Short Circuit Current Ratings

Overcurrent protective device interrupting ratings (IR) and equipment short-circuit current ratings (SCCR) are key considerations for the safety of commercial and industrial electrical systems. Inadequate overcurrent protective device IR or equipment SCCR can create a serious safety hazard. The National Electrical Code (NEC®) and Occupational Safety and Health Administration (OSHA) have requirements around these important ratings and have resulted in changes to equipment designs and specifications.

The NEC® identifies the overcurrent protective device IR and equipment SCCR marking requirements. It also addresses the installation requirements for proper application of overcurrent protective device IR and equipment SCCR.

KNOWING THE DIFFERENCES

There are probably many in the industry that don’t quite understand the difference between interrupting rating and short circuit current rating. As they say, the devil is in the details. It is important to understand these terms as they are critical to the proper use and application of electrical products. Interrupting ratings are commonly associated with overcurrent protective devices such as fuses and circuit breakers, and equipment SCCR is commonly associated with equipment such as devices, appliances, apparatus, and machinery.

The Interrupting Rating is defined in the NEC 2014 Article 100 as, “the highest current at rated voltage that a device is identified to interrupt under standard test conditions.” Therefore, IR simply is the highest current that an overcurrent protective device is rated to safely clear. According to NEC Section 110.9, the IR of the overcurrent protective device must be no less than the current available at the equipment’s line terminals.

The NEC requires the marking of the interrupting rating of fuses per Section 240.60(C) and circuit breakers per Section 240.83(C).

NEC 2014 Article 100 defines SCCR as, “the prospective symmetrical fault current at a nominal voltage to which an apparatus or system is able to be connected without sustaining damage exceeding defined acceptance criteria.” Therefore, SCCR simply is the highest current that equipment is rated to safely withstand.

Section 110.10 of NEC 2014 requires that the equipment SCCR “be selected and coordinated to permit the circuit protective devices to clear a fault, and to do so without extensive damage to the electrical equipment of the circuit.” Notice that Section 110.10 indicates that a specific circuit protective device (fuse or circuit breaker) might be required to provide proper protection.

Section 110.10 also says that the protective device must protect the equipment from extensive damage. This implies that damage can occur to equipment after a fault, but it can’t result in a shock or fire hazard outside of the enclosure.

If a violation of Section 110.9 or Section 110.10 occurs, and the fault current exceeds the IR of the overcurrent protective device or the SCCR of equipment, a catastrophic and violent failure of the overcurrent protective device or equipment can occur.

OSHA 1910.303(b)(4) and 1910.303(b)(5) contain similar language to that found in Section 110.9 and Section 110.10, so both new and existing overcurrent protective devices and equipment must have adequate IR and SCCR.

SCCR MARKING REQUIREMENTS

In the past, equipment such as HVAC, industrial control panels and industrial machinery was considered “utilization equipment” and was not subjected by the NEC to SCCR requirements and the ability to withstand fault currents. It was NEC 2005 that added new requirements for marking equipment SCCR to correlate with the product standards. This version of the NEC also added SCCR marking requirements for motor controllers in NEC 430.8; HVAC equipment in 440.4(B); industrial control panels in 409.110; and industrial machinery