Grounding System and foundations

[lume7006]

Hello everybody,

I would like to receive some feedback related to:

The grounding system of a very large factory is formed by:
a grounding grid (buried on the soil) and apparently all of the foundations (concrete) have a conductor that joins the "rebar" with the metalic structure and all metalic structures are tied together but, they are never in touch with the soil.

My question is:
Do these structures help to disipate the potentials caused by a ground fault?
Is it a normal practice to do this or it is better to "ground" these structures and foundations with a local electrode in touch with the soil?
Does "rebar" assit to disipate the potentials caused by a ground faultdespite not being directly in contact with the soil?

If you could provide some ideas or references to read about it, I will thank you!

Best Regards

 

[7anoter4]

The foundation grounding -if it is well executed- may reduce the touch potential within a building to almost zero.
According to DIN VDE 0141 if a facility is located in a large industrial zone the touch potential is probably good.
But, as far as I know, it is no possibility to calculate the foundation grounding contribution to the general switchyard GPR reducing and in turn the influence on touch and step potential. So in this case one has to use grounding rods and wires and calculate GPR as per IEEE-80

 

[Marmite]

The grounding grid is designed to carry the earth fault currents back to the source in the event of a fault. The conductor joining the metallic structures of the building and the rebar in concrete floors etc is for equipotential bonding, so as to minimise touch and step potential differences in the event of a fault. A fault may well give rise to small currents flowing in the rebar grounds but they are incidental.
Regards
Marmite

 

[rbulsara]

Concrete mass is a good conductor of electricity, especially when it is moist. It also provides a large surface area in contact with earth. This helps in providing every effective and low resistance path to earth.

That is why they are even preferred type of grounding electrodes.

Do web search on 'ufer' grounding for more info.

Rafiq Bulsara

http://www.srengineersct.com

 

[pwrtran]

Concrete foundation does have overall effect to lower the grid resistance to a certain degree. However, we do not consider rebars as a part of return current path for our application. They are meant for equipotential purpose only.

 

[lume7006]

Hello,

I thank your comments, They are very useful.

I understand that this kind of grounding should be used together with a "normal grounding grid" that it is buried on the soil to give a path to the current, in case of a ground fault and of course, reducing the transfered potentials.
Does anybody know some references to be read about this topic?

Best Regards

 

[cuky2000]

A good reference are:
• NFPA 70, National Electrical Code, Section 250-50(c).NEC, Article 250-81(c),
• IEEE std 80. Check the section 14.6 “Concrete-encased electrodes”.
• IEEE std 665 Guide for Generating Station Grounding
For industrial applications may be this job is withing the jurisdiction of the NEC. Below are some ideas:

Q1] Do these structures help to dissipate the potentials caused by a ground fault?
A1] Yes, the structures steel columns will help to dissipate the current injected into the ground since together with the foundation, rebar and anchor bolts will act as a large electrode. The concrete slab and associated rebar usually is isolated to the ground by a non-conductive vapor barrier. Therefore, the slab may not have significant contribution dissipating the current into the ground unless is a direct contact with the earth.

Q2] Is it a normal practice to do this or it is better to "ground" these structures and foundations with a local electrode in touch with the soil?
A2] Ground the steel frame, columns and foundations is a common practice in the USA supported by the NEC and building codes. The performance of the steel column as ground electrode may depend on the on the fault current, the clearing time, local soil resistivity as well if a true ground. “The bolts shall be substantially connected to the base plate of the steel columns supported on that footing. Alternately, the copper cable may be brought out of the concrete”

Q3] Does "rebar" assist to dissipate the potentials caused by a ground fault despite not being directly in contact with the soil?
A3] Although the rebar is embedded in a conductive concrete with low resistivity (30-90 Ohm.m), there is a good conduction to the surrounding earth and assist to dissipate the current injected into the ground during a ground fault conditions. In fact, for medium and highly resistive soils metallic rods (rebar) has lower resistance than similar electrode buried directly in the earth.

 

[lume7006]

Dear cuky2000,

I thank your clear answers!

We will read with detail NEC the sections you are mentioning.

Best Regards

 

[7anoter4]

Referring to cuky2000 Q1

:
"The concrete slab and associated rebar usually is isolated to the ground by a non-conductive vapor barrier. Therefore, the slab may not have significant contribution dissipating the current into the ground unless is a direct contact with the earth"
As usually it is a bitumen strata under the foundation one has to build another reinforced concrete plate beneath the
bitumen strata and to connect this rebar with the actual foundation rebar in order to have a good contact with the surrounding earth or alternately to connect the foundation rebar to the Grounding Grid.

 

[cuky2000]

For your reference, enclosed are a few snap shots of a project consisting of an outdoor substation interconnected with a new switchgear building.

The rebar of the building is connected to the substation ground grid. To avoid the ferromagnetic effect of the rebar and reduce the current on then, a large ground copper loop was recommended inside the building.

Please notice in the last slide that the building acts as a quasi equipotential surface even if the external surfaces may not meet the safe criteria for touch potentials.

 

[cuky2000]

Enclosed is the file mentioned above.

 

[7anoter4]

I very much appreciate your grounding design cuky2000, indeed. But, as I had no experience using bare copper in vicinity of uncoated rebar I think one has to be careful as the copper may corrode quickly the steel. Also the cement may attack the copper and even in some cases- as the concrete shrinks when dry- could break the copper. We are using lead-or tin- covered copper in the power station yard. But I suppose if the earth resistivity is high it is not so important.

 

[rcw retired EE]

I recall an IEEE article or standard stating that the corrosion potential of rebar in concrete is about the same as copper. This means the potential for corrosion is not as great as might be expected.

I'll see if I can find the reference.

 

[dpc]

the corrosion potential of rebar in concrete is about the same as copper

I believe this to be correct. In a previous life, this subject came up from clients on several occasions and our in-house corrosion specialists indicated that the galvanic potential between copper and rebar in concrete was not a significant issue.




David Castor

http://www.cvoes.com