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.

 

[Engineer54]

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.

Someones misinforming you here, it's the overall system value that needs to be 10 ohms or less, NOT each and every down conductors earth rod. Does this metal shed/container contain explosives by any chance?? There may well be other requirements if this is the case...

As i suggested maybe you can test each LP rod using the other disconnected LP system rods as the potential and current test spikes. with those test spikes being 3m long the results will give you far more accurate values than relying on weighted contact plates on wet concrete!! Just depends on the distances between the earth rods in a straight line or at right angles as you move around the building....

By the way, how far does the surface concrete extend beyond the building under test?? And is there any open ground beyond that concrete, where test spikes can be used?? I have often had to go a couple of hundred metres or so, and take several readings to get an accurate assessment of Ra values....

 

[ohthatshowyoudo]

cheers..
its got to be a least 400 to 500 meters to open field, there are 3 rods located around building with more than 3mts between rods but as you said on angles, so if i was to disconnect tape, and utilize earth rods as temp spikes this would be acceptable as long as results are satisfactory, the container has flares and is considered explosive.

 

[Engineer54]

cheers..
its got to be a least 400 to 500 meters to open field, there are 3 rods located around building with more than 3mts between rods but as you said on angles, so if i was to disconnect tape, and utilize earth rods as temp spikes this would be acceptable as long as results are satisfactory, the container has flares and is considered explosive.

I'd certainly give it a try on one mate, then try the weighted wet contact plate method on the same rod tested and see what sort of results you are obtaining from both methods.

3 Down conductors seems to be an odd number for an oblong building, i would have expected to see 4 in this case!!

 

[davesparks]

This all sounds very familiar! Is this fireworks storage by any chance?

The following is the new rules which came in to effect on the 1st October this year:

Lightning protection
29 Suitable lightning protection should be installed in explosives stores except where the store:
• is temporary (for example, for no more than a few weeks on a seasonal basis) and holding HT
4 pyrotechnic articles;
• is used to keep less than 75 kg of HT 4 explosives;
• is used to keep less than 25 kg of HT 3 explosives;
• is used to keep no more than 100 g of HT 1 or HT 2 explosives;
• contains only HT 4 small arms ammunition;
• is made by excavation and is thereby inherently protected from lightning; or
• is exempted under the terms of a licence issued by HSE or ONR;
• is within domestic premises, is not otherwise exempt and is being used for the storage of
explosives in accordance with regulation 27(3).
30 Suitable lightning protection should be installed in manufacturing and processing facilities that
cannot be emptied of explosives upon the approach of a thunderstorm.
31 Lightning protection will need to be based on the requirements set out in an appropriate
relevant standard. The method of protection will depend on the nature of the area to be protected and
includes the use of a suspended air termination network at an adequate height above the area to be
protected and/or any vertical conductors. All metallic reinforcement, crane and railway rails which
enter explosives buildings must be bonded to the nearest point of the lightning protection system.
32 Inspections and tests should be conducted at intervals recommended by a competent
person that will ensure the continuing effectiveness of the system. Where earth resistance is expected
to vary during the year tests should be conducted at intervals that will allow any seasonal earth
resistance to be assessed, eg every 11 months.
33 All main structural metalwork in and on the explosive building (including the lightning and antistatic
protection systems), should be connected to a common system of earthing and equipotential
bonding. Metallic enclosures of electrical switchgear, motors, starters and other electrical appliances
must be suitably earthed. Metallic cable sheaths or armouring, metal projections through walls (pipes,
rails etc) should be suitably bonded to the lightning protection system. Provision should be made to
allow access to the earth electrodes for testing purposes.
34 Steel-framed structures with metallic cladding may be regarded as self-protecting provided
the individual earth resistance of each stanchion, in a stand-alone condition, does not exceed 10ohms. The metallic cladding should be bonded to the structure by suitable metal fixings and electrically bonded with straps of the same cross-sectional area as the main down conductor, and at
least 50 mm2. Where these conditions cannot be met, a ring conductor, bonded to each stanchion and
with earth electrodes at each end of the structure, should be provided.
35 Steel ISO or similar containers used for the storage of explosives can be regarded as self protecting
provided that:
• the walls are lined with wood (or other appropriate lining) or the explosives are kept at least
150 mm away from the container’s walls;
• the panels and doors are electrically bonded with straps of a cross section of at least 50 mm2;
• two earthing points connected to earth rods are provided at opposite corners; and
• resistance from the top of the container to earth is less than 10 ohms.

 

[Engineer54]

I've made a note of that Thanks!!

Still doesn't help the OP out very much on how he's going to test his existing ground rods, but he'll get some brownie points giving these new requirements to his boss!! lol!!