caravan park tt system and earth rods

[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.

 

[richy3333]

I've read the thread with interest, particularly as I've just spent considerable time TT'ing static vans that were on TNCS system.

Caravan park systems manifest themselves in various guises with statics, mobile vans, tents etc. And then there are the outbuildings to be earthed

I'm surprised nobody has mentioned GN3 yet, which describes Eng54 and Wirepullers opinions on the different options (and gives diagrams). Both are equally valid and I guess it's horses for courses etc. Personally I prefer separate TT'ing for each static.

One other issue is this magical 200 ohms. The BGB/GN3 et al doesn't say that the Ra has to be below 200 ohms, it just states that over 200 ohms a system can be unstable. Ground conditions also play a huge role in TT'ing, particularly if the ground you have to use isn't great?

 

[Deleted member 9648]

I've read the thread with interest, particularly as I've just spent considerable time TT'ing static vans that were on TNCS system.

Caravan park systems manifest themselves in various guises with statics, mobile vans, tents etc. And then there are the outbuildings to be earthed

I'm surprised nobody has mentioned GN3 yet, which describes Eng54 and Wirepullers opinions on the different options (and gives diagrams). Both are equally valid and I guess it's horses for courses etc. Personally I prefer separate TT'ing for each static.

One other issue is this magical 200 ohms. The BGB/GN3 et al doesn't say that the Ra has to be below 200 ohms, it just states that over 200 ohms a system can be unstable. Ground conditions also play a huge role in TT'ing, particularly if the ground you have to use isn't great?

Yes,this was discussed on another thread....and as we know,if a 30ma RCD is in use a final Zs of 1667 ohms will still provide the required disconnection time,so in theory an Ra of 1667 - R1+R2 would be accaptable......(note I said 'in theory'!):earmuffs:

 

[GLENNSPARK]

I've read the thread with interest, particularly as I've just spent considerable time TT'ing static vans that were on TNCS system.

Caravan park systems manifest themselves in various guises with statics, mobile vans, tents etc. And then there are the outbuildings to be earthed

I'm surprised nobody has mentioned GN3 yet, which describes Eng54 and Wirepullers opinions on the different options (and gives diagrams). Both are equally valid and I guess it's horses for courses etc. Personally I prefer separate TT'ing for each static.

One other issue is this magical 200 ohms. The BGB/GN3 et al doesn't say that the Ra has to be below 200 ohms, it just states that over 200 ohms a system can be unstable. Ground conditions also play a huge role in TT'ing, particularly if the ground you have to use isn't great?

As i pointed out above....<200 ohms as long as you can guarantee stability..........

 

[Engineer54]

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.

I think we somehow got past the OPs PIRs, and on that score, your dead right. He just needs to go through his work methodically and noting down his considered discrepancies, and checking against actual Regs if they are indeed non- compliances or not before issuing his certs.

Yes, much of our disagreements have been about single point, and single rods as i remember. But i think this situation is quite different as the only earthing conductors, apart i think from one or two instances, are the SWA of the distribution cables!!

Though i do take issue that the current trend in the UK of using these short 3/8'' thin rods, can in anyway, shape, or form be considered able to provide a perfectly safe and therefore sound earthing system. Not when that top metre, will be doing little to nothing in the UK's adverse weather conditions, such as freezing temperatures and high temperatures where the soil just dries out. That really doesn't leave much of the rod left, to provide a safe earthing system.

I agree with you completly on the costs of providing a full blown TT system that will deliver the goods come what may!! lol!! Unlike you and the other self employed sparks here, where costs to the client via your quotes and estimates can mean the difference of getting the job/contract, i have no such concerns/worries on the projects i'm involved on. So i can at times go a little overboard on things! ...lol!!!

But we'll leave it there, as we have done in the past, and agree to disagree on certain aspects...

 

[topquark]

Yes,this was discussed on another thread....and as we know,if a 30ma RCD is in use a final Zs of 1667 ohms will still provide the required disconnection time,so in theory an Ra of 1667 - R1+R2 would be accaptable......(note I said 'in theory'!):earmuffs:

Until you start testing at 1x and it doesn't trip eh?

 

[Monkey]

Good morning boys, its a good subject this one,:6: having read all the posts here s one( if i can load it on the screen) is a picture from a site I tested the last electrician decided to cut off all the bajos that had been joined together thus removing the earth path of the ring yes PME site with plenty of earth rods so he got confused about earthing leaving swa cables unprotected (204 was replaced )

 

[Deleted member 9648]

Are you sure you have not introduced a PME earth by joining them up again?

 

[Monkey]

Hi wirepuller I have introduced back and stopped it at the point were the caravans join the supply!

 

[richy3333]

Monkey - can you clarify. Do you mean that the SWA supply is on the PME and each hook-up box is TT'd (that's what I think you are saying)?

 

[Monkey]

Hi richy3333

yes it is now

but before the swa had no earth parth

 

[Engineer54]

Hi wirepuller I have introduced back and stopped it at the point were the caravans join the supply!

Yep, ....All your doing there is, maintaining the continuity of the PME's SWA earthing system, your not incorporating any fly leads from those banjo link connections...

 

[richy3333]

Thanks for clarifying Monkey. Sounds good to me.

I label my Hook-up boxes with a warning about the earthing type, just in case some muppet 'looks at it' sometime in the future - can't say they weren't warned about what has happened!

 

[GLENNSPARK]

Fancy goin round croppin all the banjo`s??????????