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

 

[telectrix]

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.

 

[malcolmsanford]

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?

 

[Guest123]

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???

 

[Engineer54]

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

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....

 

[mcmainelectric]

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

 

[Engineer54]

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!!!

 

[malcolmsanford]

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.

 

[GLENNSPARK]

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....

So what you would be effectively creating here eng is a large "grid" of rods with a surefire way of getting a low Ra............

 

[GLENNSPARK]

Depth = stability though?...........

 

[Monkey]

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

 

[GLENNSPARK]

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

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..........

 

[Engineer54]

So what you would be effectively creating here eng is a large "grid" of rods with a surefire way of getting a low Ra............

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!!!

 

[Engineer54]

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..........

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!!!

 

[GLENNSPARK]

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!!!

Yeah i have read one of your posts before eng and you arn`t right keen on this 200 ohm thing are you.....:cool3:

 

[GLENNSPARK]

I mean, isn`t an RCD "additional protection" and shouldn`t be relied on purely as additional AND fault protection....

 

[Engineer54]

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!!!

 

[GLENNSPARK]

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!!!

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.............

 

[GLENNSPARK]

This setup would give you some discrimination n all wouldn`t it?.........

 

[Engineer54]

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.............

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...