Test of ohms for ufer ground

[dereckbc]

Anyway after I wired the house the homeowner showed up and claimed he did a ohms test on the ufer ground and it came back at 800 ohms. I'm not sure what he did to test it but I wouldn't think that could be correct.

OK this raises Red Flags. To accurately measure Ground Electrode Impedance takes some very specialized and expensive test equipment and training to use. I highly doubt the HO has the equipment or knowledge to use it. Heck not many electricians or engineers know how to do it correctly. Don't take offense guys, just stating the facts.

Second thing is for a UFER ground it is almost impossible to use a 3-point dead fall potential test using a meggar. For a 3-point test to be accurate 3 criteria has to met to be valid.

1. The test field needs to be a minimum of 8 times large than the area being tested.
2. The test line cannot pass over any metallic buried objects like a water or sewer line.
3. Must be made from multiple directions.

So for example let's say the foundation is 30 feet by 30 feet. Your test line then has to be a minimum of 240 feet. In a urban environment it would be almost impossible to meet those conditions. Only way to really do it and get somewhat accurate would require a Clamp-On loop tester and those cost $1700 for a decent one.

Even if 800 Ohms is correct, so what? It means nothing.

 

[mwm1752]

Check the continuity between the 2 UFURS to see if they are tied together -- If your continuity is bad then it is a good sign the steel was just inserted in the concrete -- Simple fix is drive 2 ground rods & move on -- do not know if you have a water pipe ground?

 

[acrwc10]

There is the question of, if the 2 pieces of rebar sticking out of the foundation are at least 10' apart, what did the HO use to extend his tester leads and how did he connect them to the rebar? was the rebar cleaned first?
I will bet the HO did the test wrong.

 

[GeorgeB]

Well one thing often over looked today is most concrete foundations have VAPOR BARRIER installed between the ground and concrete with 6 mil thick plastic sheathing is used. That pretty much renders the CCE to useless status as a ground electrode.

I'm neither a builder nor a concrete installer, but do like to watch houses being built. While concrete slabs here (upstate South Carolina) usually have a vapor barrier, I've never seen it extended under the perimeter foundation which usually must be dug in undisturbed soil to below the frost line. There often isn't any steel in the slab (residential), but usually is in the foundation.

 

[mwm1752]

There is the question of, if the 2 pieces of rebar sticking out of the foundation are at least 10' apart, what did the HO use to extend his tester leads and how did he connect them to the rebar? was the rebar cleaned first?
I will bet the HO did the test wrong.

Gennerally, The footer steel is continuous around the entire building -- if indeed you have 2 UFUR's (only 1 is required & is a stand alone electrode) then the logic that follows is both pieces connect to the footer steel. The point being continuity at least confirms a possible tie to footer -- There is no requirement I know of that says you need to use both UFUR's or that there be spacing between them ---
210.52(A)(3)(2) "If multiple concrete-encased electrodes are present at a building or structure, it shall be permissible to bond only one into the grounding electrode system."

 

[GoldDigger]

IMO in the situation originally described you only have two possible connection points to one CEE, not two CEEs!

Tapatalk!

 

[gadfly56]

IMO in the situation originally described you only have two possible connection points to one CEE, not two CEEs!

Tapatalk!

If both pieces of rebar do, in fact, tie into the foundation rebar. I think the HO's claim, as I see it, is that because of the high resistance between the two, they aren't tied together and may not, in fact, be tied to the foundation. Other than excavating the rebar back to the foundation and cracking it open, I don't know how you might prove this one way or the other.

 

[GoldDigger]

On the other hand, the OP stated that the inspector would check for continuity between the two points as evidence that the CEE was properly configured in lieu of an actual visual inspection of the rebar and stub before the pour.
If the owner later measured just the resistance between the two rebar stubs and got 800 ohms, that indicates that either the resistance changed since the inspection or the connection should not have passed inspection in the first place.
If the owner made a measurement that should have been related to the ground electrode impedance and that reading was 800 ohms, then something is wrong.
One problem is that the test used by the inspector does not in fact test whether the CEE was installed in accordance with the code.
An analogous situation would be if the inspector checked to confirm that there was raceway in a position to act as the EGC in a circuit but deliberately did not check whether the raceway was metallic or not.

Tapatalk!

 

[crtemp]

Just to clarify a few things, I'm not sure what he used or how the homeowner tested this. I got this information from the builder. Also no electrical inspections have been done to this point. Even though the inspectors require 2 pieces of rebar to stick up for a continuity test I don't even think they bother doing them. Also this is not a slab, just a regular foundation. No vapor barrier that I know of.


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[mwm1752]

Just to clarify a few things, I'm not sure what he used or how the homeowner tested this. I got this information from the builder. Also no electrical inspections have been done to this point. Even though the inspectors require 2 pieces of rebar to stick up for a continuity test I don't even think they bother doing them. Also this is not a slab, just a regular foundation. No vapor barrier that I know of.


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The plot thickens & I cannot be sure if this even qualifies as a UFUR in the first place -- Footers are always required below the frost level -- drive ground rods & move on

 

[acrwc10]

There is the question of, if the 2 pieces of rebar sticking out of the foundation are at least 10' apart, what did the HO use to extend his tester leads and how did he connect them to the rebar? was the rebar cleaned first?
I will bet the HO did the test wrong.

Gennerally, The footer steel is continuous around the entire building -- if indeed you have 2 UFUR's (only 1 is required & is a stand alone electrode) then the logic that follows is both pieces connect to the footer steel. The point being continuity at least confirms a possible tie to footer -- There is no requirement I know of that says you need to use both UFUR's or that there be spacing between them ---
210.52(A)(3)(2) "If multiple concrete-encased electrodes are present at a building or structure, it shall be permissible to bond only one into the grounding electrode system."

I think you missed my point, the leads on my OHM meter are not 10' long, the HO had to some how extend his TESTER leads. This would be a point the OHM meter could get a high reading and the TESTER connected to the rusted or concrete residue covered end on the rebar.

 

[GoldDigger]

The plot thickens & I cannot be sure if this even qualifies as a UFUR in the first place -- Footers are always required below the frost level -- drive ground rods & move on

If there really is no correctly installed CEE, and the inspector was lax in not noticing that, driving a few ground rods and moving on would be a solution in straight NEC states, but not under the WA amendments.

Tapatalk!

 

[mwm1752]

If there really is no correctly installed CEE, and the inspector was lax in not noticing that, driving a few ground rods and moving on would be a solution in straight NEC states, but not under the WA amendments.

Tapatalk!

The OP states that if the inspector does not see it installed then 2 pieces 10' away is how the UFUR is determined(I think) -- So, if there are "2 pieces 10' away" there is insinuation an inspection was not done -- Inspectors do not call for inspections the tradesmen do -- We are also talking about a poured slab not a footer or structural element -- Metallic components shall be encased by at least 50 mm (2 in.) of concrete and shall be located horizontally within that portion of a concrete foundation or footing that is in direct contact with the earth or within vertical foundations or structural components or members that are in direct contact with the earth. --
If a footer was pour for this construction then the contractor is in fault for not attaching the GEC to it as it was an available when the footer was formed. If indeed the inspection was done that should be enough for the owner to be satisfied -- if the inspector was lax then he should not be inspecting the job.

 

[don_resqcapt19]

...
If a footer was pour for this construction then the contractor is in fault for not attaching the GEC to it as it was an available when the footer was formed. If indeed the inspection was done that should be enough for the owner to be satisfied -- if the inspector was lax then he should not be inspecting the job.

The problem with that is what contractor has to install the GEC?...in many cases an electrical contractor has not even been selected at that point of time.
Our city code requires that the concrete contractor provide and install the GEC, and the inspection is by the building inspector that inspects the footing.

 

[crtemp]

The problem with that is what contractor has to install the GEC?...in many cases an electrical contractor has not even been selected at that point of time.
Our city code requires that the concrete contractor provide and install the GEC, and the inspection is by the building inspector that inspects the footing.

The concrete guys install it. I'm not sure if the building inspector looks at it or not. I would have to assume that it would be in before any rebar inspections though and a building inspector would at least see it.



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[mwm1752]

I understand the logistics in who when & how for install but the code for compliance is still NEC -- Maybe the responsible party is the GC as a EC has not been named but it is the EC that will get his inspection failed -- We also will mark on prints during the footer steel where the electrode is placed for future reference -- there is an advantage for this enity to have a Combination inspector as I inspect all aspect of building -- some dept do not have electrical inspectors & it is done by the state so how would the inspection be verified unless there was an agreement between the two enities on procedure. I will also state that most reliable contractors are aware of the UFUR to be in place. For a building inspector to inspect the NEC could be the reason for the fault in this OP. Inspecting a UFUR is not rocket science but as I noted in a prior rant what we are talking about may not quailify as a UFUR installation being a slab and the Building inspector may not have had a complete understanding other than seeing rebar steel.

 

[Cavie]

In Florida vapor barrier is used under the slabs and footers. Ufer is required. If it is missed in the slab, dig a 20' trench and install a UFER per code. The thinking is, the slab has exposer on the sides of the foundation. Ohms are not tested. Not a code requirement. (I wish someone would explain that to the FDOT) Try grounding and measuring 250 light poles!!:rant: