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 in 670.3(A).
In addition, Sections 409.110(3) and 670.3(A) also contained fine print notes (changed to information notes as part of NEC 2011) that UL 508A, Supplement SB was an approved method for determining equipment SCCR for industrial control panels and industrial machinery.
NEC 2008, Section 409.110 was changed to add an exception that SCCR wasn’t required to be marked on industrial control panels that contain only control components to correlate with the requirements of UL 508A. Therefore, if the industrial control panel contains only control circuit components (components that don’t supply loads such as motors, lighting, heating, appliance, or receptacles), then an SCCR marking is not required.
SCCR INSTALLATION REQUIREMENTS
Changes to the NEC SCCR equipment- marking requirements were designed to help draw attention to the withstand capabilities of equipment and, combined with the existing requirements of NEC 110.10, prevent installation of underrated equipment.
NEC 2011 included additional requirements for industrial control panels and industrial machinery that complement and reinforce the requirements of Section 110.10. Specifically, a new section, 409.22, was added to require that an industrial control panel not be installed where the available fault current exceeds its SCCR as marked in accordance with Section 409.110. Similar wording was added for industrial machinery per a new section in 670.5.
The added text in 409.22 and 670.5 draws attention to the fact that industrial control panels and industrial machinery must be designed and manufactured with an SCCR that is adequate for the installation.
Therefore, if equipment such as an industrial control panel is being installed in an industrial facility, the system designer must communicate the maximum fault current to the equipment supplier so the supplier can design the equipment with SCCR no less than the fault current where installed in accordance with Sections 409.22 and 110.10.
The process would be similar for HVAC equipment in commercial and industrial building facilities, although not specifically stated other than the requirements of Sections 440.4(B) and 110.10.
Another requirement highlighting proper equipment ratings was added in NEC 2011. A new section, 110.24, requires commercial and industrial service equipment to be marked with the maximum available fault current, and to be remarked if modifications of the electrical installation resulted in an increase of the maximum available fault current. The intent of this change was to assure compliance with Sections 110.9 and 110.10 for service equipment.
NEC requirements around IR and SCCR have helped raise awareness of available fault currents and proper equipment SCCR by system designers, installers, and authorities having jurisdiction (AHJs). In many jurisdictions, AHJs have responded to these changes by requiring the available fault current to be documented at equipment and by rejecting equipment if found with an inadequate SCCR. System designers and installers must identify the fault current at equipment and communicate these requirements to equipment manufacturers.
System designers and installers also must realize that once the equipment is installed with inadequate SCCR, there are no easy fixes. The only options are equipment modification and recertification or reduction of fault current (through use of additional conductors, isolation transformers, or reactors). Additionally, equipment might be relocated within a facility or to another facility that has increased fault currents, causing additional concerns for flexibility of application for equipment. This can result in costly delays and increased equipment cost.
As a result, equipment suppliers should be requested to provide equipment with high SCCR to avoid problems in the field.
As always, keep safety at the top of your list and ensure you and those around you live to see another day.
Thomas Domitrovich, P.E., is a National Application Engineer with IEC Platinum Partner Eaton Corporation in Pittsburgh, Pennsylvania. He has more than 20 years of experience as an Electrical Engineer and is a LEED Accredited Professional. Domitrovich is active in various trade organizations on various levels with IEC, International Association of Electrical Inspectors, Institute of Electrical and Electronic Engineers (IEEE), National Electrical Manufacturer’s Association (NEMA), and the National Fire Protection Association (NFPA). He is involved with and chairs various committees for NEMA and IEEE and is an alternate member on NFPA 73. He is very active in the state-by-state adoption process of NFPA 70, working closely with review committees and other key organizations.