In what unit is incident energy measured?

• A. Joule per square meter.
• B. Watts per square meter.
• C. cal/cm2.
• D. Volts per centimeter.

Answer: (C)

Incident energy is the amount of energy delivered to a surface by an arc flash per unit area, which helps determine heat exposure and PPE requirements. The standard unit used to express this energy density is calories per square centimeter (cal/cm^2). This unit has long been used in arc flash standards like NFPA 70E to quantify how much energy would affect a surface.

Other units describe different quantities: joule per square meter is energy per area in SI terms but is not the conventional way incident energy is reported; watts per square meter is radiant flux density (power per area) and reflects instantaneous power, not total energy; volts per centimeter is an electric field strength, not energy transfer. Therefore, cal/cm^2 is the appropriate unit. For reference, 1 cal/cm^2 is equivalent to about 41,840 J/m^2.