TT system main bonding CSA?

[davesparks]

Tt max value is 200 ohms

Where did you get this from? It is not in bs7671 as a max permitted value

 

[Farmelectrics]

Haven't got books with me but thought tt was 200 ohms tns 0.8. And tnc-s. 0.35. Sure it's in on site guide certainly that in my head lol

 

[richy3333]

Haven't got books with me but thought tt was 200 ohms tns 0.8. And tnc-s. 0.35. Sure it's in on site guide certainly that in my head lol

Thats not right mate. The IET state that in excess of 200 ohms can be unstable. As others have said you should be looking for as low as figure as possible.

I'd guess that 180 ohms now will exceed 200 ohms at other times of the year - and I ain't a betting man!

OP - I'd bend over and be ready to be well and truly spanked by Eng54 when he reads your post. At present he's getting his butt handed to him on a plate by the Yanks

 

[davesparks]

Haven't got books with me but thought tt was 200 ohms tns 0.8. And tnc-s. 0.35. Sure it's in on site guide certainly that in my head lol

Nope, no max value for TT systems stated.

200 ohms is suggested as a value above which Ra may not be stable.

0.8 for TNS, 0.35 for TNCS and 21 for TT are the values that a DNO will quote as the typical maximum values for each supply.

 

[jon4381]

If I remember rightly, it does say in the guidance note 3 any reading over 100 ohms would be considered unstable. But it has been a while since I been in the books

 

[7029 dave]

As stated, 21ohms is the DNO recommendation.

 

[davesparks]

As stated, 21ohms is the DNO recommendation.

Where can this be found?

And why are the DNO recommending any values for a customers earth electrodes?

 

[7029 dave]

Where can this be found?

And why are the DNO recommending any values for a customers earth electrodes?

You tell me, your post 19 lol

 

[Leesparkykent]

If you are rewiring the property, why not just stick 10mm bonding in and a 16mm main earthing conductor. either reposition the earth rod and see if you get a better Ra value or add another until you get a better value, If I was rewiring the property I would be inclined to install my own rod anyway as for all you know the rod that's already been installed could of been cut down in size as hit hard ground etc when it was installed.

 

[Deleted member 9648]

The regulations state that the value of Ra should be as low as practicable,and that an Ra exceeding 200 ohms may be unstable (table 41.5 note 2). Assuming earth fault protection is by 30ma RCD then in theory an Ra of 1667 minus the maximum value of R1+R2 on the installation would comply with Bs7671. This clearly leaves the whole thing open to interpretation.I would point out again though that in the event of an earth fault,assuming 30ma RCD protection,the outcome will be no different whether the OP's install remains at 180 ohms,or is improved to a 'stable' 20 ohms.
This is where Bs7671 fails miserably.On one hand it is implying that only sub 200 ohms readings are acceptable...and yet potentially readings far in excess of 1000 ohms will still meet the requirements.
Personally i can see no advantage at all in improving 180 ohms to 20 ohms and nobody on here has ever given a satisfactory explanation as to what 20 ohms will do that 180 wont. If you are going to improve it then the regulations should require readings approaching TN values,and if you cant accept higher values then you have to accept nothing less than TN values on your TT installations.

 

[7029 dave]

Actually, values of over 100 ohms can apply.

 

[Deleted member 9648]

Actually, values of over 100 ohms can apply.

Apply to what?

 

[davesparks]

You tell me, your post 19 lol

No it wasn't!

In post 9 I said it was their quoted typical maximum values, you first stated that it is their recommendation.

 

[7029 dave]

Apply to what?

TT that's the thread gezzz.

 

[Deleted member 9648]

Isnt 100 ohms the NIC Guru's self made regulation?