connect caravan to TT system

[jean1972]

Hello
I need connect caravan to CU in house with TT system.
I have got 10mm 3 core SWA cable.
Do i need separate earth electrode for caravan, or connect armour and 3th core to CU in house ?
Distance from house to caravan is 15m.

 

[Engineer54]

The Ra of the existing TT system is not going to be very good to start with if going by other posts here is anything to go by. So why don't you drive another couple of rods in via a coupler to give you a bit of depth. Then couple/link the new rod to the existing rod to improve the overall system Ra.

 

[nickblake]

what i would do is to install a caravan hook up point at the position of the caravan with an earth rod to the pen conductor of the socket outlet as described in the regs , the hook up points should consist also of an RCD and an MCB max value of 16 amps as per the socket outlet , your Zs reading should be below 200 ohms this will ensure that the supply is stable , and doing it this way also keeps the earthing to the caravan independant i know you could use the house TT supply but depending on the earth mass of the ground you may find the readings could exceed the recommended limits

 

[Engineer54]

what i would do is to install a caravan hook up point at the position of the caravan with an earth rod to the pen conductor of the socket outlet as described in the regs , the hook up points should consist also of an RCD and an MCB max value of 16 amps as per the socket outlet , your Zs reading should be below 200 ohms this will ensure that the supply is stable , and doing it this way also keeps the earthing to the caravan independant i know you could use the house TT supply but depending on the earth mass of the ground you may find the readings could exceed the recommended limits

Second time this week!!!! So go on then, please explain Why you would separate a TT system or would even Want an independent TT system ?? And NO a Ra of 200 ohm will NOT ensure stability, stability is not based on Ra values!!

Hook up points are a minimum requirement for travelling caravan sites, which is not what the OP is talking about here!!!

 

[Deleted member 9648]

In order once again to provide a balanced view on Ra values I will offer standard |UK practice which I consider perfectly adequate.
In the UK RCD's are considered the preferred means of earth fault protection for a TT system...A 30ma RCD will provide earth fault protection up to a max Zs of 1667 ohms,the 200 ohms working max value is to allow for a considerable increase in soil resistivity and still meet the requirements.
There has been much debate on here over standard UK 1m rods as being inadequate during periods of ground freezing or drying. While I would accept that certain soils may experience drying at times and need a long rod ,IMO freezing in most of the UK is not an issue....During the previous two harsh winters here I have found soil never froze more than a matter of inches down....(noted while digging leeks on my allotment at the height of the big freezes)....based on this IMO the standard UK rod is perfectly adequate for all but exceptionlly dry soils or the extreme north where freezing may be an issue.
Regarding the practice of linking rods....assuming this achieved an Ra of 10 ohms....on a 230v system this would give a PEFC of 23a....not much use. You are still reliant on the RCD to provide adequate earth fault protection...which means the supposed advantage over an Ra of 200 ohms is what??

RCD's do fail...and my preferred method here would be an S type 100ma main switch on the house CU feeding the distribution circuit...with 30ma RCD protection in the caravan CU to final circuits...that way there are two RCD's...back up in the unlikely event of one failing to trip.
I can see no advantage on normal UK soils to multiple rods,or to a separate rod at the caravan....as long as the rod to the MET is properly installed and has an Ra complying with Bs 7671 the method I describe is perfectly adequate.

 

[Engineer54]

Just wrote a long rebuttal of your post, and lost it!!! I can't be arsed to rewrite it...
Save to say that most of what you've stated does not say much for UK TT systems, that are the lowest that I know of, ...even compared with some 3rd world countries. Be patriotic by all means, but don't try and defend the indefensible... Complying with BS 7671 will NOT in itself give you an adequate or stable TT system, ...far from it in fact!!

Oh, and the UK is just a little bigger than your allotment mate, UK soils, especially clay type soils freeze in severe winters, just like every other country in Northern Europe...

 

[Deleted member 9648]

My purpose E54 is not to start another argument on this but simply to provide a balanced opinion as I fundamentally disagree with your views on small TT systems in the UK....(as you know).....
FYI my allotment is on weald clay,and I can assure you the ground has not frozen more than 6 inches down in very severe weather...I am in the south east and accept the situation may be different in the north...although I doubt it would freeze even half way down a standard UK rod even there.
In 30 years I have never heard of a problem or safety related incident on a UK TT system installed in accordance with Bs 7671 which could be attributed to the standards of Bs 7671 being inadequate...and I base my opinions on that.

 

[nickblake]

Who mentioned Ra values i didnt ,the reason i would do it as i have said is you are not permitted to install caravans of a TNCS system ,it may be differant where you are but round here during meter upgrades they are changing some supplies from TT to TNCS , i know we cannot plan for the future but , also increasing the rods by a house can be also an impossability with the foundations paths etc rock etc ,this could also be the same at the point of the caravan, all caravans are supplied with a 16amp in put so putting a hook up point out side means if the caravan is required to be replaced it can easily , the reading of 200 ohms is given in the regs table 41.5 note 2 the resistance of the earth electrode should be as low as practicable a value greater than 200 ohms may not be stable hence my answer

 

[Engineer54]

Who mentioned Ra values?? ....Errr, ....You did!!!

Quote ....your Zs reading should be below 200 ohms this will ensure that the supply is stable , and doing it this way also keeps the earthing to the caravan independant ....Quote


But then, one has to ask, ...WHO mentioned anything about a PME system being involved, YOU again, no-one else has!!!
Just like no-one has mentioned anything about difficultly in placing rods due to site conditions either. And you don't place Rod positions by the side of a house where all the building rubble is going to be, ...That's a sure fire way of not even getting your magical 200 ohms.
What are you suggesting here, that the OP separates a Known TT system for another TT system on the same property, JUST in case the DNO convert the present system to PME??
You have got to be joking surely!!

The OP is talking of connecting this caravan to the properties CU!! Doesn't sound to me if this caravan is going to be moving off site as a travelling caravan, so why would you restrict it's supply to a 16A plug and socket arrangement?? We also have no idea of the type and size of this caravan, it could well be a static mobile home for all we know, Your 16A plug and socket ain't gonna be able to supply one of those things adequately.

All the OP has asked, is about the TT earthing arrangement, nothing else!! I'm sure the OP has the supply arrangement/design well in hand...

 

[Engineer54]

My purpose E54 is not to start another argument on this but simply to provide a balanced opinion as I fundamentally disagree with your views on small TT systems in the UK....(as you know).....
FYI my allotment is on weald clay,and I can assure you the ground has not frozen more than 6 inches down in very severe weather...I am in the south east and accept the situation may be different in the north...although I doubt it would freeze even half way down a standard UK rod even there.
In 30 years I have never heard of a problem or safety related incident on a UK TT system installed in accordance with Bs 7671 which could be attributed to the standards of Bs 7671 being inadequate...and I base my opinions on that.

I most certainly disagree with your view on TT systems too, especially your total acceptance of 200 ohms being suitable/adequate, based purely on what is written in BS7671. That section of the Reg's is clearly in need of revision. Iv'e always stated that getting a good Ra value on a domestic installation can sometimes be an expensive exercise. But in many cases the use of 3 or more rods coupled together for depth can bring you sub 10 ohm Ra values. Which WILL give a form of protection, even if not meeting disconnection times, for when that RCD(s) fails. I have also stated many times in this forum that whenever possible a PME conversion is always going to be the better route to go in the UK!!

Ground freezing in the UK has always been a problem, even in Essex where i spent most of my time in UK. Just go ask groundwork's companies, i would imagine it's even more common for water retention type soils, the further North you go... Same goes for drying of general soils, especially in free draining type soils, even in what UK call summers these days...lol!! And remember, you only have a metre rod, ...''Any'' reduction in resistivity caused by freezing or drying of the soil, is going to have grave affects on the stability and Ra value of that rod...

I'm glad you mentioned previously about the ''UK's standard'' 1 metre rods... I'm being honest, when i tell you, that i've never seen a 1m rod in my entire working career, even when i was training/working in the UK!! I have seen 1m extension rods threaded one end only. The smallest earth rods i've seen or worked with have been 2.4m 5/8''. Normally and the vast majority of the rods i have used on my projects are 3.1m 3/4'' , and i am talking literary thousands of them over the years, several hundred on my present project alone...
To be totally honest with you, those 1m 3/8'' rods should be discontinued/scraped as ''unfit for purpose'' ...I've seen bigger earth test kit electrodes than those totally usless things!!

One thing i will say about your 1m rods ....never on god's earth will a 1m 3/8'' or 5/8'' rod give you a stable TT earth, in a general/typical soil condition, but you may just be lucky in a salt marshland location... lol!!!

I base my opinions on what i've learnt over the years and the early mistakes i've made and seen. along with the many projects, where that experience was put into practice. It is 100% NOT based on the standards of BS7671. Maybe a better standard to follow in the UK, would be that found in the ''Lightning Protection'' standard , (can't remember the BS/EN number)

 

[Deleted member 9648]

Thankyou for the reply E54....there's no point in continuing to go over old ground..(dried and frozen)....so I wont bother.
You do have a habit though of disagreeing in a way that is belittling and unnecessary.....I thought Nicks post was a perfectly reasonable reply to the OP...definately not deserving of your ridicule.

 

[Engineer54]

Thankyou for the reply E54....there's no point in continuing to go over old ground..(dried and frozen)....so I wont bother.
You do have a habit though of disagreeing in a way that is belittling and unnecessary.....I thought Nicks post was a perfectly reasonable reply to the OP...definately not deserving of your ridicule.

Agreed, we both have completely different views on this subject, and will be pointless to continue the toing and throwing...


Just re-read my reply to Nicks post, and again agree, a mite OTT. All i can say is, It wasn't my intention to belittle or in the way it sounds. Unfortunately, i was in a foul mood earlier this evening on getting home from work, having just had to deal with a couple of corrupt officials trying it on.... Perhaps i should of waited a while, till i had my coffee and calmed down some before opening up the laptop and replying to Nicks post...


Nick,

My apologies to you Nick, as i say, it wasn't meant to sound the way it came accross in my reply..