Underground mine environmental impact on RF coupling to electric blasting caps

The 2006 Mine Improvement and New Emergency Response Act (MINER Act) requires all U.S. underground coalmines to install wireless communications and electronic tracking equipment. An RF transmitter (Tx) may induce currents in nearby conductors and hence, may interact with the lead wires of a blasting cap generating sufficient current in the leads to cause an inadvertent firing of the cap. There are standards and guidelines that prescribe a minimum separation distance between transmitters to ensure that such induced currents or powers are below an assumed threshold. Generally these prescriptions account for the possible enhancement of the electric fields at the receiver (Rx) (cap) due to ground bounce. In underground mines, there may be additional reflections off the walls and roof that can further enhance the superposed fields indicating that the minimum separation distances recommended by the standards may not be adequately conservative in the underground mine environment. This paper presents analysis and corroborating measurement results to indicate that the presence of additional reflecting surfaces as in a mine environment can enhance the transmitted electric fields experienced by an Rx beyond the field levels predicted in the standards.