Figure 1: Leaky feeder versus node-based post-accident communications systems.

The 2006 Mine Improvement and New Emergency Response Act of 2006 (MINER Act) required all underground coal mines in the U.S. to have a plan to provide post-accident communication and electronic tracking for any mine workers trapped underground.

In response, post-accident communications and electronic tracking (emergency CT) technologies designed to meet MINER Act requirements have been developed by various manufacturers and approved for permissibility by the Mine Safety and Health Administration (MSHA), meaning they may be used safely in coal mines that may have gassy or dust-laden atmospheres. As a result, more than a dozen different emergency CT systems have become available for U.S. underground coal mining.

The National Institute for Occupational Safety and Health (NIOSH) recently conducted a review of the latest MSHA-approved Emergency Response Plans (ERPs) for each active underground coal mine on file as of February 2014 to identify and characterize the types of emergency CT systems installed. This review pertains only to CT systems installed underground for emergency purposes, and does not include any CT systems installed or used for non-emergency situations, such as communications for everyday operations.

Note that identifying emergency CT systems and their system components can become complicated because some manufacturers use components manufactured by other companies.
In addition, the systems and their various components will often have different MSHA approval numbers associated with them. Accordingly, for simplicity, the overall emergency CT system name listed in each ERP has been utilized for this review.

The key requirement for MINER Act-compliant communications systems is the capability to provide two-way, post-accident communication with the surface for all underground personnel via a wireless medium. This usually involves providing untethered portable communication devices, sometimes referred to as handsets, carried by or available to mine workers as part of the system. Other requirements include battery backup for line-powered equipment, and permissibility for use in conditions where mine ventilation has been compromised.

The MINER Act also requires electronic tracking systems that determine the location of all personnel underground within defined accuracies stated in Program Policy Letter (PPL) No. P11-V-13; for example, to within 200 ft on working sections. Such requirements may necessitate a wireless device for each miner. Requirements for backup power and permissibility are similar to those for communications systems.

Figure 2: Communications and tracking systems by the same or different manufacturers.
Figure 2: Communications and tracking systems by the same or different manufacturers.

POST-ACCIDENT COMMUNICATIONS
As of February 2014, there were 306 active underground coal mines in the U.S., all using either leaky feeder or node-based post-accident communications systems.

Leaky feeder systems have been around for many years, and have been adapted since the MINER Act was passed to serve as post-accident communications systems. They account for 44% of the installed systems (Figure 1). Leaky feeder systems employ a special type of coaxial cable to enable communications by acting as both a transmission line and an antenna for communication with handsets and other portable devices.

Node-based systems are communications networks installed in a mine; each node is a processor-based device that can link to untethered portable devices such as handsets, as well as to other nodes. Node-based systems may be wired or wireless. As the names imply, wired systems primarily use physical connections between nodes for transmission of communication signals, while wireless systems primarily employ radio links between nodes. Although not common before the MINER Act, node-based systems currently account for 56% of the systems installed in U.S mines. Furthermore, a little more than 60% of these node-based communications systems are wireless. These systems usually do not rely on any physical wired connection for communication from underground to the surface. Also, among node-based systems in service are examples that use both wired and wireless links between nodes, as well as some systems capable of operating as a wired network under normal operating conditions and automatically transitioning to wireless operation if the wired connections are compromised.