Caravans - yes really - going round in circles....

[Octopus]

Caravans – yes really!


Customer asked me to provide a new board and 3 x distribution circuits for “caravans” they will be living in while they redevelop their house.
Initial spec was that the incomer would be TNS and I had planned to fit a main board with MCB’s and then run 3 distribution circuits to each of the vans which were anticipated to be pre-wired with a plastic CU/RCD incomer. This solution would rely on the earth at source

I’ve now discovered that the “supply” is actually a temporary supply and is a TT supply – so I need to think again (fortunately for me I had declined to give a firm price until the incoming supply and caravans had arrived!).


So my thoughts are a bit all over the place and I could do some input as I haven’t done a caravan install like this before – nor have I touched a TT refurb / rewire for about 5 years!

Option A
100ma RCD in main board (its metal clad for SWA termination) but can’t risk a 30ma to cover the 3 vans!
TT earth for main board
MCB’s for 3 circuits
2 core swa with the armour connected at main board only
TT each caravan so 3 more earth stakes

Option B
100ma RCD in main board (its metal clad for SWA termination) but can’t risk a 30ma to cover the 3 vans!
TT earth for main board and caravans
MCB’s for 3 circuits
2 core SWA but with the armour connected at both ends so caravans rely on single earth stake(s)

Other info!
Also, judging by the cu cables they were previously connected via a blue cable with a commando socket but the loads on 2 of the vans will be above 16 amps so a more robust supply is needed. Each van has a 32A RFC and a 6A lighting circuit.

Thoughts and input appreciated.

Thanks

 

[telectrix]

if you rod the supply end and rod the 3 caravan points, parallel the rods using the SWA armour earthed at all points, you might be able to get a decent Ra .

 

[telectrix]

did one a few weeks ago. 6 pitches on a gypsy site. 7 4ft' rods gave me a Zs at each pitch of 4 ohms.

 

[Deleted member 26818]

4ohms, very nice.

 

[richy3333]

How about doing your first rod at the origin to see what Ra you get? Then you can decide if adding rods at the vans would help? You can also get 32A hook-ups boxes if your distribution circuit will cope?

In my experience the DNO won't ever supply an earth on a temporary supply (van issue aside). The permanent supply might end up TNS though?

Ill post to the Arms some pictures of a site I tested last week. It will give you a good idea of what not to do!

 

[Octopus]

Bump

Anyone?

 

[telectrix]

i'd start bybanging in acoupled rod at the origin and then seing what Ra/Ze you get. If you can achieve sub 1 ohm then you can dispense with RCD at origin


sent from my caravan using tripe ad'

 

[Leesparkykent]

what Tel said is the best option IMO. A couple of 5/8 earth rods coupled together at each pitch and at the DB all in parallel for a good stable Ra. 100mA S type at main DB and 30mA at hook up points.
.

 

[Octopus]

UPDATE

So I did the work today - with 2 meters of 5/8 rod - 1233 ohms - 3 meters of 5/8 rod it came down to 168 ohms, 4 meters of 5/8 it came down to 113 ohms.

Added a 2nd rod at one of the vans (2 meters) and it didn't improve, and it was getting dark - so I'm going back next week with more rods and more couplers!

 

[HandySparks]

Did you use the SDS?

 

[Octopus]

Did you use the SDS?

Some of the time!

Going to take my 230v unit next week! My battery one wasn't heavy enough but the smallest core drill was perfect!

 

[Marvo]

How low do you need to get ie what's your target Ra?

What was the ground like? How difficult was it to get the 4m rod in?

I'd be tempted to say rather go another 2-4 meters deeper with the first rod if you can.

 

[Octopus]

How low do you need to get ie what's your target Ra?

What was the ground like? How difficult was it to get the 4m rod in?

I'd be tempted to say rather go another 2-4 meters deeper with the first rod if you can.

UK likes to see sub 100 ohms

Ground was indifferent - easy then hard, then harder!

Going deeper is an option!

 

[Marvo]

Looking at the figures you gave you should get 50 ohms with another 2-3 meters depth and it should settle at maybe 70-80% of that figure over time with some good UK rainfall .

 

[Guest77]

UK likes to see sub 100 ohms

Ground was indifferent - easy then hard, then harder!

Going deeper is an option!

hmm? are you sure?

 

[Guest77]

sorry if I have missed something, but why cannot you have a s type RCD at the supply and 3 double pole 30 ma individual RCDs for the 3 vans?

 

[Octopus]

hmm? are you sure?

Well should I say the NICEIC like to see sub 100

I just think the lower the better!

 

[Guest77]

Well should I say the NICEIC like to see sub 100

I just think the lower the better!

ah yes, I totally agree it is the NICEIC who like sub 100 ohms, quite true mate.

 

[Octopus]

ah yes, I totally agree it is the NICEIC who like sub 100 ohms, quite true mate.

Thing is that these caravans are going to be lived in from mid Jan '16 to late summer '16 - if the Ra is difficult now, just imagine after a few dry summer months!

 

[Guest77]

Thing is that these caravans are going to be lived in from mid Jan '16 to late summer '16 - if the Ra is difficult now, just imagine after a few dry summer months!

thing is if the the supplies are via double pole RCDs and your readings are under 200 ohms for example then it will never exceed 1667 ohms will it? and that will trip within the required disconnection time, on a TT double pole RCD protection is essential for caravans.

 

[Marvo]

I think the 1667 ohm ceiling is purely safety related and derived from disconnect times in the case of a fault. I'd suggest it would be a professional approach to consider the functionality aspect of the earth as well which would involve looking how well it would perform for surge arrestors and noise suppressors etc in electronic equipment that might be plugged into the final circuits. On this occasion I'd be in the NICEC camp and striving for a stable sub 100ohm Ra or even lower if possible.

 

[Guest77]

I think the 1667 ohm ceiling is purely safety related and derived from disconnect times in the case of a fault. I'd suggest it would be a professional approach to consider the functionality aspect of the earth as well which would involve looking how well it would perform for surge arrestors and noise suppressors etc in electronic equipment that might be plugged into the final circuits. On this occasion I'd be in the NICEC camp and striving for a stable sub 100ohm Ra or even lower if possible.

Obviously a reading under 1 ohm is what we would like to achieve, the NICEIC do not write the regs book though and 200 ohms satisfactory to the book, personally I would not be anywhere near satisfied at that reading but the regs are the regs.

 

[GEOFF S]

UPDATE

So I did the work today - with 2 meters of 5/8 rod - 1233 ohms - 3 meters of 5/8 rod it came down to 168 ohms, 4 meters of 5/8 it came down to 113 ohms.

Added a 2nd rod at one of the vans (2 meters) and it didn't improve, and it was getting dark - so I'm going back next week with more rods and more couplers!

Sounds fine to me, i wouldnt get to hung up on acheiving a really low Ra, stable will do, by the way, the more you bang in the less effect it has of lowering the Ra, it all becomes a little fruitless.

Cheers

 

[Octopus]

UPDATE: A couple more rods and the reading is now 80 ohms.

So I gave up!

 

[Guest77]

so what is wrong with 80 ohms? obviously not the reading you would like but well within the limits to the regs Book

 

[GEOFF S]

UPDATE: A couple more rods and the reading is now 80 ohms.

So I gave up!

Perfectly fine, well done.

You can only work with the ground you have. Now if naturally the resistance is high then you have little chance of acheiving low values, remeber high values can be stable too and stable is the important bit.

Cheers

 

[telectrix]

the problem with the TT is that although each pitch is protected by a 30mA RCD, the distribution cable ( even if SWA) requires a RCD upfront to comply with disconnection times.

 

[GEOFF S]

the problem with the TT is that although each pitch is protected by a 30mA RCD, the distribution cable ( even if SWA) requires a RCD upfront to comply with disconnection times.

A problem in what sense Telectrix?

Cheers

 

[telectrix]

discrimination between RCDs.

 

[GEOFF S]

discrimination between RCDs.

Surley a time delayed will sort that issue out?

Cheers

 

[telectrix]

probably, not always.

 

[GEOFF S]

probably, not always.

Is "probably" not good enough, "not always" could you be more specific?

Cheers

 

[Geordie Spark]

did one a few weeks ago. 6 pitches on a gypsy site. 7 4ft' rods gave me a Zs at each pitch of 4 ohms.

So while you were nicking their wheels they were nicking your tools??

 

[Geordie Spark]

UPDATE: A couple more rods and the reading is now 80 ohms.

So I gave up!

A Spark using proper rods got 80 Ohms??

Me - a plumber (sort of) - got exactly that from a bit of 15mm copper tube hammered into the lawn!! :sailor:

 

[telectrix]

A Spark using proper rods got 80 Ohms??

Me - a plumber (sort of) - got exactly that from a bit of 15mm copper tube hammered into the lawn!! :sailor:

trust you to try and bypass your water meter. :juggle2:

 

[Geordie Spark]

trust you to try and bypass your water meter. :juggle2:

Funny you should say that. I've just had one fitted. It should save me about £100/year - and that's before I by-pass it! :joker:

 

[Geordie Spark]