TT help needed!!!

[eskimo39]

Can anyone help. Me and my boss are currently doing a full wiring job on a barn conversion. Due to the location of the barn, a TT system has been used. There is an earth rod outside with a 2mm cable running to it. We ran a test yesterday and got a result of 368 ohms, been used to working on domestic installations we are under the impression that you need a result of under 0.7 ohms to pass. Is this correct?

Today we have installed 2 x 4ft rods and upgraded the earth cable to 16mm, we are yet to test it but my boss is not holding out much hope of it been much lower. I did a search on here and a few people have mentioned that the reading should be inder 2000 ohms!!!! If this is the case we are well under with our 368 but am I misunderstanding the regulations?

Does anyone have any experience of TT rural installations or know the exact requirements.

Sorry to be a pain and thanks in advance.

Forgot to mention that on the old installation there was a 60A fuse to the meter, then connected to a crabtree ELCB, however we are replacing this to a 100A fuse via the local supplier and changing the ELCB to a 100A Isolator, we are then installing 2 Proteus CU's will full RCD protection except for the boilers.

Due to high Ze readings on TT systems, are we conforming by making everything RCD protected? If so are we breaking regs by leaving the boilers (x2) unprotected?

 

[beaver74]

should try to get under 200 to garentee stable reading

 

[eskimo39]

Anyone selling a small car we can bury

Just had a quick look in my onsite guide and it states we can use :

• private water mains (No good as it's all plastic)
• reinforcing steel in concrete (none used)

Do you think it is possible to get down from 368 to under 200 using just rods?

Thanks for the quick reply

 

[beaver74]

yes never had a problem before usualy 1 rod will give me <100 but this will depend on where you are and soil type but if a problem more than 1 rod will lower reading

 

[KFORDHUNTER]

My understanding is that the Ze reading should be 200ohms or under, the minimum size cable to the earth rod should be 6mm, also that the main incomer should me a 100a 100ma time delay rcd, then you protect ALL the circuits with 30ma rcds, conforming to the 17th edition requirement by splitting the circuits on dual rcd board or with rcbo on each circuit.

On a TT system all circuits are required to be rcd protected.

Hope i'm right and this helps.

 

[uksel]

Is the 200 Ohm reading for just Ze, or is it Zs? but Kfordhunter is spot on there, some good advice i think. i would personally use 10mm earth cable, and if i was dropping another 2 earth rods in i would put one on each corner of the barn (incase your hitting different soil properties) and connect them like a ring circuit, loop out to each and the two rods at the front corners run straight back to the CU.

use 100ma RCD with time delay and 100A overcurrent protection, and then use the standard 30ma protection across both sides of the 17th CU for discrimination.

 

[KFORDHUNTER]

200 is for Ze, according to 7671 Zs has to be below 1667 ohms for TT.

 

[uksel]

thanks for clearing that up

it's mad to think how an R1+R2 reading of 1447 Ohms is acceptable for an installation

though i suppose for a calculation using ohms law a phase short to earth with a Zs reading like that would only result on a fault current of approx 1.3A, or 130mA. which i guess is where the 100mA main RCD would come in handy

 

[eskimo39]

uksel, could you tell me how to do that calculation with ohms law?

Cheers

 

[jeremy]

Have a look at what sort of soil there is in the area. Dry sandy soil will conduct FA but wet stuff will be good. The more "earth" you can get into the ground the better so if you're having probs with 2mm!!! Get the resistance down as much as poss thicker earthing cables, more or longer eart rods. You may get better results with longer earth rods as the soil may be damper further down. If all else fails some form of earth mat which is a gridded sheet of copper may be required but I can't see that this would be necessary. I don't have a lot of practical experience with this sort of problem, but maybe ask the supplier where the transformer is what sort of results everyone else in the area has. If it's a barn con it must be fairly close to an existing property ask to test their Ze. Keep going , you'll get there!!

 

[uksel]

ha ha, i said that knowing i'd get a bite, i couldn't help myself

i've said ohms law is a good way to get a ballpark figure, but i don't really want to drag other topics and gatecrash other people's threads.

now if you want to have a decent discussion and educate me with everything you learnt from your 6 week domestic installer course please feel free to take this up on the other thread

cheers

 

[eskimo39]

uksel, it wasn't a dig or me thinking I know it all from my course (Which is 3 years not 6 weeks). I was just genuinly interested as to how to work it out. Ohms law is our third module starting in January and was just curious as to how I could use it.

 

[uksel]

Ohms Law

v = I x R

Voltage (V/Volts) is equal to Current (I/Amps) multiplied by Resistance (R/Ohms)

the forumla will transpose correctly to leave you the option of

I = V/R and
R = V/I

 

[SCOL1]

I had a similar problem with a TNS supply this week.

Ze reading was too high,so a rod was installed (no stand alone rcd as we had 30ma protection on all circuits),rod resistance was 360 ohms,the local authority Contracts administrator was insistant that we got the reading down to less than 200 ohms.
When it was pointed out to him that the 'touch test ' voltage when calculated (360 x 0.03) equated to 10.8 volts,where maximum voltage must be less than 50 volts,he renaged and accepted the 360 ohms reading.

16th edition.....

Max rod resistance----500 ohms
NIC recommended max----100 ohms
IEE recommended max----200 ohms

17th edition......

Max rod resistance-----1666 ohms
NIC recommended max----STILL,100 ohms
IEE recommended max----STILL, 200 ohms ! ! !

....................comments !

 

[beaver74]

uksel, it wasn't a dig or me thinking I know it all from my course (Which is 3 years not 6 weeks). I was just genuinly interested as to how to work it out. Ohms law is our third module starting in January and was just curious as to how I could use it.

3 years with out learning ohm law this is the back bone of electric