TNS to TT for better or worse

[TimmyB]

Hi all

A recent board change on TNS, found the Ze at 1.35ohm So made into TT but only getting 210ohm
thru rod. Board is all RCD so would it not be a better installation if we were allowed to use the TNS + the RCDs
After all we have the magic number of 1667ohm

cheers

 

[Spudnik]

Indeed but there is guidance that TT Ze should be below 200 but preferably around 100 (according to NICEIC).

Anything over 200 would be considered to be unstable.

 

[Guest123]

Hi

The 1667 max applies to a circuit protected with a 30mA RCD, not the external Ze / Ra. If the maximum stated by the supplier is exceeded then contact them as it's their duty to maintain their earthing system.

Failing that you need an extra rod(s) to get your Ra reading down below the 200ohm mark.

 

[scotsparky]

The Supplier will say as its a TT sysytem it is th cuctomers earth rod not theres, In AScotland anyway the supply authority has no obligation to supply and earth.Probbably easier to drop in another rod.

 

[Guest123]

I agree with what your saying about TT being the customers issue to resolve but as the original was TN-S and the Ze was over their given value of 0.8 then surely they should be contacted to try and rectify it??

9 times out of 10 they will just tell you to install a rod and convert to TT though.

 

[scotsparky]

Are the supply authority obliged to give you an earth at all? Or is it they were but now a tt its a case of. Not My Problem

 

[TimmyB]

Thanks for your replys

Yes i will hopefully imrove the Ra reading in this case

But do you not think that if the rules were different it would be a better
installation if we could use the TNS at a higher max value if all circuits are protected
by RCDs