What unit is used to express incident energy in arc flash calculations?

• A. cal/cm^2
• B. joule per cubic meter
• C. volt-ampere
• D. lumens

Answer: (A)

Incident energy in arc flash calculations is a measure of the heat energy flux reaching a surface, expressed per unit area. The standard unit used is calories per square centimeter, which lets us quantify how much energy would be incident on skin or protective equipment during the arc. This unit is convenient because PPE ratings and incident energy thresholds are often specified in cal/cm^2, making it easier to assess protection levels.

For a sense of the scale, 1 cal/cm^2 is about 41.84 kJ per square meter (since 1 cal = 4.184 J and 1 cm^2 = 1e-4 m^2). The other options don’t fit: joule per cubic meter is energy density per volume (not per area), volt-ampere is a unit of electrical power, and lumens measure light output, not heat energy.