GLOSSARY



Short-Circuit Current Rating (SCCR)

The Short Circuit Current Rating (SCCR) refers to the short-circuit current withstand capability of electrical systems and control panels according to North American safety standards. The SCCR value indicates the maximum short-circuit current that electrical equipment can safely withstand under specified conditions.

Relation to UL Requirements

NFPA 79 requires the use of a surge protection device (SPD) under certain conditions, particularly when safety-critical control components could be affected by power surges.

The requirements for such surge protection devices are specified in UL 1449. The surge protection device (SPD) can be installed either directly in the control cabinet or in the upstream electrical installation, provided that the applicable standard requirements are met.

SCCR – Short-Circuit Current Rating for the North American Market

For industrial control panels in the United States, determining and labeling the short-circuit current rating (SCCR) is an important safety requirement. With the publication of the 2005 edition of the National Electrical Code (NEC), corresponding requirements for industrial control panels were introduced in Article 409. In Canada, too, there are requirements for short-circuit current rating that are derived from the applicable standards and regulations there.

The purpose of these regulations is to improve the electrical safety of machines and systems and to reduce hazards caused by short circuits, particularly those resulting from arcing.

The SCCR rating indicates the maximum prospective short-circuit current that a control cabinet can safely withstand under specified conditions. The specified short-circuit current rating must be at least equal to the short-circuit current available at the installation site to ensure that the electrical equipment is suitable for the fault currents expected there.

Relationship Between SCCR and UL 508A

The requirements for short-circuit current withstand capacity of industrial control panels are specified in the National Electrical Code (NEC), Article 409. To ensure these requirements are taken into account in the design and evaluation of industrial control panels, Supplement SB was added to the UL 508A standard.

This appendix describes a standardized procedure for determining the short-circuit current rating (SCCR). In particular, it takes into account the short-circuit current withstand capability of the components used as well as the characteristics of the existing protective devices.

For a control panel certified to UL 508A, the SCCR value is determined and marked in accordance with the applicable requirements. This makes it possible to verify whether the UL-certified control panel is suitable for the short-circuit current available at the installation site. This is an important prerequisite for electrical safety and regulatory approval in the United States.

How is the short-circuit current rating (SCCR) determined?

The operator of a machine or system is responsible for ensuring compliance with applicable occupational safety regulations. This includes ensuring that the electrical equipment used has sufficient short-circuit withstand capacity.

For each relevant industrial control cabinet, a separate SCCR (Short Circuit Current Rating) value must therefore be determined and labeled in accordance with applicable regulations. The short-circuit current rating of the individual control cabinets also plays an important role in the evaluation of the entire machine or system. As a general rule, the component with the lowest short-circuit current rating can limit the SCCR value of the entire system.

In addition to a thorough, case-by-case review and evaluation of the specific control cabinet design, there are two other methods available for determining the SCCR value:

Coordinated Selection – Coordinated Component Selection

In Coordinated Selection, components from a single manufacturer that are designed to work together are used to build an industrial control cabinet. These components are specifically intended for use together and are matched in terms of their electrical characteristics and short-circuit behavior.

Manufacturers provide tested and documented component combinations for this purpose, whose short-circuit current rating (SCCR) has been verified under defined conditions. Product catalogs, selection tables, and specialized design software facilitate the design process and simplify the calculation of the SCCR value.

A major drawback of this method is the limited flexibility in component selection. To be able to use the documented short-circuit ratings, the combinations and operating conditions approved by the manufacturer must be adhered to. If alternative components are used, a reassessment of the short-circuit current withstand capability may be necessary.

SCCR Calculation for Control Panel Construction in Accordance with UL 508A

UL 508A, with its Supplement SB, provides a standardized procedure for determining the short-circuit current rating (SCCR) of industrial control cabinets. This allows manufacturers to determine the SCCR value as early as the design phase, without having to perform a time-consuming and costly individual short-circuit test for each control cabinet.

A key advantage of this process is the high degree of flexibility it offers in control cabinet design. Manufacturers can combine different components to meet their customers’ technical requirements, provided that the applicable UL 508A specifications are followed.

Important: Unlike Coordinated Selection, this method allows the use of components with different short-circuit current ratings. When determining the SCCR value for the entire control cabinet, the component with the lowest short-circuit current rating is always the determining factor. Under certain conditions, however, suitable protective devices and tested component combinations can contribute to a higher rating.

What happens if the short-circuit current rating is lower than the potential fault current?

If the short-circuit current rating test reveals that the tested component does not have the required short-circuit withstand capability, it may still be installed under certain circumstances. To do so, a suitable overcurrent protection device must be installed in the system in question. Such a device interrupts the circuit if a predetermined current level is exceeded for a certain period of time. Here, too, the provisions of Annex SB of UL 508A must be strictly observed.

Short-Circuit Current Rating (SCCR) Labeling

The National Electrical Code (NEC) requires that the short-circuit current rating (SCCR) of industrial control panels be permanently and clearly marked. The corresponding value is typically indicated on the control panel’s nameplate.

The label must be affixed in such a way that the SCCR value can be clearly determined during installation, inspection, and maintenance. For control cabinets compliant with UL 508A, the normative requirements regarding the affixing and design of the nameplate must also be observed.

Important: The requirement to specify short-circuit current rating applies regardless of whether the control cabinet has a UL label. UL certification therefore does not replace the required SCCR marking.

Important to Note: Regional Differences in SCCR Requirements

This article explains the short-circuit current rating (SCCR) and the associated requirements for industrial control cabinets as specified by the National Electrical Code (NEC) in the United States.

The NEC is not a standalone federal law, but rather a nationally recognized set of regulations for the safe installation of electrical systems. Its legal enforceability stems from its incorporation into the regulations of the individual states and the relevant local authorities.

Regional differences may arise in this regard: states, counties, and cities may apply different editions of the NEC or establish additional requirements. The relevant authority—known as the Authority Having Jurisdiction (AHJ)—is generally responsible for the evaluation and approval of electrical installations.

German machinery and plant manufacturers who wish to export their products to the United States should therefore familiarize themselves early on with the regulations in effect at the intended location of use. This applies in particular to the determination, documentation, and labeling of short-circuit current rating (SCCR), as well as any additional requirements for electrical equipment.

Working with an experienced partner like Breimer-Roth can help ensure that regional specifics are taken into account in a timely manner and that control cabinets are designed in accordance with standards for the North American market.


Conclusion


The Short Circuit Current Rating (SCCR) refers to the short-circuit current resistance of industrial control cabinets and specifies the maximum short-circuit current they can safely withstand under specified conditions. In the United States, the National Electrical Code (NEC), Article 409, requires the determination and labeling of the SCCR value for such control cabinets. Similar requirements also apply in Canada under the applicable regulations there.

To standardize the determination of short-circuit current withstand capability, UL 508A was supplemented with Appendix SB (Supplement SB). This allows control cabinet manufacturers to determine the SCCR value based on the components and protective devices used, without having to perform an individual short-circuit test for every control cabinet design.

Another option is what is known as “coordinated selection,” in which combinations of components that have been coordinated and tested are used. This method simplifies planning but limits flexibility in component selection.

In contrast, when building control cabinets in accordance with UL 508A, components from different manufacturers and with different short-circuit current ratings may be combined. The SCCR value of the entire control cabinet is determined in accordance with the specifications in Appendix SB. In general, the component with the lowest short-circuit current rating limits the overall value.

When exporting machinery and equipment to North America, it is also necessary to take into account the applicable regional regulations and requirements of the relevant authorities. An early SCCR assessment supports compliance-based planning and facilitates the subsequent acceptance of the electrical equipment.