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

 

[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...

 

[Monkey]

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

 

[mcmainelectric]

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

 

[GLENNSPARK]

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...

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...........

 

[GLENNSPARK]

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

its a bit like "daisychaining" then innit.........

 

[Engineer54]

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

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...

 

[GLENNSPARK]

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...

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.........

 

[Monkey]

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

 

[Engineer54]

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

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 ....

 

[Monkey]

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

 

[GLENNSPARK]

But surely if you rod it (1 deep rod for a lower Ra shared between 4 pitches) then whats PME got to do with it anyway?..........