eps magazine, august 2011
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DESCRIPTIONAugust 2011 Issue of EPS Magazine
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PUBLISHERDanny J. Salchert
OFFICE MANAGERAnita Salchert
NATIONAL SALES MANAGERJerry DiChiara
CREATIVE DIRECTOR Derek Gaylard
CONTRIBUTING WRITERSJohn Olobri Michael DaishJoe Bono John Gavilanes
CIRCULATION DIRECTORPam Fulmer
PRESIDENT Danny J. Salchert
Executive and Advertising Offices3591 Cahaba Beach Road
Birmingham, AL 35242toll free: 800.981.4541 phone: 205.981.4541
fax: 205.981.4544www.epsmag.net firstname.lastname@example.org
Electrical Products & Solutions is publishedtwelve times a year on a monthly basis by ABDCommunications, Inc., 3591 Cahaba Beach Road,Birmingham, Alabama, 35242, USA. ElectricalProducts & Solutions is distributed free to qualifiedsubscribers. Non-qualified subscription rates are$57.00 per year in the U.S. and Canada and $84.00per year for foreign subscribers (surface mail). U.S.Postage paid at Birmingham, Alabama and addi-tional mailing offices.
Electrical Products & Solutions is distributed toqualified readers in the electrical contracting industry.Publisher is not liable for all content (including edito-rial and illustrations provided by advertisers) of adver-tisements published and does not accept responsibilityfor any claims made against the publisher. It is the ad-vertisers or agencys responsibility to obtain appro-priate releases on any item or individuals pictured in anadvertisement. Reproduction of this magazine inwhole or in part is prohibited without prior written per-mission from the publisher.
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P R I N T E D I N T H E U S A
FEATURES6 Understanding Soil Resistivity Testing (part 2)
By John Olobri
18 Hot Stuff! Power Monitoring on Live Circuits Can Be Done SafelyBy Michael Daish
26 How Zero-Down Leasing is Changing the Solar LandscapeBy Joe Bono
WHITE PAPER12 The Importance of Oil Resistant Cables
By John Gavilanes
2 Electrical Products & Solutions August 2011
30 Industry News36 Product Focus48 Ad Index
ON THE COVERPhoto courtesy of Summit Technology, Inc.
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FEATURE AEMC Instruments
Last month we discussed theprocess of taking soil resistivitymeasurements. We chose the Wen-ner method as it is easiest to employ whenmeasurements are taken at one or twodepths. We took readings in 5 different di-rections in the area of interest (north,south, east, west and the diagonal insidethe area. Then we took the average of thefive measurements to calculate Rho ()expressed in ohm centimeters or ohm me-ters. In our test example we arrived at anaverage of 6.408 ohm-meters. Convertingthis to ohm-centimeters we get a value of6,408. It should be noted that this valueof soil resistivity assumes a homogenoussoil condition which may or may not betrue. In either case the results will assistus in the next phase of our work. Knowingthe soil resistivity can benefit you in twoways: First and the most often used ofthese values is to calculate the depth nec-essary to drive your ground rod or groupof rods to achieve the desired systemgrounding resistance for the site being de-veloped. Second, this value can also beused to determine the depth of an existingground rod in order to properly test itseffective resistance to earth.Lets look at the process of calculating
the depth needed for a new ground rod in-stallation. For this we will use a calculat-ing tool called a nomograph (See sidebaron page 8).To begin with we need to make a few de-
cisions. First what is the desired groundingelectrode resistance needed? Second whatis the diameter of the ground rods we will
Using the nomograph (available on theTech Info section of the AEMC website)we first put a dot at 10 ohms on the R scaleas it is our desires resistance.Next we put a dot at 6,408 on the p scale
representing our soil resistivity measure-ment. We will have to do our best to ap-proximate the location of this point betweenthe 5000 and 10000 hash marks.Next we take a straightedge and draw a
line between the dots we placed on the Rand P scales and let the line intersect withthe K scale and place a dot on the intersect-ing point.Now we again take a straightedge and
draw a line from the 5/8 hash mark on theDIA scale representing our rod diameterthrough the dot on the K scale and continuethrough to intersect with the D scale andplace a dot on the D scale at this intersectingpoint.
UnderstandingSoil Resistivity Testing
By John Olobri, Director of Sales and Marketing, AEMC Instruments
be using? With these two answers plus themeasured soil resistivity we can use thenomograph to calculate the depth requiredto achieve our objective. Lets say we needa resistance from this grounding system tobe no more than 10 ohms and that we choseground rods that have a 5/8 inch diameter.Looking at our nomograph, we have five
scales to work with: The R scale representsthe desired resistance needed, for our work(10 ohms). The p scale represents soil re-sistivity. Our average value is 6408 ohm-centimeters obtained using a 4 pole groundresistance tester employing theWenner testmethod. The D scale represents depth and iswhat we will use to find our answer. The Kscale contains constants that will assist usin finding the depth. Lastly the DIA repre-sents the diameter of the rods used. We willcomplete several simple steps to get ourdepth answer.
6 Electrical Products & Solutions August 2011
Continued on page 8
2 PART A
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can be used. There are several companiesthat specialize in solving these types ofproblems that can be consulted.
Taking soil resistivity measurementsprior to installing a grounding electrodesystem can save a lot of time and effort inplanning the system properly. Using a fewsimple tools and procedures can give youquality results with less than one hourseffort. Bear in mind that these results arebased on homogeneous conditions thatwont necessarily exist at the site.
Further simplifying the task today isthe fact that newer testers now have theability to calculate soil resistivity inter-nally computing Rho saving further timeand effort.
John Olobri is Director of Sales &Marketing for AEMC Instruments.
He can be reached by calling508-698-2115; Fax 508-698-2118 or
8 Electrical Products & Solutions August 2011
The value at this point is the depthneeded to drive a 5/8 inch diameter rodto achieve 10 ohms of grounding elec-trode resistance given the soil resistivitymeasured.
Looking at the completed nomograph infigure 1 we see that a single rod wouldneed to be driven 30 feet deep to meet our10 ohm objective. The completed nomo-graph is shown in the sidebar article.
In many cases this is not practical todrive deep rods. The alternative is to drivetwo or more rods to get the desired results.
There are a few important points to con-sider when driving multiple rods. First isthat driving additional rods will notachieve linear results. For example three10 foot rods will not yield the same resultsas a 30 foot rod. We need to apply an ad-justing factor. Secondly to achieve the besteffect of additional rods they should bespaced at least equal to the depth andpreferably at twice the depth. For example
FEATURE AEMC Instruments Continued from page 6
multiple 10 foot rods should be spaced 20feet apart to avoid being in the sphere ofinfluence of each other (See figure 2).
The adjustment factor required for mul-tiple rods is shown in figure 3. If we wereto use three 10 foot rods in parallel insteadof one 30 foot rod we would expect eachrod to contribute 1.29 times the theoreticalvalue. Stating it another way, if we di-vide the 10 ohms needed by 3 to find theexpected value of each rod we get 3.33ohms. Applying the adjustment factorfrom the table for 3 rods in parallel we get3.3 x 1.29 or 4.25 ohms contributed byeach rod for a total of 12.75 ohms. In thiscase we would need to drive a fourth rodto get below our desired 10 ohms.
Sometimes the final results cannot beobtained by adding additional rods. Theresimply may not be enough real estate toaccomplish it or the area is too rocky etc.In these cases soil enhancement tech-niques can be employed or chemical rods
Step 1Select the required resistance on the R scale
Step 2Select the measured soil resistivity on the P scale
Step 3Take a straightedge and draw a line between the values placed on the Rand P scales and let the line intersect with the K scale.
Step 4Place a dot at the intersecting point on the K scale
Step 5Place a dot on the desired rod diameter hash mark on the DIA scale