What's the difference between a short-circuit current rating (SCCR) and an interrupting rating (IR)? If you're not sure, clearing up the confusion could be as simple as brushing up on your definitions and how they're applied to electrical installations.
Section 110.9 of the 2005 NEC requires that equipment intended to interrupt current at fault levels shall have an interrupting rating (IR) sufficient for the nominal circuit voltage and the current that is available at the line terminals of the equipment. Section 110.10 requires the overcurrent protective devices, the total impedance, the component short-circuit current ratings (SCCR), and other characteristics of the circuit to be protected, to be selected and coordinated so as not to allow extensive damage to the electrical components of the circuit during a fault. Prior to the 1999 edition of the Code, SCCR was known as the “withstand rating.”
SCCRs of components and equipment represent the maximum level of short-circuit current that the component or equipment can withstand, and are used for designing and installing equipment and devices in compliance with 110.10. This rating can be marked on individual components (i.e., motor controllers, circuit breakers, etc.) or assemblies (i.e., industrial control panels, etc). See Sections 409.110(3), 440.4(B), and 670.3(A)(4) of the 2005 NEC. Assembly ratings (i.e., industrial control panels and assemblies, air-conditioning and refrigeration equipment, and industrial machinery) take into account all components contained within the equipment rather than just the main overcurrent protective device.
To assist inspectors with determining the safety of an electrical installation, the SCCR should be posted in a clearly visible location. SCCR markings apply to new installation, additions, renovations, repairs, or relocation of equipment.
Why the change? This new requirement will help eliminate uncertainty previously associated with short-circuit interrupter markings (i.e., on overcurrent devices) that were assumed to protect individual components, when, in fact, they really only indicated the IR of the overcurrent protective device protecting an entire assembly (as may be the case in an industrial control panel). By providing an SCCR for an assembly (i.e., industrial control panel), you can ensure compliance with 110.10 while not exceeding the SCCR of any individual component, thereby preventing damage to components, equipment, and systems.
The SCCR for an individual component or piece of equipment shall be equal to or greater than the available short-circuit current on the system you're placing it in. The idea is to design for, and install, a protective device, such as a fuse or circuit breaker, to clear a fault quickly enough to prevent damage to equipment or systems. System protection is essential for the safety of personnel and the reliability of electrical supply.
The isolation of short circuits and overloads requires the application of protective equipment that will both sense and remove the affected portion from the system. In some types of protective equipment, the sensing device and the interrupting device are completely separate, interconnected only through external control wiring. In other types, the sensing and interrupting functions are combined in the same device. In still other types, the sensing and interrupting devices, although actually separate, are included in the same equipment and mechanically coupled so as to function as a single device.
No matter what type of equipment or system you're working with, the end game should be the same with all. Select and coordinate the overcurrrent protective device(s) to interrupt a fault current condition quickly enough to allow a minimum amount of let-through current. This let-through current should be less than the SCCR of all components being protected (including the lowest short-circuit current rated component). Take into account the short-circuit current carrying rating of equipment being protected, type of overcurrent device, and clearing time of overcurrent device.
Owen is the owner and president of National Code Seminars in Pelham, Ala.
Sidebar: Know Thy Definitions
Short circuit (power switchgear) — An abnormal connection (including an arc) of relatively low impedance, whether made accidentally or intentionally, between two points of different potential. Note: The term fault or short-circuit fault is used to describe a short circuit. (Source: “IEEE Standard Dictionary of Electrical and Electronic Terms, 4th Edition”)
Short-circuit current — An overcurrent usually defined as being in excess of 10 times the normal continuous rating. (Source: “IEEE Standard Dictionary of Electrical and Electronic Terms, 4th Edition”)
Short-circuit current rating — The maximum short circuit current a component, assembly, or equipment can safely withstand when protected by a specific overcurrent protective device, or for a specified time interval. Short-circuit current rating pertains to protection of components, multiple component assemblies, or entire control panels. (Source: Cooper Bussmann, “2005 National Electrical Code Changes Affecting Overcurrent Protection” PowerPoint presentation)
Short-time current rating — The short-time current rating specifies the maximum capability of a circuit breaker to withstand the effects of short-circuit current flow for a stated period, typically 30 cycles or less, without opening. This provides time for downstream protective devices closer to the fault to operate and isolate the circuit. The short-time current rating of a low-voltage power circuit breaker without instantaneous trip characteristics is equal to the breaker's short-circuit interrupting rating. Most molded-case circuit breakers are not provided with a short-time current rating; however, some higher ampere rated molded-case circuit breakers are provided with a short-time current rating in addition to the short-circuit interrupting rating. Circuit breakers with an instantaneous trip function should not be used where continuity of service requires only long-time and short-time delay functions. (Source: “ANSI/EEE Standard 242-1986, Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems,” the Buff Book)
Interrupting rating — The highest current at rated voltage that a device is intended to interrupt under standard test conditions. (Source: “NFPA 70: National Electrical Code — 2005 Edition;” Art. 100 Definitions)
Overcurrent — Any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short circuit, or ground fault. (Source: “NFPA 70: National Electrical Code — 2005 Edition;” Art. 100 Definitions)