ADP EARTH ELECTRODES FOR GROUNDING NEMP TYPE LIGHTNING. David P. Mi Hard Georgia Institute of Technology, Atlanta, Georgia

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ADP002182 EARTH ELECTRODES FOR GROUNDING NEMP TYPE LIGHTNING ^ David P.

faster than 2 micrsecnds) pwer spectrum.

1 ADP EARTH ELECTRODES FOR GROUNDING NEMP TYPE LIGHTNING ^ David P. Mi Hard Gergia Institute f Technlgy, Atlanta, Gergia ABSTRACT An investigatin was cnducted int the prperties f earth electrde systems ver a frequency range cvering the NEMP type lightning (rise times faster than 2 micrsecnds) pwer spectrum. Since this pwer spectrum extends well int the VHF regin, cnventinal lw frequency (less than 100 H) and histric impulse measurement techniques were nt adequate t describe the electrde system's respnse t NEMP type lightning. Therefre, initial emphasis was placed n the develpment f a measurement technique that culd be used t bth: (1) Assess the perfrmance f a given grund electrde; and (2) cnduct a site survey t determine the best lcatin fr grund electrdes. Then the technique was used t evaluate the perfrmance f varius cmmn grund electrde cnfiguratins (the subject f a cmpanin paper t be published at a later date). \ M '* V -«_V_VJW. ^ \ - - -J

THE RESPONSE OF AN EARTH ELECTRODE t an NEMP type lightning pulse is determined by the prperties f the sil and the electrical characteristics f the particular electrde system gemetrical cnfiguratin.

(This step is imprtant because it dictates the gemetrical cnfiguratin (i.e., number f grund rds, methd f intercnnectin, etc.) required t establish a gd grund in a given area.

2 THE RESPONSE OF AN EARTH ELECTRODE t an NEMP type lightning pulse is determined by the prperties f the sil and the electrical characteristics f the particular electrde system gemetrical cnfiguratin. Thus, the first step in establishing an adequate grund is t determine the dielectric prperties (cnductivity, permittivity, and permeability) f the sil. (This step is imprtant because it dictates the gemetrical cnfiguratin (i.e., number f grund rds, methd f intercnnectin, etc.) required t establish a gd grund in a given area.) Histrically, sil parameter identificatin started with a labratry analysis f sil samples and then mved t sphisticated n-site analyses f the sils and underlying strata at the planned lcatin. Recent advances in n-site techniques have been made. One f these, the Resnant Linear Antenna Methd [1] appears t be the mst suitable fr NEMP type lightning grunding studies. (The methd is accurate ver a brad frequency range, is easily transprtable, and is generally in use by gephysicists fr gelgical surveying.) Frm the input admittance and the gemetry f a prbe antenna, the sil parameters can be calculated. This methd requires nly a resnant mnple antenna, a signal surce cvering the desired frequency range, and a display device cmplete with necessary caxial vltage and current prbes. An adaptin f this technique was emplyed t examine the behavir f selected earth electrdes up t frequencies reflective f fast risetime respnses applicable t NEMP type lightning wavefrms. The grunding f pwer circuits (25-60 H) and grunding fr lightning prtectin (impulse) is the primary cncern fr structures and pwer lines. Thus, many studies were perfrmed t determine the vlt-ampere characteristics f a driven grund rd using direct current r lw frequency ( < 100 Hi) alternating current instruments t determine pwer frequency prperties {2 ]. Impulse generatrs were used t determine the respnse t lightning strkes [3]. The typical impulse generatr was capable f prducing 50 KV and 800 A with a 1 t 2 micrsecnd rise time. Frm these tests, a resistive, inductive and capacitive (RLC) mdel ul a grund rd was develped [4] (Figure 1) that reflected the gemetry f the rd, the sil parameters, and the climatic cnditins at the time f the test. Within the last several years lightning pulses faster than the traditinal 2 micrsecnd rise time have been recrded [5]. Thus, develpment f a new technique fr measuring grund electrde impedances at these extended frequencies was necessary. After careful cnsideratin f existing instrumentatin capabilities, it was determined that a technique culd be devised t display the respnse f an electrde system up t 500 MH which cvers the NEMP type lightning and much f the NEMP spectrum (Figure 2). Three different measurement techniques must be used t cver the entire frequncy range frm DC up t 500 MH. The frequency ranges cvered by each are: (1) lw (DC t 100 Hert), (2) medium (100 H t 500 KH), and (3) high (500 H t 50 MH). The design and cnstructin f a standardied test prbe alng with a descriptin f the test techniques fr these three frequency regins are discussed in the next sectins. STANDARD TEST PROBE A rd f 1.25 cm (0.5 inches) in diameter and 81 cm (32 inches) in length was chsen fr the standard test prbe. (Brass was used althugh steel, cpper, r any ther metal f sufficient strength is adequate.) This length is lng enugh t prvide effective sil cntact but nt s lng as t require extensive wrk t place the rd in the grund. An adapter was then cnstructed t interface the grund rd t the test instruments (see Figure 3). The adapter cnsists f a tapered caxial line transitin with a male type N cnnectr n the tp. The taper maintains 50 hms impedance dwn t the pint f attachment t the rd. (The impedance characteristics f the cnnectr frm 28-2 ***»-«-«-»- -Ä-*. - 1\

The analyer was cnnected t the type N cnnectr as shwn in Figure 3, calibrated (Figure 5), and en impedance plt f the prbe in earth was phtgraphed (see Figure 6).

3 *.' - - *-1-»-* 0.5 t 500 MH are shwn in Figure 4.) The adapter is fastened t the grund rd via a threaded cnnectin. where Z L- R + j u L 1 HIGH FREQPEMCY MEASUREMENTS P In 2i> hms The high frequency impedance characteristics f the test prbe were measured with the aid f a General Radi 1710 RF Netwrk Analyer. The analyer was cnnected t the type N cnnectr as shwn in Figure 3, calibrated (Figure 5), and en impedance plt f the prbe in earth was phtgraphed (see Figure 6). It is nted that there is a great deal f ringing assciated with bth the magnitude and phase f the rd impedance. The ringing is due t the nn-unifrm imaging f the rd with the sil, the inductance and capacitance f the test leads t earth, and the standing waves at the surface [6]. By increasing the reference plane area f the prbe, the standing waves and reflectins were reduced, yielding a mre acceptable plt f the grund rd impedance. The reference p'.ane area was increased by attaching auxiliary grunds and an aluminum plate t the test cnrectr shield (see Figure 7). A series f plts, Figures 8 t 13, were taken with different auxiliary grunds. The figures reveal that the rd impedance ringing decreased and displayed an verall capacitive nature at high frequencies as expected. These results indicated that this apprach can be used t determine the impedance f the reference prbe up t frequencies f 500 MH. Frm this impedance characteristic, determinatin f the equivalent circuit f the prbe can be made 171. A cmputer run was made t calculate the input impedance f the equivalent circuit (Figure 1) with the test rd gemetry and sil cnditins f the particular test area. The input impedance f the grund rd equivalent circuit is given by: Z L Z C Z - (1) Z L + Z C C - I - a * P - 10 < 21 In 2,1 a i r 10" 9 21n 21 9 length f rd radius f rd Ü m e < 13 A gram f the results is shwn n Figure 14. Fr this secnd rder system, the resnant frequency is apprximately 15 MH. The respnse f the final test cnfiguratin (Figure 13) reveals a resnance arund 8 t 9 MH with ringing frm 100 t 500 MH. LOW FREQUENCY MEASUREMEHTS The impedance f an earth electrde at lw frequencies is dminated by the prperties f the sil. Analytically the resistance f a grund rd can be determined if the sil resistivity, P, and the rd gemetry are knwn, i.e., P 2 ^ In hm (2) where I length f the rd and a» its radius. Experimentally the grund rd resistance was accurately measured by the Fall-f-Ptential Methd (see Figure 15) [8> This is a simple vltage dr measurement relating the current injected t the resistance f the grund rd. (The injected current usually has a frequency f 70 t 100 H s as nt t be cnfused with stray 60 H grund currents.) The grund rd resistance 28-3 *»-, - - -' - -»~-»-«"-»"- *JU±

- -*-- was fund by recrding the resistance n a Biddle Meggar-Earth Tester as distance, d (distance between the grund rd and prbe C_), was varied. The ptential prbe, P.

Frm this graph the test grund rd resistance was determined t be 107 hms. The fur prbe technique can als be used tc find sil resistivity, P.

4 - -*-- was fund by recrding the resistance n a Biddle Meggar-Earth Tester as distance, d (distance between the grund rd and prbe C_), was varied. The ptential prbe, P., must be placed 622 f d, between the grund rd and prbe C-. By using this methd a plt was made f grund rd resistance versus separatin distance, d (see Figure 16). Frm this graph the test grund rd resistance was determined t be 107 hms. The fur prbe technique can als be used tc find sil resistivity, P. The resistivity f the sil at the test site was measured using the test setup shwn in Figure 17. This resistivity was determined t be hm-meter. With this resistivity, the resistance f the grund rd shuld be 98.6 hms, which is within 10% f the Fall-f-Ptential Methd. (This result is cnsidered t represent reasnable accuracy, given the high degree f dependence f the tests n envirnmental cnditins.) MID-FREQUENCY MEASUREMENTS The mid-frequency range, 100 H t 500 KH, impedance measurement prved t be the mst difficult t btain because f instrumentatin limitatins. Mst ff-the-shelf impedance measuring devices require that the bject f the test nt be grunded. (Specifically the HP 4800A Vectr Impedance Meter has "DO NOT GROUND" printed under the input terminals.) Therefre, measuring the impedance f a grunded rd prved impssible with this type f instrument. An apprach was develped which relied strictly n netwrk thery and the fact that the magnitude f the impedance is equal t the rati f the magnitude f the vltage acrss and the current thrugh the rd. Several attempts were made t btain a suitable measurement. Since mst scillscpes use "grund" as a reference and have a high impedance input, there was n prblem with making vltage measurements n the surce end f the grund rd. A prblem arse in trying t James G. Biddle C., Plymuth Meeting, PA measure the current int the grund rd, hwever. The first apprach was t measure the vltage drp acrss a ne hm resistr in series with the grund rd. Prblems were encuntered when the scillscpe prbe shield was cnnected t the terminal f the resistr thereby adding anther "grund" t the circuit. Attempts were made t islate the scillscpe frm grund, but this nly served t increase the nise in the measurement. Obviusly, a methd f measuring the current was needed which wuld prvide islatin frm grund and prvide nise rejectin. An HP current prbe and amplifier prvided just such a slutin (see Figure 18). This test setup wrked well in measuring the grund rd impedance ver the mid-frequency range. The results match the lw and high frequency impedance measurement i the rd and allwed measurements f impedance ver the lw end f the NEMP type lightning spectrum. Figure 19 is a plt f the test prbe impedance t grund using the midfrequency setup. The highest frequencies are cmpared t the high frequency test and the results are within the measurement errr. The test prcedure cnsisted f varying the frequency f the HP 651A Test Oscillatr, while maintaining cnstant utput vltage, and measuring the vltage at the terminal f the grund rd and the current thrugh the lead t the grund rd. The test prbe cnfiguratin was the same as fr the high frequency measurements technique. The grund plate and auxiliary grund pints were used t prvide an effective reference cntact with the sil. SUMMARY This research has demnstrated hw difficult it is t predict and measure the high frequency characteristics f a grund electrde. Withut the aid f a netwrk analyer the impedance f a grund electrde is hard t measure at high frequency. The impedance will vary greatly with the parameters f the sil and the envirnmental cnditins. At high frequencies the cupling mde 28-4 V ^"j. y -_.*_* v^.' «_~ y'. w. '

fr the earth electrde ia capacitive, therefre, the cntact area at the grund surface shuld be as large as pssible (relative gemetric mean area) t dissipate an, transient energy.

Experiment Statin f the Gergia Institute f Technlgy, At

5 fr the earth electrde ia capacitive, therefre, the cntact area at the grund surface shuld be as large as pssible (relative gemetric mean area) t dissipate an, transient energy. ACKNOWLEDGEMENTS This wrk [9] was spnsred by the Defense Nuclear Agency under RDT&E RMSS Cde B G52AAXEX40602 H2590D cntract number DNA C and perfrmed by the Engineering Experiment Statin f the Gergia Institute f Technlgy, Atlanta, Gergia Denny, H. W., Hlland, L. D., Rbinette, S. L., and Wdy, J. A., "Grunding, Bnding, and Shielding Practices and Prcedures fr Electrnic Equipments and Facilities," Vlume 1, Final Reprt, Cntract DOT-FA72WA-2850, Engineering Experiment Statin, Gergia Institute f Technlgy, December 1975, AD A Denny, H. W., Acree, D. W., Melsn, G. B., and Millard, D. P., "Supplemental Grunding f Extended EMP Cllectrs", Gergia Institute f Technlgy, Engineering Experiment Statin, fr the Defense Nuclear Agency, 31 January REFERENCES 1. Smith, G. S., and King, R. W. P., "The Resnant Linear Antenna as a Prbe fr Measuring the In Situ Electrical Prperties f Gelgical Media", Jurnal f Gephysical Research, Vl. 79, N. 17, June 10, 1974, pp DOWN CONDUCTOR 2. Curdts, E. B., "Sme f the Fundamental Aspects f Grund Resistance Measurements", AIEE Transactins, Cmmunicatins and Electrnics, Vl. 77, N. 39, Nvember 1958, pp Gaddy, 0. L., "A Simple Methd f Measuring Fractinal Millimicrsecnd Pulse Characteristics," IRE Transactins n Instrumentatin, Vl. 1-9, N. 3, December 1960, pp ROD ROD 4. Mukhedkar, D., Gervais, Y., and De Jean, J. P., "Mdeling f a Grunding Electrde", IEEE Transactins n Pwer Apparatus and Systems. Vl. PAS 92, N. 1, January-February 1973, pp Weidman, C. D. and Krider, E. P., "Submicrsecnd Rise Times in Lightning Radiatin Fields", Prceedings f the Lightning Technlgy Sympsium, spnsred by the FAA and NASA at NASA Langley Research Center, Hamptn, VA, April 22-24, ROD X Fig. 1 - Grund rd mdel. 6. Lee, K. M., and Smith, G. S., "Measured Prperties and Insulated Antennas in Sand", IEEE Transactins n Antennas and Prpagatin, Vl. AP-23, N. 5, September 1975, pp Lytle, R. J., "Prperties f the Grund Inferred frm Electrmagnetic Measurements", IEEE Transactins n Antennas and Prpagatin, Vl. AP- 27, N. 6, Nvember 1979, pp ' v -

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7 -S2J1'-...CA5.:-. i -»" ilniili -'- " ' ' i TYPE "N" MALE CONNECTOR DIAMETER 4.2mm INSIDE DIA. 1.4cm CENTER CONDUCTING ROD INSIDE D!»,. 2.85cm JIAMETER 1.23cm DIAMETER 1.27cm V_i_ Fig. 3 - Standard test prbe details. 28-7

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2 Q Z iu -,00 100000-.5 ~i 1 r 5 50 500 FREQUENCY IN MH Fig. 6 - Simple grund rd impedance. RF NETWORK ANALYZER (.

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$> \> \»~» - T" it* *".""- *> "* _^. -*& * tim '---- - -T -jyv»^ 100-100 «0 1000-10000- XL H 2 UJ h-i $ 100000. T.5 n 1 r 6 50 500 FREQUENCY IN MH Fig- 8 - Impedance f grund rd and ne auxiliary grund.$

10 > \> \»~» - T" it* *".""- *> "* _^. -*& * tim ' T -jyv»^ « XL H 2 UJ h-i $ T.5 n 1 r FREQUENCY IN MH Fig- 8 - Impedance f grund rd and ne auxiliary grund I-+100 CO i ' - 0 O 5 k-i Jg T.5 "i 1 r FREQUENCY IN MH* Fig. 9 - Impedance f grund rd and tw auxiliary grund«

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11 100. X Z N CO tu tu E 0 O w -100 < I I 5 50 FREQUENCY IN MK 500 Fig Impedance f grund rd and three auxiliary griunds. i CO tu tu E 0 w tu O O "T.5 ~i 1 r FREQUENCY IN MHi Fig Impedance f grund rd and fur auxiliary grunds ' VL.'..»» Amm. '{-» - «'-"»..'_V.» -* «' '-*-«-» *-*-* ~ ^.»_ ^ _

Fig. 12 - Impedance f grund rd, fur auxiliary grunds, and an aluminum gurnd plate.

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Module 4: General Formulation of Electric Circuit Theory

Module 4: General Formulation of Electric Circuit Theory Mdule 4: General Frmulatin f Electric Circuit Thery 4. General Frmulatin f Electric Circuit Thery All electrmagnetic phenmena are described at a fundamental level by Maxwell's equatins and the assciated