Earth grid construction
Earth grid construction
(OP)
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
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






RE: Earth grid construction
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.
RE: Earth grid construction
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.
RE: Earth grid construction
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?
RE: Earth grid construction
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.
RE: Earth grid construction
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.
RE: Earth grid construction
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).
RE: Earth grid construction
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.
RE: Earth grid construction
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.
RE: Earth grid construction
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/TE/bin/TE.Connect?C=11052&F=0&M=CINF&N=2&LG=1&I=42&RQS=C~11052^M~FEAT^G~G TERMI-FOIL general description
RE: Earth grid construction
In response to SooryaShrestha question regarding the values of allowable step & touch potentials in lower resistivity surface, allow me to use the following references:
Estep = (1000+6Cs.Rs).k See IEEE Std 80 eq. 29 &30
Etouch = (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.
(See Std 80 Appendix B, step 3 for example).
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
RE: Earth grid construction
quick question should the rebar be brought above the surface and connected above ground, or does anyone connect below ground?
regards
RE: Earth grid construction
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
RE: Earth grid construction
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/neptco.nsf/93d3dd583cccd69d8525693c0064cc91/abdd3392e37278db85256938004c2536?OpenDocument
etc. for more info.\\\
RE: Earth grid construction
RE: Earth grid construction