Mining consists of both extraction and processing. North American mine extraction areas have used resistance grounding successfully for a long time. Historically, the plant/processing area of mines has used solidly-grounded distribution of power identical to those used in other commercial and industrial designs. The simple schematic on the right is extracted from an MSHA information Circular 9258 Mine Power Systems By Lloyd A. Morley.
Similar designs are still prevalent today, with solid grounding on the plant side. There can be an improvement in safety and productivity if resistance grounding is adopted throughout the mine. The high-voltage distribution brought in from utility is typically not high-resistance grounded as the voltages are not compatible with continued operation under fault. They must trip on a ground-fault condition as a single phase-to-ground fault could quickly escalate to a phase-to-phase fault. Some customers have used Low Resistance Grounding (LRG) on the incoming high-voltage distribution. It offers some benefits over solid grounding – primarily lower fault current and, therefore, less damage at the point of fault. There is a risk of the system becoming ungrounded if the resistor or its connections between ground and the transformer neutral were to fail. Bender recently launched the NGRM550 and NGRM750 to mitigate the risk and provide protection for LRG systems.
In many jurisdictions around the globe, extraction areas (both underground and surface operations) are required by law to use high resistance grounding. These systems differ from continuous process areas since they are also required to trip on the occurrence of a ground fault. In some jurisdictions, monitoring the continuity of the grounding resistors is mandatory. Bender NGR monitors for high resistance grounded systems (NGR500, NGR700) have been adopted by many customers. Ground monitors, ground conductor monitors, or ground check relays such as the RC48C are also used to improve safety and help prevent electric shock or electrocution. As you can see by the image below, during a ground fault, there is a potential for current flow through the human.
Tripping on the occurrence of a ground fault limits the probability of an incident. Engineering controls like the protective relays, lower resistance (larger ground wires) to the portable equipment, and restricting ground-fault current with a high resistance grounding resistor (HRG) help control the potential current flow. The probability of human interaction with a fault is minimized because when a fault is detected the contactor or/breaker starts to interrupt the power. It is a common misconception that all HRG systems are alarm only – they are not.