AN ACTIVE PROXIMITY WARNING SYSTEM FOR SURFACE AND UNDERGROUND MINING 

APPLICATIONS 

W. H. Schiffbauer 

Ntnl Inst for Occuptnl Sfty and Health

Pittsburgh, PA, USA 

ABSTRACT 

NIOSH has developed an active proximity warning 

system called HASARD (Hazardous Area Signaling 

and Ranging Device) for warning workers as they 

approach known dangerous areas around heavy 

mining equipment and other dangerous work zones. 

HASARD is composed of a transmitter and a 

receiver. The transmitter generates a 60-kHz 

magnetic field using one or more wire loop 

antennas. Each antenna is adjusted to establish a 

magnetic field pattern for each hazardous area. The 

receiver, worn by the worker, is a magnetic field 

strength meter. The received signal is compared 

against preset levels which are calibrated to identify 

levels of danger. The receiver outputs can include 

visual, audible, and vibratory indicators and it can 

also be made to disable machine functions. 

HASARD was field tested on a Joy 12 continuous 

miner, a Komatsu 210 M Haulpak, and on a highwall 

launch vehicle. Slight modifications were made for 

each application. HASARD provided warnings as 

designed and proved to be rugged enough to 

withstand the harshest of production environments. 

INTRODUCTION

Researchers at the Pittsburgh Research 

Laboratory (PRL) of the National Institute for 

Occupational Safety and Health (NIOSH) have been 

investigating proximity warning systems 

[Schiffbauer, 1999] that can be used to warn and 

protect persons working close to heavy machinery in 

surface and underground mining operations. Their 

efforts are based on statistical data which has 

identified the need for the investigation. The Mine 

Safety and Health Administration (MSHA) collects 

data for surface and underground mining operations. 

The MSHA database shows that there has been a 

high number of surface and underground personnel 

that have been killed or permanently disabled as a 

result of working near machinery and powered 

haulage. In surface mining operations, an average 

of 13 mine workers are killed each year by being run 

over or pinned by mobile mining equipment. In 

underground mining, twenty-four fatalities were 

associated with continuous mining machines during 

the six-year period between 1992 to 1997. In most 

cases the miners are well aware of the dangers, but 

at times they become preoccupied with operating 

their equipment, and they fail to notice when they or 

their co-workers stray into or are exposed to 

potentially hazardous situations. Part of the problem 

is that the workers attention can be divided among 

many tasks such as maintaining production goals, 

watching out for their own safety or that of their coworkers, processing visual, touch, and auditory 

information. The operator may find himself in a rush 

or mentally overloaded making him less able to 

process the information and to make safe decisions 

[Cornelius, et al, 1998]. When these situations 

occur, the machine operators and their co-workers 

can be injured or killed. 

HASARD DESCRIPTION 

NIOSH developed the HASARD system to reduce 

the risks of working near heavy mining equipment. 

HASARD is an active proximity warning system. 

The active feature is a very important part of the 

system and is somewhat unique since most 

proximity warning systems [Jurgen, 1998] are of the 

passive type. Passive types of proximity warning 

systems are triggered by all objects they detect 

within their range which is not always desirable. 

This is especially true in underground continuous 

miner (CM) coal operations. There, operators and 

helpers work in proximity to CM’s as well as the 

walls (ribs) of the mine. A passive system would be 

triggered every time it detected a wall or a person. 

In surface mining operations large haul trucks 

frequently run over large pieces of earth, rock, and 

other debris. If the system triggered with every large 

piece of material it detected, it would become a 
 

nuisance to the driver of the truck, causing him/her 

to tend to ignore the system. HASARD, being an 

active type system can minimize or eliminate 

nuisance alarms. 

HASARD requires a transmitter on one object and 

a receiver on another object. In this way, objects to 

be avoided are positively identified and can be 

avoided, and objects that don’t matter can be 

ignored. In the underground mining example with a 

transmitter on the CM and the receiver on the 

person, the system could keep the person from 

being crushed, but would let the operator use the rib 

to steer. With a receiver on a large haul truck, and a 

transmitter on a small truck, the large truck operator 

would know that a small truck was in the vicinity, 

which could effectively prevent an accident. A 

transmitter and receiver combination could be put on 

a wide range of objects such as people, edges of 

roads, poles, etc. 

As mentioned above, HASARD consists of two 

parts, (i.e., a transmitter and a receiver). 

Accessories have been added to the system to 

provide remote alarms, data logging, and shut-down 

features. The transmitter consists of a lowfrequency (60 kHz) source with two independently 

adjustable power output channels. The transmitter 

creates a current through a properly tuned loop 

antenna, producing a magnetic field about the loop. 

Figure 1 shows how a magnetic field is produced 

around a conductor. The loop acts like any lowfrequency transformer. The magnetic field 

generated is held constant due to its constant 

current design. By measuring the resultant magnetic 

field with a calibrated 3-axis receiver, the distance 

between the transmitter loop antenna and the 

receiver can be determined. The 3-axis receiver 

compensates for fluctuations in the detected 

magnetic field due to changes in orientation that 

could otherwise be interpreted as changes in 

distance. These properties are the essence of the 

HASARD system. The receiver can be made to 

provide some action when ever a magnetic field 

signal of a certain level is detected. From the 

HASARD perspective, as a person enters a 

dangerous zone some signal or action can be 

triggered. 

Figure 2 shows a top view of the HASARD system 

as applied to a CM. The HASARD transmitter puts a 

signal into a loop of wire (the antenna) that is 

positioned around the periphery of the machine. A 

HASARD receiver is being held by a miner. As 

shown, the magnetic field generated by the loop 

conforms to the shape of the loop. The zone 

indicated is defined by a certain level of magnetic 

signal generated by the transmit loop antenna. The 

receiver contains circuitry for two threshold detectors 

which are calibrated to trip at signal levels which are 

relative to specific distances from the transmit loop 

antenna. These threshold detectors are calibrated 

to identify the zones around the CM. The receiver 

(figure 3) includes a vibrating motor. The motor is 

triggered by the output of one of the threshold 

detectors when a signal of a certain amplitude is 

received, which causes the entire receiver to vibrate 

alerting the wearer. The vibrating motor was chosen 

as the alert indicator, since a buzzer probably would 

not be heard by a miner because of the noisy nature 

of underground mining. A visual indicator was not 

used since the receiver would most likely be worn on 

a belt or in a pocket. Figure 4 shows how the 

HASARD system is typically used with a CM. 

Included in the receiver design is a short-range UHF 

transmitter that can convey the receivers status to a 

remote data collection point. The data conveyed 

includes radio status, as well as safe, caution, and 

risk signals. 

Figure 1. Magnetic fields. 

Figure 2. Top view of HASARD on CM. 
 

Figure 3. HASARD receiver. 

A CM-mounted data receiver and light display 

intercepts the data from the receiver on the worker, 

and provides a remote visual indicator of the workers 

position with regard to zones around the machine. 

The combination is called a Data Link Receiver with 

Machine Mounted Indicator Lights (DLRL) (figure 5). 

The DLRL consists of four explosion-proof 

enclosures with Lexan caps. One enclosure houses 

an antenna and receiver, and the other house a red, 

yellow or green, light bar. The green light bar 

indicates the worker is in a safe area and the radio 

link from the worker to the DLRL is functioning. No 

lights indicate that the radio link has failed. 

Normally, when the green light goes out, a yellow or 

red light is illuminated. The yellow light indicates the 

worker is in an area where his safety will be 

compromised. The red light indicates the worker is 

in peril. 

Figure 4. HASARD installation on CM. 

A second device designed for the system, called a 

Data Link Receiver with Data Logger (DLRD) (figure 

6) also intercepts the data from the receiver on the 

worker. It was primarily designed to act as a 

research tool for our investigations. It provides a 

remote visual indication of the status signals, and it 

archives that data in a data logger along with a time 

stamp. This device was created so that an observer 

could remotely see the worker’s status as he goes 

about his daily routine. The data logger is initiated 

at the beginning of the work shift, and is unloaded at 

the end of the shift. The DLRD provides a 

continuous recording of the workers position relative 

to the zones. A typical scenario for the DLRD’s use 

is shown in figure 7. 

Figure 5. Data link receiver with machine mounted indicator 

lights. 

Figure 6. Data link receiver with data logger. 

Figure 7. Typical installation of data link receiver with data 

logger. 

DESIGN AND INTEGRATION ISSUES 

The HASARD system was originally designed to 

work in an underground coal mine. It was designed 

in compliance with the Title 30 Mineral Resources 

Code of Federal Regulations (30 CFR) Part 18, 
Section 18.82 as required by the Mine Safety and 

Health Administration (MSHA) and the Pennsylvania 

Department of Environmental Protection, Bureau of 

Deep Mine Safety (DEP). HASARD was assigned 

an Experimental Permit by MSHA and DEP. 

HASARD is an intrinsically safe prototype system 

which was designed to be attached and removed 

from machinery as it evolved. HASARD is battery 

operated so that it would not have to be attached to 

the CM’s power system, minimizing the time 

required to install and remove it. Ultimately, 

HASARD would be permanently attached to a CM’s 

power system. The output power of the HASARD 

transmitter was limited to 3 watts at 60 kHz as a 

result of conforming to the MSHA and DEP approval 

criteria. Although the power is very low, an effective 

magnetic envelope can still be produced. 

High frequency magnetic fields can be influenced 

by large metal objects. Since the system is being 

applied to large metal machinery, a lower frequency 

was chosen to minimize interference. The large 

metal machinery can also pose another problem for 

magnetic fields. Usually large machinery is 

electrically driven and can generate a tremendous 

amount of electrical noise. If the electrical noise is in 

the same frequency band as the frequency of the 

magnetic signal, the receiver may not be able to 

distinguish the signal from the noise. To account for 

this possibility, field tests were performed around the 

machines to determine their electrical noise 

characteristics. It was found that most of the noise 

tapers off above 30 kHz. Another potential source of 

interference to HASARD are communication devices 

that operate in the same band of frequencies. One 

communication device identified in mines that 

comes close in frequency is trolley phones, which 

typically operate at 88 kHz. Tests showed that 

HASARD worked well near large metal machines, 

with electrically noisy drive systems and 88 kHz 

trolley phones. 

Designing transmit antennas for HASARD was 

particularly challenging because the antennas had to 

be protected from general abuse such as running 

into walls, roofs, and other mining equipment. The 

antennas would also be exposed to falling coal, 

shale and rock. In order to provide strength and 

protection heavy metal shrouds were specially 

constructed to protect the antennas. Installing the 

antennas on the top of large metal structures 

attenuates the signal, decreasing its range. 

Installing the antennas inside metal shrouds further 

reduces the range of the signals. In spite of those 

constraints, HASARD was capable of producing 

enough of a magnetic envelope to effectively mark 

the dangerous zone. 

The HASARD receiver, although relatively simple, 

is the most complex part of the system. This is 

primarily due to the nature of the transmit antenna. 

The loop antenna magnetic field is predominant in 

one axis. A single receiver would receive the 

maximum field as long as it was oriented in the 

same plane as the transmit antenna. The received 

signal is therefore dependent on the orientation as 

well as the distance from the antenna. To 

compensate, the receiver contains three loop 

antennas, with three separate amplifiers. The 

antennas are positioned orthogonally in order to pick 

up the maximum possible signal for any orientation. 

The received AC signals are converted into DC 

voltages which are then summed together. The 

resultant DC voltage is calibrated to relate to the 

distance the worker is from the machine. A multilevel threshold detector provides a trigger for a builtin vibrating motor and encoder. The vibrating motor 

lets the worker know that he has entered a 

hazardous zone. The encoder is used to convey 

data to an ultra high frequency (UHF) 418 mHz 

transmitter that provides remote acknowledgment of 

the workers position relative to identified zones. The 

loop antennas used in the receiver employ small 

ferrite rods which concentrate the magnetic signal, 

effectively reducing the physical size requirements of 

the loops. This helps to keep the size of the receiver 

practical. 

Although HASARD was primarily designed for use 

at coal mines, its use could be extended to include 

applications in metal/non-metal mines. The 

transmitter signal will be minimally affected by strata. 

But, large metal machinery will affect the transmitter 

signal in the same way as it does in a coal mine. 

Radio frequencies used for communications could 

be an issue but typically metal/non-metal mines use 

leaky feeder systems that employ frequencies of 150 

mHz or 450 mHz. These frequencies are far 

removed from any of those used by HASARD so 

there should not be any interference issues. 

LABORATORY TESTING 

HASARD was tested at a PRL surface test facility 

and also in the PRL Safety Research Coal Mine 

(SRCM). Most tests centered on a Joy 14 CM 

(Mention of specific products or manufacturers does 

not imply endorsement by the National Institute for 

Occupational Safety and Health) in the surface test 

facility. Some tests were performed on a shuttle car. 

Other tests were performed on a Jeffrey 102 CM in 

the SRCM. The testing characterized how magnetic 

fields were affected by large metal structures. Many 

different antenna configurations and shapes were 

built and evaluated. A simple set of antenna design 

methods were developed that could be easily 

adapted to most machine types. All antennas were 
constructed from MSHA-approved three-conductor, 

No. 14 AWG type, SO cable. The loops consisted 

of one to three or more turns of SO cable. The 

number of turns was dictated by the required size of 

the magnetic field. The magnetic field produced by 

the antenna is generally referred to as the magnetic 

moment (m) [Fink, 1975]. m is the product of the 

current (I), the number of turns in the loop (N) and 

the area (A) of a current carrying loop. There is 

however, 

m = INA (1)

a practical limit on the number of turns of wire in 

the loop. As the number of turns of wire increases, a 

point will be reached where the power begins to 

decrease due to long cable lengths and increased 

inductance which limits the size of the magnetic 

moment. 

Antenna protection schemes also required much 

laboratory testing. An antenna on a CM would be 

exposed to tremendous abuse. The laboratory 

prototype antenna cover consisted of 1/8 inch thick 

“C” channel with a series of holes in it (figure 8). 

The holes allowed the magnetic signal to emanate 

but still provided protection for the cable. 

Experimentation proved that a repeatable and a 

predictable magnetic moment could be developed 

around the machine with the metal cover. 

Figure 8. C channel on CM. 

HASARD UNDERGROUND FIELD TRIALS 

A coal mine near Indiana, PA agreed to provide 

NIOSH the opportunity to test HASARD in a 

production environment. A test plan was approved 

by MSHA and DEP, and each inspected the system 

after its installation to insure that it met all applicable 

regulations. The HASARD tests were primarily 

designed to determine if the system would be able to 

survive an intensive production operation. This mine 

was a good choice for extreme conditions due to the 

type of mining method employed and to the tonsper-man-hour of coal produced (10.97). The mine 

used a Joy 12 CM with a ripper cutting head. This 

particular cutting head had a large and relatively 

smooth surface. The HASARD antenna for the 

cutting head had to be placed on top of this smooth 

surface. The coal seam was 12 feet high and it had 

a 6 to 12 inch shale parting at 4 feet from the floor. 

The mining technique employed involved cutting the 

top 8 feet of coal. Next the cut would be bolted. 

Since the mine didn’t have a preparation plant they 

would go back into the cut and take out only the 

parting. The parting would be loaded separately and 

then used throughout the mine for road bed 

construction. Finally, the bottom 4 feet of coal for 

the cut would be mined. This unique mining method 

was very destructive to any object that would be 

attached to the smooth part of the cutting head 

because the parting would come off in large sheets 

that would tend to shear off any attached object. 

Figure 9. Antenna on cutting head. 

The HASARD transmitter was bolted close to the 

center of the machine. One loop antenna was spot 

welded to the cutting head and along the conveyor 

box down the center of the machine. This antenna 

consisted of one long cable with two multi-turn lobes, 

encased in a protective metal channel, each 

attached to the surface of the cutting head (figure 9). 

The second antenna was spot welded to the 

conveyor. It consisted of two multi-turn lobes, one 

on each side of the conveyor. With this configuration 

the transmitter was able to generate a magnetic 

envelope which extended a number of feet around 

the periphery of the machine. A receiver was 

arbitrarily calibrated so that at two feet from the 

machine a “Caution” signal was triggered, and at 

one foot from the machine a “Risk” signal was 

triggered. The receiver was worn by a miner 

operator for a full production shift. His actions were 

remotely observed both visually and with a DLRD. 

As the miner moved into and out of the different 

zones, the indicators on the DLRL and DLRD 

performed as designed with no major failures. After 

the tests, the transmitter was removed from the 

machine but the antennas were left in place for long 

term exposure to production. These first antennas 

lasted for about one week of production which 
consisted of 10 shifts. The remnants of one lobe of 

the cutting head antenna is shown in figure 10. The 

antennas on the conveyor were ripped off. More 

rugged antennas were designed and installed. The 

new cutting head antenna cover was constructed 

from 1/4 inch thick angle iron. The conveyor 

antenna was protected by 1/8 inch thick “C” channel. 

The cutting head antenna was largely encased in 

the angle iron with some of the cable being attached 

along the body of the machine. The antenna on the 

conveyor was recessed into the “C” channel. Again 

the system was tested and the antennas were left on 

the machine. This set of antennas lasted about one 

month. The conveyor antenna suffered minimal 

damage. One lobe of the cutting head antenna was 

ripped away , but the second lobe remained intact. 

In the final phase of antenna testing, only a new 

cutting head antenna was evaluated since there was 

confidence that 1⁄2 inch “C” channel would protect the 

conveyor antenna. The last cutting head antenna 

cover design consisted of 1⁄2 inch thick angle iron 

(figure 11). A wider weld was used to hold the angle 

in place more firmly. The cutting head antenna was 

installed, tested and left in place for over three 

months. The antenna showed no signs of failure 

(figure 12). 

Figure 10. Remnants of cutting head antenna. 

Figure 11. 1⁄2 inch angle iron antenna. 

Figure 12. Cutting head antenna. 

The HASARD field trials showed the system could 

withstand the abuse of a production environment. 

The environment in which it was tested was probably 

a worst case scenario due to the unique mining 

method employed. 

The signal from the transmitter was degraded each 

time thicker steel was used for the antennas, but the 

signal was able to penetrate all the protective 

coverings and provide a large enough magnetic 

envelope to identify a hazardous zone. The area in 

front of the cutter head was the hardest to cover with 

magnetic signal because the antenna was two feet 

or more from the tip of the picks on the cutter. All 

other areas around the machine perimeter were 

easily covered with the signal. A more powerful 

transmitter and a more finely tuned antenna would 

certainly achieve a more substantial magnetic 

envelope beyond the cutter head. Using a higher 

power transmitter would probably require housing 

the transmitter and antennas in explosion proof 

enclosures in order to meet the MSHA and DEP 

regulations. 

HASARD SURFACE FIELD TRIALS 

The HASARD system was tested for potential use 

in surface mining operations. In one case it was 

evaluated in combination with researchers at the 

Spokane Research Laboratory (SRL) of NIOSH. 

SRL researchers were investigating various sensing 

and warning system technologies [Ruff, 2000] that 

could be used to detect the presence of on obstacle 

in the blind spot of a large haul truck. The obstacle 

to be avoided in the SRL evaluation was a small 

pickup truck. HASARD was compared with other 

types of systems including radar, and Radio 

Frequency Identification Systems (RFID). The SRL 

study was limited to radio frequency devices. For 

purposes of comparison, this test classified 

HASARD as an RFID type of system, although it 

might have been more appropriately identified as a 

magnetic sensing system. The testing centered on a 

50-ton Komatsu 210 M Haulpak at an SRL surface 

test facility. While this truck was much smaller than 
 

most trucks used in surface mining operations, it 

provided an adequate platform for the tests. 

A more powerful version of the HASARD 

transmitter was configured for the SRL tests in order 

to provide more range between the transmitter and 

receiver. The underground version of HASARD was 

severely limited in power because of the stringent 

MSHA and DEP regulations required for electrical 

equipment. The HASARD transmitter was capable 

of developing about three watts of signal in an 

antenna. The surface version of HASARD was 

designed to develop 50 or more watts. The same 

receivers used for underground tests were also used 

in the surface test. 

Figure 13. Roof antenna. 

The system configuration for underground 

applications of HASARD consisted of putting the 

transmitter and antennas on the CM and placing the 

receiver on the person. For the surface tests, the 

transmitter was placed on a small truck, and the 

receiver was placed on the large haul truck. The 

experiment consisted of placing a rectangular loop 

antenna (approximately 49 by 53 inches) on the roof 

of the small truck (figure 13). The antenna was 

composed of seven turns of 14-gauge wire encased 

in PVC pipe. The receiver (figure 14) incorporated a 

simple LED bar graph display that indicated the 

signal strength from 1 to 10, with 10 indicating the 

strongest signal. The receiver was used to map the 

detection area. The results of the tests showed that 

a very uniform detection zone of about 30 feet could 

be provided at each corner of the large haul truck. 

In retrospect, if the transmitter and antenna were 

placed on the haul truck and the receiver was placed 

on the small truck, the range of operation would 

have increased substantially due to the fact that a 

larger antenna generates a bigger magnetic 

envelope. The selected test configuration proved 

that HASARD could be used as an effective warning 

system for surface mining operations. 

The HASARD system was only tested between a 

large haul truck and a small pick up truck. To 

include protection for people working in blind areas 

around the machine, it would make sense to have 

the transmitter and associated antenna installed on 

the large truck and have a receiver on the person. 

The primary advantage for this configuration would 

be increased system range. 

Figure 14. Receiver with LED bar graph. 

Using a higher powered transmitter for HASARD 

raised some safety concerns should it be employed 

in an area where explosives were used or stored. 

No known MSHA regulation could be found to 

determine a safe operating distance from a HAZARD 

transmitter/antenna to an explosive material. The 

Institute of Makers of Explosives [Safety Guide, 

1981] provided recommendations for frequencies 

above 535 kHz, and powers above 4,000 watts. At 

that frequency and power a suggested separation 

distance from an antenna to a blasting cap was 750 

feet. A second known recommendation was 

provided in a mining research contract report 

[Franklin Research Center, 1985]. The report 

suggests that a minimum distance of 10 feet should 

be maintained from antennas and signals of the type 

used in HASARD, from any blasting cap or wiring 

attached to a blasting cap. 

A second surface mining application of HASARD 

was on a highwall mining system. This system 

consisted of a continuous mining machine, a 

multiple-car haulage system, and a large launch 

vehicle (figure 15). The problem addressed by 

HASARD was on the launch vehicle. The vehicle 

included a long and wide conveyor belt that ran 

through the center of the machine and located just 

below worker foot level (figure 16). In most cases 

the conveyor belt area was covered by a stacked 

conveyor section, but there were times when that 

was not so. There was a fear that an operator may 

trip and fall on to the belt, and be transported off the 

machine. To address the potential problem, a 

HASARD transmit loop antenna was placed below 

the belt and along the length of the launch vehicle. 

The loop antenna established a danger zone over 

the whole length of the belt. Workers on the launch 

vehicle were required to wear a receiver. The 

system was designed so that if anyone got close to 
 

or fell on the conveyor belt, a radio remote switch 

would be activated via the workers receiver and the 

belt would be shut off. The system was placed on 

the machine in early 2000 and is still in operation on 

the machine. Plans include adding the system to 

the remainder of the companies launch vehicles. 

Figure 15. Launch vehicle. 

Figure 16. Conveyor belt. 

CONCLUSIONS 

NIOSH has created an innovative safety device 

called HASARD that has the potential to not only 

warn workers around dangerous machine areas but 

can also shut down the machine should it pose a 

danger to the worker. The system has been 

installed in a very harsh production environment. In 

that environment the antenna and its protective 

cover has proven itself to be capable of surviving 

even when exposed to tremendous forces. 

HASARD has been tested and compared to other 

warning systems and has demonstrated its ability to 

provide a uniform and reliable marker in blind spots 

around heavy trucks. HASARD has also 

demonstrated that it can be applied to heavy 

equipment where a uniform marker and machine 

shutdown capability is needed to keep workers out 

of harms way. 

NIOSH has demonstrated that HASARD is a 

rugged and reliable tool that can be adapted to alert 

workers when they approach hazardous work areas 

and can also provide remote machine shutdown 

should the worker be in imminent danger. 

REFERENCES 

Cornelius, K. M., Steiner, L. J., and Turin, F. C., 

1998, “Using coal miners’ experience to identify 

effective operation cues,” Published in the 

Proceedings of the 42 Annual Human Factors and 

Ergonomics Conference, October 1998. 

Fink, D. G., McKenzie, A. A., 1975, “Electronic 

engineers’ handbook,” McGraw-Hill, Inc. ISBN 0-07-

020980-4. 

Franklin Research Center, 1985, “A study of RF 

hazards at low and medium frequencies to blasting 

in underground coal mines,” Bureau of Mines 

contract JO318023, January 1985. 

Jurgen, R. K.,1998, “Object detection, collision 

warning and avoidance systems,” Published by 

Society of Automotive Engineers, Warrendale, PA, 

ISBN 0-7680-0226-5, 300 pp. 

Ruff, T. M., 2000, “Test results of collision warning 

systems for surface mining dump trucks,” NIOSH, 

Report of Investigation, RI 9652, 44 pp. 

Safety Guide for the Prevention of Radio 

Frequency Radiation Hazards in the Use of Blasting 

Caps, 1981, Institute of Makers of Explosives 

publication No. 20, September 1981. 

Schiffbauer, W. H., 1999, “A workplace safety 

device for operators of remote controlled continuous 

mining machines,” Am J. of Ind. Med., Suppl. 1. 

Sept. 1999, pp. 69-71.