Arc-Flash Hazard Calculations in LV and MV DC Systems Part—II: Analysis

Based on the results of the short-circuit calculations in Part I of this paper, the calculation of the arc-flash hazard in dc systems is demonstrated. Much like ac systems, the arcing currents are not the same as the short-circuit currents. The National Fire Protection Association 2012 methodology recommends that the arcing current be taken as 50% of the calculated dc short-circuit current and that the arc-flash hazard be calculated for a time duration of 2 s. Both these concepts are much flawed and will give inaccurate results. It is also imperative to appreciate the short-circuit protection in dc systems. Semiconductor devices have very little energy let-through withstand capability and are protected with fast-acting semiconductor fuses or dc circuit breakers that 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 the calculations in low-voltage and medium-voltage dc systems.