Arc flash hazard calculations in LV& MV DC systems Part II - Analysis

Based on the results of short-circuit calculations in Part 1 of the paper, the calculation of arc flash hazard in DC systems is demonstrated. Much like AC systems, the arcing currents are not the same as short-circuit currents. NFPA 2012 methodology recommends that arcing current can be taken as 50% of the calculated DC short-circuit current and the arc flash hazard can be calculated for time duration of 2 seconds. Both these concepts are much flawed and will give inaccurate results. It is also imperative to appreciate the short-circuit protection in DC systems. Semi-conductor devices have very little energy let-through withstand capability and are protected with fast acting semi-conductor fuses or DC circuit breakers which may arrest the prospective short-circuit current and limit the incident energy release to a small value. This paper calculates the arc flash hazard in specimen cases to demonstrate the methodology of calculations in low-voltage and medium-voltage DC systems.