The Short Circuit Current Rating (SCCR) is a U.S. term used to describe the testing of the short-circuit withstand capability of electrical systems.
Relation to UL
The UL 1449 standard, in conjunction with NFPA 79, stipulates that a surge protection device (SPD) is required when safety components are installed in the control cabinet. The SPD can be installed either on-site or inside the control cabinet.
The SCCR: Short-Circuit Resistance for the U.S. Market
All control cabinets and other electrical systems intended for use in the United States or Canada must be rated for short-circuit current in accordance with Article 409 of the 2005 National Electrical Code (NEC 409). This rating is referred to as the Short Circuit Current Rating (SCCR).
The goal of the amendment to the law was to improve workplace safety, particularly by preventing short-circuit arcs. The maximum value of the short-circuit current is referred to as short-circuit withstand capacity. This must be at least as high as the highest possible fault current, or higher.
What does the SCCR have to do with UL 508A?
The requirements for short-circuit protection are defined in NEC 409. In response to the amendment to NEC 409, Underwriters Laboratories has updated its UL 508A with Appendix SB. This appendix incorporates the requirements of NEC 409 into the UL standard. This ensures that a UL-certified control cabinet meets all legal requirements, including those for short-circuit resistance.
How does a short-circuit current rating work?
The operator of a system is responsible for complying with occupational safety regulations. This also includes ensuring that its electrical equipment has sufficient short-circuit withstand capability. The law requires that a separate short-circuit current rating be performed for each control cabinet that controls a machine or part of a machine. The results of the SCCRs for the individual control cabinets are, in turn, essential for determining the short-circuit resistance of the entire machine or system (“The weakest link determines the strength”).
In addition to the very time-consuming process of individually analyzing and designing each control cabinet configuration, there are two methods available for determining a control cabinet’s short-circuit withstand capability:
Coordinated Selection
Under Coordinated Selection refers to the design and manufacture of a control cabinet using components exclusively from a single manufacturer. The components are optimized by the manufacturer for use together, which primarily means that they all have the same short-circuit withstand capacity and are backed by documented testing (SCCR). To simplify the design process, the manufacturers in question provide catalogs of the relevant components and design software. The main drawback of this approach is the loss of all flexibility. Only those components designated by the respective manufacturers for use together may be installed.
Control Panel Construction in Accordance with UL 508A
Since UL 508A, as specified in the aforementioned Annex SB, also requires short-circuit withstand testing, a control cabinet designed in accordance with this UL standard also meets the SCCR requirements. A UL-certified control cabinet manufacturer can thus design cabinets flexibly according to its customers’ requirements without having to perform a time-consuming and expensive individual test on each cabinet every time. Important to note: Unlike Coordinated Selection, while components with different levels of short-circuit withstand capability can be installed in a UL-certified control cabinet, the short-circuit withstand capability of the “weakest” component always determines the SCCR of the entire control cabinet.
What happens if the short-circuit withstand capacity is less than the maximum fault current?
If the short-circuit current rating test reveals that the component under test 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 Resistance Rating
The NEC requires that the short-circuit withstand rating be clearly marked on all control cabinets. The law does not specify exactly what “clearly visible” means, which is why the marking is often found inside the control cabinet in an easily visible location. This marking requirement applies regardless of whether a UL label is present.
Important to Note: Regional Differences
This article describes the Short Circuit Current Rating (SCCR) and the associated requirements for control panels under the National Electrical Code (NEC). The NEC is a national occupational safety law in the United States. Unlike in Germany, federal law in the U.S. takes a back seat to state laws regarding occupational safety, which in turn are subordinate to the regulations of the respective counties or cities. German machinery and plant manufacturers who wish to export to the U.S. should therefore be sure to familiarize themselves in advance with regional specifics and variations in short-circuit current resistance (SCCR) testing —or simply partner with an experienced provider such as Krayer.
Zusammenfassung
The SCCR (Short Circuit Current Rating) is the term used to describe the rating of control panels based on their short-circuit current capacity. According to NEC 409, this rating is mandatory for all electrical systems intended for use in North America. In response to the requirements of the 2005 revisions to NEC 409, UL added Appendix SB to the UL 508A standard. If components from a single manufacturer are used exclusively during the design and manufacture of a control panel, these components have the same short-circuit withstand capability, which can be verified through a documented test. However, this method, known as “Coordinated Selection,” results in a loss of flexibility. If, however, the control cabinet is built and certified according to UL 508A, components with different short-circuit withstand capabilities can be installed, since Annex SB specifies a short-circuit rating (SCCR) in any case. Nevertheless, regional specifics should always be taken into account when implementing the SCCR.
