Understanding what incident energy levels are at various locations in an electrical distribution system is important for electrical safety. This information can also be input into a reliability review that indicates the extent of damage to expect should an arc flash event occur. An important arc flash analysis output is guidance on the requisite PPE when justified, energized work is conducted. The steps necessary to conduct an arc flash study starts with these steps:

Step 1: Collect system information. Information accuracy is important for any analysis. Whether it be an existing installation or new construction, the power distribution system must be properly evaluated and documented. The foundation of power system analysis studies is to collect accurate information and the conservative nature of any assumptions made. New and existing systems present challenges unique to each, with existing installations, that have not been maintained, posing the biggest challenge.

Existing facilities may need to receive a walk down of the facility to update or verify one-line diagrams. New facilities under construction may go through study phases; the first of which is based upon many conservative assumptions until as-built drawings are obtained after the construction is complete.

Step 2: Operation modes. Once the system configuration is understood, one-line diagrams are updated and accurate, a review must be made to determine the various power distribution system configurations that will impact the available fault currents.

Examples of the varying configurations include:

  • Number of utility feeders that are, or could be, in and out of service
  • Unit substations that can be supplied by one or two primary feeders
  • MCCs with more than one feeder having the ability to energize one or two feeders
  • Large motors that may or may not be running during fault conditions
  • Generators running in parallel with the utility supply, or in standby.

Step 3: Fault current study. The fault current study is critical to the incident energy calculation (Ref.  4.1). This fault study must include both maximum and minimum expected fault currents at each major piece of electrical distribution equipment. When performing incident energy calculations, minimum fault currents (when no motors are connected as an example) could result in higher incident energy values due to longer clearing times at lower fault currents.

Step 4: Coordination study. The coordination study is critical because arcing fault current clearing times are determined by comparing the arcing currents with the TCC curves of the OCPDs in the system. The TCC curve is selected as part of the selective coordination study.

Step 5: Calculating arcing currents. Arcing currents are calculated based on the equations of IEEE 1584-2018, “IEEE Guide for Performing Arc-Flash Hazard Calculations”. The total arcing current at each piece of equipment must be determined, as well as the arcing current level that passes through the upstream OCPD. The total arcing current is used to calculate the incident energy. The arcing current that will pass through the upstream OCPD is used to determine that upstream device’s clearing time.

Step 6: Calculating incident energy. Equations available from IEEE  1584 are used with the assembled information.  Multiple calculations are made for the various identified power system configurations to determine which configuration provides the highest calculated incident energy. The calculated incident energy will depend upon voltage, equipment types and working distances.

Step 7: Determine flash-protection boundary. The flash-protection boundary is determined through iterative calculations based on the same equations used to calculate incident energy. The iterations are designed to determine the distance from the arcing source at which the onset of a second-degree burn could occur. Most programs include the flash-protection boundary based on an incident energy of 5.0 J/cm2 (1.2 call/cm2). To convert from J/cm2 to call/cm2 divide J/cm2 by 4.184.

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