Tieing foundation rebar to ground grid 1

[livewire9]

Does anyone know if it is a requirement to bond foundation rebar to the ground grid?

 

[waross]

Pro: the re-bar may make a very effective Ufer-Ground.
Con: Fault currents may cause some concrete to explode. (A very low probability.)
Note: A vapour barrier under a concrete slab defeats the effects of a Ufer-Ground.
There may be an indirect requirement:
When the codes require that the resistance to ground be below a given value, and this is difficult to achieve due to unfavourable ground conditions, a Ufer-Ground may be an effective way to lower the resistance to ground.
Specs for a Ufer-Ground:
a bare copper conductor not less than 6 m in length, sized in accordance with Table 43 and encased
within the bottom 50 mm of a concrete foundation footing in direct contact with the earth at not
less than 600 mm below finished grade;
Table 43 lists #3 AWG as suitable for a 200 Amp service.
This may be spec'ed as a recognized grounding method and may be connected to the re-bar.
This mitigates the possibility of fault current caused explosions.
For 20 feet of cable this is a safe and effective grounding method.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[dik]

Thanks didn't know that... I thought that's what the conductor was for (not being sarcastic).

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[7anoter4]

You may use the rebar as grounding conductor. Our experience is to bond every steel rebar to a 1/2" diameter steel wire in each foundation column and by welding -at least 2" long welding-all these steel wires together .Where such a steel wire will be close to surface a lug could be welded and use as grounding point for grounding grid or other grounding connection.

 

[dik]

Next time I do one... I'll ask... I never considered it to be part of the grounding system and no one brought it to my attention.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[waross]

Hi Dik.
Canadian Electrical Code:
Section 10, Grounding and bonding.
Subsection 10-700 Grounding electrodes.
Rule 10-700 Grounding electrodes (see Appendix B)
(1) Grounding electrodes shall consist of
...........
(3) A field-assembled grounding electrode shall consist of
(a) a bare copper conductor not less than 6 m in length, sized in accordance with Table 43 and encased
within the bottom 50 mm of a concrete foundation footing in direct contact with the earth at not
less than 600 mm below finished grade; or
(b) a bare copper conductor not less than 6 m in length, sized in accordance with Table 43 and directly
buried in earth at least 600 mm below finished grade.
Dik and 7anoter4.
Excerpt from Wiki re Ufer Ground.
The NEC can go to Texas.

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 rods to be driven into the earth to create a low 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.
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.[2] 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]

If you have read this far you will probably enjoy this link to the entire Wiki article. Ufer Grounds

Note re; Vapour barriers.
A vapour barrier beneath a slab will both insulate the slab and prevent the doping of the soil below.
I can't find it now, but I remember seeing somewhere, an injunction against using Ufer grounds in concrete poured on top of a vapour barrier.

<Rant alert>
While the scientific community has, over the years, replaced the original descriptive names of many effects and constants with the names of scientific pioneers, the NEC has gone the other way and now calls Ufer Grounds CEEs, Concrete Encased Electrodes.
The CEC describes Ufer grounds but does not use the term Ufer in the code.
I will continue to remember and honour Mr. Ufer by calling CEEs Ufer Grounds.

<Rant off>

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[dik]

thanks...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dpc]

It's an NEC requirement - Article 250.52. Referred to as concrete-encased grounding electrode.

 

[waross]

NEC has gone the other way and now calls Ufer Grounds CEEs, Concrete Encased Electrodes.
....
I will continue to remember and honour Mr. Ufer by calling CEEs Ufer Grounds.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!