EARTHING DESIGN CONSIDERATIONS

[kalbur]

Hey everyone,
I'm looking for interesting and possibile solutions which came out with my practice.

#1. SOIL MODEL
During the preparation of the soil model (one or two layer soil model), I have to pass through a multilayer soil model. According to the local country rules I suppose to consider and increase the upper specific soil resistivity value at depth “hc” (usually 0,5-1,2m depending of the soil freezing depth and it’s duration) with coefficient “qc” (1,2 – 2,5 and over depending of the weather conditions when the measurement was done; 1,2=wet soil after rain) as shown on attached picture.

http://files.engineering.com/getfile.aspx?folder=a6a30f5f-9a35-44a6-ae8b-15c4bc0a2c74&file=Ro.TIF

I’m interesting do you know about such considerations and how they’ve been solved in the different countries.
Also I’m looking for a simple and easy for use software tools for calculating from field measurements (Wenner method) to multilayer soil model, and additional one from input multilayer soil data (after the described over manipulations) to the one and two layer data (which can be used as input data for the regular earthing system design softwate).

#2. SOIL CHEMICAL ACTIVITY
Do you know about good methodology for considering an increase of the earthing conductor minimal cross-section depending of the project live and the soil chemical activity (pH measurements). I met one, but it’s only for steel conductors, and even of that I’m looking for a comparison?.

#3. OLD CU WIRE FOR EARTHING
In one of my last earthing design projects, according to the Investor requirements I used an old Cu conductor (removed from OHTL after 30 years of use). It supposes to be oxidized, and even after brush cleaning I’m considering about its conductor-ground resistance. Does any one have experience with such problem? Do you know about comparison results or research result of new and oxidized copper wires used as earthing conductor? Do you know some thing about the old Cu conductor-ground resistance?

#4. Cu COATINGS
Typically I’m using uncoated soft drawn copper wire for earthing system design, but I have seen also coated ones to be used. What is your usual practice? In what cases I suppose to use coated one and what kind of coating is suggestible?

#5. EARTHING CLAMPS vs. EXOTERMIC WELDINGS
Does any one have made or know about comparison between earthing system resistance built with clamps and the same one build with weldings (if they work in the same external conditions)? With which one is reached lower earthing system impedance (if there is difference as I expect)? Which solution is better if we have to consider the earthing system transient over voltages as result of direct lightning?
About the last one I think that with the welded one the overvoltage will be lower and the distribution of it will be on wider area then the builded with clams.

I’m sorry for the bad English, but I believe that the technical part of this post will be more interesting for you?.

 

[jghrist]

#1. SOIL MODEL
Software tools are available to deal with seasonal variation of resistivity. See

http://www.sestech.com/Products/SoftUtil/SoilModelManager.htm

#2. SOIL CHEMICAL ACTIVITY
I can't help with this. Maybe hire a corrosion consultant.

#3. OLD CU WIRE FOR EARTHING
I see nothing wrong with using old copper wire. Copper oxide will not significantly affect the grid performance. How long do you think new copper wire will remain oxide free?

#4. Cu COATINGS
You might want to consider this if you are worried about galvanic corrosion of buried steel structures. Tin coating will reduce the cell potential with respect to zinc an steel by about 50%.

#5. EARTHING CLAMPS vs. EXOTERMIC WELDINGS
The reason for using exothermic welds instead of clamps is to maintain the integrity of the connection. The connections have to maintain proper conductivity to handle fault currents and be able to withstand forces during faults and during construction. I don't think that the choice of connection will affect earth impedance because this is determined by soil characteristics and the extent of the grid more than by small differences in the resistance of portions of the grid.

 

[cuky2000]

#1 SOIL Model:
• In the US two layer soil model provides acceptable accuracy in most case.
• Increasing the number of soil layer will improve theoretically the accuracy of the soil model. However, this accuracy require fairly large amount of field data not only to characterize the heterogeneous nature of soil using average values of the apparent soil resistivity but also a good statistical analysis to filter the bad data with error analysis.
#2. SOIL CHEMICAL ACTIVITY:
• Earth (ground) conductor should be sized to avoid a melting the earthing conductor and associated connectors. The conductor will be exposed to degradation over time and therefore will be advisable considering a larger size (typically 2/0 or 4/0) to account for corrosion and other future degradation.
• There are few soil enhancement and corrosion inhibitor available in the market (ex Sanearth, GEM, Bentonite, etc.). However, this should be careful evaluated because the cost and reduction of effectiveness over time. I will consider that as last resource if not other solution is available.
• Soil resistivity <25 Ohm-m is considered severely corrosive, >100 very mildly corrosive. Copper is very resistant to corrosion
#3. OLD CU WIRE FOR EARTHING: New conductor buried in earth will develop a layer brown cuprous oxide (Cu20). A guess an old conductor will provide an equivalent result.
#4. Cu COATINGS
Coating could improve slightly the corrosion resistance and usually is used when there is presence of dissimilar metallic material or in close proximity or embedded in concrete. Coating conductors are not often used for grounding application.
#5. EARTHING CLAMPS vs. EXOTERMIC WELDINGS
Connectors are rated for max. temperature as follow: Bolted (250 oC), Crimped(450 oC) and Brazed (650 oC). Fussing current should be considered during the selection of connector. The main parameters to determine maximum allow temperature is a function of time current injected in the grid, the time to clear the fault and the size of the component. Please note that the fault current through the grid conductor is usually half or less than half of the fault current to ground because the current splits in at least two directions in the grid.
Clamps could save labor time. Exothermic weld is more expensive but more current resistance than all options.