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A requirement for short-circuit current rating was added in the 2005 National Electrical Code (NEC) to Section 430.8 covering motor controller marking, to Section 440.4(B) covering controller marking for air conditioning and refrigerating equipment, and to 409.110 covering controller marking for industrial control panels. In the 2008 NEC, a new definition was added to Article 100 explaining that “short circuit current rating” of electrical equipment is the “prospective symmetrical fault current rating at a specific voltage that electrical apparatus or systems can be connected without sustaining damage during a fault that would exceed a defined acceptance criteria.” In other words, the short-circuit current rating of the electrical equipment would permit the equipment to clear an electrical fault and be re-energized without major repair to the equipment.
Electrical equipment that is designed to open under fault conditions, such as a circuit breaker and a fuse as covered in 110.9, must be able to withstand the amount of available fault current at the line terminals of the equipment and clear a fault downstream from the line terminals without major damage to the circuit, as covered by 110.10. Limiting the damage of the circuit and the electrical components is accomplished by using the impedance of the circuit and using overcurrent protective devices upstream from the fault that will react fast enough to protect the electrical components from damage.
Based on changes that have occurred in the past few Codes, electrical equipment, such as motor controllers and industrial machinery, must now have a short-circuit current rating marked on the equipment. This marking ensures the designer of the electrical installation, the electrician and the electrical inspector can easily recognize the level of fault current that the component can withstand without sustaining damage that would unnecessarily incapacitate the electrical system, cause injury to personnel, and possibly initiate a fire within the facility.
To ensure proper operation of the overcurrent protective device and protection of the electrical circuit during a fault condition requires understanding of fault current and the operation of the protective device. In a fault, the impedance of the circuit drops to a much lower value than normal, causing the current in the circuit to rise dramatically. The fault current between the source and the point of the fault is limited by the amount of impedance in the circuit between the source of the circuit and the fault. Where a bolted fault occurs, the maximum current flows in the circuit and the overcurrent device operates within a specific time period, protecting the circuit. Damage to the circuit and surrounding area is limited, since a bolted fault does not involve an electrical arc with the resulting high heating of the air and components in the arc path so heat damage in the bolted fault is minimal. However, magnetic lines of flux in a bolted fault can cause extensive damage to the circuit and any electrical equipment connected to the circuit. With an arcing fault, high heat is involved, often at levels at or above 35,000 degrees Fahrenheit, with extensive damage to the electrical circuit as well as anything in the surrounding area.
Overcurrent protective devices provide protection for the electrical components of the circuit. However, if the fault current is extremely high, a current-limiting overcurrent fuse or circuit breaker may be used to clear the fault during the first half-cycle of the fault. This fast-acting device limits the fault current downstream, commonly called “let-through current,” to a much smaller energy than a normal overcurrent protective device since the current-limiting device operates in a small fraction of a second and limits the amount of fault current as well as the time of the fault. The amount of “let-through current” in a particular circuit is dependent on the types, sizes and location of all overcurrent protective devices in the circuit, since a current-limiting device will not clear in the same fraction of a second if another overcurrent protective starts to open. When the other device starts to open, the arcing between the opening contacts in that other device can decrease the level of current through the current-limiting device resulting in a longer time frame for the arc and more heat damage to the circuit.
Be careful in the types of overcurrent protective devices designed and installed in the circuit, since the longer the arcing fault exists, the more thermal and magnetic damage can occur to the electrical circuit and, thus, to the facility.
ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 or at [email protected].