Using steel wire mesh as ground

[onsiteeng]

Hi There,
First let me state I am a Structural Engineer so not up date on the electrical requirements for grounding. We have a warehouse building that is being built on slab on grade which has steel mesh through out the slab. The mesh comes in 5'x10' segments and is tied together with galvanised wire. Is it possible to use this element instead of running a 4/0 ground wire around the perimeter of the slab (The electrical engineer specified this). The mesh has a steel area of 252mm2 per meter each direction.

 

[dpc]

If the concern is meeting the installation codes (assuming NEC), then neither concept is strictly necessary. Refer to Article 250. Reinforced concrete is a reasonably good conductor as long as the concrete remains moist. If the concrete dries out, it is not nearly as good a conductor. Also, the galvanized wire ties will not be effective electrical connectors over time due to corrosion and contact resistance.

Reinforced below-grade concrete footings are a good ground and in fact, the NEC requires a connection to these in addition to a ground rod or other "made" electrode. Google "Ufer ground".

For a typical warehouse, I'd expect a ground rod, a ufer ground to a footing steel, if available, and the useless water pipe ground connection. The perimeter copper is not a bad idea, but not typically done for a warehouse that has modest power requirements.

Cheers,

Dave

 

[7anoter4]

At first I agree with Dave. However, the NEC requirements are not so expensive-in my opinion. See art.250.52 for instance (3) Concrete-Encased Electrode. The steel rod has to be 1/2 " diameter, indeed, but only 20 ft. long [or a copper conductor of 4 awg only].

 

[onsiteeng]

Thanks for the replies above. So I mentioned using the Ufer to my electrical Engineer and got this reply

"Not in this case. NEC also does not apply.
We have to have ground per the transformer section to ensure equipotential bonding and no possibility of electrocuting anyone. What you do not understand is that we have both North American AND European breakers in this building. This building employs both North American Neutral breakers and European real ground ELCBs.
The resistance to Earth must be lower than an Ufer method to avoid electrouciton because the breaker will not trip when there is a fualt in the other system, - it simply wont detect the fault because the ground floats in North America."

This still confuses me as I thought all we needed to do was use enough concrete encased reinforced steel to produce a low enough impedance to match that of the 4/0 copper cable. as I have 7000 SF of heavy mesh in this slab I thought that would do it.

 

[wayne440]

IMHO- This is a situation where you are better off to build to spec and then let the electrical "engineer" justify his/her reasoning to the bean counters.

 

[onsiteeng]

I have done a bit more reading and I think the Electrical Engineer has specified a Ufer grounding system. He basically has specified the following.

1)continuos 4/) copper wire placed in the slab just inside the perimeter of the building(125' x 150ft) so the cable is about 350ft long.
2)"Bond reinforcing steel @ 4points(2 each bay).Using #4/0 exthermic bonding method.
3) Extend 18" ground pig tail for future connection(3 locations as shown)

So each of the 3 pigtails are shown next to 3 column located at the ends and center of the building.
I am assuming the 4 connections are connected to the foundation steel in 4 quadrants of the building plan to distribute the grounding evenly.

As there is no ground rod system then this appears to me to be a Ufer system. However as I have one foundation with 14 5/8" bar that run continuous top and bottom and it is 125ft long couldn't I just connect the pig tails to that and make sure the rebar is bonded properly. Would this save a lot of money on copper cable? The client is very cost conscious.

 

[dpc]

The resistance to Earth must be lower than an Ufer method to avoid electrouciton because the breaker will not trip when there is a fualt in the other system, - it simply wont detect the fault because the ground floats in North America."

This sounds confused - or perhaps I'm confused. Sounds like he is describing two separate electrical systems, one of which is ungrounded. Seems like a complex system for a "warehouse". Most systems in the US are solidly grounded.

Where is your EE from? And where is the project?

Again, rebar in concrete is a fairly good ground as long as the concrete is below grade and stays moist. Copper in the slab is fine, but not strictly a Ufer ground.

 

[waross]

WIKI is your friend;

https://en.wikipedia.org/wiki/Ufer_ground

You may want to diplomatically share this with your EE.

Ufer ground
From Wikipedia, the free encyclopedia
(Redirected from Ufer Ground)

The Ufer Ground is an electrical earth grounding method developed during World War II. It uses a concrete-encased electrode to improve grounding in dry areas. The technique is used in construction of concrete foundations.

Contents

1 History
2 Construction
3 External links
4 References

History

During World War II, the U.S. Army required a grounding system for bomb storage vaults near Tucson and Flagstaff, Arizona. Conventional grounding systems did not work well in this location since the desert terrain had no water table and very little rainfall. The extremely dry soil conditions would have required hundreds of feet of copper rods to be inserted into the ground in order to create a low enough impedance ground to protect the buildings from lightning strikes.

In 1942, Herbert G. Ufer was a consultant working for the U.S. Army. Ufer was given the task of finding a lower cost and more practical alternative to traditional copper rod grounds for these dry locations. Ufer discovered that concrete had better conductivity than most types of soil. Ufer then developed a grounding scheme based on encasing the grounding conductors in concrete. This method proved to be very effective, and was implemented throughout the Arizona test site.

After the war, Ufer continued to test his grounding method, and his results were published in a paper presented at the IEEE Western Appliance Technical Conference in 1963.[1] The use of concrete enclosed grounding conductors was added to the U.S. National Electrical Code (NEC) in 1968. It was not required to be used if a water pipe or other grounding electrode was present. In 1978, the NEC required rebar to be used as a grounding electrode if present. The NEC refers to this type of ground as a "Concrete Encased Electrode" (CEE) instead of using the name Ufer ground.

Over the years, the term "Ufer Ground" has become synonymous with the use of any type of concrete enclosed grounding conductor, whether it conforms to Ufer's original grounding scheme or not.[2]
Construction

Concrete is naturally basic (has high pH). Ufer observed this meant that it had a ready supply of ions and so provides a better electrical ground than almost any type of soil. Ufer also found that the soil around the concrete became "doped", and its subsequent rise in pH caused the overall impedance of the soil itself to be reduced.[3] The concrete enclosure also increases the surface area of the connection between the grounding conductor and the surrounding soil, which also helps to reduce the overall impedance of the connection.

Ufer's original grounding scheme used copper encased in concrete. However, the high pH of concrete often causes the copper to chip and flake. For this reason, steel is often used instead of copper.

When homes are built on concrete slabs, it is common practice to bring one end of the rebar up out of the concrete at a convenient location to make an easy connection point for the grounding electrode.[4]

Ufer grounds, when present, are preferred over the use of grounding rods. In some areas (like Des Moines, Iowa) Ufer grounds are required for all residential and commercial buildings.[5] The conductivity of the soil usually determines if Ufer grounds are required in any particular area.

An Ufer ground of specified minimum dimensions is recognized by the U.S. National Electrical Code as a grounding electrode.[6] The grounding conductors must have sufficient cover by the concrete to prevent damage when dissipating high-current lightning strikes.[7]

A disadvantage of Ufer grounds is that the moisture in the concrete can flash into steam during a lightning strike or similar high energy fault condition. This can crack the surrounding concrete and damage the building foundation.

Bill
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"Why not the best?"
Jimmy Carter