caravan park tt system and earth rods

M

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
 
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not sure on this one, but i would prefer to see individual reods, if only to counter the problem if a sub-main earthing system were to fail due to corrosion or other cause, that would take out the earthing for 50 caravans.
 
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Is that 8ohms at the Ze or as you say at the hook up, it reads to me that your getting a Zs of 8ohms ? What Ra are you getting at the electrode?

Reg 708.553.1.14 just tells us that each socket must be connected to an earth electrode, not that each socket or hook up needs it's own earth electrode so the one electrode is OK, providing it is giving you a good and stable value.

I'm a little dubious of this 8 ohm reading your getting, especially as there are so many hook ups and therefore there must be distances involved. Are you getting this 8 ohms through parallel paths?
 
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There will be only one Ze measurement for the install at the origin.

What you have been recording is Zs for the sub-main circuits from the origin to the hook-up points.

A single electrode whilst technically conforming to regs is a bit pants IMO, as Malc has said the whole installation relies on this and should it go pear shaped........!! The more earthing points that exist the better be they rods, tapes, mats etc.

What is the supply earthing system??? what OCPD's & fault protection do you have at the origin protecting the sub-mains???
 
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mcmainelectric said:
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
Click to expand...

Personally i would be using the DNO's earthing system for the mains and sub-main distribution locations, only having the TT system supplying the caravan pitches. If these pitches are grouped in say 4s then i would certainly have a minimum of one earth electrode position serving that group. Better still, linking those group electrode positions with the other group electrode positions. Depending on your soil type and it's resistivity and electrode depths, you could well find you have a site earthing system comparable with some TNS systems....
 
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just to clarify any ambiguity, I'm only testing the caravans themselves not the park the ZE for the caravan at the hook up is 8-9 ohms (or the ZS for the submain depending on which way you look at it) Main incomming supply to park is TT, which then filters out to 4 to 5 submain points where the earth rods are 1 per submain (each submain feeding 50 ish caravans) then either a looped out in swa to each hook up point or a single cable to individual hook up. all hook ups are on there own with 1 van per hook.
from the submain the cpc is provided by a swa armour or a cable core usually 2.5mm
at the hook up is either a 16a double pole rcbo or a rcd and mcb 16a.
It has opened a mine field in my mind as the blue hook up plugs/sockets are only splashproof but not weatherproof although the regs 708.553.1.8say this is allowed surely they do not meet the ambient conditions , no discrimination of mcbs with caravans haveing a 32amp ring circuit inside but with a 16amp supplymcb outside, and 30m/a rcds on the inside of the van and on the hookup box which offer no discrimination as to which one will trip first or both under a fault condition,
so does the concensus appear to be that the individual caravans should have individual earth rods for the sake of a better more reliable system but a common rod for 40-50 vans is acceptable
 
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That system has been installed by an electrician that doesn't understand TT systems/distribution. If your testing the caravans themselves, i take it theses are residential type caravans and not the typical touring type?? Though it would be beneficial, there is as i understand, no requirement to provide individual earth electrodes to each pitch location.

It does sound like a bit of a mish mash, in the way the overall installation has been distributed around this site, as well as the protection and discrimination that's been afforded!! From what your telling us here, the overall design of the distribution is abysmal, and it's a big site by anyone's standards.

Any chance of some photo's, say of the main incomer and distribution panel, a typical example of one of the sub-main distribution panels, and one or two of the pitch hook up panels?? What sort of size are these distribution cables, supplying the caravan pitches, and what sort of distances are we taking about??

More importantly for you, what would you say the chances are, of the owners spending any money putting things right on this site??
If the answer is None, ....then just carry on with the work you've already been given, noting all the discrepancies as you go. As the saying goes ...you can lead a horse to the water but you can't make him drink the water...lol!!!
 
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As we said your doing the Zs/zdb not the Ze. As I said I would be doing a proper Ze, at the origin to see what that electrode is reading, and I would be totally amazed if you got 8ohms.

IMO your getting parallel paths that is giving you such a good Zs/Zdb. My first job would be the Ra, see what you have and decide from there.

As your doing a PIR and the sockets do conform to reg 708.553.1.8 then you can not really code it. If though you feel that the sockets only meet the minimum requirement and you recommend that they need changing you will ahve to accompany the report with a letter of these recommendations.

There is not a lot you can do about the discrimination part. The regs tell us in 708.553.1. 14 that each socket must be protected by it's own RCD. Then reg 721.411.1 tels us that you also need a double pole RCD of 30mA for the caravan installation.
 
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Engineer54 said:
Personally i would be using the DNO's earthing system for the mains and sub-main distribution locations, only having the TT system supplying the caravan pitches. If these pitches are grouped in say 4s then i would certainly have a minimum of one earth electrode position serving that group. Better still, linking those group electrode positions with the other group electrode positions. Depending on your soil type and it's resistivity and electrode depths, you could well find you have a site earthing system comparable with some TNS systems....
Click to expand...
So what you would be effectively creating here eng is a large "grid" of rods with a surefire way of getting a low Ra............
 
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Hi there First You say the main in comer is TT
If this is correct Y would you require more earth rods
from this point all Earth continuity is no different If you were designing or testing
ZS is = TO ZE + R1+R2 YOUR ZE WILL be high cause its TT but your r1 and r2 and volt drop all the same
you either chosse to start with TT or end it with TT but cost will come in to it testing these places can be nightmare
 
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Monkey said:
Hi there First You say the main in comer is TT
If this is correct Y would you require more earth rods
from this point all Earth continuity is no different If you were designing or testing
ZS is = TO ZE + R1+R2 YOUR ZE WILL be high cause its TT but your r1 and r2 and volt drop all the same
you either chosse to start with TT or end it with TT but cost will come in to it testing these places can be nightmare
Click to expand...
So what you are saying is go for 1 deep rod at the incomer and use the supply cables to each pitch as given cable csa impedence (Ze) should be sufficiently low and this arrangement should also make it easier for testing purposes..........
 
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GLENNSPARK said:
So what you would be effectively creating here eng is a large "grid" of rods with a surefire way of getting a low Ra............
Click to expand...

In a way Yes, but it wouldn't be classified as a ''Grid'' as such!! You would just be linking out the earth electrode positions at the pitch groups. It'll be more of a linear installation than an actual grid. Much the same as they did in the past with TN-S, DNO distribution supply cables, they used to rod some of the joints on long strings many years ago...

I only think in terms of multiple electrodes at a earthing position, So yes, depth equals stability and generally lower Ra levels.
Can't think of the last time i saw a single rod being used!! ...lol!!! To be honest, i can't remember ever using 5/8'' rods either, let alone those 3/8th things they have the cheek to call earth electrodes!!!
 
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GLENNSPARK said:
So what you are saying is go for 1 deep rod at the incomer and use the supply cables to each pitch as given cable csa impedence (Ze) should be sufficiently low and this arrangement should also make it easier for testing purposes..........
Click to expand...

I think he's saying, he would be basically ignoring the Ra level if under 200 ohms and be totally relying on the RCDs for any earth fault protection. Not my idea of a working TT system at all!!!
 
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Engineer54 said:
I think he's saying, he would be basically ignoring the Ra level if under 200 ohms and be totally relying on the RCDs for any earth fault protection. Not my idea of a working TT system at all!!!
Click to expand...
Yeah i have read one of your posts before eng and you arn`t right keen on this 200 ohm thing are you.....:cool3:
 
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Not on TT systems, because of the very low UK requirements for a TT system , RCD are the ONLY protection against earth faults...
And god help you when they Fail!!! lol!!!
 
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Engineer54 said:
Not on TT systems, because of the very low UK requirements for a TT system , RCD are the ONLY protection against earth faults...
And god help you when they Fail!!! lol!!!
Click to expand...
No probs eng. Ta for clearing that up mate. So you would go for a multiple rod (1 for 4 pitches) as you posted on here earlier to reduce the chances of an RCD failure at the incomer. You could rely on the RCD at the hookups/caravan board that way couldn`t you..........still would need to be a deep rod though.............
 
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GLENNSPARK said:
No probs eng. Ta for clearing that up mate. So you would go for a multiple rod (1 for 4 pitches) as you posted on here earlier to reduce the chances of an RCD failure at the incomer. You could rely on the RCD at the hookups/caravan board that way couldn`t you..........still would need to be a deep rod though.............
Click to expand...


A good TT system, won't reduce the chances of RCDs failing, but with a low enough Ra it can still give a means of protection, if or when an RCD device fails.

Because TT systems are now totally based on RCD protection, you will rarely be able to achieve Reg stated disconnection times on a RCD failure via the TT systems Ra value (but it's by no means ''impossible'')!! But with a a decently low Ra value, the protection device will trip on an earth fault, depending on Ra values, protection type, and protection ratings. To my mind, if a protection device tripping in 3 or 4 secs or even more, .....it's got to be far better than no chance of any tripping at all. So the lower you can get your Ra values down too, will allow the higher protection devices to trip, and the lower rating devices to trip faster. I maintain, that it perfectly plausible to get Ra values down to 3 to 5 ohms, IF your soil resistivity is not that bad, are prepared to spend time finding the best electrode location(s) and some money on driving a number of earth electrodes in the ground. (Commercially in most instances, that isn't viable, in today's climate. But in the case of the above senerio perfectly viable. ..lol!!! )

In most cases you are also connecting parallel paths, via main bonded gas and water pipes, and although you should not rely on such earth paths, when coupled to a good stable Ra value, the overall Zs value can disconnect protective devices in some pretty reasonable times...
 
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simple just use earth rods as near to the end as pos this way you have les chance of rcd falilure remember Esqcr does not per mit the use of pme on buildings made of matel and caravans be it static or portable Rcd should be checked more now than ever
 
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so to sum it up its not against the regs but it is poorish design, that on a tt supplied site the individual caravans do not need to have a seperate earth rod conductor and can rely on a multiple used rod futher up the installation
 
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Engineer54 said:
A good TT system, won't reduce the chances of RCDs failing, but with a low enough Ra it can still give a means of protection, if or when an RCD device fails.

Because TT systems are now totally based on RCD protection, you will rarely be able to achieve Reg stated disconnection times on a RCD failure via the TT systems Ra value (but it's by no means ''impossible'')!! But with a a decently low Ra value, the protection device will trip on an earth fault, depending on Ra values, protection type, and protection ratings. To my mind, if a protection device tripping in 3 or 4 secs or even more, .....it's got to be far better than no chance of any tripping at all. So the lower you can get your Ra values down too, will allow the higher protection devices to trip, and the lower rating devices to trip faster. I maintain, that it perfectly plausible to get Ra values down to 3 to 5 ohms, IF your soil resistivity is not that bad, are prepared to spend time finding the best electrode location(s) and some money on driving a number of earth electrodes in the ground. (Commercially in most instances, that isn't viable, in today's climate. But in the case of the above senerio perfectly viable. ..lol!!! )

In most cases you are also connecting parallel paths, via main bonded gas and water pipes, and although you should not rely on such earth paths, when coupled to a good stable Ra value, the overall Zs value can disconnect protective devices in some pretty reasonable times...
Click to expand...
That being said that a parallel resistance/impedence will bring the overall value of said resistance/impedence down....a bit like an IR test at the tails...........
 
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Monkey said:
simple just use earth rods as near to the end as pos this way you have les chance of rcd falilure remember Esqcr does not per mit the use of pme on buildings made of matel and caravans be it static or portable Rcd should be checked more now than ever
Click to expand...


On an installation such as this one, that would be a very bad idea. Any loss of that distributed earth, by alterations to the distributed supply cables or such, and you are left with no earth past the break. In these circumstances, it's far better to not to rely on the SWA of a cable and have earth electrode positions around the site. Nothing to stop you using the SWA as an additional means of transporting your earth system mind...
 
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Engineer54 said:
On an installation such as this one, that would be a very bad idea. Any loss of that distributed earth, by alterations to the distributed supply cables or such, and you are left with no earth past the break. In these circumstances, it's far better to not to rely on the SWA of a cable and have earth electrode positions around the site. Nothing to stop you using the SWA as an additional means of transporting your earth system mind...
Click to expand...
This seems a better setup to me for the reasons you have stated. After all we used to have a static caravan and when you decide you want to move it to a new site (as we did with ours) they will use a great big tractor to yank it free and the mess of the ground it made.........easy to disturb buried cables etc here.........
 
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But the loss of earth is what the PME is about not being used if the natrual goes down in the street you ve lost it all and the earth rod becomes the earth path also any alterations to it would be recorded and it would not be pos to lose the earth path because of testing. Caravan parks need to be addressed make it more clear you have camping(tents)
caravans that come and go you have static sites and static sites that people can live in all year round and only two pages cover it. sorry soap box time again
 
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Monkey said:
But the loss of earth is what the PME is about not being used if the natrual goes down in the street you ve lost it all and the earth rod becomes the earth path also any alterations to it would be recorded and it would not be pos to lose the earth path because of testing. Caravan parks need to be addressed make it more clear you have camping(tents)
caravans that come and go you have static sites and static sites that people can live in all year round and only two pages cover it. sorry soap box time again
Click to expand...

Not so!! it's the loss of a neutral that's the big problem on a PME system ....

But were not talking about PME here were talking about achieving and maintaining a good TT system.
Anyone will tell you, the more deep driven electrode rods you can link together the better your system will deliver the goods. If your going to have a single area where your rods are going to be driven on an installation such as a caravan site, then you really need to distribute that earth system with its own earthing conductors, and not solely by a cables SWA ....
 
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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
 
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Monkey said:
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
Click to expand...

?????? ...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!!!!
 
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Engineer54 said:
?????? ...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!!!!
Click to expand...
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 :)
 
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topquark said:
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 :)
Click to expand...
Same here what i think he`s on about is the PEN conductor.................
 
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topquark said:
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 :)
Click to expand...


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
 
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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.
 
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wirepuller said:
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.
Click to expand...
so on that basis then lets assume its a new install job......Multiple rod system or single?............
 
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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.
 
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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......
 
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caravan park tt system and earth rods
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