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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
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