Short-Circuit Current Ratings: Key Considerations for the Safety of Commercial and Industrial Electrical Systems

The interrupting rating (IR) and short-circuit current rating (SCCR) of equipment is a key consideration for the installation of equipment in electrical distribution systems. To emphasize this consideration, the 2005 National Electrical Code (NEC) added SCCR marking requirements for various types of equipment that previously was not required to be marked. The marked SCCR of equipment is determined by the product standards as part of the listing and labeling process of the equipment or an approved method, such as UL 508a, UL Standard for Safety for Industrial Control Panels. UL 508a supplement SB is an analytical (nontested) method of determining the SCCR of industrial control panels. This method basically determines the "weak link" of all power circuit components in the industrial control panel and the lowest-rated component then determines the assembly rating. Additional changes to the 2011 NEC have been made to ensure that equipment has sufficient SCCRs for the available short-circuit (fault) current where the equipment is installed.