Calculation of arc flash energy

The potential scope of damage from arc flash short-circuits is described using the concept of “incident energy”. Incident energy is stated in calories per square centimeter (cal/cm2) and it shows how much energy is released while the arc lasts, and the distance from the arc flash.

The calculations of arc flash are based on voltage level, short circuit current, protection settings, and the active users’ connections. The incident energy is usually calculated at a defined distance, often an “arm’s length” (90 cm) from the busbar to where the arc flash occurs. The time the arc flash continues to burn is crucial to the level of the incident energy. In principle, you should attempt to reduce incident energy by disconnecting equipment and adjusting protection measures to keep the cut-out time as short as possible.

Incident energy, arc flash:

• PPE 0 ≤ 1.2 cal/cm² (no requirement for arc flash-safe clothing)
• PPE 1 ≤ 4 cal/cm²
• PPE 2 ≤ 8 cal/cm²
• PPE 3 ≤ 25 cal/cm²
• PPE 4 ≤ 40 cal/cm²
• PPE x > 40 cal/cm² (extreme danger)