TECHNICAL 

REPORT 

Final Report 

C5490 

SAFE DISTANCES FOR BLASTING WIRING FROM 

COMMONLY ENCOUNTERED UNDERGROUND ELECTROMAGNETIC ENERGY SOURCES 

by 

Ramie H. Thompson 

September 1983 

Prepared for 

U.S. Bureau of Mines 

Department of Interior 

Washington, D.C. 

Contract No. 50328023 

mnklin Research Center 

A Division of The Franklin Institute 

The Benjamin Franklin Parkway, Phila., Pa. 1 9 103 (2 15) 448- 1000 
The views and conclusions 

contained in this document are 

those of the author and should 

not be interpreted as necessarily 

representing the offi cia1 policies 

or recommendati ons of the Interior 

Department's Bureau of Mines or of 

the U.S. Government. 
ACKNOWLEDGEMENTS 

This project was cd cmt by personnel of the Applied Physics 

Laboratory of the Franklin Research Center. The mine measurements were 

carried out principally by the author, Mr. Ramie He Thompson, Principal 

Scientist, Mr. James Stuart, Research Scientist, and Mr. A. W. Cipkins, 

Technical Associate. 

Thanks are due to Messrs. Harry Dobroski and James Means of the U.S. 

Bureau of Mines in providing specialized equipment and actual help in the 

preliminary measurements. 
Section 

CONTENTS 

Title 

ACKNOWLEDGEMENTS 

1.0 INTRODUCTION 

1.1 Background 

1.2 Early Efforts 

1.3 Final Measurements 

2.0 MINE MEASUREMENT DESCRIPTIONS 

2.1 The Mines and the Measurement Locations 

2.2 Apparatus 

2.3 Apparatus Check Out 

2.4 Measurement Procedure 

3.0 MINE MEASUREMENT RESULTS 

3.1 Loop Measurement Results 

3.2 Transmission Line Results 

4.0 ANALYSIS OF MEASUREMNT RESULTS 

4.1 Loop Data ~nalysis 

4.2 Transmission Line Data ~nalysis 

5.0 CONCULUS ION FROM MEASUREMENTS 

6.0 SAFE DISTANCES FROM SOURCES 

6.1 Critical Values of the Field 

6.2 Free Space Values of the Field From Various Sources 

6.3 Safe Distances From Trolley Phone Line Conductors 

6.4 Safe Distances From Loop Sources 

6.5 Summary of Safe Distances and Comment 

Page 

7.0 REFERENCES 
1.0 INTRODUCTION 

1.1 BACKGROUND 

In many underground coal mining operations the use of electromagnetic 

field equipment, particularly communication systems, not only increases the 

overall efficiency of the mining operation but can be directly linked with 

mine safety considerations. The use of electromagnetic field producing 

equipment in underground coal mining operations can only be expected to grow. 

Several new communication systems have already been proposed. In addition, 

high-frequency producing equipments for many diverse uses in mining operations 

are under study. 

The use oi these equipment in underground mining operations is hampered 

by the possibility of their electromagnetic fields interacting with the 

electric blasting cap operations commonly carried out in the mines. Such 

interactions can have at least two results bearing directly on mine safety: 

o premature initiation of the cap, either in its normal shot location or 

during hookup or transportation; 

o dudding of the cap so that normal firing operations do not cause 

irritation, thereby leaving unexploded high explosives after normal 

firing. 

Since the consequences of a premature initiation could, particularly in 

coal mines operations, result in catastrophe involving human life, the overall 

approach to prediction of possible hazard must be, as far as possible, of a 

"worst case" nature. 

The general problem of predicting possible RF hazards for any 

electroexplosive device (EED) is best treated by reference to Appendix A of 
this report. Appendix A considers the general problem from a military/space 

agency viewpoint but blasting caps are the primitive models for most EEDs so 

the material is directly applicable to blasting cap problems. 

In brier, the specific problem of analytically predicting possible RF 

hazards to blasting caps can be reduced to finding answers to the following 

three questions: 

(1) How much power, at the frequency of concern, is necessary to 

function the cap? 

(2) How much power is coupled to the cap by a given field existing in 

the vicinity of the cap and its associated wiring? 

3) What fields will exist in the vicinity of the blasting cap and its 

wiring for a given source, physical environment, frequency and 

separation? 

A considerable body of work exists that deals with questions (2) 

and (3) for aboveground blasting operations and sources. Although some of the 

results of this work are directly applicable to underground mining operations, 

the factors of minimum distance between transmitting sources and caps, tunnel 

geometry, unknown electrical properties of the surrounding minerals, etc., 

that are inherent in underground operations limit complete application of 

495 these methods and results. Some limited theoretical investigations , 

tailored to a specific underground communication system, have been performed. 

In the end, electromagnetic hazard evaluation of the system was forced to rely 

on experimental data taken in a tunnel of the mine in which the communication 

system was to be installed. 

Under the U.S. Bureau of Mines Contract H0210068 The Franklin Institute 

undertook a program aimed at producing a worst case model for electromagnetic 

energy/blasting cap interactions in underground workings. The results were 

presented in FIRL Report F-C3102, "Evaluation and Determination of Sensitivity 

l~u~erscri~ts refer to numberea references of Section 7. 

1- 2 
of Electromatnetic Interactions of Commercial Blasting Caps. 116 R.H. 

Thompson, August 1973. 

The experimental program showed that, for blasting caps of American 

manufacture, the application of 40 mW of average RF power, either continuous 

wave or with a typical radar pulse signature, to the input impendance of the 

cap, either pin-to-pin or pin-to-case, could be expected to result in an 

initiation less than once in one thousand applications with a 95% confidence 

level. Thus the use of a 40 mW "no-fire" level for underground caps, a level 

that had been tentatively established for all American-made blasting caps, was 

confirmed. 

Under U.S. Bureau of Mines Contract H0252015, the Applied Physics 

Laboratory of The Franklin Research Center undertook measurements in coal 

mines that, coupled with the 40 mW "no-fire" level, previously developed worst 

case wiring pick up antenna models, published electromagnetic mineral 

parameter data and a "lossy layer" model for propagation in the mine, could be 

used to predict safe distances for blasting operations underground. This work 

is reported in "RF Hazards to Blasting Caps in Coal Mines. lt7 The predicted 

safe distances in that report for frequencies less than 10 MHz were quite 

large. They are given in Figure 1-1. This was the direct result of the 

demonstration by our measurement results that, at these frequencies, 

underground sources coupled well to longitudinal conductors along the 

drifts. Thus, the energy from the source could propogate long distances with 

relatively small attenuation. The analyses assumed a large coupling factor 

between the source and any nearby conductors, thus large values of the fields 

were predicted at very large distances. We recognized at the time that the 

assumption of very good coupling between the source and the conductors in the 

mine was in error but the measurements taken provided no way to produce a 

valid worst case estimate of smaller magnitude. 

The present project is aimed directly at this difficulty. Our overall 

effort was directed at source frequencies of 10 MHz, or less. In this 

frequency regime coupling between the source and the blasting cap can be 

considered to be completely by magnetic induction. If the magnetic field, B, 

is known in the vicinity of the blasting wiring, then an upper limit on the 
10 100 

FFIEQUENCY (MHz) 

Figure 1-1 Safe Distances 
power absorbed by a blasting cap in the wiring can be made. Since the 

magnetic field can be thought of as having its source in currents driven by 

electromagnetic equipment, and these equipments can be related to the maximum 

currents they can produce, we see that a knowledge of the B field in the 

vicinity of known curents would allow us to predict safe distances for these 

currents. The experimental work on this project was directed to determining 

the B field distribution around typical underground current carrying conductor 

configurations. 

In particular, we chose loop current sources to approximate the "back 

pack" antennas and bandolier antennas being used for communication in coal 

mines. We also made measurements on a configuration approximating the 

conventional trolley line communcation systems. 

Our overall meausurement scheme was guided by the following approach to 

computing safe distances in the frequency band below 10MHz. 

The maximum voltage induced in a loop of blasting wiring containing a cap 

can be written as 

/- 

where 19 is the RMS open circuit voltage (volts) 

is the RMS magnetic field (maxwells) 

linking the loop and 

2 4 is the area of the loop (meters ) 

Total worst case power to the cap (W ) is R 

4 = /~/~-x~~ 

4, *. z/ ,n-.[ .-y- &--d ;/- 749. C.;f , where W is the worst case power (wattsbe z6 +W R c-----

- 6-4. 

is the resistance of the cap -- '/ 

about 1 ohm. 

It the received power is less than 40 milliwatts, we assume the cap is 

safe in its environment. Now, we could easily compute B at the blasting 
wiring location if we knew the direction, magnitude and location of all driven 

currents in the vicinity and the overall conductor configuration were located 

in free space. The actual fact is that the driven currents exist underground 

and there are other conductors present in the mine. The direction, magnitude 

and location of the driven currents can be estimated -- in a worst case sense 

-- from the equipments used in the mine and the conductors they drive, thus we 

need a correction factor that can be applied to the free space calculation of 

the B field that compensates -- in a worst case sense -- for the presence of 

the additional conductors in the mine and the presence of the minerals that 

make up the overall mine environment. 

The determintion of such a correction factor was the object of our 

measurements. 

1.2 EARLY EFFORTS 

Various loop antennas and transmission line simulating antennas were 

enlployed in late 1981 in a gallery of the U.S. Bureau of Mines Bruceton 

experimental mine. Measurements were taken of the variation of magnetic field 

with distance from the known currents in the transmitting antennas. The 

measurements, as expected, showed values larger than those predicted from free 

space calculations. The various conductor locations -- metal roof mesh, water 

pipes, electrical conduit, pipes, etc. -- were closely determined in the 

mine. A complete mockup of the mine -- constructed of wooden 2x4's was 

erected in an open field in Millville, N.J. 

All conductors were simulated in this mockup. The overall idea was to 

determine if the conductors or the minerals of mine contributed most to the 

elevated B tield measurements obtained in the actual mine. Hindsight is much 

better than foresight. We found our aboveground measurements to be completely 

useless. All our measurements were contaminated by large pickups -- large in 

relation to our previous measurements -- from AM & FM radio stations, even 

though these stations were many miles away. In the main, this entire result 
was due to the fact that the 300 foot long conductors were excellent antennas 

for the commercial radio stations. 

This entire phase of our efrort provided no insight into our problem but 

did produce improvements in our source equipment and measurement techniques. 

1.3 F INAL MEASUREMENTS 

The measurements presented herein were aimed at the determination of B 

field variation -- in coal mine settings -- from known straight line and loop 

driven current sources. The measurements were performed at three locations in 

each of three mines. The individual locations were selected to provide a wide 

variation in local conduction density and degree of wetness. 
2.0 MINE MEASUREMENT DESCRIPTIONS 

2.1 THE MINES AND THE, MEASUREMENT LOCATIOhS 

The measurements were performed in two mines in New Raton, New Mexico and 

a mine in Ashland, PA. In the data presented the measurement locations are 

given by a number 1 to 9. 

The York Canyon Mines 

Two underground coal mines called respectively "~ain" and "~entral" are 

in York Canyon near Raton, New Mexico. Franklin Research Center personnel 

made electromagnetic measurements in three locations in each of these two 

mines. The mines were temporarily shut down at the time of measurments with 

only maintenance and supervisory personnel present. 

The mines are owned by the Kaiser Steel Co. The cooperation of the 

following people, among others, was greatly appreciated: Mr. Harry Elkin, 

Manager; also Fred Revera, M.A. Bertola, Paul Starkovich and Joe Romero. Help 

and hospitality from Mike Sepich and Larry Stolarczyk of A.R.F. Products 

Incorporated, manufacturers of mine-communications equipment, is also 

acknowledged. 

(1) Location 1 was in York Canyon Main Mine, number nine left intake, in 

front of the entrance to cross-cut thirty-six. This was a dry 

location. Density of metallic conductors present was high. A 

three-inch steel water-pipe ran along one wall along with many heavy 

electric cables. Thin steel plates called "mats", approximately 

fifteen inches wide by fifteen feet long, with their long dimension 

oriented across the tunnel, were bolted to the roof at four-foot 

intervals. The circular antenna loops were hung with the top of the 

loop approximately eighteen inches below the roof. This location 

was about three miles in from the mine portal and close to a 

long-wall continuous-mining apparatus. 
Location 2 was not far from the previous location: York Canyon Main 

Mine, number nine left intake, in crosscut twenty-seven. This was a 

dry location. Density of metallic conductors present was medium. 

There was just one electric cable hung along the side of the 

crosscut, and no pipes. There were steel mats on the roof like 

those described previously. Transverse steel I-beams were used as 

roof supports. The antenna-loop was hung with its top two feet 

below the roof. 

(3) Location 3 was York Canyon Main Mine, number nine left intake, in 

crosscut 26. This was a dry location. Density of metallic 

conductors present was low. There were no cables or pipes running 

along the side of the crosscut. Steel mats were on the roof in an 

irregular pattern. The roof was held up by steel I-beams going from 

one side of the crosscut to the other, supported at the ends by 

sturdy wooden posts. These I-beams were about eight feet apart. 

The loop antenna was hung so its top was about two feet below the 

root, in a spot fourteen feet into the crosscut from the place where 

it opened into the haulageway. Thirty feet from the loop in the 

other direction was a concrete-block wall with a small door leading 

to a beltway. 

(4) Location 4 was York Canyon Central Mine, main intake, first south, 

in the haulageway between crosscuts eleven and twelve. This was a 

dry location. Density of conductors was medium. There were two 

three-inch-diameter high-voltage electric cables running along the 

side of the tunnel; these were the only longitudinal conductors. 

Steel I-beams ran across the roof every eight feet, supported by 

vertical logs at the tunnel sides. Between these I-beams were one 

or two of the "mats" described previously, running across the tunnel 

and bolted to the roof. The antenna-loop was hung with its top 

three feet below a roof mat, in the center of the haulageway. 
(5) Location 5 was York Canyon Central Mine, first south intake, near 

crosscut thirteen. This was a wet location. Density of conductors 

was high. Because this was close to the face there were a great 

many longitudinal conductors, pipes and electric cables running 

along the sides of the tunnel. The roof, about five feet five 

inches high, was steel mats, oriented transversely, bolted to the 

roof, spaced about four feet apart. There were steel I-beams 

running across the roof, supported by vertical logs at the ends. 

These I-beams were about twenty feet apart in this tunnel. The 

floor was covered with puddles. Our radiofrequency apparatus was 

put onto duckboards. The roof dripped water everywhere. Icicles up 

to three feet long hung from the roof; we had to clear a small space 

through the icicles so we could set up the antennas to make 

measurements. The loop antenna was hung with its top approximately 

eighteen inches below the roof, one foot away from a steel I-beam 

which was at roof-level. 

(6) Location 6 was York Canyon Cental Mine, first south intake, in the 

haulageway near crosscut twelve. This was a wet location. Density 

of conductors was medium. The only longitudinal conductors were a 

few electric cables and a telephone wire. There were puddles all 

over the floor. Water dripped from many places on the roof, and 

icicles hung all around. The steel roof mats were like those in 

location 5. 

Pioneer Tunnel 

This is a 70-year-old coal mine in ~ennsylvania's anthracite 

region. The mine has three levels, three hundred vertical feet 

apart. All the tunnels have steel rails on the floor, and many 

places have electricity and compressed-air pipe. The mine was not 

operating, i.e. not producing coal at the time we were there. The 
author would like to express their gratitude to George Staudenmeier, 

manager, and to William Whyne, foreman. 

(7) Location 7 was the so-called Little Buck Gangway. There were steel 

rails underfoot. Two wires ran overhead, along the center of the 

tunnel, attached to insulators nailed to logs or to the roof. Those 

were the only conductors present except for water, which dripped 

slowly from many places on the roof, but did not form puddles on the 

floor. The "Little Buck" vein of anthracite coal was two or three 

feet wide and slanted up at about a forty-five-degree angle. Above 

and below this vein were layers of hard rock. The bottom layer of 

rock had been blasted and excavated to to make the tunnel. It was 

ten feet wide, with a horizontal floor and a smooth, sloping rock 

roof which was nine feet from the floor at its highest point. The 

vein of coal could be seen clearly. It continued up on one side, 

and down on the other side of the tunnel, parallel with and just 

below the tunnel's slanted rock roof. In most places no 

roof-supports of any kind were necessary. Infrequently (about 

fifty-foot intervals) there were a few vertical logs at the walls, 

and horizontal logs across the top, to give support at places where 

a chute led into this gangway from levels further up. The 

transmitting-loops for our measurements were hung with their centers 

about five feet from the floor, in the middle of the tunnel. 

(8) Location 7 was called "West number two Buck Mountain Gangway." 

There were no conductors a t a l l here. The rails and electric wires 

had stopped farther back, and the roof did not drip water. The roof 

was twelve feet off the floor at its highest point, sharply slanted 

over a two-foot-wide coal seam. At the end of the gangway, where we 

were, the coal seam pinched down to one foot wide. In other 

respects this location was like the one described above. The 

transmitting coils were hung with center approximately 

four-and-a-half feet above the floor, in the center of the tunnel. 
(9) Location 8 was called "West Orchard Gangway." The roof dripped much 

water, and there were puddles on the floor. There were rails 

underfoot, and spare rails, pipes, tools and metal cans were 

stacked against the wall. This gangway had logs against the walls 

about every two feet, supporting horizontal logs overhead. 

Evidently the roof here needed more support than the roof in the 

other two gangways where we took data. This gangway was about ten 

feet wide, seven feet high, and seventy feet long. The transmitting 

coils were hung about twelve feet from the face at the end of this 

gangway. The gangway ended abruptly at a face or vertical wall 

where the coal seam, three feet wide here and tilted up at 

forty-five degrees, could be seen clearly in cross-section. 

2.2 APPARATUS 

Measurement of the B field was performed using a loop antenna source at 

frequencies of approximately 0.175, 0.5, 1.0, 5.0 and 10.0 MHz in each mine 

location. Measurements were also made using a rectangular loop 1 by 10 meters 

at approximate frequencies of 0.175, 0.5 and 1.0 MHz. 

All loops were one-half meter in diameter. At the lower frequencies 

(175 KHz, 500 KHz, 1 MHZ) a loop with fifteen turns was used, with appropriate 

matching-circuits which were different for each frequency. At 5 MHz a twoturn loop was used, and at 10 MHz a one-turn loop. Typical total antenna 

currents (number of turns times current) were up to six amperes. 

Current flowing in the antennas was measured by a clip-on current probe, 

Hewlett-Packard type lllOA. Peak-to-peak current, displayed on the 

oscilloscope described below, was entered by hand in a notebook. The transfer 

impedance of this current probe was 1 ohm, so that voltages displayed on the 

oscilloscope were of equal magnitude to currents enclosed by the probe jaws. 
The mobile transmitter which drove the antennas consisted of a low-power 

oscillator (General Radio 1211-B serial number 1215, or Hewlett-Packard 204B 

serial number 416-08150) and a 35-watt RF power amplifier (E.N.I. 440LA serial 

number 141 1. 

The electromagnetic-field sensor consisted of a one-turn pickup loop of 

10 cm diameter, with a 160-ohm load paralleled by the 50 ohm measurement cable 

leading to the oscilloscope. This small size and low "Q" eliminated resonance 

effects over the entire frequency band of interest. The coaxial cable, 

terminated in its characteristic impedance, carried the pickup signal without 

reflections to a small battery-powered oscilloscope (Leader LBO-308S, serial 

number 1080193). Peak-to-Peak voltage, displayed on this oscillscope, was 

entered by hand in a notebook. 

The low-frequency transmitting-antenna, a 15-turn loop, is shown in 

Figure 2-1. Figure 2-2 shows the apparatus for measurement of RF current on 

the single-turn 10-MHz loop. 

The rectangular loop or long wire antenna was one meter high and ten 

meters long, with the lower conductor right on the ground or floor of the 

tunnel. This long-wire running down the center of the tunnel was intended to 

serve as a model or mockup of a trolley wire carrying radiofrequency signals. 

The RF magnetlc field was then measured at points near the center of this 

wire, far enough from the ends that this mockup might be approximated 

theoretically by a wire of infinite length carrying uniform current. 

Naturally this long-wire could only be used at the lower frequencies (0.175, 

0.5, 1 MHz) where the current was approximately uniform over its length. The 

B field measurements were made using the same 10 cm probing loop described 

above. Preliminary measurements were made to insure that the current in this 

loop was approximately uniform. 
Figure 2-2 Measurments of 10-MHz RF Current 

2-7 
2.3 APPUTUS CHEcK OUT 

In order to determine if the source antennas and the measurement 

apparatus were functioning properly we performed all anticipated measurements 

aboveground in conductor free environments. These check out measurements were 

performed on an open field here in Philadelphia outside our laboratory and 

again on the grounds of the Bruceton experimental mine. 

The pickup results were very close to those calculated by free space 

assumptions. 

2.4 MEASUKEMENT PROCLUUKE AT EACH MIhE LOCATION 

A circular loop antenna was hung in the center of the tunnel, at eye 

level, with its axis pointing along the tunnel. Current in the loop was 

driven by the transmitter set to that antenna's tuned frequency. Loop current 

was measured by the current probe. The strength of the radio~requency 

magnetic field was first measured at points along the axis of the loop, and 

then at points in the plane of the loop. Approximately twenty-four 

measurements were made, at distances up to three meters from the center of the 

loop. This was a practical limit because at greater distances the signal 

level was generally too low to measure. 

Then the loop was rotated so its axis pointed across the tunnel. This 

was important because we expected the tunnel-loop coupling to be dependent 

upon the relative orientation of the loop and the tunnel. Another twenty-four 

measurements were then made like those described above. 

Having thus finished with the first frequency, the loop antenna was 

exchanged for another one, tuned to another frequency, and the whole procedure 

described above was followed again. In all, data was taken at five 

frequencies (approximately 0.175, 0.5, 1, 5, 10 MHz) in each mine location. 
After the loop measurement the long-wire antenna was set up. The wire 

was suspended on wooden forms so that it was 1 meter above the flow for its 10 

meters of length. The rectangular loop was driven by the same transmitters 

used for the circular loops and current in the source loop measured with the 

current probe. The magnetic field was measured at a point 5 meters (half way 

down) from the driven end at heights of 0.5 and 1.0 meters at various 

distances from the plane of the rectangular loop. Measurements were carried 

out at approximately 0.175, 0.5 and 1.0 MHz. 

Note that all data were recorded for B field measurements as millivolts 

-- (peak-to-peak) displayed on the oscilloscope. All source antenna currents 

were also recorded as total amps (number of turns times current, peak-to-peak) 

so that no conversion from RMS to peak-to-peak was needed for our calculations. 
3.0 MINE MEASUREMENT RESULTS 

3.1 LOOP MEASUREMENT RESULTS 

The raw loop data is given in Appendix B. The loop is always assumed to 

be in the X,Y plane with the axis of the loop perpendicular to this plane. 

Further, the loop is assumed to be suspended along the Y axis. Thus if the 

axis of the loop is pointed along the drift, Z is measured along the drift 

from the center of the loop. X is then measured to the drift wall from the 

center of the loop. This orientation, with the axis of the loop in the 

direction of the drift axis is referred to in Appendix B as direction 1. 

Direction 2 is associated with the axis of the loop pointing to the drift 

wall. In this case also X and Z are defined as above. Thus, the X,Y,Z 

cordinate system is associated with the loop, not its orientation in respect 

to the tunnel. 

In particular, if X=O for a data point, then the measurement is at a 

point along the axis of loop and if Z=0, the measurement point is in the plane 

ot the loop. Note that both X and Z values can be greater than or less than 

zero. Measurements were made in both directions along the axes. 

In Appendix B the column headed "current" gives the total (number of 

turns times single turn current) current in the source loop, the column headed 

"~ickup" is the peak-to-peak voltage recorded from the oscilloscope connected 

to the 10 cm search coil. 

The order of measurements in Appendix B is the Actual order in which the 

measurments were taken. 

3.2 TKANSMISS ION LINE RESULTS 

Appendix C gives all the raw data for the transmission line 

measurements. Notation is the same as in Section 3.1; note that here, 
however, we have no direction notation since the long line always runs down 

the driit. The coordinate system is a bit different here. With the 

transmission line measurement data were always taken at the midpoint of the 

line with the two dimensions taken as vertical height from the floor (2). The 

Y direction is measured toward the mine wall from the middle of the drift (the 

locatlon of the transmission line). A sketch of the coordinate system is 

given as the first page of Appendix H. 
4.0 ANALYSIS OF MEASUREMENT RESULTS 

4.1 LOOP DATA ANALYSIS 

The raw data listing of Appendix B is difficult to interpret even if 

exactly the same frequencies were used at each location and exactly the same 

loop source currents were obtained. In order to compare the results at 

differing locations we have lumped all frequencies that are close together, 

e.g. all 0.173, 0.174 and 0.175 frequencies can be lumped together for 

clarity. Also, we expect all results of these experiments to be linear in 

relation to source current. Therefore, all source currents can be normalized 

to a single value if the "pickups" are changed by the same factor. Applying 

these rules and normalizing all the data to a 10 ampere source current we have 

plotted the results for all frequencies (1.73, 0.5, 1.0, 5.0 and 10.0 MHZ) for 

both possible directions of the loop axis and for both X=O and Z=0 data. 

These plots are given in Appendix D. Note that some individual curves show a 

sawtooth pattern. This is the result of our plotting both the result at a 

negative coordinate and a positive coodinate at the same positive value of the 

coordinate. Each individual curve on a given graph thus presents all data for 

a single location and direction along the indicated coordinate. 

The curves of Appendix D, then, present a good visual picture of the 

variation from location to location for the same conditions. Our overall 

interest is in determining how far our measurement data deviates from a 

computed free space value. The free space values of induced voltage in a 

10 cm diameter loop at various distances from a 0.5 meter loop along both the 

X and 2 axes can easily be computed for a frequency of 1 MHz. These data are 

shown in Figures 4-1 and 4-2. Note that Figure 4-l(b) expands 4-l(a) at the 1 

meter point where our first data point in the 2 direction was taken. Figure 

4-2 provides the same data for the X direction. Figures 4-3 and 4-4 present 

the same data in tabular form. Thus, we know the calculated data for all the 

data points at 1 MHz. 

The pickup values at all frequencies are assumed to be proportional to 

3 JP , thus all pickups can be scaled to any other frequency. For the 10 MHz 

A' r 

data we can simply divide the measured pickup by 10 to scale it to 1 MHz. 

Appendix E presents all the loop data scaled to 1 MHz and 10 amperes source 

loop current. It also presents the free space calculated value for data and 

the ratio of measured over calculated values. This ratio of measured to 
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PKGP LOOP R= ij ,QStJ: PICKUP VRS QIST+MCE FROW LOOP GEFjTEg: F)jZ= 1, 
Figure 4-3 

COMPUTED PICKUP FOR A 0.5 CM RADIUS LOOP 

SOURCE CURRENT IS A 10 AMPERE LOOP OF 0.25 M 

DISTANCE Z 

(METERS) 

PICKUP 

(VOLTS ) 

Figure 4-4 

COMPUTED PICKUP FOR A 0.5 CM RADIUS LOOP 

SOURCE CURRENT IS A 10 AMPERE LOOP OF 0.25 M 

DISTANCE X 

(METERS ) 

PICKUP 

(VOLTS) 
calculated values is precisely the "correction factort' mentioned in the 

In t roduc t ion. 

Comparison of data from location to location or direction to direction is 

difficult in Appendix E. Figure 4-5 gives a breakdown of the ways the ratio 

of measured to calculated pickup vary with direction. Here, all measurements 

along a given axis, at a given frequency, are lumped together. Note the 

figure suggests that changing the direction of the loop axis has only a small 

effect on the data obtained. 

Figure 4-6 gives much the same data as 4-5 but here direction of the axis 

is ignored. 

Figure 4-7 breaks down the variation in the ratio for each measurement 

distance and direction. Note that high values of the ratio seems to be 

associated most frequently with high values of X or 2. 

Appendix F investigates this association in detail. Here, each ratio 

larger than 2, 3, 4, 5 ... is listed so that distance, location, direction 

can all be evaluated. There were approximately 193 data points taken at each 

location. Study of Appendix F shows that these were 294 points greater than 

2. Of these, 103 occurred at Location 5 -- our most conductor rich location. 

Study of the locations and conditions for values of the ratio greater than 5 

shows that there were 29 such values and, of these, only 3 occur at distances 

less than 2.5 or 3 meters. At these distances (2.5 and 3 meters) we are close 

to our noise level and small changes in field can make large changes in our 

ratio. Also, note that 16 of the 29 values are from location 5 and location 1 

-- our high conductor density locations. 

For ratios equal or greater than 7 all 10 occur close to the wall of the 

drift -- except one occurring in location 5. The values of the ratio greater 

than 8 all show the same pattern. We checked these against the data notebook 

and some show marginal notations showing that they were taken close to 

conductors. 

Figures 4-8 and 4-9 are bar graphs of the occurrence of various values of 

the ratio on differing scales. We see that we had only 39 of 1744 occurrences 

of the ratio larger than 5 -- approximately 1.7% -- and it seems that most of 

these are associated with measurements close to conductors and relatively far 

(2 to 3 meters) from the source loops. 
Figure 4-5 

RATJfj LlF Nni2.!4ALXZEP DATA Tf! CUivlPUTED VALiJE 

FOR THE DATA t\iOfii*iALIZEl: 1B FF;EQUEI!CY APlu I)ktlJL CdKHE;\rl' 

ru 1 I~HZ AXD io,o AMPS 

FREQ (Mt;lZ) PICKUP RATIO B OF 1TEPS SIGMA 

3 A X PI I ill AVG, 

,173 2m4 7 1m4 9 3 3 

,530 5m4 ,I 1 mb 107 9 

1 .OOO 2,7 5 1 3 108 3 

S,OOC, 2,7 6 1.3 8 1 4 

1OmO00 3*7 .4 1A 104 00 

Pi[?'rE THAT SIGI-iA (S) IS CO,rIPUl'ED FI~CJP?: 

S**~=(~/(K-~))*(S~~~$((X-MEAN)**~) 

/dHEREt I\;=# OFITEIPS; X IS PICKUP; AI;D PEAIS IS rHE VEAfJ f)E' TFiL PfCkliVS 

FRFQ (PiHZl PICKUP HATIU ;r !If; 1'I'E;::S SIGMA 

MAX 14 1 N AVG, 

,173 2mC 5 I m2 8 5 3 

0530 4m8 9 1*7 8 8 b 

1 ,000 Ll 4 1 ,b 8 7 7 

5,OOO 4,2 mb 1.4 6 7 .5 

10,000 3,s 6 1 ,4 6 6 5 

KITH Z=0,0 AND DIRECTION= ALU?IG THE DRIFT 

FHEQ (iJrh'Z) PICKIJP RATIO # UF ITEhiS SIGMA 

VAX t4 1 t4 AVG, 

,173 3,2 a 1.7 7 1 .4 

530 7,l 7 2,O 8 7 l,j 

1.0(30 2,8 1.0 1 ,6 8 5 .4 

5 ,Oi)O 8.0 4 1,9 6 3 1,2 

1G,OUO 13.0 4 2,9 92 2.4 

WITH Z=O,U AbL) DIRECTION= ACROSS THE l1HIF.l' 

FREQ(:.lhZI PICKUP PATJO # Ilk' 1 Ik&S - S IGa+A 

"t~i( V 1 id AVG, 

173 3.3 9 1,6 8 9 • 4 

,530 5.8 b la8 100 (4 

1 .GOO 5 3 , 9 1.8 104 .i? 

5,nc)r) 6*1 .5 2,o 7 3 1 1 

10,000 7,5 . S 2.4 2 1.5 
Figure 4-6 

RATIO OF MUHpJAL12ED DAI& TO CIIMPUTED VALGE 

FOR TkiE OATA IJOHI*!ALIZED I?! E'RE()IJE:.ICi[ AtdI) DRIVE CLJHAEI,~' 

TO 1 MMZ AND 13,0 AMPS 

FREQ (PIiZ) PICKUP RATIO # DF ITEhS SICAA 

FAX 1-1 1 ;YAVG 

,173 2,4 l 5 1-3 184 . 3 

.530 5 , 4 l 1 1.6 195 . 9 

1 ,000 4,1 . 4 1-4 195 . 5 

5,000 4.2 6 1 w4 148 (4 

10,001) 3-7 l 4 le4 170 l 5 

FOH ALL DATA 

.I73 3,3 . 5 1.5 344 l 4 

.530 Tei l 1 1-8 382 leG 

1 .OCO 5-3 l 4 lwh 3fl4 l 6 

5,000 8.0 l 4 la6 290 • 9 

10,000 13eO 4 4 2 w 0 344 1.6 

NUTE THkT SISl4.A (Sl IS CilMPUTFU FROr4: 

S**2=( I/ (K-1) )*(SUP( (Xo84EAl'i )**2) 

hHERE: N=tt DFITEIbiS; X IS FICKllP; ANil VEAN IS THk, viEA!q ~JL; TtiE. PlCki-lt'S 
Figure 4-6 (Cant'nued) 

F-C5490 

FREQ (l4HZI PICKUP RATIO # OF 1TEi4S SLGMA 

YAX M 1 N AVG, 

.1'13 1.5 l 7 102 18 • 2 

.530 4.7 le1 1 7 18 100 

1.000 3.1 1.1 1.6 18 6 

5.000 2.3 . 7 1.3 16 . 4 

1(1,000 1.7 6 102 15 • 3 

1 WITH X=3.0 ; Z=2.0 AND DIRECTIUN= ALONG THE DRIFT 1 

FHEQWHZI PICKIIP RATXU R OF XTEbjS SIGMA 

hITH X=O.O ; 2=2.0 AND DIRECTION= ACROSS IHE DRIFT 

FP.F:Q ( MtiZ 1 PICKUP RATIO # Gk' ITEi4S SIGMA 

YAX MI .\ AVSo 

.I73 1.9 .S 1.3 14 4 

.530 3. b 1.1 1.6 15 . H 

1.000 3.5 OR 1.6 15 .I 

5.000 i ,tj l 6 1.3 8 4 

10 ,000 2.4 09 1.4 10 05 
Figure 4-6 (Conti nucdj 

FHEQ (lblHX 1 PICKUP r A'I'IO # uk' ITE;qS SI(;=4k 

14 I IW AvG, 

0173 1 . 9 • 7 1.3 11 • 4 

.530 3.4 • 9 1.7 13 • 8 

1,000 40 1 1.0 1.8 1 I 3 

5,000 2 . 2 . 9 1.5 7 . 5 

10.000 2.3 6 1.5 b . 7 

FPEQ ( FiHZ 1 PICKI1P RATIO # OF ITEXS STGXA 

YAX kl 1 AVC, 

.I73 2.4 1.4 1.8 8 • 4 

0536 5.4 1.1 1.8 17 1m2 

1,OUO 1.7 100 la3 18 • 2 

CJOOOO 2.7 .6 1.6 12 • 7 

100000 3.7 • 7 2.2 12 .9 

kjIT1-i X=Q,O ; 2=3,0 AND DIRECTICri= ACROSS THE DKlF'T 

FREQ (NHZ I PICKUP RATIU # dF 1TEi.15 STt;;.IA 

QAX PIIIU BVG. 

,173 2.0 1 1 1.3 6 .4 

.530 300 1.1 1.7 6 • 7 

10000 2.5 .4 1.6 7 • 8 

5,000 4.2 . 7 2 1 4 1.5 

10oOOO 3.5 101 2.1 3 102 

LClTti X= .S ; Z=U.O APID UIiiECTIOrl= ALOr1(; TtiK UHIk'T 

FHEQ(MI-TZ1 PICKIJP RATIO P UF STEnS SIGW, 

YAX r! 1 ;; AVG 

0173 3.2 8 1.8 17 • t, 

.530 7.1 100 2.1 18 1.5 

1 .OCiO 2.5 1.0 106 1 b • 4 

5.000 2.7 . 9 1.7 I h .4 

100000 3.4 • tl 2.0 2 9 .8 

~ITH X= .5 ; Z,=O,0 AtiD OIdECTIUIt= ACROSS THE DRIF~ 

FRi!,Q (;4llZ 1 PICKUP HAT10 4 ilk* ITEXS SIG'vjA 

M AX i4 I ri num 

0173 2.4 9 104 18 11 

,530 3.b • * 1.6 1 X • d 

1,000 5.3 1.2 2 2 15 101 

5.000 2.5 1 1 1-7 1 b 4 

lO,r300 4 0 1.3 16 . 3 
Figure 4-6 (Continued) 

FREQ(WZI PICKUP RATIO # UF ITEwS SIGiqA 

".AX rk1 I N AVG, 

el73 200 1 • 0 1.5 17 3 

.330 4.6 1.1 lod 18 1.1) 

1.000 2 2 1.0 1.4 1 ;J . 3 

5.ooo l,a . 9 1.4 16 3 

lCiaOOO 2.2 • 4 104 2 0 • 5 

4ITH X=1.O ; Z=0.0 AND D.lAECTIUN= ACROSS THii uH1k-T 

FREQ [MHZI PICKUP FAT10 fl (IF ITEMS SIG?!A 

',I A X M I iu AvG, 

,173 2.4 1.1 1,s 1 S 3 

.530 4.1 1mb 1.6 18 ,8 

1 ,000 4.3 1.2 1.8 1 d . d 

5.000 1.3 5 1.3 15 4 

IOoOOO 2m1 5 1.5 f 6 • 4 

FREQ (FIHZ 1 PTCKUP PATIO $ OF XTEr-IS SIGMA 

'4 AX !4 1 (4 AVG. 

01 13 1.9 101 lob 17 m2 

.S3O 5.0 • 7 1.8 I i3 1.1 

1.000 2.3 1.1 1,s 1 d . 4 

5.009 4 (2 . 4 1.7 13 9 

10mOi)O 3.6 8 200 19 '1 

FHEQ(MHZ) PICKUP HPl'l'IU # UF ITEI-iS S.K(;brA 

YAX !-I 1 R AVGm 

0173 2.7 .3 105 18 4 

.530 4.1 1.1 1.5 l il • 6 

t .ooo 3.8 . 9 1.6 1 a . -1 

5.009 3.5 7 1.8 14 7 

10~000 2.9 101 1.9 15 06 
Figure 4-6 (Continued) 

FREQ (MHz) PICKUP PATIO ff Uf: TTEl*!;; $lG?tA 

4 A X PI 1 12 AVG, 

,173 2,7 1.8 2eo 6 .4 

,530 6.1 1.1 2*3 13 1e0 

le000 2*4 1.3 13 13 3 

5,000 L1 1,4 2.9 b 1 ,7 

lO.030 8-3 2e4 5.2 11 leg 

FRKQ (KHz) PICKUP Fih'ff3.U 4 UF ITEdS SIGi9A 

YAX Id1 K AVG. 

,173 2,6 1e3 13 13 4 

,530 4,8 12 1.9 18 1,1 

1.000 4.3 1 .Q leb 17 7 

5,000 3.5 I,O 2,2 11 9 

10,000 6,2 1.6 3,s 11 l ,i 

FREQ (i4iiZ 1 PICKUP FAT10 i# UF ITEN5 SIG+*iA 

'4 ?'i X i4 1 54 AVG. 

,173 1.3 1-9 1e9 2 

,530 6-3 ieQ 23' 5 2,4 

l,OOO 223 1.4 leg 3 . h 

5,000 8.u 1,4 4.0 '4 3,1 

10,000 13.0 3e3 8.1 7 3 'J 
Figure 4-7 

RATIO OF YOPMALTZED DATA TO C(1MYUTED VALJE 

Frm Tdk: DATA tJC)F,FIALIZED IN k'REQuE1.4Ck' A1.U Diil J< C:IHi?EkT 

TC! I 6iiZ ANv 10,O AMP;; 

FHLC, (i=!ilZ 1 PICKUP F?ATIO # ilF ITLMS SIG;IA 

MAX NI 19; AVG, 

.I73 1,7 .9 1,2 19 2 

,530 4,2 1.1. 1,6 l i3 ij 

1 ,GO0 1,7 , 8 L3 16 , 3 

5,000 1 ,9 1 -2 1,4 lb , 2 

10,000 1,7 o 1-3 2 0 .3 

NOTE THAT SIGVA (S) IS COFIPUTED FHUb:: 

S**2=(l/(tJ-l) )*(SlJi4((X-f4EAN)**2) 

$HERE: rd=e OF1TF;flS; X 1s PIZKUP; AFU tlEAPJ IS THE @WAb! uk' Thh, PICKUPS 

WITH X=O,O ; Z= -5 APJD DZRECTI~~J= ACHUSS T:lE DP1h.T 

FHEQ (!-\HZ) PICKUP RATIU 1 OF XTE:>+IS SLG9A 

VAX 1'4 I ti AVG, 

e1.73 1m8 7 1e2 18 • 3 

,530 4,4 1,O 1-7 1 i3 3 

1,000 "21 1,O l,6 I4 , 7 

5,000 1.7 1,3 1,5 lb , 1 

10,000 2e1 1e1 1,s lb , 3 

WITH X=O,O ; Z=1,0 AN[) DIRECTILlF'= ALONG C:iZ DRIFT 

FRKQ ( YIiZ) FICKIIP RATIO # Gk" ITEMS SIGti?A 

%AX b! 1 !: AVG, 

17.3 1 e6 9 1,'L 19 2 

,530 3,6 l,O leb 18 -7 

1.000 2e3 ,ti 1.3 I rj 4 

5,000 1 ,5 , 6 1 -2 10 2 

10,000 1,8 . 6 12 2~ 3 

FPEQ (MHZ) FICKUF HATLO # Oh- lTEMS SIGYA 

!4 A X ?I 1 pi 4VG, 

,173 1 ,b , 7 1,2 1 t3 , 2 

.53U 4.8 lel 1*7 I 8 • Y 

1.000 3,2 ,7 1,s 18 7 

5,000 2,l 9 1,3 16 .3 

10, (300 1.h .C) 1-2 16 .2 
Figure 4-8 

'1)F VALUES 

- 1.000 

- 2,000 

- 3,000 

- 4,000 

IL 5.000 

- b.000 

- 7,000 

- 8,000 

- 16,OCi 

FkcQ PtE,d'l' +----+----+-m-m+-0--+ 

184 l0.b IXXX 

1282 73,5 IXXXXXXXXKXXXAAAXXK 

157 9,O IXX 

5 2 3.0 IX 

40 2.3 1X 

I0 0,o I 

10 0.6 I 

4 0.2 I 

5 0,3 I w-m- +--am+----+----+----+ 

1'744 C) a A A 

20.0 40,G 60,~ 30.~) 

PRt.,Ct(:ili fAi;F, 
Figure 4-9 F-C5490 

***** tillti GliAPli FOR VtiRIASLb;: PtiiJP ***** 

PZEAiT +----+----+*---+---a+ 

[)*I 1 

1.1 IX 

2e.4 IX 

15,6 XXXXXXXXX 

19.9 IXXXXXXXXXX 

29.7 IXXXXXXXXXXXXXXX 

13.9 IXXXXXXX 

3,5 IXX 

3.U dX 

2.0 IX 

1.8 IX 

1.0 X 

1.3 1X 

0.9 I 

0.5 I 

0.8 I 

0.5 I 

0,5 I 

oe7 1 

0.0 I 

oe3 I 

Oe1. I 

0.2 I 

0.3 I 

0.0 I 

0.1 'I 

0.2 1 

0.1 I Oe1 I 

0.0 I 

Oeb I 

0.1 I 

0.0 1 

0,i) 1 

0.1 1 

om0 1 

0,o I 

0.0 I 

0.0 I 

OeO I 

1 1 

0,~) 1 

0,o I 

0,o 1 

0 1 

0 , 0 i 

Uec, i 

0,U 1 

0,o I del i 

+--a-+*---+--a-+-o-w+ 
Figure 4-10 is the output of a commonly available statistics program 

(STP) that evaluated the basic statistics for the 1744 values of the measured 

to collected ratios. Figure 4-11 is an evaluation of the same data by the 

program that prepared the previously presented breakdowns. We present it here 

to show the agreement. 

4.2 TRANSMISSION LINE RESULTS 

We have processed the transmission line data in much the same manner as 

the loop data. In order to compare data at different mines the nearby 

irequencies are first grouped, e.g. 0.175, 0.74, 0.72 MHz, all go to 0.170 

NHz, then all pickup data is normalized to a line current of 10 amperes. The 

data are all also normalized to a pickup for 1.0 MHz. The resulting data is 

plotted in Appendix G. Note that plots are presented for both Z = 0.5 and Z = 

1.0 meters. This allows a comparison of the spread in the data for both mine 

location and 2 distance. 

We next computed the free space pickup from a 10 amp., 1 MHz current. 

This computation was performed by a specialized computer program. The results 

are given in Figure 4-12. The computer program only computes the magnitude 

and direction of the H field. Pick up for the 10 cm diameter search loop was 

computed irom: 

Pickup (volts) = A k,ci/H/ /' 

where A = area of the pickup coil (radius 0.05 m) 

LW'= 2 irfrequency = 217x 106 for f = lo6 

/ = 4 ?x h/m 

/H/ = magnitude of the H field linking the search coil 

Appendix G gives the computer program output. 

Using the calculated pickup and the current normalized data we can 

compare the measured results to the calculated free space values. The 

comparison is given for each measurement in Appendix I. Figure 4-13 breaks 

down the data by location and measurement point. Figure 4-14 gives the basic 
Figure 4-10 

I.~IORP?ALIZED PICKUP FilR ALL MINES 

MERE PKE I VARIABLES AND 1744 OBSERVATIONS 

'.J A f3 . MEANS STD. DEV. VARIANCE 

PKUF 1.69G101 1 .C106G5 1 ,021 444 

'JAR. MEDIAN H0DE MAXIMUM t4INIMUM 

PKUP 1.4r31;OOiS 1.400030 13 COOOC1 C.IOCOOOGE+OC 

'4 A R . STD ERR OF MEAN SKEWNESS COEF. OF VAR. 

PKUF 0.242C104E-01 3.968334 59.58756 

Figure 4-11 

RATLO OF ;Gr'lhbiPl,IZf.:D L,:3t3PDr\ I'A l'rj C[.IPPUTI*:U VALlJk 

FOP THE IIATA !JrlF;i'~AI-,IZEL, IN FRE!JIJEP.JCY AtJD LV F. CIJFPE:iJ' 

T~I i r-=uz ti^) 13.0 A*+IPS 
Figure 4-12 

Free Space H Field and Pickup From Transmission Line 

Wire Configuration -- 10 Amps at 1 MHz 

Z 

Meters 

Y 

Meters Pickup 

(mv) 

Figure 4-14 

Transmission Line Data Basic Statistics 
statistics for all the transmission line measured/calculated ratios and 

Figures 4-15 and 4-16 are bar graphs of the distribution. 

Note that there are only 4 of the 128 ratios more than 3 (about 3.2%) and 

of these, 2 occur at location 7 and 2 at location 8. Study of the notebook 

entries -- and Appendix C -- shows that all three values greater than 4 are 

associated with a measurement at 170 or 500 KHz at a point 0.5 meters from the 

plane of the rectangular loop and 1 meter from the floor. This measurement 

point is the one nearest to the driven wires of the loop. The field varies 

more rapidly at this point than at any other mesurement point. Errors in 

pickup measurement due to pickup loop location uncertainty would be expected 

to be maximum at this point. 
Figure 4-15 

Bargraph of Measured/Cal cul ated 

Ratio Distri bution-Course Division 
Figure 4-16 
5. CONCLUSIONS FROM MEASUREMENTS 

Study of our data indicates that, for frequencies of 10 MHz and below, an 

adequate worst case approximation for the magnitude of the magnetic field, for 

use in blasting safe distances, can be computed from the free space 

predictions of the magnetic field if we use a multiplying factor of 5. 

Only one of our transmission line measurements showed ratios as high as 5 

and of the 1-72 of our loop measurements that were greater than 5, most seem 

to be associated with measurements taken very close to conductors or in -- or 

close to -- our noise level for the measurement. Further those ratios greater 

than 5 that were not associated with measurement near conductors or close to 

the noise level are isolated, i.e. the neighboring values are not greater than 

five. This indicates a "hot spot" of small extent or errors in the 

measurements. For use in the calculation of blasting safe distances, "hot 

spots" of very localized extent will not contribute unduly to the overall 

pickup and they can be safely ignored. 
6.0 SAFE DISTANCES FROM SOURCES 

6.1 CkITICAL VALUES OF THE FIELD 

In order to evaluate safe distances for blasting wire from transmission 

line and loop sources we assume -- as a worst case estimate -- that the 

shorted loop of blasting wiring is arranged normal to the B field produced by 

the source and that it is formed as a circle of area A square meters. This 

assures maximum area for a given lead length. Further, we assume that the /B/ 

linking the loop is the maximum /B/ anywhere in the loop. Thus, if the edge 

of the loop is at a given maximum of the source field and the field is less at 

other portions of the loop, we will use the maximum value of /B/ everywhere in 

the loop. This is a worst case assumption. We further assume that the 

blasting wiring loop is loaded with only one cap and that its resistance is 

Rd c 

and Rdc equals 1 ohm. This, too, is a worst case assumption in that 

more resistance lessens power pickup and the nominal resistance of American 

made caps is 1.2 ohms or greater (~eference 7, p. 4-3). 

The maximum open circuit voltage included in the blasting wiring is then 

where = 2 x 10 6 

'MH~ = frequency in megahertz 

/Bm/ = the maximum B field magnitude linking the loop. Note that 

we will use this notation throughout this report, i.e. /A/ 

is read as "magnitude of the vector (or phasor) A." 

A = area of the blasting cap 

VA = open circuit voltage and 
In terms of the H field (B =LL H) 

./ 

where/( = 4 ;Y"X henrys/meter and 

/H/ = maximum /H/ linking the loop - amps/meter 

The total current /I / that will flow in the blasting wiring loop is B 

where 2 is the total impedance of the blasting wiring loop. This impedance b 

includes K -- the blasting cap resistance -- any reactance of the cap, any dc 

internal drop of the wire forming the loop and the external inductance for the 

loop. The external inductance term is of considerable importance for our 

application. It represents the reduction in current flow due to the reduction 

of the linking B field by the field produced by the current in the loop. 

Reference 8, Chapter 5, gives an excellent outline of these points. 

- Thus, Zb - Rdc + Zi + j~cL I + j X 0 cap 

where 2 is the internal impedance of the blasting wire forming the loop, b 

L is the external inductance of the loop and, 

0 

X is the reactance of the cap. 

cap 

We will assume that the blasting wire that forms the pickup loop is 20 

gauge (B 6 W) copper wire. Caps are usually supplied in 22 gauge for short 

leads (30 feet or less) and 20 gauge for longer wires. 20 gauge wire has a 

diameter of 31.96 mils (0.8118 millimeters) and a dc resistance of 
10.15 ohms/1000 feet. Figure 6-1 is from reference 8, p. 297. It shows the 

variation of the internal impedance terms for round wires in terms of R 

0 

(the dc resistance), r (the radius of the wire) and dg (the skin depth). 0 

Figure 6-2 gives the same data in a more accessible way for the internal 

resistance term. 

Using these figures, we compute that 

For a circle of this wire of area A square meters, 

I 

The X term is of the form .&/L where L is the external 

cap c' C 

inductance of the small rectangular loop (approximately lmm x 1 cm) formed 

where the cap leads enter the base of the cap through the cap plug. The 

external inductance for a circular loop of the same area can be evaluated 

using (see reference 8, page 311) 

-7 where r is the radius of the loop,,,L/= 4 ?x 10 and a is the 

0 C -3 radius of the wire (.81 mm). Since the area of our loop is (10 x lo-') - = 10 m2, and r =fiFT for a circle, we compute 

L = 0.00196 microhenrys C 
Figure 6-1 

Solid wire skin effect quantities 

compared with d-c values. 
Figure 6-2 

can be reduced by sketching in a smooth curve fro;] a point 0.835f 

1.15fbp for each individual curve. (See example cn iha 1 mil curve.) 

For circular wires with pR or pR # I, multiply frequency breakpoints 

by pR IpR ; slope of curve above breakpoint wil I not change. 

EXAMPLE: If pR = 10, pR = 2' d = I, we multiply 60.8 Mc 

(as read from the curve below or as determined by the 

breakpoint formula ) by 1012 to obtain the break 

R DC is DC Resistance = (0.340 p /d2 ) (ohms /cm ) = ( 10.37~~ /d2 )(ohmslf t ) 

R is Actual Resistance d is the diameter of the circular wire in mils I THE FORMULAE ABOVE ARE A BREAKPOINT APPROXIMATION TO THE CURVE ON P248, RAM0 AND WHINNERY, FIELDS AND WAVES IN MODERN RADIO j 

I V 

10-2 10-I lo0 10' lo2 lo3 lo4 lo5 

FREQUENCY (Mc) 

(R/R~~ -0.25) vs. Frequency for a Circular Wire = = 1 
At frequencies as high as 10 MHz and areas as large as 10 square meters, 

all of the terms can have magnitudes comparable to Rdc = 1 so we will retain 

them all in our calculations. 

The no-fire power level (0.1% probability) with 95% confidence) for 

American made caps is 0.04 watts. For a 1 ohm cap this gives a no-fire 

current level of 0.2 amperes. If we substitute this value for current in 

equation 6-3 we form an expression for /H/ that will just produce the no-fire 

current in the cap. Thus 
where here we have written /H / to call attention to the face that this is 

C 

the critical value of /H/ and we have indicated that the magnitude of the 

impedance Z is to be used. Note that Zb is given completely in terms of b 

irequency and area (A) of the blasting cap wiring. We have used a computer 

program to evaluate equation 6-11. Figure 6-3 lists the values of /Hc/ for 

trequencies up to 10 MHz and areas up to 10 square meters. Figure 6-4 shows 

/H /for a finer gradation of frequencies for the 0.1 to 1 MHz range. C 

If we can now calculate the free space values of /H/ due to various 

sources; we can determine safe distances for the blasting wiring 

coniigurations. 

6.2 FREE SPACE VALUES OF THE FIELD FROM VARIOUS SOURCES 

Our main concern is with small loop antennas such as those used in "back 

pack" applications and bandolier configurations. Also we are interested in 

the typical "trolley wire" communication systems. 

For the trolley wire configuration, we consider the geometry of Figure 

6-5. Figures 6-6 through 6-10 give, for a current of 1 ampere, the magnitude 

of the H field along various lines indicated in Figure 6-5. Figure 6-11 

through 6-13 give the same sort of data for the lines indicated in Figure 

6-14. Study oi these plots shows the magnitude of the H field (/HI) around 

the 2 meter separation transmission line varies from about 0.318 half way 

between the driven lines, to about 0.15 at 1 meter away along the y axis. 

Figure 6-15 shows some of the values. For the 0.5 meter radius loop the /H/ 
Figure 6-3 

Frequencies from 0.01 to 10 MHz 

\.; 1.; 1 ,.! t j j c: :>.,45i/lYI 1. 3i?-73ijtc 20. tiq)h4 7t. 

t .i:'3tt~Z3 0 3.3,+SAti 3.')7Sb'2.k ~G.O 1~023 

(8 1 I)!~~:(.II) 1).31351.14 12.291247 14.5nJi3.li 

(1.3 I f?>!:::j l.1 3 9 '1 t* 4 t.> jii .3c1'f01 ci ti>. ~;1(!2d3 

1 -i>(t:ti ,IIJ (1. :3:jf~i 3 1 120 .ii:+ijt~y(.l LL.a. lt:Si>51 

i . 1 t>'2>! 1;; 3 .37?.26;L51 t! l<Jo,:.4i+~,t(,!i 

1 I.! I.! :> t? c*(!l. . 1 J 1 l',Jfi.t)i;~>i)~! 1 :4 ~LI ~1 4 2 1 .i 

'b.! 7 i1 :) ti 11 I.! 

\,I . i> (> 1,) i) fi 

4.1 (j [) ,;!I) (,- 

1 !! 0 lj i' 0 I.! 
Figure 6-4 (Con 't. ) 
Figure 6-4 (Con 't. ) 

Frequencies. from 0.1 to 1 MHz 
Figure 6-4 (ConJt.) 
Trolly Wire Coordinate System For H Field Sampling 
TROLLY WIRE, 2 M SEP., 1 A, I MHZ 

ALONG X= 0.00 Ye 0.00 2- 1.00 TO X= 0.00 Ye 3.00 Z= f.OO 

0.0 0.5 1.0 1.5 2.0 2.5 3.0 

FMHZ= 1.008000 UNITS OF H ARE AMPS/MET€R 

DISTANCE ALONG THE LINE <METERS3 

ALONG X= 0.00 Y* 0.25 Z= 0.00 TO X= 0.00 Y= 0.25 Z= 3.00 

TROLLY WIRE, 1 A, 1 MHz, 2 METERS SVERATION 

0.0 0.5 I .0 1.5 2.0 2.5 3.0 

FMHZ= 1.000000 UNITS OF H ARE AMPSIMETER 

DISTANCE ALONG THE LINE CMETERS) 
ALONG Xu 0.00 Y= 0.00 2- 0.S TO X= 0.00 Y= 0.00 Z= 1.95 

TROLLY WIRE, 2 M SEP . . 1 A, 1 MHZ 
m 

I 

IV 

I-' 

ALONG X= 0.00 Y= 0.25 Z= 0.00 TO X= 0.00 Y= 3.00 Z= 0.08 

TROLLY MIRE, 2 M SEP., 1 A, I MHZ 

FMHZ= 1.000000 UNITS OF H ARE AMPSIMETER 

DISTANCE ALONG THE LINE <METERS> 
ALONG X= 0.00 Y= 0.00 Z= 8.00 TO X= 3.00 Y= 0.00 Z= 3.00 

LOOP, RADIUS- 0.5 METERS, I A, 1 MHZ 
ALONG X= 0.00 Y= 0.00 Z= 0.05 TO X= 3.00 Y= 0.00 Z= 0.05 

LOOP, RADIUS- 0.5 METERS, 1 A, 1 MHZ 

0.0 0.5 1.0 1.5 2.0 2.5 3.0 

FMHZ=: 1.000000 UNITS OF H ARE AMPSIMETER 

DISTANCE ALONG THE LINE (METERS) 
LOOP, RADIUS- 0.5 METERS, I A, 1 MHz 

1 2 3 4 

1.000000 UNITS OF H ARE AMPSIMETER 

DISTANCE ALONG THE LINE <METERS> 
The loop is in the x, y plane, centered at the origin. 
Figure 6-15 

H Values at various points around the transmission line. 

TRANSM 

I 

ISSION LINE WIRES 
LOOP CONDUCTORS 
varies as shown by the points in Figure 6-16. For the moment we can note that 

/H/ = 0.32 for a 1 meter separation from the transmission line current 

carriers and /H/ = 0.1 for a 1 meter separation from the loop source. 

6.3 SAFE DISTANCES FROM TROLLEY PHONE LINE CONDUCTORS 

A. D. ~ittle's Mr. Robert LaGace has supplied us with data on mine 

trolley carrier phones -- see Appendix J -- that indicates that the carrier 

frequencies are no more than 0.190 MHz and the maximum short circuit current 

for the transmitters -- and therefore the maximum current on the trolley line 

-- is approximately 2.6 amperes RMS. 

For a trolley phone operating at the worst-highest-frequency, say 200 

KHz, and running into a shorted line with the short located precisely right -- 

about a half wavelength away (a distance somewhat smaller than 3000 meters or 

about 2 miles) with no loss in the lines, then the maximum current close to 

the end of the line near the short will be 2.6 amperes. The values of /H/ 

given in Figure 6-15 multiplied by 2.6 and again multiplied by our correction 

factor oi 5 (to account for the difference between free space and the mine 

environment) indicate that the maximum H field one meter from either 

transmission conductor line would be 4.16 or less. Indeed, since the return 

would be through the track, the 4.16 a/m figure would be high for a 1 meter 

separation from either rail. 

If we consult Figure 6-4, we see that a blasting wiring loop of area 

0.0316 square meters has a critical H field value, /Hc/, of 5.53 a/meter for 

200 KHz. Thus, blasting wiring loops of less than 0.0316 square meters would 

be safe at .1 meter from any trolley phone conductor. This is the area of a 

square about 17.8 cms (about 8 inches) on a side or a circular loop formed of 

63 cm (about 2 feet) of wire. 

With the "worst case" trolley phone and a loop of this size at a point 

one meter from the trolley phone conductor wire we would still need to orient 
the loop correctly to recieve maximum power. At that orientation we assume 

that the "no fire" current -- the current that will function the cap with 0.1% 

probability at 95% confidence, i.e. 1 in 1000, 95% of the time, -- will flow 

in the cap. Our analysis has "worst cased" several other factors however; we 

assume that the maximum /B/ field can be normal to our loop over its total 

area of the blasting wiring; we assume that the cap lead wires are only as 

long as the loop periphery -- they are in general longer and will tend to 

reduce the current due to inductance and resistance --; and most important, we 

assume that our correction factor of 5 applies at all times and all places in 

the mines. 

Each of these "worst case" factors, however, cannot be shown to be 

improbable. At some time they may all occur naturally together. So our 

overall result is that blasting cap wiring with areas of greater than 0.0316 

sq. meters -- a square about 8 inches on a side -- not be brought closer than 

1 meter to anv trollev hone wire or track return. 

Lest we convey the idea that our assumptions are so "worst case" as to be 

restrictive of cap use, consider a cap with six foot leg wires and the short 

still on the wires. If the leg wires were pulled apart in the middle forming 

an approximate 1 sq. meter loop and this loop were placed coplaner (plane of 

the loop in the plane of the trolley phone conductors) with the conductors of 

a normally operating trolley phone at 200 KHz with about 1.3 amps in the 

conductors, we can see from Figure 6-8 that the H field would be more than 0.3 

x 1.32 = 0.4 amplmeter over the loop assuming no correction factor of 5. 

Figure 6-4 shows the /H / for this area to be about 0.85 aim. So we would 

C 

be about a factor or two from the "no fire" level even if we were in free 

space. It is also wise to consider that the spread in currents for blasting 

caps from the "no fire" level to the "all fire" level -- 99.9% probability 

with 952 confidence -- is about 1.2 to 1.4, e.g. 1.25 times the "no fire" 

current level for at least one American-made cap gives the "all fire." So the 

total safety factor 
separating us from a "sure fire" would be about 2 x 1.2 = 2.4. Study of the 

data taken in our mine measurements shows that, under some conditions, 

"correction factors" of much greater than 2.4 has been measured. This would 

convince me to leave the area quickly. 

6.4 SAFE DISTANCES FROM LOOP SOURCES 

Study of Figures 6-11 and 6-13 shows that the magnetic field of the loop 

along the loop's axis is larger -- at a given distance from the center of the 

loop -- than the magnetic field in any other direction. The field variation 

along the axis has a particularly simple form. 

where H is the H field on the axis, a 

Z is distance (meters) from the center of the loop, 

R is the loop radius in meters, and 

I is the loop current in amperes. 

For 2)) R 

where A is the area of the loop. -4 

If we define the magnetic moment of the loop as 

where M is the magnetic moment , m --- 

then for 2) R 
This definition allows us to treat small loop antenna sources without 

regard to the geometrical differences and actual number of turns. Note that 

equation 6-14 is valid only for Z >) R . How much bigger is "much, much" 

bigger. Figure 6-17 plots H field variation for a R_c~. = 0.5 meter loop along 

the Z axis. On this "log log" plot the slope of -3 at distances such that 

Z ) 2R is readily apparent. We therefore write, with only very small error 

To determine safe distances from loops we need only compare 5.0 times 

H in (6-15) to the /H / values of Figures 6-3 and 6-4 for various values 

z C 

of blasting wiring loops. 

Mr. Larry Stolarczyk of the ARF Company of Raton, New ~exico, has 

informed us, by private communication, that the "back pack" and vehicular 

communication systems, that they manufacture for underground use, operate at 

frequencies of less than 400 KHz and that the magnetic moments of the antennas 

are 1.5 ampere turns meter2 for the "back pack" system and 11.09 (6.3 x 11 x 

0.16) for the vehicular system. Table 6-1 shows the results of substituting 

these values for M in 

m 

f 

and solving for Z. The values of H (A)/are taken from Figure 6-4. Note 

d~,,~ : y.o~/rs 

that we have included the "correction factor" of 5 in (6-16) 

Transmitting systems of this type are usually tuned for peak current and 

any malfunction ok the system results in less current flowing in the antenna 
ALONG Xu 0.00 Y= 0.00 Z= 0.00 TO X3 0.00 Y= 0.00 Z~20.00 

LOOP, RADIUSa0.5 METERS, 1 A, t MHZ 

FMHZ= 1.000000 UNITS OF H ARE AMPSIMETER 

DISTANCE ALONG THE LINE CMETERS) 
Table 6-1. Values for Safe Distance (2) for AJAF Transmitter Antennas 

I3 las t ing Wire 

Area (sq. m) 

Safe Distance (2) 

(meters) 

/Hc / 

amp /me t er 

Vehicular 

System 

~x=ll .O 88 

"~ack pack" 

System 

%=1.5 
than the design value. We therefore feel that the above distances are valid 

even for equipment malfunction conditions. 

Note that a blasting wiring configuration having a 10 square meter area 

is large indeed. This is the area of a typical blasting face layout -- the 

maximum we would expect to see underground. Table 6-1 shows that we would be 

safe -- even for 10 sq. meters of blasting wiring loop area -- if such 

portable transmitter equipment is always kept about 3 meters from any blasting 

wiring. This is a convenient number in that any electronic or electrical 

equpipment should always be kept five (about 1.55 meters) or ten feet (about 

3.06 meters) from any portion of a blasting wiring layout to eliminate 

potential premature firings due to dropping the equipment on the wiring or due 

to a man carrying (body mounted) equipment falling on or into the wiring and 

having the equipment come in contact with the blasting wiring. 

Safe distances from other underground loop sources such as "bandolier" 

antennas can be calculated by the same methods as used above if the frequency 

exceeds 400 KC and/or the magnetic mount exceeds 11.09 amp-turns-meters 

squared. 

6.5 SUMMARY OF SAFE DISTANCES AND COMMENT 

Section 6.3 recommends that blasting wiring with areas of greater than 

0.0316 square meters -- a square about 8 inches on a side or a circule formed 

irom 2 feet of wire -- not be brought closer than 1 meter to any trolley phone 

wire or track return. Certainly no blasting wiring should approach even this 

close to a "hot" dc trolley wire. 

This recommendation can be implemented most easily by prohibiting any 

blasting operation in a haulage way containing an operating trolley phone. 

The recommendation would allow, however, a shot line from a remote blasting 

wiring configuration to be run into (or along) a haulage way that contains an 
operating trolley phone. Thus firing could be done from the haulage way if 

care is taken to keep the shot line conductors close together so that any 

shorted loop is less than 64 sq. inches in area. 

Section 6.4 recommends that the "back pack" and vehicular antennas 

(operating at less than 400 KHz) made by ARF be kept respectively 5 feet and 

10 feet from blasting wiring of 10 square meters area to preclude RF hazard. 

In essence this recommendation says that as long as these antennas are 

restrained from being within 10 feet of any blasting wiring configuration we 

eliminate the possibility of RF hazard. Since the hazard separation distance 

would be smaller for smaller areas, one might conclude that smaller 

separations are desirable for smaller blasting wiring areas. We do not 

believe so. Some minimum distance should be provided to eliminate accidental 

contact between the blasting wires and the transmitting equipment. 

Note that we have considered here only the shorted blasting wire 

configuration. For frequencies and sources below 10 MHz at the distances 

considered, the entire possibility of RF hazards to blasting caps is 

eliminated by keeping the blasting circuit open. We do not recommend this 

procedure however. Caps are shipped shorted with the leg wires configured so 

as to form a minimum coupling area. Good blasting practice -- dictated by 

consideration of electrostatic hazards and stray currents -- requires that 

blasting circuits be kept shorted. We agree. 

Also note that we have considered only the pin-to-pin firing mode of the 

caps. American-made caps require very high voltages to function in the 

pin-to-pin case mode for the frequency range up to 10 MHz. The open circuit 

voltages produced by the sources considered in the blasting wiring pin-to-case 

loops is extremely small for the separations recommended and thus safety for 

the pin-to-pin mode insures -- for the conditions considered here -- safety 

for the pin-to-case mode. 
7 .0 REFERENCES 

1 . "~nvestigations of the RF Hazards to Electric Blasting caps" - FIRL 

Technical Report F-B2256, R.H. Thompson, P.F. Mohrbach, et a1 -- prepared 

for the Institute of Makers of Explosives, 1965. 

2. "KF Pickup of Antenna Simulating Blasting Wiring Configurations, 

Measurement ~esults" -- FIRL Technical Report F-B2256-2, R.H. Thompson -- 

prepared for the Institute of Makers of Explosives, 1966. 

3. "~easurements of the RF Coupling Between an Antenna Simulating a Blasting 

Wiring Configuration and Nearly Mobile ~ransmitting Antenna," FIRL 

Technical Report F-C1951, R.H. Thompson, K. Fertner -- prepared for the 

Institute of Makers of Explosives, 1967. 

4. he Posible RF Hazard to Blasting Caps Caused by a Lossy Communication 

Systemt' -- FIRL Technical Report F-C2399, R.H. Thompson, P.F. MOhrbach -- 

prepared for Motorola, Inc., 1969. 

5. "RF Pickup of Blasting Face Wiring" -- FIRL Technical Report F-C2942, 

R.H. Thompson -- prepared for Motorola, Incorporated, 1970. 

6. valuation and Determination of Sensitivity and ~lectromagnetic 

Interaction of Commercial Blasting caps" -- FIRL Technical Report 

F-C3102, H0210068, August 1973. 

7. "KF Hazards to Blasting Caps in Coal ~ines" -- FIRL Technical Report 

F-C4075, R.H. Thompson -- prepared for U.S. Bureau of Mines under 

Contract H0252015, October 1976. 

8 . "Fields and Waves in Communication Electronics," Ramo, ~hinnery & Van 

Duzer, 1965, John Wiley & Sons, NY, NY-