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One particular observation on the Slope Method is that if the calculation of µis greater than that given in the table, the distance C must be increased.Secondly, before the measured values for R1, R2 and R3 can be acceptedwith a degree of confidence, it is recommended that a curve be plottedwhich will identify any localized effects and eliminate uncharacteristicreadings from the calculations. Thirdly, it is also suggested that the test berepeated in different directions and with different spacings. The variousresults should exhibit a reasonable degree of agreement.Table VII: Values of D P/D cfor Various Values of µµ D P/D cµ D P/D cµ D P/D c0.40 0.643 0.80 0.580 1.20 0.4940.41 0.642 0.81 0.579 1.21 0.4910.42 0.640 0.82 0.577 1.22 0.4880.43 0.639 0.83 0.575 1.23 0.4860.44 0.637 0.84 0.573 1.24 0.4830.45 0.636 0.85 0.571 1.25 0.4800.46 0.635 0.86 0.569 1.26 0.4770.47 0.633 0.87 0.567 1.27 0.4740.48 0.632 0.88 0.566 1.28 0.4710.49 0.630 0.89 0.564 1.29 0.4680.50 0.629 0.90 0.562 1.30 0.4650.51 0.627 0.91 0.560 1.31 0.4620.52 0.626 0.92 0.558 1.32 0.4580.53 0.624 0.93 0.556 1.33 0.4550.54 0.623 0.94 0.554 1.34 0.4520.55 0.621 0.95 0.552 1.35 0.4480.56 0.620 0.96 0.550 1.36 0.4450.57 0.618 0.97 0.548 1.37 0.4410.58 0.617 0.98 0.546 1.38 0.4380.59 0.615 0.99 0.544 1.39 0.4340.60 0.614 1.00 0.542 1.40 0.4310.61 0.612 1.01 0.539 1.41 0.4270.62 0.610 1.02 0.537 1.42 0.4230.63 0.609 1.03 0.535 1.43 0.4180.64 0.607 1.04 0.533 1.44 0.4140.65 0.606 1.05 0.531 1.45 0.4100.66 0.604 1.06 0.528 1.46 0.4060.67 0.602 1.07 0.526 1.47 0.4010.68 0.601 1.08 0.524 1.48 0.3970.69 0.599 1.09 0.522 1.49 0.3930.70 0.597 1.10 0.519 1.50 0.3890.71 0.596 1.11 0.517 1.51 0.3840.72 0.594 1.12 0.514 1.52 0.3790.73 0.592 1.13 0.512 1.53 0.3740.74 0.591 1.14 0.509 1.54 0.3690.75 0.589 1.15 0.507 1.55 0.3640.76 0.587 1.16 0.504 1.56 0.3580.77 0.585 1.17 0.502 1.57 0.3520.78 0.584 1.18 0.499 1.58 0.3470.79 0.582 1.19 0.497 1.59 0.341Appendix VIFour Potential MethodThe Four Potential Method is another method based on fall-of-potentialthat allows the user to overcome the problems posed by complexgrounding systems where the electrical center of the ground system isdifficult to locate. This method was first introduced by Dr. G.F. Tagg in a1964 IEE paper 15 . For the purpose of this booklet, we will cover the testmethod and the formulae by which the resistance of the ground systemcan be determined. The theory behind the method will not be covered,although it links the resistance values obtained by measurements at sixdifferent positions for the potential probe in four formulae with the trueresistance (R ∞ ), which would occur with an infinite distance to the currentprobe.The test probes are set out as shown in Figure 32, with measurementmade from the edge of the electrical system. The current probe is set asuitable distance away from the edge of the ground system. The potentialprobe is then placed at distances equal to 20%, 40%, 50%, 60%, 70%and 80% of the distance to the current probe and a test is made at eachlocation. The resistance values obtained (R 1 , R 2 , R 3 , R 4 , R 5 , and R 6 ) arethen entered into four formulae.The formulae used follow:1] (R ∞ ) = -0.1187R 1 – 0.4667R 2 + 1.9816R 4 – 0.3961R 62] (R ∞ ) = -2.6108R 2 + 4.0508R 3 – 0.1626R 4 – 0.2774R 63] (R ∞ ) = -1.8871R 2 + 1.1148R 3 + 3.6837R 4 – 1.9114R 54] (R ∞ ) = -6.5225R 3 + 13.6816 R 4 – 6.8803R 5 + 0.7210R 615 Reference 24 (page 77)64 1-866-254-0962 www.megger.com Getting Down to Earth 65