TT systems and CCU

Spudnik · Dec 23, 2009

And you chaps would be happy to leave a TT installation with a high Ze albeit within 1667?

Not sure i would.

Deleted member 9648 · Dec 23, 2009

I've never had a reading anything like as high as 1667 ohms and if I did I would probably try to improve it.But it would be a case of taking into account the ground conditions at the time of the test ,and deciding whether the resistance would be likely to improve or deteriorate further with a change in the ground moisture levels.
In the end as long as the reading is less than 1667ohms...(less the R1R2 reading of the longest final circuit) the requirements of the regulations are satisfied.

Guest123 · Dec 23, 2009

NIC require an Ra of 100ohms or less.

Deleted member 9648 · Dec 23, 2009

Our inspector has never picked up readings on certs greater than 100 ohms....and if he did I would ask him to justify it in the light of what the regulations permit. Have to say though most TT Ze's we get are well under 200 ohms....but I do remember one around 350 some time back.

xraytek · Dec 23, 2009

wirepuller said:
The maximum permissable resistance on an earth electrode with 30ma rcd protection is 1667ohms.....the 200 ohms value quoted is only advice that anything over that value may be unstable...... anything under 1667 ohms will comply with the regulations.I'd only be worried if I had a very high resistance when the ground was already very wet and likely to dry out further.

The 1667 figure quoted in the regs book is for maximum Zs not earth rod resistance.
It is recommended that the earth rod be <200 ohms for conformance with 542.2.2

TPES · Dec 23, 2009

xraytek said:
The 1667 figure quoted in the regs book is for maximum Zs not earth rod resistance.
It is recommended that the earth rod be <200 ohms for conformance with 542.2.2

Just what i was going to say, I put 1667 in my "Max Zs permitted by BS7671" on my certs as agreed by Elecsa and 17th Regs tutor for TT systems.

<200 for your Rod reading!