A mine trailing cable is used to power mobile mining equipment, such as draglines, shovels, drills, bucket wheels, conveyors, etc. These cables are usually rated between 5kV and 25kV, and in some cases, can reach as high as 72kV. They carry not only high voltage but also a significant amount of current, providing a tremendous amount of energy measured in megawatts or horsepower. Unfortunately, the immense power they carry can lead to dangerous situations that have resulted in the deaths of many miners over the years.
To counteract this danger, cable design, manufacture, and integrity are crucial considerations. Modern cables feature a shielded sheath of grounded metal around each conductor. Additionally, there may or may not be an outer shield surrounding all the conductors, which would also be grounded. The insulation on the cables has a high safety factor against electrical failure but is not designed to withstand mechanical abuse. In the event of insulation failure, a current blast will be directed from the phase conductor toward the shield, which should trip the protection system.
Protective relays continuously monitor these cables for ground faults or phase-to-phase faults and trip the supplying circuit breakers accordingly. Moreover, a wire known as a “pilot wire” (also referred to as a ground check) is included in the cable. It is monitored in series, and if a break occurs, it will trip the system, assuming that the cable has been damaged or detached.
To reduce the voltage to the ground during an electrical contact, high-impedance grounds are commonly used. Prudent companies install high-speed protection systems due to the inherent dangers associated with these high-voltage systems.
After installation, safety procedures, safe working practices, personal protective equipment (PPE), and training are employed to further minimize the risk. The movement of the cables introduces additional hazards for workers.
Our experience has shown that human contact with these cables elicits two reactions: either a strong emotional response, such as “These cables are dangerous and foolish; I shouldn’t touch them,” or a rational reaction based on the aforementioned safety factors and the necessity of relocating the cables. Incidents involving these cables are rare.
I encountered an incident involving an experienced miner who was shocked by a 5kV cable operating at 4160V phase-to-phase and 2400V toward the ground. The cable was in close proximity to a moving conveyor, and the conveyor belt rubbed against the insulation. Unintentionally, the worker touched the cable near the damaged area and received a severe shock.
Technically, it is unlikely that the belt directly rubbed against the conductor, as that would have created a bridge between the conductor’s insulation and the ground sheath. The most plausible explanation is that there was still a thin layer of insulation surrounding the conductor, allowing a charge to develop. This was the cause of the reduced shock.
Even momentary contact with the skin would result in burns. However, in this case, the worker did not suffer burns, indicating that there was no direct contact. It is possible that an alternate path caused heart failure.
Be prepared, exercise caution, and stay safe until next time.
This post was written by Justin Tidd, Director at https://beckerwmsusa.com/ For over 15 years, Becker Communications has been the industry’s leader in increasingly more sophisticated electrical mining communication systems. As they expanded into surface mining, railroads, and tunneling they added wireless communication systems, handheld radios, tagging, and tracking systems, as well as gas monitoring.
