Fall of potential

[ohthatshowyoudo]

Please can any one advice on how to take a measure of the fall of potential on 3 mtr ground rod when surrounding area is concrete, i have been told to secure test probes on p1 and c2 at the appropriate distances and cover in water, any help is greatly appreciated.

 

[Knobhead]

Do you know if the concrete has rebar in it?

 

[Richard Burns]

So long as you have good contact with the concrete then you should get a low enough resistance value to be able to get reasonable results.
If there is rebar in the concrete this is almost certain but the rebar will also work to prevent an accurate reading as it will affect the resistivity of the ground.

 

[Engineer54]

To be honest the results you'll obtain using that method would at best only give a very rough as old boots value and to my mind not worth the time and effort involved. Your best bet here is conducting a standard ELI test at the supply intake.

As Tony suggests, if that value is cac, try getting a connection to the buildings foundation rebar or even the surrounding concreted areas rebar. Only problem with the later is, that the rebar is unlikely to be connected together as it would be in foundation reinforcement....

 

[sammyshooter]

Can cover your probes in chains and soak with water.

 

[ohthatshowyoudo]

yes i believe it has but can not be 100% sure,

 

[ohthatshowyoudo]

cheers, do you reckon i best with an earth ground clamp on meter ? just i have been told these can give false readings.

 

[Engineer54]

Can cover your probes in chains and soak with water.

Using this system on reinforced concrete is going to potentially give erroneous results, also the surrounding concrete grid is unlikely to be solidly connected and could well compromise the earth rods zone of influence anyway.

I'm not even sure of the reasoning behind wanting to test the earth rod using a dedicated earth tester under these conditions?? Is this an existing earth rod thats being tested if so, is there any past Ra values recorded?? Is there a problem with the Ra value being obtained?? Is it a newly installed 3m earth rod??

 

[ohthatshowyoudo]

Hi cheers for the comments,
the earth rod under test is for a LPS and there is no supply to the building., i have to achieve 10ohms or less, i was just looking to see if any body eles has had similar problems dealing with concrete ?

 

[Engineer54]

cheers, do you reckon i best with an earth ground clamp on meter ? just i have been told these can give false readings.

They can indeed give false readings if you don't know how to use them. These earth clamp testers work best with linked multiple earth electrodes. With single rod location installation they are not the best bit of kit to use!!

 

[Engineer54]

Hi cheers for the comments,
the earth rod under test is for a LPS and there is no supply to the building., i have to achieve 10ohms or less, i was just looking to see if any body eles has had similar problems dealing with concrete ?

Just one earth rod for a LP installation, what is the building a garage, garden shed?? lol!!

 

[ohthatshowyoudo]

no not one earth rod several. and yes its basically metal shed lol. its connected via 25mm 3mm bare copper tape. i will disconnect tape from rods then test using the 3 point fall of potential test, but am asking for help on how i can overcome the concrete any help appreciated.

 

[Engineer54]

Do you have any previous rod Ra results for this LPS?? To be honest you are on a hiding to nothing if you need to rely on results gained by using weighted contact plates on wet concrete. I doubt very much if you could clamp an earth tester onto the rod, not usually anywhere near enough room in the inspection pit to get the clamp on.

Is your involvement on this job to test or to achieve the 10 ohm minimum value. If it's a matter of conducting tests, you can only record the results obtained, if they fall way outside minimal accepted values that's another matter, and will be outside your remit....

In the UK it is far better to have a LP specialised contractor in to conduct system condition reports, they also carry the insurance to cover themselves when things go wrong, that they should of picked up on. etc!!

 

[Engineer54]

ohthatshowyoudo, it's the overall system that needs to be 10 ohms or less not the individual earth rods. All you can really do is use the weighted contact plate on saline water method of testing these rods, just don't expect them to be accurate. What other tests are being conducted to this LP system installation if any??

You could try using the earth rods themselves as the voltage and current test spikes, for the rod under test, if there are enough of them around the building. got to be better than wet contact plates on concrete!! ...lol!!

 

[ohthatshowyoudo]

No there's no previous results, i am to test LP and record results, the earth rods have been situated around container already, attached to bare copper tape covering roof , attached to two air terminals, the test has to be conducted with connections removed and 10 ohms achieved on each rod, but if this is not achievable on one rod adding multiple rods will assist in lowering resistance. or so im lead to believe. also testing the air terminals on disconnected system via a low resistance ohm meter,should give me results required to establish the systems stability.