Published in the December 2006 American Longwall Magazine
Presenting at the National Coal Show 2006, Joy’s global certification engineer, regulatory affairs David Thomas said the improved safety benefits of using high voltage continuous miners included a reduced exposure to energized conductors, increased reliability, lighter weight trailing cables, reduced risk of overheating of the motors and increased sensitivity of the ground fault systems.
The Mine Safety and Health Administration also agreed that using high voltage machinery would translate into improved electrical safety.
“We have recognized that high voltage electric equipment and circuit design improvements in combination with sensitive electrical circuit protections reduce the potential for fire, explosion and shock hazards,” the administration said in late 2004.
Also in 2004, the MSHA proposed new requirements for the approval of high voltage CMs operating in face areas of underground mines. The requirements, which are expected to become law by 2010, would allow type acceptance of CMs to avoid the petition for modification process that currently has to be lodged for each high-voltage continuous miner placed into service.
MSHA also proposed new mandatory electrical safety standards for the installation, use, and maintenance of high voltage CMs used in underground coal mines. These provisions would enable mines to use type-accepted high voltage CMs with enhanced safety protection from fire, explosion, and shock hazards.
But in his presentation, Thomas said the proposed requirements did not utilize research that was conducted to address the major issue with proposed regulation concerning cable installation and handling.
He said as they are currently proposed, the regulations hamper operator acceptance and limit the greatest benefits in both safety and productivity.
Joy continuous miner engineering supervisor James Haughey said at one Southern Illinois coal mine, a high voltage Joy 14CM27 operating at 2300V was compared to a Joy 14CM15 operating at 950V and was found to be more efficient, increasing the loading rate by 30%.
Continuing the trend, a southern West Virginia coal mine compared the same two CMs and reported an 8% increase in tons per shift and 6% increase in feet of advance per shift with the high voltage machine.
According to Joy, the improved performance potential is due to the decrease in percent voltage drop for a given current occurring in a trailing cable when higher voltage is induced on that cable. Since motor torque varies with the square of the voltage, any decrease in machine voltage has a drastic effect on motor performance.
High voltage CM machines increase productivity with a minimal increase in machine size. Higher voltages also require less current, resulting in the use of smaller cables, which are easier to handle, and can reduce injuries to miners.
In his portion of the presentation to the National Coal conference, Virginia Polytechnic Institute and State University’s Mining and Minerals Engineering department head Dr. Thomas Novak shared the results of his research into the safety aspect of high voltage trailing cables compared to cables used on medium and low voltage.
Novak said special design, use and maintenance precautions are needed to ensure an equivalent level of safety when high voltage systems are used in permissible areas.
He said in the current mining environment, high power requirements have reached the point where 1000V is inadequate for CM applications where there are excess fault currents, massive cable sizes or reduced motor torque from excessive voltage drop. High voltage cables also feature a double jacket with different colors used to increase visibility and damage detection.
Novak’s presentation concluded that a higher voltage system provides a greater level of protection against electric shock because of its enhanced cable construction, very low ground fault current limit of 0.5A and very low ground fault pickup setting of 0.125A.
