Grounding-Slope Method-Additional Methods for Tight Spaces-Part1-Intersecting Curves.pdf

7/27/2019 Grounding-Slope Method-Additional Methods for Tight Spaces-Part1-Intersecting Curves.pdf

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In previous columns, we reviewed the fundamental fall-of-potential

test method for ground electrode resistance. This is the most gen-

eral, most thorough, most explicit, and most reliable technique. Its

principle shortcoming is that it is a lot of work and takes a lot of time.

But it also has another limitation, working space. In order to construct a

fall of potential graph, one must clearly separate the electrical fields of

the test electrode and current probe, so that the plateau of maximum test

electrode resistance can be clearly recognized. If not, the superimposed

resistance associated with the current probe (which is of no interest toanyone, as long as it is low enough to allow the test set to meet its mea-

surement accuracy) will overlap that of the test electrode and obscure

the point at which the test electrode resistance ceases to rise. The graph

will reveal nothing other than that the current probe is too close. Stan-

dard procedure is to take the probe further out and repeat. But what if

there simply is no more room? This is a common problem. The bigger

the grid being tested, the larger its electrical field in the soil, and the

further away one must go with the current probe.

Fortunately, methods have been devised to deal with this problem.Possibly the most popular, the slope method, was outlined in the previ-ous column. But there are others. Why? Because when the electrical sys-tem is tied to a buried electrode, the entire planet Earth becomes, in ef-fect, a part of the system and that leads to a lot of variables! Addi-tional methods provide something to fall back on when a given proce-dure just is not yielding satisfactorily consistent or intelligible results,when additional verification is suggested, or when adapting to atypicalsituations and environments. One of these methods is called intersectingcurves.

The generic fall-of-potentialgraph is typically shown startingfrom the junction of the x and yaxes; that is to say, the test electrodeis taken to be a point ground, likea single rod. From this, it is rela-tively easy to measure precise dis-tances to the probes in order to lo-cate the popular 62 percent posi-tion for the potential probe or toperform all the other distance mea-surements required to completethe simple mathematical proofs

that are associated with a numberof methods. But most commercialground testing does not afford thisdegree of specificity. Grids can belarge, with numerous individualelements interconnected, and ofhighly irregular configurations. Tobe fully specific, one would haveto make probe-spacing measure-ments from the electrical center. Butthis is by no means necessarily co-

by Jeff JowettMegger

Tech Tips

Part 1 Intersecting Curves

Additional Methods for Tight Spaces


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Figure 3

incident with thegeometric center. Unless one wantsto bring in a research team from MIT, the position ofthe electrical center must remain an unknown. Stan-dard practice is to hook up the test lead(s) to a conve-nient access point and work from there, whatever theimplications. Given limitless working space, one couldcarry out the current probe far enough to see the con-stant reading before the final upturn of the graph.Having some part of the initial rise obscured wouldbe of no practical consequence.

But in the absence of sufficient working space, onecan start by visualizing the grid as being concentratedinto one deep-driven rod a point ground ratherthan spread out as an array of parallel grounds. Thiswould represent the electrical center. By connectingthe test set at any convenient point, a fall-of-potential

graph is then constructed. For the method to work,the distance to the current probe should be no morethan twice the maximum dimension of the grid. [Thisfits nicely with our stated space limitation.] Otherwise,

the graph tends to be too flat, and the point of intersection will be indefinite. The resulting curve will tendto be rising constantly, as would be expected with insufficient current probe spacing, but the correct reading will be situated somewhere on the curve. Nextreposition the test setup and construct another graphThis could be done by moving off at another anglebut in field situations it is quite common for there tbe no more than one clear sight-line available. There

fore, the current probe would simply be moved oufurther for the repeat test.The testing could be considered complete at thi

point, but for clarity it is best to make a third graphfrom yet another position. Now, we must determinwhere the correct value is on these two or three curves

We know by the 62 percent rule that the correcvalue can be read at 0.618 times the distance from thtrue electrical center to the current probe. We do noknow where the electrical center is, but we can designate it as a distance x from the point of attachmenof our test lead(s). Therefore, if we designate thknown distance to the current probe as d

cand th

known distance to the potential probe as dp, we knowthat:

x + dp= 0.618 (x + d

c)

Solving for dp, we get:

dp= 0.618d

c 0.382x

Upon assigning x a number of values (for our firscurve) and calculating for dp, the corresponding resistances can be taken from the curve. These are usedto construct a second graph of x versus resistance. Th

calculation is repeated using dcfor the second curvand some appropriately assigned values for x. Anothe

Figure 1

Figure 2


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x-versus-resistance graph is plotted. Where these two

graphs intersect is the true earth resistance value. It isthe only point they hold in common, while the othersdrift according to the arbitrary selection of incorrectvalues for x. If a third test had been made, it too isplotted as above. In theory, its graph should intersectat the same point, but this is not likely in practice be-cause of the nonhomogeneous nature of most soil com-position and a host of other possible extraneous fac-tors that can influence readings away from the idealmodel. In these latter cases, a triangle will be formedby the three curves, and its center is taken as the cor-rect value. The smaller the triangle, the more accuratethe test result. There is little point in constructing any

additional curves, as they only tend to make the re-sulting intersection more difficult to interpret.

For the skeptical or meticulous operator, furtherconfirmation can be gained by working backward:find the value of x for the point of intersection, putthis into the equation for d

pfor one of the chosen val-

ues of dc, and perform an actual physical test at that

point to see if the measured value agrees with the onedetermined from the graph.

Figure 5

Figure 4

The intersecting curves method affords another tool

to defeat unusually demanding situations. It also il-lustrates how specialized procedures have been de-rived from the broader fall-of-potential method in or-der to either save time and effort or to deal with un-compromising environments. As a means of locatingthe 62 percent point, it traces directly to fall-of-poten-tial and can, therefore, be related to the industry-stan-dard IEEE #81 for conformance purposes. Other meth-ods exist that do not rely on fall-of-potential for theirtheoretical basis. We will examine one of those nextissue.

Jeffrey R. Jowett is Senior Applications Engineer for AVO In-ternational in Valley Forge, PA, serving the manufacturing linesof Biddle, Megger, and Multi-Amp for electrical test andmeasurement instrumentation. He holds a BS in Biology andChemistry from Ursinus College. He was employed for 22 yearswith James G. Biddle Co. which became Biddle Instruments andis now AVO International.

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Grounding-Slope Method-Additional Methods for Tight Spaces-Part1-Intersecting Curves.pdf