Resistivity Testing Of Substation
Testing Substation Yardstone-Lessons LearnedFIVE YEARS OF NATIONWIDE TESTING SUMMARIZEDJOHN EDLEBECKCOLEMAN ENGINEERING COMPANY
IEEE Standard 80–2000, Section 12.5 suggeststhat the resistivity of the surface materials used ingrounding systems should be tested.IEEE 80-2000, Section 17.3 includes a sample calculation using a value of 3,000 Ωm for theresistivity of surfacing material. Mistakenly, this value is often utilized as a typical value forsurfacing material when values are unknown.3,000 Ωm is often cited in specifications as a minimum resistivity for surfacing material.Table D.1 presents a range of resistivity for a variety of surfacing materials. The informationlacks standard terminology for describing the materials and too general to assign values.
Brief Background and History2012 to 2013 ATC inquired about possibilityof testing yardstone surfacing. 2014 “Napkin” sketches of testapparatus and “garage”bench scale tests.Started testing surfacingmaterial at substationsthroughout the ATC footprint. Coleman retained to developstandard apparatus and testprocedure.Presented the proposed testprocedure and data atprofessional conferences. Published findings in IEEETransactions October 2013. Acquired significant data onvarious surfacing materials.Started noticing that 3,000 Ω mwas elusive and not easilyachieved.2015 to Present PES IEEE Working Group G6Task Force 1 formed to addressstandardized testing andprepare technical report. Trafficability over surfacingmaterial prompts a look atcomposite surfacing sections. Continued testing is beingcompleted to gather moreinformation about surfacingmaterials.
Existing Recognized StandardResistivity Test Procedures ASTM G57 – Discusses the 4 pin Wenner array for field determination of soil resistivity.The procedure extends to laboratory testing using a small test box with 4 pinmeasurement.ASTM G187 – Discusses a 2 pin method in a small test box. Generally utilized toevaluate soils for corrosion potential for embedded steel construction components(culverts, guy anchors, steel pile, etc.).AASHTO T288 – Presents 2 pin and 4 pin methods referring to ASTM methods.Other standard test methods, developed by various electrical utilities, are notuniversally recognized. Some of these methods were discussed in an informationalreport published in IEEE 80-2012.
ASTM G57/AASHTO T288 Revisited G57 and T288 provide a reasonable approach to measuring electrical resistivityin soils. The approach does not consider coarse materials such as yardstonesurfacing materials. The standards do not address variables that affect resistivity other thanindicating the minimum resistivity is considered when sample is saturated. Conventional test apparatus does not accommodate coarse size particlesgenerally included in surfacing material. Test procedure does not include discussion on variables that will affect resistivitysuch as particle geology, grading, compactness and moisture. The test could easily be modified to include documentation and control ofvariables as well as accommodate larger particles. Agencies are reluctant to change test procedures.
Examples of ASTM/AASHTO Test Boxes
What’s Wrong with this Picture? Rather than modify the test box,laboratories have modified thesamples or used existing equipment. Top example shows removing orcrushing larger particles to prepare asample that can accommodate thetest box. Bottom example tries to utilize a testbox that is much too small toaccommodate the maximum particlesize.
Make the Box Fit the Material Larger dimensions allow representingcoarse particles in the test procedure. Non conductive materials. Rigid construction to allow rough service. Easily modified to incorporate 2 pin and 4pin test methods. Meets the intent of ASTM and AASHTO.
Gradation alone is not enough ASTM Size 57 (Granitic Particles) ASTM Size 57 (Limestone/Dolomite Particles) Resistivity – 2,000 to 3,500 Ω m Resistivity 1,000 to 2,000 Ω m Saturated Moisture - 0.5 to 1.5% Saturated Moisture 2.0 to 3.5 %
Geology plays an important role The geologic origin of surfacing materialscontribute to the resistive properties ofthe particles in surfacing materials. Granites, basalts and othermetavolcanic rock are dense and lesslikely to absorb water contributing tohigh resistivity. Limestone, dolomite, shale, sandstone,and other sedimentary or metasedimentary rock generally absorb morewater contributing to low resistivity. Have also tested caliche, coral andcoconut straw.
Geographic LimitationsBedrock Geology of North America Red oldest; yellow youngest. Reds generally very dense volcanic andmeta volcanic granite and basalt, verydense (Canadian Shield). Blues and purples are moderately denseand consist of meta volcanics andmetamorphic rock created fromsediments (shales, dolomites, slates). Yellows are sedimentary rock very softand absorptive(limestone, sandstone).Map courtesy of Kate E. Barton, David G. Howell, José F. Vigil on behalf of the U.S.Geological Survey in collaboration with the Geological Survey of Canada and theMexican Consejo Recursos de Minerales.
What Does 3,000 Ω m Material Look Like?Maximum Density:101.6 pcfSaturated Moisture:1.1%Test Compaction:100%Test Method:Moisture ConditionWenner 4 Pin and Modified Test Box1/3 Saturated2/3 SaturatedSaturatedVoltage (V)13.8415.4414.90Current (A)0.0001480.000260.00048Resistance (Ω)93,51459,38531,042Box Factor0.1310.1310.131Resistivity (Ω m)12,2507,7793,104
TrafficabilityParticle shape and range of sizes affect the support of traffic. Rounded particles all of uniform shapeand size reduce trafficability. Angular particles with range of sizes improve trafficability. Some designersare considering composite layers to maintain resistivity and improve trafficability.
What we are seeing to improvetrafficability Dense graded aggregate placedto enhance traffic support. Saturated resistivity 150 to 300 Ω m
Current Movements in Testing IEEE Working Group G6 Task Force 1 iscurrently preparing a technical report todiscuss electrical resistivity of surfacingmaterial and considers standardized testmethods currently being developed. ASTM Standard Guide being consideredwhich will provide guidance to designerand testing agencies to considervariables when measuring the resistiveproperties of soil and constructionmaterials. Vertical oriented test to allow compositelayers of surfacing material beingconsidered to evaluate resistivity.
Design and Construction Consider a resistivity value that isachievable given local constraints ofmaterials available. Consider regional limitations ofavailable surface materials. Establish submittal process to testsurfacing materials to confirm thatminimum resistivity criteria is met orachievable. Do not rely on gradation alone.Gradation is only one variable.Consider geologic composition andshape of particles and absorptioncharacteristics. Consider trafficability of surfacingmaterial. Include a minimum crushedparticle content. Consider constructing traffic laneswhich can accommodate lowerresistivity materials which will supporttraffic.
Questions?THANK YOU
that the resistivity of the surface materials used in grounding systems should be tested. Ω. m for the resistivity of surfacing material. Mistakenly, this value is often utilized as a typical value for surfacing material when values are unknown. 3,000 . Ωm is often cited in specifications as a minimum resistivity for surfacing material.
6 Resistivity Profiling for Mapping Gravel Layers, Amargosa Desert Research Site, Nevada resistivity soundings and multielectrode resistivity profiling. Models selected from the resistivity data are presented and interpreted, with particular attention to resistivity sections produced from the multielectrode transect measurements.
surface resistivity testing. This method helps in determining key mixture parameters such as fly ash content and w/cm of placed concrete. Based on the gain in resistivity over time, it was found that the slope of the surface resistivity versus time curve could be used to differentiate fly ash content. And, the resistivity value
Resistivity is also sometimes referred to as "Specific Resistance" because, from the above formula, Resistivity (Ω-m) is the resistance b Soil Resistivity In the USA, a measurement of -cm is used. (100 -cm 1 -m) 1.3 MAKING A MEASUREMEN the soil is required. The procedure and result interpretation. 1.3.1 PRINCIPLES Soil resistivity va
the materials show very less resistivity change even at high pressure i.e. 10 GPa. The change can be calculated by subtracting resistivity value at 10 GPa and at atmospheric pressure. This difference can be divided by the original resistivity value (at atmospheric pressure) and multiplied by 100 to get the percentage resistivity change.
The direct-current resistivity method is used to determine the electrical resistivity structure of the subsurface. Resistivity is defined as a measure of the opposition to the flow of electric current in a material. The resistivity of a soil or rock is dependent on several factors that include amount of
Electrical Substation Configuration Effect on Substation Reliability . Courtney Otania and Kurt Vedros a. a Idaho National Laboratory, Idaho Falls, ID, USA . . Substation Level, Generic 4.931E-05 Busbar, Substation [7] 132 kV Busbars 1.507E-05 Control Panel Switchgear [2] C4-300 2.845E-06 .
Moisture content, temperature and salts also affect soil resistivity. Soil that contains 10% moisture by weight will as much as five times lower soil resistivity than that which contains 2.5%. Soil at room temperature will be as much as four times lower in resistivity than that at 32 degrees. So the time of year that you conduct the test can
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