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eskimo39

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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?
 
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beaver74

  • Thread Starter Thread Starter
  • #2
should try to get under 200 to garentee stable reading
 
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eskimo39

  • Thread Starter Thread Starter
  • #3
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
 
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beaver74

  • Thread Starter Thread Starter
  • #4
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
 
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KFORDHUNTER

  • Thread Starter Thread Starter
  • #5
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.
 
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uksel

  • Thread Starter Thread Starter
  • #6
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.
 
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KFORDHUNTER

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  • #7
200 is for Ze, according to 7671 Zs has to be below 1667 ohms for TT.
 
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uksel

  • Thread Starter Thread Starter
  • #8
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
 
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eskimo39

  • Thread Starter Thread Starter
  • #9
uksel, could you tell me how to do that calculation with ohms law?

Cheers
 

jeremy

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Mentor
Arms
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!!
 
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uksel

  • Thread Starter Thread Starter
  • #11
ha ha, i said that knowing i'd get a bite, i couldn't help myself :p

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
 
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eskimo39

  • Thread Starter Thread Starter
  • #12
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.
 
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uksel

  • Thread Starter Thread Starter
  • #13
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
 
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 ! ! !:confused:

....................comments !
 
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beaver74

  • Thread Starter Thread Starter
  • #15
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
 
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eskimo39

  • Thread Starter Thread Starter
  • #16
Beaver74 I am only on the first year of a three year course.

As stated in the post "Ohms law is our third module starting in January and was just curious as to how I could use it" As I am sure some of you will be aware there are 9 modules alone in our first year before they allow us on to NVQ 2 & 3.

The comment in question to uksel was of genuine interest. I joined this forum for support and (hopefully) guidance. I am not some guy who has done a six week "Intensive" course and now think I know it all, I am a member on here to assist my development and hope in the future I will be able to post answers to help others.
 
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KFORDHUNTER

  • Thread Starter Thread Starter
  • #17
I have a question about TT systems, i have just done a rewire on a bungalow, and have installed an earth rod, all readings are good, have installed a high integrity 17th edition board, single rcbo for smoke alarms then split 30ma dual rcd for rest of bungalow, i think that is all you need to do, am i right to say that due to every circuit being protected by 30ma rcd i do not need to oversee the main incomer on a 100ma time delay.

Replies appreciated.
 
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montybaber

  • Thread Starter Thread Starter
  • #18
Beaver74 I am only on the first year of a three year course.

As stated in the post "Ohms law is our third module starting in January and was just curious as to how I could use it" As I am sure some of you will be aware there are 9 modules alone in our first year before they allow us on to NVQ 2 & 3.

The comment in question to uksel was of genuine interest. I joined this forum for support and (hopefully) guidance. I am not some guy who has done a six week "Intensive" course and now think I know it all, I am a member on here to assist my development and hope in the future I will be able to post answers to help others.
dont worry Eskimo asking questions will make you a much better sparkie, and I know when you are you will be a lot more helpfull than a few posts on this thread have been;)
 
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prideofengland

  • Thread Starter Thread Starter
  • #19
1) Ze needs to be less than 200 ohms or 100 ohms if your in the NIC.
2)Cable size should be half the size of the phase conductor or calculated using the adiabatic equation: S=_/I²t [FONT=Times New Roman, serif]٪ k[/FONT]
3) Generally its better to go deeper to lower your reading than adding more rods. You can get rods from the wholesalers with a threaded end that allows you to add another rod to it, a ferrule protect the thread from hammer dammage, someone posted a link to it about a month ago.

hope this helps
 
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andy pandy

  • Thread Starter Thread Starter
  • #20
if your strugling to get a good earth path for a rod try putting marconite round it this will enhance the earth reading. Try google it will tell you all about it. Allso the chap who has used rcbos to protect smoke alarms, i think rcbos are not required unlless any of your wiring on the smoke circuit is burried less than 50mm below the surface.
 
I have a question about TT systems, i have just done a rewire on a bungalow, and have installed an earth rod, all readings are good, have installed a high integrity 17th edition board, single rcbo for smoke alarms then split 30ma dual rcd for rest of bungalow, i think that is all you need to do, am i right to say that due to every circuit being protected by 30ma rcd i do not need to oversee the main incomer on a 100ma time delay.

Replies appreciated.
You are right,if you were installing a metaclad board then you would have to install an RCD on tails to board.As you are using a high integrity board then you dont need to as you already have 30ma protection on all circs.
 
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eskimo39

  • Thread Starter Thread Starter
  • #22
Why do you need 100A Time delay on the tails to a distribution board? I thought if the board was fully protected by RCD and RCBO's the 100A breaker would never trip before them.

Is it to protect any metallic parts of the board becoming dangerous? Does a high integrity board have no metallic parts?

Sorry for been so dumb :eek:
 
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uksel

  • Thread Starter Thread Starter
  • #23
i think he is concerned asbout covering the requirement to have a TT system to be protected by a 100mA time delay RCD

i would suggest installing the 100mA time delay RCD to comply, as although in practice it is highly unlikely each circuit could in theory have a leakage of 20mA, no RCD's will trip yet there could be 240mA running through earth in a 12 way CU.

Eskimo, my apologies if i was rude, i assumed you were making a response to something said in another thread.
 
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eskimo39

  • Thread Starter Thread Starter
  • #24
Don't worry about it. I just really was curious. I have been having a good read up on ohms law this morning. Have found 2 different charts. 1 a triangle with V,I,R and the other a circle with P,I,V,R. Not sure what the difference is but it is certainly interesting reading.
 
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uksel

  • Thread Starter Thread Starter
  • #25
one allows for Power calculation (measured in Watts, like a lightbulb)

the other is the basic ohms law V=I.R
 
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beaver74

  • Thread Starter Thread Starter
  • #26
Beaver74 I am only on the first year of a three year course.

As stated in the post "Ohms law is our third module starting in January and was just curious as to how I could use it" As I am sure some of you will be aware there are 9 modules alone in our first year before they allow us on to NVQ 2 & 3.

The comment in question to uksel was of genuine interest. I joined this forum for support and (hopefully) guidance..
eskimo39 i was'nt being rude i was just statting that ohms law in my apinion should be one of the first thing to learn as almost every thing revolves around this in some way
and i did miss read the 3 years
 
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