Grounding (earthing) via rebar - any comments?

[agulesin]

Hello everyone,

I've just read about rebar (reinforcement in concrete foundations) being used as an electrical earth in the USA. Apparently it's in their equivalent of the 17th Edition:

"it turns out that rebar in concrete footings or the foundation for a house is actually a more effective grounding system than the ground rods we've been using for decades. So if there's rebar in the new footings, that rebar needs to be used as the primary grounding electrode. In a nutshell, if a new home has footings with at least 20 ft. of 1/2-in. rebar, the rebar embedded in those footings needs to be used as the primary grounding electrode. This new provision in the National Electrical Code"
http://www.familyhandyman.com/elect...common-electrical-code-violations-diyers-make (no.

From my scant knowledge on the subject, that seems strange; rebar is steel and liable to rust easier than copper grounding rods. OK when it's embedded in concrete it shouldn't rust; but then is it contacting the earth? how conductive is dry concrete? (let's assume the concrete is isolated from the surrounding earth by a plastic membrane).

Also what about the connection between the copper used in building wiring and the steel rebar; isn't this going to corrode (more) quickly due to the potential between different metals (see https://en.wikipedia.org/wiki/Galvanic_corrosion)?

I would like to hear your opinions on this which, as it's in the "Code", must have been discussed by our colleagues across the pond...

Thank you in advance!

 

[OwainDIYer]

The concrete-encased electrode has proven that it offers optimal performance and longevity. The footing or foundation of any building will typically be there as long as the building is. Because all of the rebar in the bottom perimeter of the building footing is usually tied together with tie wires, the electrode acts similar to a ground ring, only it has much more surface area in the concrete connection to the earth. The footing is present around the bottom of the building perimeter, which means there is significant ground (earth) contact from concrete-encased electrodes. Concrete retains moisture and is continuously absorbing moisture through the bottom of the footing. This ensures an effective connection between the footing and the earth.
http://www.ecmag.com/section/codes-standards/what-ufer-ground

The technique Mr. Ufer came up with was necessary because the site needing grounding had no underground water table and little rainfall. The desert site was a series of bomb storage vaults in the area of Flagstaff, Arizona.
The principle of the Ufer ground is simple, it is very effective and inexpensive to install during new construction. The Ufer ground takes advantage of concrete’s properties to good advantage. Concrete absorbs moisture quickly and looses moisture very slowly. The mineral properties of concrete (lime and others) and their inherent pH means concrete has a supply of ions to conduct current. The soil around concrete becomes "doped" by the concrete, as a result, the pH of the soil rises and reduces what would normally be 1000 ohm meter soil conditions (hard to get a good ground). The moisture present, (concrete gives up moisture very slowly), in combination with the "doped" soil, make a good conductor for electrical energy or lightning currents.
http://www.psihq.com/iread/ufergrnd.htm
(that page also notes that the rebar may be stainless steel or copper-clad to prevent corrosion)

A Ufer Earth is not used alone, a conventional earth rod is still needed as usual.
http://wiki.diyfaq.org.uk/index.php/Ufer_Earthing

 

[bernardgreen]

Rebar that has been correctly set into concrete is for practical purposes insulated from the ground. Concrete is a very poor conductor of electricity.

Rebar is ( should be ) protected from corrosion by being totally enclosed in concrete to prevent moisture amd oxygen getting to the rebar once the concrete has set.

That said the Americans seem confident that concrete will retain / absorb enough moisture to be conductive. Given the large surface areas specified it seems it will work.

http://www.ecmag.com/section/codes-standards/what-ufer-ground

( beaten to the post by Owain )

 

[vinn]

The technique works perfectly well, and is also used in various structures/projects here in the UK, and across the world.