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Fault Current

Fundamental to an arc-flash hazard analysis is determining the available 3-phase bolted short-circuit (fault) current to be used in the calculations. This must be done at every point in the system where workers may be exposed to electrically energized (hot) system components.

Short-circuit calculation methods used to determine required interrupting ratings of overcurrent protective devices provide the maximum available bolted short-circuit current. These methods assume: that the fault is initiated at a point in the voltage wave that will produce the maximum current; that the fault is caused by a zero impedance bolted connection; and that the system is at room temperature of 25 degrees Centigrade (worst case scenario).

These conditions are not common when functioning systems are maintained or repaired. Conductor temperature, and therefore resistance, may be increased by current flow. Faults may occur at peak or decreasing voltage. If available fault current is less than maximum, protective devices may respond more slowly, greatly increasing the arcing time. Since incident energy is a linear function of arcing time, damage and hazard may increase. Therefore after calculating available bolted-fault current, users need to make conservative estimates about what lesser fault current could be available. NFPA 70E states that the generally accepted minimum for a sustaining arcing fault is 38 percent of the available three-phase “bolted-fault” current. Fault hazards should be analyzed at both maximum and minimum values.



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