Using a Virtual Main to Mitigate Your Arc Flash Hazard

What Is an Arc Flash?

In electrical power systems, unwanted electrical discharge between conductors results in what is called an “arc flash”—the rapid release of energy in the air. These arc flashes release heat that is 35,000°F, creating a major risk of injury for those working with these systems. In today’s new electrical power system designs, addressing potential arc flash hazards with the latest technologies has become a common consideration when specifying new equipment.

Unfortunately, the risks of arc flashes were not considered in the design of older electrical distribution systems. Recently, however, the Occupational Safety and Health Administration (OSHA) and the National Fire Protection Association (NFPA) have emphasized electrical safety and arc flash protection. As a result of this new language, companies have been searching for solutions to comply with these standards and improve the safety of their employees.

Updating Older Systems to Reduce Arc Flash Hazards

Arc flash mitigation
Figure 1 – Typical Substation Configuration

In this example, the primary medium voltage fuse provides the protection for the substation, which results in an incident energy result well above 40 cal/cm2—creating the environment for arc flash hazards. This value is high due to (1) the speed at which the medium voltage fuse can clear the fault and (2) the available fault current at the point of calculation.

To reduce arc flash hazards, it is necessary to lower the calculated incident energy. Since incident energy is a result of available fault current and fault clear time, reducing either one of these values would reduce our results. One way to accomplish this is to create a virtual main protection scheme.

With a virtual main scheme, the system is able to sense high levels of fault current at the low voltage side of a transformer and initiate a trip of the primary medium voltage device as quickly as possible. Typically, a replacement medium voltage circuit breaker requires an operation time of 3-5 cycles to achieve this desired fault clearing time.

An engineering study of the arc flash mitigation can help determine the available fault current as well as the maximum fault clearing time required to achieve the desired results.

Depending on the configuration of the existing electrical distribution, there are several ways to perform mitigation:

  • Replace the medium voltage fused switch with a medium voltage circuit breaker.
  • Install a medium voltage circuit breaker downstream of the fused switch and upstream of the transformer.
  • Explore retrofit options for the medium voltage circuit breaker (depending on the make and model).

Using those possible options, the circuit shown in Figure 1 was engineered to implement a concept. In this example, the specifications were to change the medium voltage fused switch to a circuit breaker that has an operating time of three cycles, along with an overcurrent relay and secondary CTs located on the low voltage side of the substation that will monitor the current. Figure 2 below shows what the one-line diagram could look like when the virtual main concept is implemented.

Arc Flash Hazard
Figure 2 – Virtual Main Substation Configuration

The implementation of the virtual main greatly reduces the risk of arc flashes, as demonstrated in Figures 3 and 4, which show fault-clearing capabilities on a Time Current Characteristic (TCC) curve. Figure 3 shows the electrical system before the implementation of the virtual main. This plot shows the arcing fault current in relation to the tripping characteristics of the main fused switch and the trip time (2 seconds) for the protective device to clear the fault. Figure 4 shows the electrical system after the implementation of the virtual main. This graph demonstrates that at the arcing fault current, the new protective device will clear the fault in just 0.05 seconds, resulting in incident energy values under 8 cal/cm2—well below the initial 40 cal/cm2 that created greater arc flash hazards.

Virtual Main
Figure 3 – Substation Prior to Mitigation
Power System
Figure 4 – Substation After Mitigation

Improving Outdated Systems Improves Safety

Outdated electrical systems can pose a number of safety issues, chief among them the risk of arc flashes. Using a virtual main scheme, we can greatly improve the fault-clearing capabilities of the system to reduce the incident energy values. The experts at Matrix can help you implement changes to your existing systems to not only bring the system within OSHA and NFPA standards but also greatly decrease the risk of injuries to your employees.

Matrix Technologies is one of the largest independent process design, power systems engineering, industrial automation engineering, and manufacturing operations management companies in North America. To learn more about our power system design capabilities and our safety services and arc flash analysis, contact Eric Allar, PE, Associate Discipline Manager in the Process & Electrical Design Department.

© Matrix Technologies, Inc.

Tags: / Arc Flash Hazard  / Arc Flash Mitigation  / Electrical Engineering  / Instrumentation Engineering  / Power Engineering  / Power System  / Virtual Main  / Safety  / Protective Relaying 

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