been caused b y insufficient lubricant on t h e rollers. Conclusion T h e connected horsepower sometimes is a n indication as t o w h a t t h e energy a n d d e m a n d requirements will b e for a specific operation such as a haulage locomotive, b u t often t h e horsepower r a t i n g is n o t r e ­ flected, as in t h e case of mining machines. I t is vital t o b e a r in m i n d t h a t each m i n e h a s certain conditions which are unique, a n d t h a t t h e d a t a presented here is based on t h e average figures for m a n y mines u n ­ less otherwise s t a t e d . I n t h e cases where specific tests are presented, i t should n o t be assumed t h a t these are necessarily m a d e in mines w i t h average conditions. On t h e o t h e r h a n d , t h e conditions af­ fecting each t e s t h a v e been m a d e a part of t h e d a t a for t h a t test. Reference 1. POWBR FOR CONTINOUS M I N B R S , W. A. MacCallA* Proceedings, Coal Mining Institute of America, Pittsburgh, Pa. No Diiscussion The Effect of Reactive Components in the Measurement of Grounding Circuits L. H. HARRISON MEMBER AIEE K N O W L E D G E C O N C E R N I N G the electrical characteristics of t h e e a r t h ' s surface is i m p o r t a n t in safe opera­ tion of electric e q u i p m e n t . Such knowl­ edge is helpful i n application of lightning protection t o power systems, protection against shock h a z a r d s , a n d protection against p r e m a t u r e blasting accidents where electric shot firing is used. Various devices are available for m e a s ­ u r i n g t h e resistivity of circuits t h r o u g h t h e e a r t h , a n d h a v e been in wide use for a n i u n b e r of y e a r s . T h e m e t h o d generally used is based u p o n work b y t h e N a t i o n a l Btu-eau of S t a n d a r d s in 1918, a n d alter­ n a t i n g c u r r e n t is used for meastuing, on t h e t h e o r y t h a t t h e readings will t h e n b e unaffected b y s t r a y or extraneous volt­ ages in t h e e a r t h . T h e devices are cali­ b r a t e d t o r e a d directly in ohms, a n d i t h a s been generally assumed t h a t O h m ' s law is applicable t o circuits t h r o u g h t h e earth. Electric c u r r e n t does n o t always flow from a power s y s t e m i n t o t h e e a r t h in ac­ cordance w i t h O h m ' s law, when t h e o h m i c values used h a v e been d e t e r m i n e d b y tests m a d e w i t h commercial groimd testers. Investigation h a s indicated t h a t t h e r e is a reactive c o m p o n e n t t h a t m u s t b e considered, a n d this involves t h e t e s t leads as well as t h e e a r t h circuit itself. Tests from Power Circuits A n investigation conducted in t h e P i t t s b i u g h district of western Pennsyl­ 340 v a n i a involved transmission of voicem o d u l a t e d signals t h r o u g h t h e e a r t h . T h e a n t e n n a system for such transmis­ sion usually consisted of a section of e a r t h s t r a t a between t w o electrodes driven i n t o t h e e a r t h ; or, in a coal mine, between t h e m i n e t r a c k a n d a n electrode in t h e roof. I t was possible t o p u t m o r e power i n t o t h e e a r t h circuit t h r o u g h t h e same elec­ trodes w i t h t h e higher frequencies. T h e curve, Fig. 1, indicates this relationship for some of t h e tests m a d e . C o m p a r a t i v e tests using direct c u r r e n t a n d alternating c i u r e n t from mine power circuits were also m a d e . D a t a typical of these tests are shown in T a b l e s I a n d I I . T h e s e d a t a were obtained w i t h c u r r e n t flowing i n t o a circuit m a d e u p of t w o g r o u n d rods installed in t h e b o t t o m of a mine e n t r y a n d the e a r t h between. Volt­ ages were measu r ed from one electrode t o various points in t h e e a r t h . W h e n m easu r ed with a commercial-type ground tester, these electrodes indicated values of 48 a n d 68 ohms, or a t o t a l for t h e e a r t h circuit t h r o u g h t h e electrodes of 116 o h m s . T h e r e was some fluctuation in t h e d-c voltage during these tests, a n d t h i s accounts for t h e variation in t h e current. T h e a l t e r n a t i n g voltage remained steady; therefore t h e c u r r e n t values remained constant. Comparison of these tables shows t h a t t h e d-c resistance checked with t h e meas­ u r e d 68 ohms a t 5V2 feet from t h e elec­ t r o d e ; however, t h e impedance of the s a m e circuits with alternating current indicated considerably lower values. Tests with Higher Frequencies T o investigate further t h e impedance characteristics of t h e e a r t h surface, tests were m a d e using frequencies from 30 cycles t o 200 kc, inclusive, as well as with direct current. Fo r these tests, t w o metal-pipe electrodes were driven i n t o t h e ground a b o u t 45 feet apart, A a n d B, Fig. 2. I n line with these, a n d ap­ proximately 11 feet from each, t w o addi­ tional electrodes, C a n d D, were driven. E a c h electrode t h e n was measured with a s t a n d a r d - t y p e groimd resistance tester, a n d t h e resistance t o ground of each elec- Paper 53-359, recommended by the AIEE Mining and Metal Industry Committee and approved by the AIEE Committee on Technical Operations for presentation at the AIEE Middle Eastern District Meeting, Charleston, W. Va., September 29October 1, 1953. Manuscript submitted June 29, 1953; made available for printing August 5, 1953. L. H . HARRISON is with the United States Bureau of Mines, Birmingham, Ala. Harrison—Reactive Components in Measuring Grounding 100 90 110 120 130 140 MILLIAMPERES-POWER INPUT Fig. 1. Circuits Frequency versus power input NOVEMBER 1 9 5 3 Table I. Test Data with Direct Current AMPS flu Distance from Electrode, Inches Amperes Flowing Voltage Readings Ohms Calculated 6 12 18 24 30 36 42 48 54 60 66 1.68 1.73 1.75 1.82 1.83 1.85 1.86 1.86 1.90 1.86 1.78 86 89 95 96 101 104.5 108.3 110 115 118 122 51.2 51.4 .54.3 52.7 55.3 56.5 58.3 59.2 60.6 63.4 68.5 I t RHEO R' I I V Μ Fig. 2 . Typical test circuit for various fre­ quencies Rheostat R' adjusted to halve the normal voltdse, then R' = R Ground tests Rod A = 9 0 ohms Rod B = 110ohms Rod C = 150 ohms Rod D = 250 ohms trode was indicated as follows: .^ = 90 ohms, J9 = 110 ohms, C = 1 5 0 ohms, a n d D = 250 o h m s . T h e ground circuit b e ­ tween C a n d D w a s measured, using direct cmrent, a n d i t indicated a resistance of 730 ohms. To m a k e t h e tests, a signal generator was then connected t o electrodes A a n d Β with a milHammeter in series with t h e circuit. A v a c u u m - t u b e v o l t m e t e r w a s connected between inner electrodes C a n d D to measure t h e voltage drop across t h e earth circuit when c u r r e n t flowed in t h e ground. T o save time in calciUating t h e resistance or i m p e d a n c e of t h e circuit, a 0- to 1,000-ohm variable r h e o s t a t w a s connected across t h e terminals of t h e voltmeter, a n d 100 milliamperes of alter­ nating current was caused t o flow t h r o u g h the ground between electrodes A a n d B, The current v a l u e was held c o n s t a n t , a n d the signal generator adjusted for various frequencies. R h e o s t a t R, Fig. 2, w a s a d ­ justed to halve t h e voltage reading, a n d thus the position of t h e r h e o s t a t indicated the value of i m p e d a n c e i n t h e circuit under test. T h e graphic curve. Fig. 3, shows t h e relationship of i m p e d a n c e t o frequency indicated in these tests. Measurements of voltage drops across a section of earth w i t h 100 milliamperes flowing in t h e circuit indicated a difference of 40 per cent in A'oltage drop, between electrodes for frequencies from 30 t o 200 kilocycles. T h e fact t h a t a b o u t 25-per­ cent difference is indicated for a variation in frequencies from 0 (direct c m r e n t ) t o 1,000 cycles appears significant in t h e consideration of ground m e a s u r e m e n t s . Fig. 4 shows t h e relationship of i m p e d a n c e NOVEMBER 1 9 5 3 t o frequency from 0 to 1,000 cycles. T e s t s were m a d e w i t h u n m o d u l a t e d carrier signals t r a n s m i t t e d t h r o u g h vari­ ous thicknesses of overbiurden from a n u n d e r g r o u n d mine t o t h e surface, w i t h o u t t h e aid of metallic conductors. I t was found t h a t stronger signals were received with transmission a t t h e lower frequen­ cies. T h e o u t p u t power of t h e t r a n s m i t ­ t e r w a s held c o n s t a n t . These results were t h o u g h t t o h a v e been accomphshed b y a greater spread of t h e c u r r e n t from t h e t r a n s m i t t e r , because of t h e increased i m ­ pedance of t h e a n t e n n a p a t h a t t h e lower frequencies. Table II. Comparative Test with Alternating Cun-ent Ground Measurements m i n e power circuit. T o obtain these d a t a , copperweld ground rods were driven i n t o t h e e a r t h 1 foot a t a time. Resist­ ance m e a s u r e m e n t s were m a d e , a n d t h e c u r r e n t was measu r ed a n d calculated for each foot of electrode d e p t h . A t a coal-gasification project in Ala­ b a m a , electrocarbonization w a s used as a m e a n s of igniting t h e coal b e d . F o u r holes a b o u t 60 feet a p a r t were drilled from t h e siuface i n t o t h e coal lying a b o u t 180 feet below. T h e holes were cased w i t h 6-inch steel pipe, a n d electrodes in­ stalled inside t h e pipe. T h e electrodes consisted of 4-inch stainless-steel cyl­ inders, 17 inches in length, t o which was a t t a c h e d a 2-inch steel pipe with a copper cable inside, b o t h of which extended from t h e coal b e d t o t h e surface. These elec- Ground-resistance m e a s u r e m e n t s a r e usually associated w i t h safety i n opera­ tion of electric e q u i p m e n t a r o u n d coal mines. Because ground m e a s u r e m e n t s m a d e w i t h commercial groimd testers usually are r e a d directly i n ohms, i t is generally t h o u g h t t h a t O h m ' s law c a n b e a p p h e d . T h i s is t r u e only w h e n t h e entire circuit is considered; even then, i t does n o t always hold t r u e . If a grounding electrode is measu r ed a n d t h e probable c u r r e n t flow calculated, using t h e ohmic value indicated a n d t h e line voltage, t h e result is likely t o b e a t g r e a t variance w i t h t h e a c t u a l c u r r e n t . T h e curves. F i g . 5, show results of typical t e s t s of t h i s kind, using power from a Distance from Electrode, Inches 6 12 18 24 30 36 42 48 54 60 66 Amperes Flowing ...1.52 ...1.52 ...1.52 ...1.52 ...1.52 ...1.52 ...1.52 ...1.52 ...1.52 ...1.52 ...1.52 Voltage Readings Ohms Calculated . . . . 1 3 . . . . . . . . 8.55 ....16 ....10.53 ....25 . . . . ....16.45 . . . . 30 ....19.74 ....35 . . . . ....23.03 . . . . 40 ....26.32 . . . . 45 . . . . . . . . 2 9 , 6 1 ....48 . . . . ....31.58 ,...50 . . . . ....32.89 . . . . 5 2 . 2 . . . . ....34.34 ....55 . . . . ....36.18 300| 200 100 Harrison—Reactive 2 5 KILOCYCLES Fig. 3 . Relationship of impedance to frequency Components in Measuring Grounding Circuits 341 7001 tLCULAl •ED ; 500 300 200 trodes were insulated from t h e steel cas­ ings b y m e a n s of t r a n s i t e pipe a n d in­ sulating-fiber centering disks a t t h e sur­ face terminals. T h e electrodes were em­ b e d d e d in t h e coal, a n d t h e 6-inch casings were in c o n t a c t w i t h t h e t o p of t h e coal seam. T h e resistance between t h e elec­ t r o d e a n d t h e casing in t h e no. 2 hole measiu-ed 7.5 ohms. T o determine t h e best value of voltage t o a p p l y for t h e initial carbonizing tests, a series of v e r y careful ground-resistance m e a s u r e m e n t s were m a d e . Because power would b e a p p l i e d t o t h e nos. 2 a n d 3 electrodes first, m o s t of t h e tests were m a d e on these holes. T h e no. 2 electrode measiu-ed 8.37 ohms, a n d t h e no. 3 elec­ trode 7.9 ohms. T h e casing in t h e no. 2 hole m e a s u r e d 2.5 o h m s t o ground, a n d t h a t in t h e no. 3 hole 2.75 o h m s t o ground. T h e circuit, including electrodes 2 a n d 3 with t h e e a r t h circuit in series, m e a s u r e d 17 ohms. L a t e r , after w a t e r h a d b e e n r e ­ m o v e d from t h e holes, t h e circuit resist­ ance was indicated a t 20.9 ohms. Alternating c t u r e n t , a t 60 cycles, from a n isolated circuit was t h e n applied t o t h e n o . 2 electrode a n d t h e no. 3 electrode in series. C i u r e n t a n d v o ltag e readings were t a k e n a t short-time intervals, a n d t h e resistance was calculated, w i t h results shown in T a b l e I I I . T h e o h m s calculated from t h e first readings varied considerably from t h e o h m s measured. T h e i m p e d a n c e of t h e circuit d r o p p e d gradually as t h e t e s t voltage was continuously applied. A 500-kva variable-voltage a u t o t r a n s former, h a v i n g t a p s for voltages from 200 t o 1,800 was installed a n d connected t h r o u g h suitable e q u i p m e n t t o t h e elec­ trodes in nos. 2 a n d 3 holes. Single phase 60-cycle power from t h i s transformer Table III. Amperes 1.95 2.15...· 2.50 2.75 342 Variation in Impedance of Circuit Throush the Coal Seam Volts 75.5 73.0 66.0 62 0 800 400 600 FREQUENCY-CYCLES r 1000 Fig. 4 (above). Change in imped­ ance, 0 t o 1,000 cycles / MEAS JRED ^ 2 4 6 LENGTH OF DRIVEN ROD-FEET Fig. 5. Impedance measured and calculated with different depths of electrode was applied a t 800 volts, b u t t h e switch­ board a m m e t e r failed t o indicate c u r r e n t flowing. I t was expected, having applied 800 volts t o a circuit which h a d m e a s u r e d 21 ohms, t h a t a c u r r e n t of 38 amperes would flow. After a check of t h e circuits a n d metering e q u i p m e n t h a d been m a d e , 800 volts was again applied, w i t h t h e s a m e results. A p o r t a b l e a m m e t e r was t h e n used t o m e a s u r e t h e c u r r e n t in t h e circuit, w h i c h a t t h a t t i m e measiu-ed 3 amperes. S u b s e q u e n t voltage a n d c i u r e n t read­ ings t a k e n a t s h o r t - t i m e i n t e r v a l s indi­ c a t e d a lowering i m p e d a n c e as t h e c u r r e n t p a t h t h r o u g h t h e coal b e d was gradually carbonized. T h e graphic curve. Fig. 6, shows t h e relationship of t h e i m p e d a n c e calculated t o i m p e d a n c e measured. I t is interesting t o n o t e t h a t , i n t h i s in­ stance, t h r e e different values of imped­ ance were indicated: 1. A value of 21 ohms when measured with a ground tester; 2. A value of 38.7 ohms when a 60-cyele voltage of 75.5 was appUed to the circuit; 3. A value of 266 ohms when a 60-cycle voltage of 800 was impressed on the circuit. T h i s a p p a r e n t phenomenon has been n o t e d in other instances, where ground m e a s u r e m e n t s were m a d e with ground testers a n d with c u r r e n t from industrial power circuits. Conclusions T h e flow of electric c u r r e n t in circuits t h r o u g h t h e e a r t h is affected b y reactive components, which m u s t b e considered in t h e accurate calculation of current flow or voltage drop in such circuits. 300 200 < 100 IMPE0AN( Ε 2 = j - IPEDANCE MEASUR iD Ohms (Calculated) 3g.7 ,...33.9 26.4 9.9. a Harrison—Reactive — - — -, 6 8 MINUTES Fig. 6. Relationship of calculated impedance to measured impedance Components in Measuring Grounding Circuits NOVEMBER 1953 Voltage drop a n d c u r r e n t values are af­ fected b y t h e frequency of t h e voltage apphed, a n d t h e m e a s u r e m e n t of g r o u n d impedance should b e m a d e w i t h power of the same frequency as t h e power circuit or equipment involved, in order t o b e rigor­ ously correct. Circuits t h r o u g h t h e e a r t h a p p e a r t o have a negative i m p e d a n c e characteristic, the impedance v a r y i n g inversely with t h e frequency of t h e a p p h e d voltage, within certain limits. G r o u n d i n g circuits t h r o u g h e a r t h s t r a t a c a n n o t b e depended u p o n t o blow a fuse or t r i p overcurrent devices installed for t h e protection of m o t o r s or o t h e r electric e q u i p m e n t , even t h o u g h t h e measm*ed ohmic v a l u e of such circuits so indicates. F u r t h e r knowledge of such charac­ teristics should b e helpful i n developing communication t h r o u g h e a r t h s t r a t a , a n d in developing b e t t e r m e t h o d s of protec­ tion against shock a n d p r e m a t u r e blasting hazards. Dii s c u s s i o n measured as such by ground testers of this type since there are no quadrature compo­ nents in t h e measuring circuit. T h e author's statement concerning test leads is also open t o question, particularly in connection with t h e current interrupter type of ground tester. I t has been demonstrated by practical field measurements with such instruments t h a t self- and mutual reactances between leads causes no significant errors, even with a very close and very long coupling. I t has been claimed t h a t lead reactance has caused errors when using ground testers of the ciurent interrupter type when measuring large grounding grids. Since t h e design features used in this type of ground tester are such t h a t reactive components are sup­ pressed, it is more likely t h a t a n y errors encountered were the result of mutual resist­ ance effects between t h e earth under test and t h e current reference, these effects being caused by t h e current reference elec­ trode not being sufficiently remote from t h e electrode under test. As is generally known, the reason why commercial ground testers use alternating or reversing current rather t h a n direct cur­ rent in making ground tests is t o eliminate for practical purposes t h e effects of electro­ lytic polarizations. Unless t h e volumes of earth surrounding t h e electrodes are entirely free of electrolytic moisture, a d-c test of one polarity would indicate higher resistance values t h a n those obtained with alternating or reversing voltages due t o gas films form­ ing and being sustained a t t h e surfaces of t h e electrodes. Therefore, under conditions of electrolytic dissociation there should be no reason t o expect t h a t measurements made with commercial and higher frequen­ cies, or with commercial ground testers, should conform t o those made with direct current. However, experience has indicated t h a t there are no difficulties in co-ordinating ground-fault protection on d-c chcuits with currents determined from commercial ground-tester measurements where natural variations in electrode t o earth resistances are given sound engineering consideration. Wide variations in conditions in t h e imme­ diate layers of earth surrounding an elec­ trode can occur in relatively short periods of time. More stable conditions can be ob­ tained by driving longer rods t o permanent moisture and by properly spaced multiple rods t o reduce current density. Mr. Har­ rison's unquaUfied statements in t h e third paragraph of t h e conclusions are both mis­ leading and questionable. it would appear likely t h a t the anomahes encountered in the cases cited by the author, a t least a t the lower frequencies, could be due t o unstable or nonreproducible condi­ tions surrounding t h e electrodes. Such in­ stabilities, not only due t o electrolytic dis­ sociation in the case of the d-c tests, can be caused by changes in electrode t o earth con­ tact conditions such as moisture variations, and thermal changes due t o the magnitude of the test current used, including t h e drying out of the earth immediately surrounding the electrodes. The geometrical charac­ teristics of an electrode t o earth contact cannot be overlooked in considering the voltage gradients t h a t exist as a result of current. I t can be simply shown t h a t the greatest voltage gradient, and therefore most of the resistance, appears in the earth layers immediately surrounding t h e elec­ trode. Thus, any physical, chemical, or thermal changes occuring in these immedi­ ate layers may greatly affect the measure resistances without any reactive properties existing. The results of the test, as shown in Fig. 5, are extremely puzzling. I t has been demon­ strated in numerous instances t h a t ground resistance measurements can be relied upon t o determine current flow unless the actual current is of sufficient magnitude and dura­ tion t o increase the earth temperature or to dry out the earth immediately surrounding the electrode. Where steep wave-front highamplitude current impulses are involved it has been found t h a t actual breakdown of high gradient layers occurs, resulting in deviations from t h e calculated current values. The calculated current values shown in Fig. 5 seem t o conform with practical and theoretical results when the resistance measurement is made t o remote earth. The actual current curve does not, but in­ stead shows a flatness with increasing rod depth which one would expect t o find if the reference rod forming t h e 2-terminal net­ work were of such high resistance, compared t o t h e rod under test, t h a t it became t h e controlling factor. Mr. Harrison attributes the very much smaller measured current to electrode t o earth reactance. Using the 7-foot rod depth in Fig.γ 5, .the calculated current is 5.5 amperes, and t h e measured current is 1.5 amperes. Calculating the relative values of resistance and reactance from these data, it is found t h a t t h e reactance component of impedance would be about 3.5 times t h a t of t h e resistance component. Such a situation would seem extremely unlikely. T h e measuring technique for a test of this kind is simple, so there should be no cause E. B. Curdts (James G. Biddle Company, Philadelphia, P a . ) : This paper is interest­ ing in t h a t it reveals t h e existence of prob­ lems in mining activities which m a y call for a better understanding of the characteristics of electrodes associated with or surrounded by earth in general. In dealing with problems involving t h e flow of electric current in t h e earth, it has been generally agreed t h a t opposition t o such flow is usually an electrolytic phe­ nomenon and does not conform t o accepted theories describing the transmission of elec­ tric charges through metals. Furthermore, the relative geometry of t h e elements of ground circuits usually does not conform with that involved in ordinary metallic cir­ cuits, requiring a somewhat different treat­ ment in reaching conclusions concerning current flow and voltage gradients. In commenting on this paper exception can first be taken t o t h e broad statements in the third paragraph concerning Ohm's law and the ohmic values obtained from com­ mercial ground testers. Commercial ground testers are logically calibrated t o indicate the ratio of voltage t o a cmrent, which is what the user wishes t o know, b u t there is no reason t o assume t h a t t h e linearity of Ohm's law must apply in such measurements be­ cause the device is calibrated in ohms. Cpmmercial ground testers of t h e more commonly used types are provided with either a-c or polarity reversing sources, and are available in t h e frequency range of from about 25 t o 100 cycles or reversals per second. I n the case of t h e crossed-coil ohnuneter type of ground tester which uses a hand-cranked generator for its source of supply, the frequency or reversals per second may be varied b y t h e cranking speed t o avoid objectionable beat frequencies with commercial 25-, 50-, and 60-cycle extraneous voltages in t h e earth. Those commercial instruments using battery-vibrators are usu­ ally designed for one fixed frequency in t h e order of 100 cycles for the same reason. The crossed-coil ohmmeter type ground tester, recognized in section 3.43 of A I E E Standard no. 550,^ does not have an alter­ nating output in t h e ordinary sense, b u t through dead zones in t h e mechanical reversers the ciurent is forced t o zero a t t h e instant of the voltage zero so there can be no phase displacement between t h e two. I n other words, a t least this particular type of commercial ground tester is intended t o measure resistance and not impedance. Even if t h e ground circuits should have any reactive properties they would not be NOVEMBER 1953 Without disputmg t h e possibilities of minor reactive properties of a circuit in­ volving a relatively small electrode in con­ tact with an infinite mass such as t h e earth, Harrison—Reactive References 1. APPLICABILITY OF RADIO TO EMBRGBNCY M I N E COMMUNICATIONS, E . W . Felegy, E . J. Coggeshall. Report cf Investigations 4294, United States Bureau of Mines, Washington, D. C , 1948. 2. SOME CHARACTBRISTICS OP THB EARTH AS A CONDUCTOR OF ELECTRIC CURRENT, M . C . McCall, L. H. Harrison. Report cf Investigations 4903, United States Bureau of Mines, Washington, D. C . Components in Measuring Grounding Circuits 343 to question t h e one used in this case. Al­ though t h e technique is not described, it would be natiural t o assume t h a t t h e same 2-terminal network was used for both t h e calculated and measured tests, which would give comparable results. However, t h e results indicate t h a t a 2-terminal network was used t o determine t h e actual 60-cycle current, and a 3-terminal network t o deter­ mine t h e resistance t o remote earth of t h e rod being tested. Any a t t e m p t t o compare a 2-terminal with a 3-terminal test would be invalid. I t is also assumed t h a t t h e power system used was either an ungrounded one, or if not, then t h e system ground was used as reference (the rod under test being connected t o t h e ungrounded side of t h e sovuce). In summarizing this particular phase of t h e investigation covered b y this paper, I cannot but help being left with t h e feeling t h a t because of t h e lack of information con­ cerning measurement technique, and be­ cause of t h e great difference in measured and calculated currents as shown in Fig. 5, t h a t there is no conclusive evidence t h a t this great ciurent difference can be accounted for by reactive effects in t h e rod t o earth contact. Mr. Harrison discusses, in the section entitled "Tests with Higher Frequencies," and shows in Fig. 2, some details of t h e technique used in making t h e frequencyimpedance measurements. Actually this test, from t h e information given, turns out t o be a measure of t h e combined impedance characteristics of two umelated parts of t h e circuit. One of these parts involved t h e mutual impedance between points A and Β when measured with a high impedance volt­ meter and with a t infinity. T h e other part involved t h e net impedance of rods C and D. T h e measurements as described in this paper actually resulted in t h e sums of t h e mutual impedance of rods A and B, and the net impedance of rods C and D, The question then arises as t o just what information was being sought in this investi­ gation. If the electrode t o earth impedance was the information desired, then an addi­ tional question arises. Were the measure­ ments which resulted in values of 150 ohms and 250 ohms for t h e C and D rods taken to remote earth; if so, what was t h e mutual resistance between these two rods? In other words, did this investigation have as its purpose the determination of t h e impedancefrequency effects of an electrode t o remote earth, or just between two rods chosen t o be spaced 23 feet apart i; Even assummg t h a t t h e measuring ch-cuit was valid for t h e purpose intended, some assurance should be given by Mr. Harrison t h a t t h e ammeter, voltmeter, and rheostat involved were acciuate over t h e wide range of frequencies used in these tests. I t is con­ ceivable t h a t skin effect alone in the rheo­ stat could account for t h e negative-imped­ ance characteristic shown in Fig. 3. Mr. Harrison makes reference t o t h e Bureau of Mines report no. 4903 on this subject, which he co-authored. However, t h e information given in t h e Bureau of Mmes report conflicts with t h a t given in this paper. A comparison of t h e two shows t h a t t h e test described in t h e paper is not represented by Fig. 3 as stated, b u t repre­ sents data from an entuely different 2terminal test as described in t h e Bureau of 344 Mines report. Fig. 4 apparently represents d a t a resulting from t h e 4-terminal test de­ scribed in t h e A I E E paper. D a t a tabulated in t h e Bureau of Mines report roughly checks t h e curve shown in Fig. 4 of the A I E E paper. Assuming t h a t t h e tabulated d a t a in t h e Bureau of Mines report are valid, t h e 30-cycle impedance is 450 ohms and t h e 60-cycle impedance is 420 ohms, a reduction of 6.7 per cent in t h e range of these com­ mercial frequencies. T h e 730-ohm figure for the d-c test can be discarded as irrelevant unless t h e measurement was made before electrolytic dissociation developed, which was unlikely. The anomalies encountered in t h e test in connection with the coal-gasification project are challenging, t o say the least. However, a study of the information given leads one t o believe t h a t these anomalies are caused entirely b y changing conditions around t h e electrode between t h e different tests. Again it must be emphasized t h a t t h e resistance of electrodes surrounded by earth is confined largely t o t h e immediate layers of earth and is greatly influenced b y conditions of mois­ ture and temperature. Summarizing this discussion, Mr. Harri­ son is t o be commended for pointing out the wide variations t h a t can occur in electrode t o earth contacts, even though the reactive effects may not be as pronounced as his paper may lead one t o believe. REFERENCE 1. MASTER TEST CODE FOR RESISTANCE M E A S ­ UREMENT, AIEE Standard No. 550, May 1 9 4 9 . Richard W. Inman (Associated Research, Inc., Chicago 18, III.): T o properly evalu­ ate Mr. Harrison's paper, one must ask several questions, any one of which has a distinct bearing on the accuracy of the state­ ments made and t h e conclusions reached. Consequently this discussion must take the nature of a question rather t h a n a comment. Very little reference is given t o the equip­ ment used in the performance of the various measurements and tests. For instance, was t h e commercial type of ground tester a d-c or an a-c unit? If it was an a-c tester, of what frequency and voltage were the out­ put? Where d-c d a t a were submitted, was any considerations given t o reversed readings to eliminate t h e possibility of error due t o electrolytic potentials or stray currents within t h e area of measurement? There are various types of commercial ground resistance testers presumedly de­ signed t o measure resistance to earth of man-made grounds and t o do so on t h e basis t h a t Ohm's law is applicable t o circuits through t h e earth. Experience has indi­ cated, however, t h a t while these testers may b e calibrated directly in ohms under various conditions t o be found in conjunction with t h e measurement of ground resist­ ance, it is impossible t o closely check t h e readings of one type of tester with another; yet it is possible t o obtain repetitive read­ ings of t h e same magnitude for t h e same tester under t h e same given set of conditions. I t is a known fact t h a t some testers are materially affected by t h e resistance of t h e auxiliary probes while others are not. T h e sensitivity and response of other testers are affected by t h e relation of the auxiliary probe resistance t o t h a t of t h e resistance of Harrison—Reactive t h e man-made ground under test. I t is apparent, therefore, t h a t considera­ tions of t h e measurements technique in­ volved, t h e relative spacing of the auxiliary probes, the depth of rod, and several other factors are essential. Equally important in evaluation of the material presented would be data as to the depth of penetration of the man-made ground rods as well as depth of penetration of t h e auxiliary probes and whether one or two were used and what the spacing of them was. I t is true that measurements with com­ mercial testers are usually made directly in ohms, but in so doing it is natural that opera­ tion of t h e over-all cucuit must be under­ stood. The same would seem t o apply to the information in Fig. 5 concerning which the statement is made t h a t power was used from t h e mine power circuit. Equally im­ portant would be the method of measure­ ment used in determining the factors making up the curves contained in Fig. 5. Here reference is made t o the question of whether consideration was given to the theory of t h e sphere of influence of a ground rod in relation t o its depth and the conse­ quent spacing of auxihary probes in the ground-resistance measurement. As the water content of the earth has a material effect upon the resistance of any ground rod driven into t h e earth, and as all too often t h e water content is changing, unless the timing of any testing is relatively short in duration, it would appear extremely difiicult t o propose absolute statements and conclu­ sions. If one compares t h e readings of Table III with t h e final reading taken after the water had been removed from the holes, then the variations are not too great and on the basis of t h e statement t h a t the impedance of the circuit dropped gradually as t h e test voltage was continuously applied one would cer­ tainly be lead to believe t h a t a change in circuit conditions resulting from the watts in t h e circuit under test was a contributing factor rather t h a n an error in reading result­ ing from t h e change in impedance. Is it not reasonable t o assume t h a t possibly due to a continuous application of current potentials t o the circuit under test a drying-out effect was taking place, which in turn would naturally affect what was assumed to be the impedance of t h e circuit rather t h a t its d-c resistance? The flow of electric current into the earth is affected not only by the apparent resist­ ance t o earth of the man-made ground con­ nection b u t also by t h e character of the earth surrounding the ground, by electro­ lytic potentials and currents in t h e earth, and by t h e earth resistivity in the area of t h e ground contact. The fact t h a t there is variation between calculated current values and actual measured current values can be considered more as a summation of these grounding factors than as a conclusion that it is entirely due t o the reactive components in ground resistance. I t would seem very possible t h a t we are dealing with a nonlinear active resistance, the value of which is fre­ quency dependent. Perhaps we are also dealing with a combination of nonlinear re­ sistances and reactances, any or all of which could be present and effect measurements made if cognizance is not taken of effect upon measurements of man-made grounds located within t h e sphere of influence of the test electrodes. Components in Measuring Grounding Circuits NOVEMBER 1 9 5 3 L. Η . Harrison: Some very interesting points have been brought out in these dis­ cussions. Mr. Ciudts' statement t o t h e effect that there is no reason t o assume t h a t the Unearity of Ohm's law must apply in such measurements is strange indeed. I t would seem t h a t where a resistance measiuement is given in ohms this would be the logical assumption. Reference is made to the crossed-coil, mechanical-reverser t5φe of ground tester by Mr. Curdts, who states that, even if t h e ground circuits should have any reactive properties, they would not be measured as such by this type of ground tester. I t is hard to conceive of phase displacement with the square-wave form of alternating ciurent produced by the mechanical reverser; never­ theless, it can be demonstrated t h a t the addition of capacitance to the resistance of the circuit under test will lower t h e reading obtained with this type of instrument in about the same proportions as t h a t of t h e battery-powered vibrator types. Many textbooks. Bureau of Standards papers, and technical bulletins on groundresistance testing give t h e equation R = p(l/2nC) for t h e resistance to the flow of current away from an electrode in the earth. where C = the combined electrostatic capacity in free space of t h e electrode and its image above the surface of the earth. If this equation is valid, then every circuit comprising one or more grounding dectrodes and a section of t h e earth's surface has capacitive reactance as one of its component parts. Mr. Inman states t h a t it would seem very possible t h a t we are dealing with a nonlinear active resistance, the value of which is frequency dependent. This is merely another way of saying t h a t we are dealing with circuits having resistance com­ ponents and reactance components, which was my original contention. I t is regrettable t h a t these discussions either ignore or pass lightly over the results of tests made at the coal-gasification proj­ ect. The theory of electrolytic dissociation is inadequate t o cover the anomalies indi­ cated, since such wide variations in imped­ ance were observed with the same frequency of applied voltage. Neither can they be ex­ plained by changing conditions around t h e electrodes in the time elapsed between t h e different tests, as the power for carboniza­ tion was first applied within a matter of minutes after t h e low-voltage 60-cycle re­ sistance test was completed. The sugges­ Wanted: A Modern Diesel-Electric Rail Car K. O. ANDERSON A S S O C I A T E MEMBER T H E self-propelled rail c a r is r e ­ t u r n i n g to a place of p o p u l a r i t y as t h e raih-oads seek t o reduce costs a n d a t t r a c t additional passenger business. Current rail-car application in t h e U n i t e d S t a t e s covers relatively long main-line r u n s with diesel-hydraulic cars, a n d b r a n c h line service w i t h light diesel-electric cars. Between these t w o extremes t h e r e lies a need for a main-line t y p e of c a r c a p a b l e of performing s u b u r b a n service. C e r t a i n characteristics of t h e electric drive a r e welksuited t o t h i s application. Trends in Rail Passenger Business A ghost h a s r e t u r n e d t o our n a t i o n ' s railroads. Considered d e a d for t h e p a s t 20 odd years, t h e diesel-powered rail c a r has come b a c k in n u m b e r s a s railroads are reawakening t o t h e a d v a n t a g e of provid­ ing a d e q u a t e passenger service for their patrons. Not all r o a d s , i t is t r u e , are t r y i n g t o better their passenger service. Almost every week t h e I n t e r s t a t e C o m m e r c e Commission is petitioned t o p e r m i t abandonment of a n o t h e r r u n . B u t t h e r e NOVEMBER 1 9 5 3 AIEE are some reversals t o this t r e n d . The N e w H a v e n , m o s t notably, in t h e p a s t y e a r h a s a d d e d m o r e t h a n 50 new pas­ senger trains t o i t s schedules. I t is plan­ n i n g even more, because i t h a s found t h e passenger business lucrative. R a i l cars are used on m o s t of these new r u n s . O t h e r r o a d s , too, are finding rail c a r s t h e answer t o m a n y of their passenger problems. T h r o u g h t h e i r use, deficit operations h a v e been t u r n e d i n t o m o n e y ­ m a k e r s , or losses h a v e been c u t down considerably. M a n y of these r o a d s a r e finding t h a t rail cars m a k e new services b o t h possible a n d profitable. T h e N e w Y o r k Central, for instance, h a s restored passenger service t o M i d l a n d , Mich., for t h e first t i m e in 25 y e a r s . As t h e n u m b e r of automobiles a n d t r u c k s in operation continues t o increase, our n a t i o n ' s h i g h w a y system becomes ever m o r e crowded. T h i s problem, ex­ isting everywhere, b u t particularly crit­ ical in our m o r e crowded areas, brings traffic t o a literal crawl, a n d m a k e s driv­ ing extremely hazardous, t o say n o t h i n g of t h e h u m a n frustration involved. T h u s t h e r e is a n a t u r a l t e n d e n c y t o Anderson—Wanted: tion t h a t the increased resistance might be due t o a drsdng-out effect caused b y the coutmuous application of current is not ac­ ceptable because it is shown in Table I I I and by the curve of Fig. 6 t h a t the imped­ ance of the circuit was gradually lowered by the continuous application of current. If, however, we can conceive of this circuit as comprising a capacitance with a high leakage dielectric, it is possible t o work out a plausi­ ble explanation of t h e phenomena observed. M y paper deals primarily with the measure­ ment of grounding circuics intended t o limit the voltages t h a t might appear on the frames of electric equipment upon occurrence of a ground fault within t h e equipment. If such ground measurements do not indicate accurately t h e impedance of this fault-cur­ rent circuit, they are valueless as a means of determining the degree of protection pro­ vided. There is need t o distinguish be­ tween ground measurements for system protection against lightning and those made t o provide protection from shock hazard. I a m indebted t o Mr. Curdts and Mr. In­ man for their valuable contributions t o the available knowledge of ground-testing pro­ cedure, and then- interest in this paper is greatly appreciated. t u r n t o t h e rails for relief, wherever a d e ­ q u a t e service is offered. T h e word a d e ­ q u a t e here m e a n s clean, comfortable equipment, operating on a fast schedule, a n d with reasonable frequency. Even crowded highways a r e preferable t o a dirty, green-plush-upholstered coach of 1900 vintage creaking along behind a s t e a m e r a t 20 miles per hour. Passengers will simply refuse t o ride in such cars. Place of the RaU Car Locomotive-hauled streamliners are suitable for long-distance runs, where enough d e m a n d exists t o fill several cars. B u t on m a n y of t h e shorter a n d interu r b a n schedules, only one or t w o cars a r e required, which m a k e s locomotive power a n d i n v e s t m e n t excessive a n d therefore uneconomical. Diesel-powered rail cars a p p e a r t o answer these requirements very well. T h e new ones being p u t into service t o d a y a r e clean a n d quiet, h a v e high ac­ celerating r a t e s a n d high t o p speeds, a n d a r e economical t o operate. T h e well-known B u d d RDC, of which 116 are in operation or on order, is used on 13 U n i t e d S t a t e s railroads a n d in t h r e e foreign countries. I t s t w o 275 gross horsepower engines a n d 63 t o n s Paper 53-361, recommended by the AIEE Land Transportation Committee and approved by the AIEE Committee on Technical Operations for presentation at the AIEE Middle Eastern District Meeting, Charleston, W. Va., September 29October 1, 1953. Manuscript submitted June 29, 1953; made available for printing August 4, 1953. K. O. ANDBRSON is with the General Electric Com­ pany, Erie, Pa. A Modern Diesel-Electric Rail Car 345