Two types of earthing

[mhar]

Been at a chalet in a holiday park. There is a 2 core swa distribution feed to the chalet glanded to an external ip rated enclosure which feeds through to a connected enclosure housing to a meter. Feed then goes from the meter back into first enclosure to din rail mounted dp isolator from which di meter tails into the (internal) cu. Below is an earth rod, the 16mm earthing conductor following a different route into the property to the cu. 6" from the rod the earthing conductor has been cut and a loop of 16mm cable crimp joined. This loop goes up to the enclosure with the isolator where it is connected to a din rail mounted connector next to the isolator and where a 16mm tail has been connected from the incoming swa banjo. Basically this 'new' earth from the swa has been inserted into the tt system and this appears to be the case at all neighbouring chalets. Zdb is .36 Ohms but when the earth path from swa is disconnected (just using the rod) it is 634 Ohms in sodden ground. Obvious that an improvement was required but puzzled as to why it had been done in this manner leaving the rod connected and at the neighbouring chalets as well.
Rang the ELECSA helpline as I have no way of determining the type of earthing system the swa is connected to and to enquire whether I should ditch the tt system and take my earth direct from the swa banjo avoiding the (poorly) crimped loop of 16mm which has been introduced. Their advice concurred with my thoughts that there should only be one type of earth system so ditch the rod and the only truthful entry on the certificate is 'lim' to describe earthing system.
Quite happy with their response until I started thinking!!! On any tn system where the Zs is lower than the Ze we effectively have two types of system anyway so, as long as the rod is in close proximity to the property so as not to vastly increase the size of the equipotential zone, is there any real reason to ditch the rod? In this particular case the rod is within the footprint of the building as it is under the roof overhang. I do not yet have a copy of gn8 to refer to.

 

[telectrix]

what's the construction of the chalets? if metal, you could have a problem if the earthing is pme.

 

[mhar]

Timber framed. Not a 100% certain about the roof though, looks like the fibreglass equivalent of metal sheeting. Unable to ascertain the earthing system but there are also many metal mobile homes on the site so I doubt pme but as stated, unable to ascertain the earthing system both by looking or by enquiry. I have asked the chalet owner to enquire of the site owners but have been advised not to hold my breath!

 

[telectrix]

OK. so what are you there for? periodic or installation work?

 

[mhar]

Alterations, additions and replacements which will probably result in cu change due to dampness and advisability of splitting loads accross two rcd's, two x humidistat fans and possible underfloor heating, elec shower replacement + smoke alarm. Got to go out for a few hours so will respond further later.
My scam providers are happy for me to certificate this install with the 'lim' on earthing system. Main query is the need to ditch the earth rod as per the last half of the last paragraph of my original post. At first I thought I should (as did ELECSA) now I may have talked myself out of it! Looking for opinions

 

[Rockingit]

I've known it in the past where the DNO have been resistant to providing PME to all timber structures (cricket pavilions are a good example), so it may have been this. Or, it might be the case that for whatever reason the Zs values were too high, so someone has added a rod in an attempt to drive the Ze down.

What I would say for certain is that until you've 100% validated the type and condition of the 'supply' earthing, I wouldn't go disconnecting that rod! Generally speaking, people only do things that involve time and materials for a reason.

 

[telectrix]

agree with above. i'd see if i could ascertain why the Ra is so high on the rods.

 

[mhar]

Thank you both.
Rockingit, your last sentence was what raised alarm bells. Doing it the way they have (everywhere) is more effort and materials, kept thinking there has got to be a reason. Be nice to find out what reason someday.
Telectrix, agree totally but can't see the owner going for it. He is THE man, with his pointy black shoes and gay hairdressers bmw convertible!

 

[Engineer54]

On any tn system where the Zs is lower than the Ze we effectively have two types of system anyway

Do you, how do work that one out, ...because i can't?? So what your saying is if you have a TN Ze of 0.30 and a Zs of 0.20 you have two types of earthing system?? Don't suppose parallel earths such as main bonding, have anything to do with the decrease then??

Why can't YOU contact the site owners to either gain access to check yourself or have them confirm earthing system, via previous site periodic test certificates??

 

[mhar]

E54 Don't understand your point. Ze of 0.30 is the connection to the distributors earth facility. By bonding to the incoming water / gas we are introducing another path to earth producing the Zs of .20 in your example. We haven't made another connection to an additional distributors earth, we have provided a separate path to earth within the footprint of the building. This second separate path is not tn so I visualise it as a tt earth (by default and fortuitous). Is this wrong? On older installs where no distributors earth was used but the main water pipe was used then this is tt, just not through a rod. I am not comfortable with the advice from my scheme provider about disconnecting the rod even though it has an poor ra, the fact that all 12 chalets in the immediate area have been treated the same makes me think there must / could well be a reason for this having been done which I should not alter without understanding the reason why, if any. I understand the reason for bonding and that it is a separate issue from earthing but the effect can be to provide another path to earth. I think I understand the reasoning why we disconnect the tt earth when a supply is changed to tn which is to limit the size of the equipotential zone, however in this case the rod is within the footprint of the building and I see little reason to disconnect it. The reason for posting is to have any flaws in my thinking pointed out.

Of course I have tried contacting the site owners, sometimes these things are not as simple as they should be. Strictly speaking I do not have an interest in the site as I do not own anything there, hence my request is now via my customer who does have an interest. Talking to others on the site we could be waiting a long time. Additionally I believe my customer to be full of sh£t and would not wholly trust any information gleaned by him unless supported by documentary evidence. However you have made me realise that the potentially vague question I have asked my customer to ask regarding the earthing system could be better, a simple direct request for the site PIR/EICR is much more likely to get the results, still not holding my breath though.

 

[Engineer54]

My point is, that main bonding of a water pipe, is not considered as a separate means of earthing.
Water and gas pipes etc, can bring extraneous earths into a property and so are bonded to the MET/EMT to limit touch voltage. Bonding and Earthing are not the same thing...

No problem to leave the less than useless existing Earth Rod in place, (it'll probably only be one of those thin sticks anyway) In fact, early council estate TN-S systems in the 50's were generally provided with a local earth rod or plate, not that any of them would be approaching anywhere near 650 ohms mind, or near a tenth of that figure.

Yep, you can always ask for a copy of the last PIR/EICR, if in fact the owners actually bother having them done regularly.... Some of these site owners seem to be able to get away with murder these days lol!!

I was not in a good mood this morning (my morning here) so might have come across as a bit short with you, my apologies if i did!!!....