Apr 12, 2017 |  ElectricalUtilitySafety


Why the Mining Industry Needs System Grounding


While working around live wires, keeping electricity grounded should be every miner’s number one priority. It stops electricity from seeking a worker’s body as the grounding path. Grounding electrical equipment is required by the Occupational Safety and Health Administration and ASTM International to promote safe work environments while electrical work is done.

Here’s why grounding is important in the mining industry and some different methods of system grounding that keep workers protected.

Grounding in the Mining Industry

Since the 1970s, grounded systems have been implemented across the mining industry to protect workers from shock hazards and lightning. These incidents are associated with the use of powered equipment in extreme environmental conditions.

In the mining industry, protection from electrical faults is a key part of mine safety. In an industry that uses a great deal of electrical power, guarding against fault hazards is necessary to prevent everything from minor accidents to workplace disasters. Approximately 14 percent of all electrical fatalities occur from improper or inadequate grounding.

The most common type of fault is typically a line-to-ground fault. These types of faults make up 98 percent of all failures and are incidents that can increase costs and production time due to damaged material. Even worse, they can lead to employee injuries and fatalities.

Methods of System Grounding

To ensure the safety of mine personnel, metal/nonmetal plants require proper grounding. Power systems can come in several designs, but there are four main types: the ungrounded system, the solidly grounded system, the low-resistance grounded system, and the high-resistance grounded system.

Here, we’ll discuss each grounding system in more detail:

Ungrounded Systems

When a system is ungrounded, no intentional grounding system (with the exception of ground-fault monitoring) is used. Though they can offer a low current flow value for line-to-line ground faults and have no flash hazard for accidental line-to-ground faults, they are prone to insulation failures and shock hazards from transient and steady-state overvoltage conditions. Monitoring phase unbalance is typically used for ungrounded systems. This requires a very good maintenance program to ensure service reliability.

Solidly Grounded System

Solidly grounded systems have no intentional added impedance in the circuit. A major advantage is that over-voltages are controlled because the system is referenced to the ground. In addition to the standard over-current protection provided by a circuit breaker, sensitive protection relays can be used to detect high-impedance ground faults.

Low-Resistance Grounded System

In high-voltage systems with a significant capital investment, or where loss of service could have a major impact, use low-resistance grounding. These systems typically limit the maximum ground-fault current from 100–1000 amps. While over-voltages are still controlled, it is not recommended for low-voltage systems because of the limited ground fault current. They are sized to limit current based on a specific value. It also reduces electric shock hazards in mines where preventing unplanned outages is paramount. As these systems are not suitable for four-wire (3 phase with neutral) loads, they are not used in the commercial market.

High-Resistance Grounding

High-resistance grounding is used in plants and mills where a fault should not normally disrupt the flow of a critical process. By limiting the ground fault current to a lower level through the resistor greater than or equal to the capacitive charging current of the system, it can control over-voltages and limit ground currents based on relay limitations or coordination requirements.

While this simplifies process maintenance, it can reduce the stress in circuits and equipment. Alarm indicators and lights are primary indicators when a fault does occur, allowing the operator to fix the issue or coordinate a shutdown while maintaining operations                

Better Protection Against Faults and Failures

While fuses can protect from phase-to-phase faults, these only make up a small fraction of failures. Only 1.5 percent of faults are phase-to-phase, while 98 percent are ground faults. For miners, the result is a need for additional safety measures, such as protection relays. Improper frame equipment grounding, a potential problem in both metal and nonmetal mines, also usually requires frequent tests to ensure that the conductor is constant in the power system.

Protection relays provide preventative maintenance with their ability to track and continuously calculate the thermal capacity of the motor, including:

• Thermal overload
Motor stall
Phase failure
Phase unbalance
No load
Overload
Earth faults
Motor overhead

Earth ground testers have an important role at mine sites. Local and engineering standards and codes are met with these testers, as well as establishing a reliable connection to the earth. As time passes, corrosive soil with high moisture, salt content, and high temperature create wear and tear on ground rods. This causes the resistance of earth ground values to heighten, becoming a danger. Using these special instruments is essential for the overall maintenance and electrical safety of the site.

Keep Your Personnel Protected

Although miners work closely with electricity, this familiarity should never override safety procedures. Establish grounding systems to keep mine personnel protected from hazardous, even fatal, electrical hazards. Stay safe and stay grounded.

Need more information? Contact WESCO today.


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