November/December 2021 | Vol. 26 No.6
by Dan Neeser, Senior Field Application Engineer, Eaton
Those who design and construct commercial buildings live by a code. It’s the National Electrical Code®, or NEC.
Many thousands of products help builders and designers stay compliant. Overcurrent circuit protection items, such as modern current-limiting fuses, help enhance safety and reliability and ensure conformity with code requirements.
The main NEC requirements include Interrupting Rating (IR), Short-Circuit Current Rating (SCCR) and Selective Coordination.
Interrupting Rating (IR) is the maximum fault current at a specified voltage that a circuit breaker or fuse can safely interrupt under standard test conditions. An Overcurrent Protection for Equipment and Conductors (OCPD) must interrupt destructive fault current energy safely. A fault current could rupture, causing damage and posing a safety hazard if it exceeds a level beyond OCPD capacity.
An interrupting rating defines the OCPD ability to maintain its integrity when reacting to fault currents. When applying a fuse or circuit breaker, it’s essential to use one that can safely interrupt the largest potential fault currents. Most modern, current-limiting fuses have a 200 kiloampere (kA) or 300 kA interrupting rating. Misapplication is rare; thus builders may use them in nearly any system. NEC 110.9 requires equipment intended to break current at fault levels to have an interrupting rating sufficient for the available fault current at the point of application.
The fuse interrupting rating is not dependent on a particular voltage when applied within its rating. For example, a 600 Vac rated fuse can have a UL Listed 300 kA interrupting rating for any voltage up to 600 Vac. Whether for the initial installation or system updates, a fusible system can maintain a sufficient interrupting rating throughout its life. There is little need for additional fault current calculation, and builders need not worry that an improper interrupting rating will cause a fuse misapplication. A short-circuit study isn’t needed, either, when applying fuses for selective coordination. In this case, meeting NEC 110.9 requirements is simple.
Additionally, high interrupting ratings help equipment achieve a high short-circuit current rating that may be limited by the installed OCPD’s low interrupting rating.
Fuses provide peace of mind as the interrupting rating is always at least equal to or, in many cases, greater than the available fault current at the line terminals.
Equipment Short-Circuit Current Rating
Short-circuit current rating (SCCR) is the ability of the equipment to withstand a certain magnitude of short-circuit current at a specific voltage in conjunction with an OCPD without causing a shock, fire, or projectile hazard external to its enclosure. SCCR may be an attribute of a single electrical device, such as a contactor, or of an electrical apparatus with multiple devices in an enclosure, such as a panelboard, switchboard, motor control center, or industrial control panel.
NEC 110.10 requires equipment to have an SCCR adequate for the available fault current. Compliance with equipment short-circuit current protection (NEC 110.10) is an analysis much different than compliance with overcurrent protective device interrupting ratings (NEC 110.9). NEC 110.10 requires all electrical equipment to have adequate short- circuit current protection. The intent of NEC 110.10 is that the equipment and OCPDs must be “selected and coordinated” so that equipment will not sustain extensive damage under fault conditions up to the available fault current value.
To assure proper application, the designer, installer, and inspector must guarantee that the marked SCCR of a component or equipment is greater than the calculated available fault current. To ensure compliance, it’s necessary to:
- Determine the available fault current at the point of installation of the component or equipment
- Ensure the component or equipment marked SCCR is equal to or greater than the available fault current
Equipment manufacturers may use current-limiting fuses to increase the equipment SCCR by using methods under product Standards such as UL 508A for industrial control panels. Specification and installation of equipment with higher SCCRs, such as 200,000 amps, make it easy to meet the requirements of the NEC. In addition, equipment with the highest ratings can be moved from plant to plant without worrying about unsafe situations that might arise from placing the equipment in a new location where the available fault current is higher than the old location and now above the rating of the equipment.
Selective coordination is critical for electrical distribution system reliability. A dependable system is not only important for life safety. It’s vital from a business perspective as nothing will stop all activity, paralyze production, inconvenience, and disconcert people more than a major power failure.
Selectively coordinated overcurrent protective devices address localizing faulted conditions on the power distribution system and often have a reliability design goal. In addition, the NEC mandates selectively coordinated OCPDs for circuits that supply power to vital loads in specific building system applications (such as multiple elevators, emergency, legally required, and critical operation power systems). A properly selectively coordinated engineered and installed system will allow only the nearest upstream OCPD to open for the full range of overcurrents (both overloads and all fault types), leaving the remainder of the system undisturbed and preserving continual service.
Correctly selected and applied fuses alleviate the design hassle of trying to achieve selectively coordinated OCPDs. Fuses must maintain a minimum 2:1 amp rating ratio for upstream and downstream and be properly chosen to achieve selective coordination. This eliminates the possibility of cascading multiple OCPD levels under fault conditions.
NEMA Members offer solutions that can help builders and designers meet code requirements, such as selective coordination. The manufacturers’ comprehensive overcurrent circuit protection products portfolio provides solutions that enhance safety and reliability and ensure compliance with code requirements.
For proper selection and sizing of an application, contact a NEMA Member: https://www.nema.org/ directory/products
NEMA Members from Eaton, Littelfuse, and Mersen contributed to this report. ei