Short-Circuit Current Rating Assignment Options for Industrial Control Panels
The short-circuit current rating (SCCR) requirement was implemented as a safety consideration to prevent potential arc flash that could cause a fire, damage to the panel or property, or personal injury or even death.
Understanding short-circuit current rating (SCCR) assignment options for industrial control panels is the first step in avoiding risk.
Article 409 of the National Electric Code (NEC) is well known to many industrial control panel builders and facility managers. Effective on January 1, 2005, Article 409 requires industrial control panel assemblies to be clearly labeled with a respective short-circuit current rating (SCCR). The SCCR represents the maximum amount of instantaneous current the equipment can withstand without compromising the physical integrity of the panel. This rating must meet or exceed the available “fault current” for the installation.
The SCCR requirement was implemented as a safety consideration to prevent potential arc flash that could cause a fire, damage to the panel or property, or personal injury or even death. It is important to note that the SCCR is not represented by the overcurrent protective device’s interrupt rating. Supplement SB, “Short Circuit Current Ratings for Industrial Control Panels,” has been added to UL 508A to provide one method of addressing Article 409 of the NEC.
All panels that include one or more power circuit components as described in 49.5 of UL 508A require an SCCR, even if they are not otherwise designed for compliance with UL 508A. Excluded from this requirement are industrial panels that consist of only a control circuit. However, if the control circuit includes overcurrent protection that is connected to a power circuit, it does require an SCCR. If the panel includes a power circuit, in addition to control circuit with a separate supply, the control circuit also requires an SCCR.
Responsibility for compliance with SCCR requirements lies with multiple parties. The electrical contractor that will install the panel must work with the respective consulting and facility engineers to determine the appropriate SCCR based upon the fault current of the installation. This should be part of their process in gathering the requirements of the panel. It is important that this information is clearly communicated to the panel manufacturer early in the design stage. The panel manufacturer is required to properly label the panel with the SCCR.
The panel manufacturer is not required to confirm that the SCCR on the panel is sufficient for the installation. In compliance with the NEC, it is the responsibility of the authority having jurisdiction (AHJ) to confirm whether or not the panel’s SCCR is suitable for installation in a facility. In most facilities, the AHJ is the local electrical inspector. In the event of an accident or safety hazard due to an inadequate SCCR requirement on a panel, ultimate responsibility will lie with the electrical contractor who specified and installed the panel. The consulting engineer also will be held responsible as they are relied upon to determine or confirm the appropriate SCCR and fault current.
Planning for SCCR When Designing or Retrofitting
Designing a panel, using the method described under supplement SB of UL 508A to the appropriate SCCR, or determining the rating for an existing panel, can be addressed in three basic steps.
First, research must be conducted to determine the SCCR of each component in the power circuit. This must be determined for the components on the line side of the branch circuit and, as a separate calculation, the components in the feeder circuit. Some exceptions to which components are required to have an SCCR considered are across-the-line components such as power transformers, and current transformers as listed in SB4.2 of UL 508A. If it is available, this can be found by researching specification sheets and instruction manuals, or it may be labeled on the component itself.
Once the SCCR for each component in the power circuit is identified, it must be done for the line side of the branch circuit as well. This can be accomplished by first identifying the lowest-rated component on the load side of the branch circuit’s protective device, then determining the rating of the control circuit overcurrent protective device. The two ratings then are compared, and the lower of the two is assigned to the line side of the branch circuit protection device.
The second step is to see if there are any feeder circuit components that will limit the fault current. This may be a power transformer, current-limiting fuses or UL-listed current-limiting circuit breaker. If there are any of these devices in the feeder circuit, the SCCR can be adjusted as described under SB4.3 of UL 508A.
The final step is to compare the SCCRs calculated by the lowest-rated feeder component, the line side of the branch circuit and the fault current as calculated from step two. The rating for the panel will be the smallest of the calculations.
The most daunting step of the process is locating SCCR values for individual components. Unfortunately, many manufacturers of power controllers, connectors, switches and terminal blocks have not caught up, or perhaps they have found it cost prohibitive to test components to determine the SCCR. UL 508A provides a table under SB4.2.14 that includes an assumed maximum rating for components that do not have an SCCR identified. Many of these components are listed with a minimal SCCR of 5 kA or less. This may result in a maximum SCCR on the panel of 5 kA or less, and this is not sufficient to accommodate the fault current at most installations because most requirements are 35, 65 and 100 kA.
An alternative method in determining an SCCR of a panel is to have the combination of components for the assembly tested by a third-party test laboratory. This method can be cost and time prohibitive as capable test laboratories are limited, and testing of the assembly will likely damage or destroy the panel. Therefore, this generally is not a viable option for customized assemblies manufactured for unique installations. Yet it may be suited for an application where multiple panels with similar designs are being built for installations requiring the same SCCR. There may be a significant return on investment by reducing the cost of panel components by having the ability to utilize components that do not include a predetermined SCCR.
If neither of the aforementioned methods of determining the SCCR can be used, or if an existing panel requires modification to comply with Article 409 of the NEC, the installer will need to take measures to limit the available fault current to the panel. This can be done by installing a transformer in the feeder supply circuit.
Article 409 of the NEC has a noble intention in improving the safety of industrial facilities. As demand increases for SCCRs to be included on power circuit components, suppliers will be expected to respond by providing those ratings. This will make assigning SCCRs to industrial control panels a much simpler task and compliance with the code more easily adoptable.
Note: This article appeared in the June 2011 print edition of Process Heating with the headline, "Be In Control."