Happy earth rod day

[pc1966]

No major question really. Screwed in a 2m long M16 stainless rod for earth today and got around 10 ohms which was better than I planned, but not desperately surprising as its clay soil and only a couple of meters above the river's mean level (i.e. water table).

Measured Zs extension lead to MET = 1.5 ohm, Zs same extension lead to rod = 11.5 ohm.

But what was odd was a DC resistance test (DMM, after low-ohm meter said > 100R) was showing around 170-200-ish ohms in both polarities. I expected I might see a DC bias, but did not expect that sort of non-linearity!

 

[Lucien Nunes]

Where was the other lead of the DMM, still the MET? Did you measure the AC voltage? That would cause a symmetrical influence on the resistance readings. I agree though that some electrolytic potential does usually show up that at least makes a difference between the two indications, or even swamps the DMM source making one of them negative.

 

[pc1966]

Yes, it was basically the same test points. It was starting to rain so I didn't spend much more time checking AC volts, etc, but as the system is not finished yet there will be a more formal test when its all done.

 

[LastManOnline]

No major question really. Screwed in a 2m long M16 stainless rod for earth today and got around 10 ohms which was better than I planned, but not desperately surprising as its clay soil and only a couple of meters above the river's mean level (i.e. water table).

Measured Zs extension lead to MET = 1.5 ohm, Zs same extension lead to rod = 11.5 ohm.

But what was odd was a DC resistance test (DMM, after low-ohm meter said > 100R) was showing around 170-200-ish ohms in both polarities. I expected I might see a DC bias, but did not expect that sort of non-linearity!

10 ohms. That must get you an automatic nomination for "rod of the year award". Well done

 

[pc1966]

I expected about 4 times that to be honest!

There will be another 4 rods going in as well, but they are all more for lightning / EMC issues than power feed grounding. But it is still nice to know that if your TN-S supply earth goes open then you can still disconnect on the 300mA incomer RCD.

 

[LastManOnline]

I expected about 4 times that to be honest!

There will be another 4 rods going in as well, but they are all more for lightning / EMC issues than power feed grounding. But it is still nice to know that if your TN-S supply earth goes open then you can still disconnect on the 300mA incomer RCD.

Absolutely. I have never heard of such a good result. Very encouraging. Yes a great plan B for TN-S. But with those soil conditions another 2 rods could quite possibly get you a plan B for a TNC-S Istallation as well. Wish we had conditions like that over here

 

[pc1966]

Wish we had conditions like that over here

Electrically it is way better than I expected. But there is a down-side to the wet clay below the local ground surface: when you get it on your shoes it behaves, in every way but smell, like you have stood in a dog turd!

 

[LastManOnline]

Electrically it is way better than I expected. But there is a down-side to the wet clay below the local ground surface: when you get it on your shoes it behaves, in every way but smell, like you have stood in a dog turd!

Bearing out the old saying "nothing in life is free"

 

[jay sean]

I think a reading below 200ohms is good and not unstable this is from the 16th ed