caravan park tt system and earth rods

[mcmainelectric]

Hello everyone,
Have been inspecting caravans statics at a 200+site. The levels of ZE are 8+ ohms at the caravan hook up and it is on a TT system (the incomming mains) to 4 or 5 submain distribution points where there is a common earth rod and from there via swa using the cable core or armourmed as the cpc to the individual hook up points.
My question does each caravan need its own earth rod? or is using the collective one at the submain distribution point acceptable?
I feel the tt system is being turned into a poor tns, but i cant get any answers from the regs to help
regards

 

[Engineer54]

Thats right thats what I am saying too thats why it has be TT other wise any parallel earth path becomes the earth further down the line TT breaks the two systems

anyway roast lamb for dinner I;m of

?????? ...Huh! No-one's suggested using PME!!!! The whole site including the main incoming supply is TT, where did you get the PME from for god's sake?? lol!!!!

 

[topquark]

?????? ...Huh! No-one's suggested using PME!!!! The whole site including the main incoming supply is TT, where did you get the PME from for god's sake?? lol!!!!

I think what he's trying to say is that's why you can't use PME; because of the chance of loosing the neutral and the corresponding consequences. But that's not how I read his post the first time (it's not very clear). I, as you, thought he was suggesting that loosing the earth was the problem with PME

 

[GLENNSPARK]

I think what he's trying to say is that's why you can't use PME; because of the chance of loosing the neutral and the corresponding consequences. But that's not how I read his post the first time (it's not very clear). I, as you, thought he was suggesting that loosing the earth was the problem with PME

Same here what i think he`s on about is the PEN conductor.................

 

[Engineer54]

I think what he's trying to say is that's why you can't use PME; because of the chance of loosing the neutral and the corresponding consequences. But that's not how I read his post the first time (it's not very clear). I, as you, thought he was suggesting that loosing the earth was the problem with PME

Yeah, .... i just read it again, myself...lol!! Must be thinking too much about his Sunday lamb roast dinner to get his thoughts straight!!:whatchutalkingabout

 

[topquark]

@monkey, have a read of your post #27 again, and you'll see what's caused the confusion: http://www.electriciansforums.net/e...n-park-tt-system-earth-rods-3.html#post355788

 

[Deleted member 9648]

Having had this discussion with E54 before,we are never going to agree on TT systems...fair enough,but I would like to make two points.
1. If the OP was carrying out a PIR on the supply system the only consideration is whether the earthing system complies with bs7671....the fact that there is a preference for multiple rods is irrelevant,a single rod with an Ra below the required value will meet the requirements,and for the purpose of a PIR that is the only consideration.
2. What is the difference between a single point of earthing via an electrode.....and a single point of earthing via a DNO supplied earth?....they are both single points of earthing and thus just as likely to be lost.

 

[GLENNSPARK]

Having had this discussion with E54 before,we are never going to agree on TT systems...fair enough,but I would like to make two points.
1. If the OP was carrying out a PIR on the supply system the only consideration is whether the earthing system complies with bs7671....the fact that there is a preference for multiple rods is irrelevant,a single rod with an Ra below the required value will meet the requirements,and for the purpose of a PIR that is the only consideration.
2. What is the difference between a single point of earthing via an electrode.....and a single point of earthing via a DNO supplied earth?....they are both single points of earthing and thus just as likely to be lost.

so on that basis then lets assume its a new install job......Multiple rod system or single?............

 

[topquark]

Agreed, that was what was trying to be uncovered, whether it was indeed the Ra of the rod leading to that low value, or "other extraneous factors".

Not a lot of difference, IMHO I think the main difference would be down to environment. If you've seen what goes on at a lot of these sites, vans moved on off, with very little room for manouver. Tourers in/out on a regular basis (often driven by Reginald Molehusband (ficticious name incase that causes offence) ). Large ride on lawnmowers.

 

[topquark]

so on that basis then lets assume its a new install job......Multiple rod system or single?............

Good question, can I vote for TNS please ... Multiple for me.

 

[GLENNSPARK]

I would go for a multiple rod system (1 deep rod for 4 caravans) as eng says. Deep rod for stability. This should give you better discrimination and as you say TQ, less likely to suffer ground disturbance as a result of heavy towing equipment, latrine diggers or whatever going round plowing up your nice cables..lol...
Downside to this: Increased cost due to time and materials.....I guess its a ballancing act this one......

 

[Deleted member 9648]

so on that basis then lets assume its a new install job......Multiple rod system or single?............

DNO to cover distribution circuits,and one rod for each final distribution board serving hookups...separated from DNO.

I do not subscribe to the multiple rods in order to achieve a very low Ra. My argument being that it is not practical to achieve a low enough and stable Ra for operation of an overcurrent device. IMO you are always going to be reliant on the RCD for earth fault protection on a TT,therefore what is the point in going to expensive lengths to get an Ra of say 5 ohms,which still will not be low enough for the overcurrent device...so you are still reliant on the RCD,therefore you may as well settle for <200 ohms with a single rod.

This is purely my opinion,and despite previous altercations on this subject...I accept that E54's view is equally valid.

 

[GLENNSPARK]

DNO to cover distribution circuits,and one rod for each final distribution board serving hookups...separated from DNO.

I do not subscribe to the multiple rods in order to achieve a very low Ra. My argument being that it is not practical to achieve a low enough and stable Ra for operation of an overcurrent device. IMO you are always going to be reliant on the RCD for earth fault protection on a TT,therefore what is the point in going to expensive lengths to get an Ra of say 5 ohms,which still will not be low enough for the overcurrent device...so you are still reliant on the RCD,therefore you may as well settle for <200 ohms with a single rod.

This is purely my opinion,and despite previous altercations on this subject...I accept that E54's view is equally valid.

<200 ohms.....as long as you can guarantee stability also, i would still be concerned about buried cables on a caravan site as the ground dont arf get chewed up when there draggin em around....would go for a 1 rod for 4 pitches to be safer than sorry and the site owners would just have to stand the cost.............

 

[Engineer54]

Having had this discussion with E54 before,we are never going to agree on TT systems...fair enough,but I would like to make two points.
1. If the OP was carrying out a PIR on the supply system the only consideration is whether the earthing system complies with bs7671....the fact that there is a preference for multiple rods is irrelevant,a single rod with an Ra below the required value will meet the requirements,and for the purpose of a PIR that is the only consideration.
2. What is the difference between a single point of earthing via an electrode.....and a single point of earthing via a DNO supplied earth?....they are both single points of earthing and thus just as likely to be lost.

Very true, and in most instances i'd be totally in agreement with you, but when were talking about an installation such as a caravan site, things become a little different. On single point TT earthing that probably already has a high value, and using the SWA of sub distribution, and final distribution cables, it's going to get even higher passing through all those termination points. The chance of losing what little you do have, over what is going to be pretty long distances, also makes distributing solely by means of cables SWA a bad choice. Couple that, with the very real chance of having maybe several instances of sheath damage to these direct buried cables, and things start going from bad to worse!! ...lol!!!

Multiple electrode points however can not only substantially alleviate the above distribution concerns, but also bring Ra levels down considerably, especially when being linked or daisy-chained. Would you not agree that this is a far better earthing distribution method than the single point method above??

 

[GLENNSPARK]

Right guys Im off for my tea (and it int roast) fish pie....yummy..........

 

[Deleted member 9648]

Very true, and in most instances i'd be totally in agreement with you, but when were talking about an installation such as a caravan site, things become a little different. On single point TT earthing that probably already has a high value, and using the SWA of sub distribution, and final distribution cables, it's going to get even higher passing through all those termination points. The chance of losing what little you do have, over what is going to be pretty long distances, also makes distributing solely by means of cables SWA a bad choice. Couple that, with the very real chance of having maybe several instances of sheath damage to these direct buried cables, and things start going from bad to worse!! ...lol!!!

Multiple electrode points however can not only substantially alleviate the above distribution concerns, but also bring Ra levels down considerably, especially when being linked or daisy-chained. Would you not agree that this is a far better earthing distribution method than the single point method above??

Yes,but not essential...and as far as a PIR goes the inspector needs to be very careful over introducing requirements which he percieves to be good practice...but is beyond the requirements of 7671. Whatever the inspector thinks his only duty is to code non compliance.
I think where I have taken issue with you before E54 is your complete dismissal..(in previous threads)...of single point TT earthing,regardless of the fact that such an arrangement properly installed complies with bs7671 and will provide a perfectly safe earthing system. I would agree that your advised earthing system makes a lot of sense...if the additional cost can be justified to the client.