No one near me wants this job - New ground rod

[FrodoOne]

Hi all,

I've failed now three times to get even a quotation out of nearby electricians to do this job.

Power comes to my house overhead, and we have a ground earth rod in the back garden. For some reason, years ago, the inspection pit was installed proud of the concrete base that it passes through and the condition is poor in the following ways:

- Rod appears heavily corroded. Perhaps it's fine under the surface rust?
- Clamp cracked
- Whole thing is a bit loose (kids play football out there).

I feel it needs replacing with new one nearby, installed right to ground level ideally with recessed pit. Connected up, and tested.

What am I missing?

If you were being asked to do this job, would you prefer if the home owner had already set the new rod into the ground and just asked you for connection & test?

At the very least, surely a new clamp is needed.

Grateful

While there have been some later discussions concerning TT Systems,
I have not seen it established whether the Earthing System involved is TN-C-S, TT or what?

Of course, I am "sticking my neck out" - once again - in commentating on something of which I do not know the actual details.
I am quite familiar with the TN-C-S system, almost universally used in Australia and North America.
(However, I am not familiar with TN-S and TN-C, often used in densely packed towns in Europe and the UK, although I do understand the principles. - See https://en.wikipedia.org/wiki/Earthing_system )

"In the UK, a common practice with TN-C-S is to connect the combined PEN supply conductor to Earth at multiple points along its length between the source transformer and the consumer installation.
This is known as protective multiple earthing (PME).
This is so common that consequently PME is often incorrectly used as a synonym.
Similar systems in Australia and New Zealand are designated as multiple earthed neutral (MEN)
and, in North America, as multi-grounded neutral (MGN),
as opposed to the singularly grounded neutral of, a TN-S or TN-C system."

Now, "Why is this so?"
It has to do with a phenomena known as "Lightning Discharge" - which is NOT the same as a "Lightning Strike".
When Lightning Strikes the Earth, (or, more accuracy), the Earth Discharges the lighting,
there will be a large current within the "conductive" earth which will produce a "potential gradient",
where there will be an almost instantaneous potential difference between points which are only a few "distances" apart'.
While a MEN/MGN system attempts to "even-out" such disparity,
it is VITAL that the Earthing-Path at any "Installation" be of low IMPEADANCE.
While one may think of a "Lightning Discharge' DC, it is an almost "Square Wave" which contains Very High Frequencies.

Hence, the Discharge Path - to any and all Earth Electrode- must be of low IMPEADANCE - NOT just Low Resistance.

Here, I must refer you (once again) to

particularly, to that from about 21:10

Good Luck.

(One further comment,
Australian/NZ and North American regulations seem to require that the "Earthing/Grounding"electrode
be as close to the building a possible,
which seems not to be so in the photographs concerned.)

 

[JohnW2]

.... "In the UK, a common practice with TN-C-S is to connect the combined PEN supply conductor to Earth at multiple points along its length between the source transformer and the consumer installation. ... This is known as protective multiple earthing (PME). ... This is so common that consequently PME is often incorrectly used as a synonym. ..."

That's all true - particularly the last bit about "TN-C-S" and "PME" being very commonly used (incorrectly) as synonymous, even though they relate to two totally different things. In the UK (and perhaps everywhere?) TN-C-S is only allowed IF there is PME (but see below about the "M"!).

Of course, in the UK (and I presume probably everywhere else), historically speaking nothing started as TN-C-S, with TN-S or TT (meaning no earth provided by supplier) being all that existed. Most TN-C-S therefore arose by 'conversion', most commonly because of failure of the cable sheath that had been providing a TN-S 'earth'.

I originally assumed tht the "M" of "PME" meant that there were multiple connections between neutral and earth along the length of the network cable. However, I was disillusioned when we were told here (by a much loved contributor who is sadly no longer with us) by someone involved with the networks that creation of PME to allow TN-C-S to be provided very often involved only the addition one extra connection to earth - at the end of the network cable run. The "multiple" would then just be "two" - one at the transformer and the other at the far end of the cable.

I don't think that there is any doubt that the 'ideal system' is TN-S, provided it is working satisfactorily, since there is then a separate, direct, connection from the neutral side of the transformer to each installation's 'earthing system'. However, as above, one problem is that the cable sheaths providing such an earth do eventually 'fail' - often, as I said, resulting in 'conversion to TN-C-S' (since that is far cheaper than replacing all of the cable).

TN-C-S is generally OK, although it does inevitably result in some potential difference between the installation's 'earth' system and the surrounding 'true earth'. The problems (potentially dangerous ones) arise if a fault (break) occurs in the neutral conductor of the supply cable, something you will often read about here.

Now, "Why is this so?" ... It has to do with a phenomena known as "Lightning Discharge" - which is NOT the same as a "Lightning Strike". ... When Lightning Strikes the Earth, (or, more accuracy), the Earth Discharges the lighting, ... there will be a large current within the "conductive" earth which will produce a "potential gradient", .... where there will be an almost instantaneous potential difference between points which are only a few "distances" apart'.

I think we've been through this with you in the past, since there seem to be some conceptual differences between our countries. At least historically, in the UK, a DNO-supplied 'earth' (TN-S or TN-C-S) is really only there the facilitate 'fault protection' in the event of faults between L and touchable parts of installations, whilst anything to do with the consequences of lightning is dealt with separately, involving local 'earth electrodes' - although that may have changed a little with the advent of SPDs. It seems that thinking may be different in your country?

(One further comment, ... Australian/NZ and North American regulations seem to require that the "Earthing/Grounding"electrode ... be as close to the building a possible,
which seems not to be so in the photographs concerned.)

As you say, no such explicit requirement exists in UK. In terms of function as an earth (fault protection etc.), what matters is the impedance of the path to the electrode, regardless of length (so, in that sense, a very long but very fat conductor would be acceptable), but there is a possible issue of potential gradients in the ground if the electrode is distant from the building.

 

[ebee]

what matters is the impedance of the path to the electrode, regardless of length (so, in that sense, a very long but very fat conductor would be acceptable), but there is a possible issue of potential gradients in the ground if the electrode is distant from the building.

I`m not actually aware of (in the UK) any specific mention of the distance of the electrode to the installtion earth reference of the supply concerned.
Of course I might be wrong.
I can see that a limit on the distance might be desireable in any event though.

 

[JohnW2]

I`m not actually aware of (in the UK) any specific mention of the distance of the electrode to the installtion earth reference of the supply concerned. ... Of course I might be wrong.

As I wrote, that is also my understanding (although I, too, 'might be wrong'!). As far as within-building fault protection is concerned, I would think that all that really matters is the impedance of path to the electrode (regardless of length) - and with any credible CSA of earthing conductor (and any reasonably credible length) that would be totally overshadowed by the impedance/resistance of the electrode itself.

I can see that a limit on the distance might be desireable in any event though.

Yes, as I also wrote, I can see that there could be issues with 'potential gradients' if the electrode was distant from the building - particularly for someone with 'one foot in and one foot out'!
Having said that, virtually every TT electrode I can recall having seen has been extremely close to the building (for fairly obvious reasons).

Kind Regards, John

 

[ebee]

there could be issues with 'potential gradients' if the electrode was distant from the building - particularly for someone with 'one foot in and one foot out'!

Well to be a bit safer it might be adviseable to not do the Hokey Cokey then !