Earth grid construction 5

[Flashover]

im am currently reviewing a complete design for earthing of a new power station. I have some small problems with the following:-
1) connection of re-bar in the piling to earth grid, have searched many standards but cant find anything definite
2)lighting protection and earting of I&C cabinets
3)any websites or recommended reading.... warmly welcomed
regards

flashover

 

[tgott]

For foundations our practice is to bond the anchor bolts to the rebar and then to the ground grid. I don't know of any standard but experience has shown that when we don't do this, our foundations sometimes get broken after a lighting stike due to the current coming down the metal structure to the anchor bolts and then trying to get to ground. (We do also ground the structure itself, but this is a backup in case the ground wire from the stucture gets broken.)
IEEE Std. "Guide for Safety in Substation Grounding" is generally recognized as one of the most authoratative guides available. See also chapter 9 of RUS Bulletin 1724E-300 "Design Guide for Rural Substations"

http://www.usda.gov/rus/electric/pubs/1724e300/1724e300.pdf.

 

[brupp]

Until recent years, IEEE 80 was the standard. However, the IEEE has finally recognized that generating stations are not substations and have issued IEEE 665 "IEEE Standard for Generating Station Grounding". It has discussion about many aspects of generating station grounding. IEEE 142 is for industrial and commercial power systems and is helpful regarding grounding of individual equipment but is not designed to cover generating stations.

As for the rebar and piling, it can certainly help to bond this to the ground grid but it should not be relied upon. Unless you exothermally weld the rebar together it is impossible to determine the connection resistance at any given future point. Even if you do weld the rebar it is likely to become corroded over time and the effectiveness of that portion of the ground grid will be reduced. If you want to connect to the rebar, go ahead and do it. But ignore it in your grounding calculations.

I&C grounding is covered by IEEE 1050 but reading this standard suggests that the authors did not agree in all respects. There are alternate applications and some design guidelines which require a lot of interpretation. It is a good guide but does not stand alone as a design guide.

 

[dpc]

To brupp,

I am interested in your comments regarding corrosion in the rebar grounding connection. As you probably know, the NEC requires this Ufer ground for all new construction, although a separate ground rod and water pipe ground is still required, I believe.

The old papers I haver read regarding Ufer grounding is that this method is generally superior to use of ground rods if deep footings or pilings are available.

The corrosion engineers I have talked with in the past have always felt that reinforcing steel that is encased in concrete is generally quite stable and not subject to much galvanic corrosion.

Also, although I agree that exothermic welded connections to rebar are preferred, I believe that there are UL-approved compression connections as well.

Can you provide some additional info on these issues?

 

[GordS]

I had to do a bit of research into rebar grounding last year. I used Google and searched for "re-bar grounding". One of the papers I found was entitled "Some Engineering Objections to using Reinforcing Steel as Grounding Electrodes" written by several senior IEEE members. It talks mainly about the corrosion possibilities and basically concludes to keep grounding separate from re-bar.

Also, I noted IEEE 80 (pg 69 of the 2000 version) states that any AC current in a rebar grounding system gets recified by the rebar/concrete junction. It doesn't explain how. The % rectification is small. However, since small currents are common on bonding conductors over long periods of time, the impacts may become significant over time.

IEEE 80 also gives guidelines for the current withstand capability of rebar electrodes. This is a function of the power system capability and protective settings. Any fault is going to split between the ground electrodes and any binding wires between the fault and source. The better the ground electrodes become, the more current they carry. When I was trying to understand this technique, I modelled a building and investgated the current split due to a fault. I could see the rebar current capability being exceeded if in soils of low resistivity; I could also show that rebar capability was NOT exceeded in souls of high resistivity. (The Ufer ground was originally proposed for obtaining low impedance grounds in high resistivity areas - it seems to have been extended in application to everywhere)

I echo the concept in one of the previous postings - use re-bar grounding as a supplementary ground but don't depend upon it. Further, do some engineering to prove that the technique will not affect structural integrity.

 

[dpc]

Thanks peterb. I do seem to remember reading something about rectification.

It will be interesting to see what, if any, problems develop with all of the Ufer grounds now being installed. As I mentioned, these are *required* by the NEC for new construction.

I have heard a lot of whining from structural engineers regarding connecting the copper ground wire to their rebar, but the corrosion guys were generally able to get them calmed down. I would avoid connecting to steel that is not encased in concrete.

Also, we are seeing a wider use of epoxy-coated rebar here in the NW, especially along the coast.

 

[cuky2000]

Dear Flashover,

Grounding the pile is an acceptable method to increase the grounding resistance. We had a similar case in a power plant application grounding the steel piles and other concrete foundations with steel rebars reinforced. Our civil and electrical group agree in this issue concluding that the corrosive effect in the steel pile were not significant the expected life design of the project.

This conclusion was based on the fat that ac current does not produce corrosion. Very small amount of ac current became rectified but very far from the threshold for corrosion limit of 60 Vdc. A good overview of this subject could be found on the articles 14.6 of IEEE 80-2000 and the list reference articles.

We compare two similar designs grounding and the conclusion favor grounding the pile to be more economical and provide additional safety.

We had extensive discusion of this issue with many engineers and authority in this field. The bast majority agree that is a good practice to ground the rebards, concrete and other metalic structures in contact with the ground.

Be aware that step and touch potentials in concrete surfaces (~50 Ohm-m) will be lower than other surface covered with gravel (2000 Ohm-m) or asphalt (10,000 Ohm-m).

 

[SooryaShrestha]

cucky2000,
How will the step and touch potentials become lower in the concrete surface(low resistivity) than gravel(high resistivity)? With my limited knowledge,if that be the case, substation would have been covered with low restivity material.

 

[cuky2000]

In reference to question #2 Lighting Protection and Grounding here are few suggestions:

To protect the DCS system again lightning should be also considered a proper grounding system. IEEE Standards 1100 , C62 and C22-1 & 2 cover lightning protection. NEC Article 250 could cover the requirement for grounding. Common sense and good engineering practice suggest to be considered the following:

- Run the communication system properly grounding the tray and conduit systems. Do not use conduit as equipment grounding; use separate isolated wire.
- Provide shielding protection for the cable and other apparatus. Connect ground I/O wiring shield at one end only.
- Install secondary arresters and staged transient voltage surge suppressors (TVSS) at the distribution panels.
- Consider design a power isolation system with isolation device grounding.
- Alternate form of transmissions to twisted pair could be fiber optic or radio communication.
- Consult the DCS manufacturer such as ABB-Symphony, Siemens-XTP or GE system.

 

[busbar]

Darn, that's a lot of free advice for one day. All I can add is look over IEEE 837 ratings [INDEXED at

http://standards.ieee.org/reading/ieee/std_public/description/subst/837-1989_desc.html

] and suggest a gnarley little connector for bonding copper control-cable shields...Amp closed-barrel crimp tape connector for bonding to copper-tape shielding without solder:

http://catalog.tycoelectronics.com/TE/docs/pdf/6/77/170776.pdf

10-12 AWG

http://catalog.tycoelectronics.com/TE/docs/pdf/1/18/170811.pdf

14-16 AWG

http://catalog.tycoelectronics.com/...0&M=CINF&N=2&LG=1&I=42&RQS=C~11052^M~FEAT^G~G

TERMI-FOIL general description

 

[cuky2000]

In response to SooryaShrestha question regarding the values of allowable step & touch potentials in lower resistivity surface, allow me to use the following references:

E[sub]step[/sub] = (1000+6Cs.Rs).k See IEEE Std 80 eq. 29 &30

E[sub]touch[/sub] = (1000+1.5Cs.Rs).k See IEEE Std 80 eq. 32 &33

For a quick assessment let’s consider k= constant.

CASE I: Crushed rock resistivity, Rrock=2500 Ohm-m and earth resistivity & Rearth=400 Ohm-m.

K1=(Rearth-Rrock)/(Rearth+Rrock) = (400-2500)/(400+2500) = -0.72 ---> Cs=0.74.
[sub](See Std 80 Appendix B, step 3 for example).[/sub]


CASE II: Concrete as surface, Rconc=50 Ohm-m and earth resistivity & Rearth=400 Ohm-m.

K1=(Rearth-Rconcrete)/(Rearth+Rconcrete) = (400-50)/(400+50) ~1 ---> Cs~1.
(See Std 80 Appendix B, step 3 for example).

Comparing both cases above:

CASE I: Cs.Rs = 2500*0.74 = 1850 CASE II : Cs,Rs=50*1 = 50

Replacing in the equation above, for lower surface material resistivity will result in lower allowable step and touch potential.


See the following thread: IEEE Std 80-2000 Equation Error

 

[Flashover]

thanks for all the help folks,
quick question should the rebar be brought above the surface and connected above ground, or does anyone connect below ground?
regards

 

[cuky2000]

The rebars should be located as require by the structural design. For grounding purposes, there are not need to bring the rebars outside the concrete encase.

You could use exothermic weld, compression or bolt connectors to attach the ground conductor to the rebars depending of fraction of total current injected into the ground. See the enclose fig 3 & 4 of the enclose article.

http://www.omegaps.com/PDF/SpecSheets/A18M.pdf

 

[jbartos]

Suggestion to Flashover (Electrical) Jul 29, 2002 marked ///\\im am currently reviewing a complete design for earthing of a new power station. I have some small problems with the following:-
1) connection of re-bar in the piling to earth grid, have searched many standards but cant find anything definite
///Reference" IEEE Std 142-1991
2.5 Outdoor Unit Substations on page 107 refers to reinforcing bars contained on the below-grade foundation structure.\\2)lighting protection and earting of I&C cabinets
///Reference" IEEE Std 142-1991
Chapter 5 Sensitive Electronic Equipment Grounding.
NFPA Std 780 Standard for Installation of Lighting Protection Systems.\\3)any websites or recommended reading.... warmly welcomed
regards
///Visit

http://www.usda.gov/rus/telecom/publications/word_files/1751f810.doc

http://www.neptco.com/website/neptc...abdd3392e37278db85256938004c2536?OpenDocument

etc. for more info.\\

 

[Flashover]

Thanks everybody for the help, have plenty to keep me going now

 

[SooryaShrestha]

Thanks for reminding the equations. I also use the same equations for determinig the safety of the designs, but was not physically aware of the situation. My interpretation was also same that the safe design is to have high resistivity layer on the top of the grounding grid. Thanks anyway and offer a star which you deserve.