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Overview

Commercial buildings need to comply with the requirements of the National Electrical Code (NEC®). The use of modern current-limiting fuses in commercial buildings can help with compliance in meeting these requirements.  The key NEC requirements include:

  • Interrupting Rating (IR)
  • Short-Circuit Current Rating (SCCR)
  • Selective Coordination

Interrupting rating

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 OCPD must be able to safely interrupt destructive fault current energy. If a fault current exceeds a level beyond the OCPD’s capability, it may rupture, causing damage and posing a safety hazard. The rating that defines OCPD’s capacity to maintain its integrity when reacting to fault currents is its interrupting rating. It’s important when applying a fuse or circuit breaker, to use one that can safely interrupt the largest potential fault currents. Most modern, current-limiting fuses have a 200 kA or 300 kA interrupting rating and can be used in just about any system without fear of misapplication. NEC 110.9 requires equipment intended to break current at fault levels to have an interrupting rating sufficient for the available fault current at 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 that has 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 or worry that a fuse will be misapplied due to an improper interrupting rating. Nor is a short-circuit study needed when applying fuses for selective coordination, so meeting NEC 110.9 requirements is easy.

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. Finally, 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 (SCCR)

Short-circuit current rating is the ability of the equipment to withstand a certain magnitude of short-circuit current at a specific voltage in conjunction with an OCPD(s) without becoming 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 which has multiple devices in an enclosure, such as a panelboard, switchboard, motor control center, or industrial control panel.

NEC 110.10 requires equipment to have a short-circuit current rating 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 be provided with adequate short-circuit current protection. The intent of NEC 110.10 is that the equipment and OCPDs must be “selected and coordinated” so that under fault conditions up to the available fault current value, the equipment will not sustain extensive damage.

To assure proper application, the designer, installer and inspector must ensure that the marked SCCR of a component or equipment is greater than the calculated available fault current. In order to ensure compliance, it’s necessary to:

  1. Determine the available fault current at the point of installation of the component or equipment.
  2. Ensure the component or equipment marked SCCR is equal to or greater than the available fault current.

Current-limiting fuses are an effective means used by equipment manufacturers to increase the equipment SCCR per methods in accordance with product standards such as UL 508A for industrial control panels. Specification and installation of equipment with higher SCCRs, such as 200,000 amps, makes it easy to meet the requirements of the NEC. In addition, if equipment is later moved within a facility or from plant to plant, equipment with the highest ratings can be moved 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

Selective coordination is critical for electrical distribution system reliability. A reliable system is not only important for life safety, it’s important 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 is quite often 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 elevator, emergency, legally required, and critical operation power systems). A properly engineered and installed system that’s selectively coordinated 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 continuity of service.

Using properly selected and applied fuses in alleviates the design hassle of trying to achieve selectively coordinated OCPDs. To achieve selective coordination, fuses simply need to maintain a minimum 2:1 amp rating ratio for upstream and downstream and be properly selected. This eliminates the possibility of cascading multiple OCPD levels under fault conditions.

Our NEMA members offer solutions that can help easily achieve code requirements such as selective coordination.  Our comprehensive portfolio of overcurrent circuit protection products offer solutions that not only enhance safety and reliability, but also ensure compliance with code requirements.

Please contact our NEMA Members for proper selection and sizing for your application.