How to Calculate the Short Circuit Current Rating of a Control Panel

How to Calculate the Short Circuit Current Rating of a Control Panel

Failing to calculate the short circuit current rating (SCCR) of a control panel could lead to catastrophe involving damage and injury if the control panel is connected to an electrical system with an available fault current higher than the SCCR of the panel. For that reason, SCCR marking on the nameplate of a control panel is required by the National Electrical Code 409.110.

There are two ways to find the SCCR of a control panel. The first method is to test the entire panel by building multiple panels and testing them by repeatedly exposing them to higher fault currents until you get a failure. This process is very costly and time consuming. Therefore, these tests are usually only done on individual components and common combinations of components rather than complete panels.

The second method is by calculation. NEC 409.110 allows for calculation of SCCR using an approved method. The most common method in the United State is UL 508A supplement SB.

UL 508A Calculation Method

Step 1: Determine the short circuit current rating all individual power circuit components by using one of the following methods:

1. Use the short circuit current rating marked on the component or on the specifications provided with the component.
2. For unmarked components, use the assumed short circuit current rating from Table SB4.1 of Supplement SB.
3. Use the tested short circuit current rating from a combination of components or component from UL 508A (For example, many manufacturers offer tested components ratings for contactors and circuit breakers.)

Step 2: Calculate the short circuit current rating of each branch circuit in the panel.  The SCCR of a branch circuit is equal to the smallest SCCR of its individual components.

Using current limiting components (fuses, circuit breakers, transformers) in the feeder circuit can increase the SCCR of the branch circuit. These components must be specified and tested as current limiting by the manufacturer.  All current limiting devices have a let-through current rating.  This is the amount of current that devices downstream of the current limiting device will be exposed to in the case of a short circuit.  If all the components on the load side of the current limiting device have higher SCCR ratings than the let-through current of the current-limiting device, the SCCR of the current limiting device can be used as the SCCR of the branch circuit.

Step 3: Determine the overall panel short circuit current rating according to SB4.4. The panel SCCR is the smallest branch circuit SCCR.

By calculating the short circuit current rating of a control panel, operators can avert potential disaster and comply with regulations. Matrix Technologies can assist you in ensuring control panels meet the standards.

Matrix Technologies is one of the largest independent process design, industrial automation engineering, and manufacturing operations management companies in North America. To learn more about our multidiscipline engineering capabilities and manufacturing process control solutions, contact Brian Haury, PE, Discipline Manager.

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Using a Virtual Main to Mitigate Your Arc Flash Hazard

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

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.

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.

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.

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