue is 'what is the point?'. Adding an earth electrode (whether one calls it that or a 'bonded extraneous-c-p') will obviously achieve nothing so long as there is an effective TN-C-S earth. In the very rare event of 'failure' of the TN-C-S neutral (hence also earth), it would leave one with a TT installation (and no neutral), but I'm not really sure to what avail.

What about a loss of the (single, unprotected) source earth?

Loss of it where? Do you mean between the supplier's cut-out and the MET of your installation - if so, that's a risk/consideration which applies in any electrical installation, with any type of earth, and we don't usually consider having a 'secondary earth' just in case that cable somehow becomes broken or disconnected.

If loss of the earth is on the supplier's side of their cutout, then the nature of TN-C-S is such that you would then have no effective neutral, either (hence no functioning electrical installation). In that situation, the (relatively high resistance/impedance) TT earth electrode would provide very little protection against anything. In particular, if (in the extremely rare scenario which so many people seem to worry about!) the supply-side fault was such that TN-C-S earth rose to a high potential relative to earth, the 'high resistance' TT earth rod would make virtually no difference to that.

That's how I see it, anyway. Others may disagree.

Kind Regards, John