High ze

[marksparky]

hi folks, testing ze on a tt system 2day and getting a reading of 730ohms! put in another rod about 1mtr away and this brought it down to around 340ohms! zdb is about 103ohms, but how can it be that wen i performed the test and had forgot to reconnect the earthing conductor to the rod i got 0.58ohms surely it couldnt have found a path to earth! earthing conductor was dissconnectrd from met so no parallel paths! i feel im missin somethin simple staring me in the face but cant think what! these are the worst tt readings iv ever seen and this is the 1st time iv ever tried to introduce 2rods to bring reading down but to no avail!

 

[malcolmsanford]

Did you couple up both rods to get that 340 reading or was it just the second rod on its own?

Sometimes you will achieve a better reading by driving 2 rods attached, therefore going deeper into the soil,as there could be a lot of rubble or sub standard back fill that the single rod could be sitting in, try doubling the length. I have driven in 3 rods once and then had to drive another, so a total of 4 before I got the result I wanted.

As for the 0.58 for just the conductor floating free, well without seeing what you done I think that must be a false reading, or you did something, as to be honest I cant explain where you got that from.

 

[marksparky]

yes malcolm that was with two rods connected together, think i will try 2 or three coupled together and go deeper. as for the floating earth test lol, thats wat really threw me was pullin hair out at this stage as with rods connected couldnt get a decent earth and with nothing connected was gettin a great readin lol even tho there was no connection to earth! must be false reading

 

[cbw]

Wrong button on the tester!

My fluke is a nightmare if I am not wearing my old man glasses!!!!

So easy not to test the ZE

 

[morph]

completey agree much better to double length, its going to get you closer to the water table and more stable soil resistivity

Did you couple up both rods to get that 340 reading or was it just the second rod on its own?

Sometimes you will achieve a better reading by driving 2 rods attached, therefore going deeper into the soil,as there could be a lot of rubble or sub standard back fill that the single rod could be sitting in, try doubling the length. I have driven in 3 rods once and then had to drive another, so a total of 4 before I got the result I wanted.

As for the 0.58 for just the conductor floating free, well without seeing what you done I think that must be a false reading, or you did something, as to be honest I cant explain where you got that from.

 

[sparky3366]

I know someone who joined about 10 rods together when rewiring a church. He belted the rod in and then it would drop 5 ft. Another rod on...same thing...yet another one and the same, drops by 5 ft. You would think he would've known about the crypt type thing below him with a very long earth rod going through 4 caskets!!!

 

[telectrix]

hi folks, testing ze on a tt system 2day and getting a reading of 730ohms! put in another rod about 1mtr away and this brought it down to around 340ohms! zdb is about 103ohms, but how can it be that wen i performed the test and had forgot to reconnect the earthing conductor to the rod i got 0.58ohms surely it couldnt have found a path to earth! earthing conductor was dissconnectrd from met so no parallel paths! i feel im missin somethin simple staring me in the face but cant think what! these are the worst tt readings iv ever seen and this is the 1st time iv ever tried to introduce 2rods to bring reading down but to no avail!

just had similar on a TT.Ze was 116, with the water bonded, it came right down to 0.57.

 

[Engineer54]

hi folks, testing ze on a tt system 2day and getting a reading of 730ohms! put in another rod about 1mtr away and this brought it down to around 340ohms! zdb is about 103ohms, i feel im missin somethin simple staring me in the face but cant think what! these are the worst tt readings iv ever seen and this is the 1st time iv ever tried to introduce 2rods to bring reading down but to no avail!

What size and dia rods are you using on this TT system?? If your rods are these 1.2m variety, then driving the second rod in 1m away from original rod isn't a good idea as they should be separated by at least the length of the rods, but better if spaced 1.5 X rod length. Better still by a big margin, is going deeper say 3 rods coupled together. Never used these short thin rods myself, in most situations there a waste of time, and barely get past soil depth weather influences that can almost nullify rod to soil conductivity....

Another commonly made mistake, is driving rods next to the house walls, ...this is where the builders bury there waste and rubble. So far better to stay at least a metre or two away from any house walls and the like!!!

EDIT... Zs is Not Ra and should never be considered as such, No matter what your Zs readings are, It's the Ra that you need to reduce substantially...

 

[IQ Electrical]

Yes, Ra, not Ze and I absolutely agree, depth is absolutely key in these situations, I'd take an Ra of 400 Ohms on a deeply driven rod over an Ra of 200 Ohms on a 1.5M rod any day, the deeply driven rod will be almost immune to soil drying and freezing and that is the object of the excercise.