Incident Energy (cal/cm²)

Incident energy quantifies the severity of an arc flash hazard at a given working distance. The IEEE 1584-2018 model calculates incident energy based on multiple variables: bolted fault current (which determines the arc current), gap between electrodes, enclosure size and type, electrode configuration (VCB, VCBB, HCB, VOA, or HOA), and — most critically — the arc duration determined by the protective device clearing time. The relationship between clearing time and incident energy is approximately linear: halving the clearing time roughly halves the incident energy. PPE categories are defined by NFPA 70E: Category 1 (4 cal/cm²), Category 2 (8 cal/cm²), Category 3 (25 cal/cm²), and Category 4 (40 cal/cm²). Incident energy above 40 cal/cm² generally means live work is prohibited — no commercially available PPE provides reliable protection at these levels. The working distance is measured from the potential arc source to the worker's face and chest — typically 455mm (18 inches) for low-voltage panels and 910mm (36 inches) for medium-voltage switchgear. Engineers can reduce incident energy through faster protection, current-limiting fuses, bus differential relays, zone-selective interlocking, or arc flash detection systems.