acceptable zs

[sparke]

Hi all , trying to get ready for elecsa assesment. planing to show new cu insall on my own home 17th ed of course. Problem is that TTearthing put in 1 rod got reading of aprox 230 ohms zs(very rocky stoney ground) on socket outlets so put in anoter linked to the first now geting a reading of 154 ohms .
cant find any clear guide lines in regs as to what is acceptable level seems a bit of a grey area. who knows ????

 

[Mr Mark Sparks]

this is how i see it the max permitted is 1666 ohms based on not allowing the voltage to rise above 50v
however no one would leave an electrode this high the old rule of thumb was not more than 200 ohms and now you should be aimimg for 100 ohms or less.
also you need to use your ze reading not the reading at your sockets just test purely beetween live on feed side of main switch/rcd and earth to stake with it removed from cu.
with everything else in the board the reading may come down even more.
hope this makes sense.

 

[Guest123]

Hey Sparke.

The max permissable RA (earth electrode resistance) (Ze) fir a TT installation is 1667 ohms.

However it is generally taken that RA should not exceed 200 ohms and there are notes on this in OSG.

This should be measured at the C.U on the supply terminals of the main switch with the main earth disconnected and main switch open, same as you would measure Ze.

As you have a reading of 154 ohms then you are under the max of 200, but as you are in rocky ground this result may be unstable and the ELECSA man may refer to this as it may change significantly and go over the 200 in different weather conditions

To absolutely cover yourself are you able to get a third rod in?? if so it would be worth doing I feel.

Hope this helps.

 

[Spudnik]

You should also try and ensure the rods are spaced at least one rod length apart.

 

[Guest123]

Hey.

Just as a bit more info, I carried out a lot of work for a pipeline construction company during a project they had in my area.

All the construction yards were TT installs and we were achieving RA's of between 4 and 5 ohms, on rocky ground also

This was with 120mm earth and somewhere in the region of 10 14mm copper rods though, in 5 sets of 2.

I know this would be too extreme for you domestic situtation but it just shows that you can achieve a low RA reading in any circumstance.

 

[ezzzekiel]

<200 ohms

 

[maddfridge]

Hi all , trying to get ready for elecsa assesment. planing to show new cu insall on my own home 17th ed of course. Problem is that TTearthing put in 1 rod got reading of aprox 230 ohms zs(very rocky stoney ground) on socket outlets so put in anoter linked to the first now geting a reading of 154 ohms .
cant find any clear guide lines in regs as to what is acceptable level seems a bit of a grey area. who knows ????

hi there

Is all the ground around you rocky ?

we ran a copper tape to a couple of square grids made out of copper buried at a couple of different depths this gave both an increased contact area with the earth and did not mean having to go deeper with constant rods all the time.

just a suggestion it may help you

we used a couple of bags of a powder which you wet once you put it in to give a better / lower resistance cannot think of the name but it sticks like **** to a blanket lol The big boys use it to ground out airfield installtions etc.


cheers