The longer an arc flash incident lasts before an overcurrent device interrupts the fault condition, the greater the level of incident energy exposure is. It is critical to have an overcurrent device interrupting a fault condition as quickly as possible to reduce the overall level of incident energy exposure.

Many large frame circuit breakers have adjustable settings so that during maintenance, the circuit breaker can be set to trip with no intentional delay in order to reduce the amount of arc flash energy potential. This is required in the National Electrical Code (NEC) in sections 240.67 and 240.87.

There are several factors that have an impact on the final amount of incident energy:

• Voltage level
• Duration of the fault condition
• Amount of available fault current
• Distance to the arc source

The following definitions can be found in Article 100 of NFPA 70E:

Below is a sample of NFPA 70E. For the complete section, see the actual NFPA 70E text at NFPA.ORG. Once there, click on the free access link to NFPA 70E.

Incident Energy. The amount of thermal energy impressed on a surface, a certain distance from the source, generated during an electrical arc event. Incident energy is typically expressed in calories per square centimeter (cal/cm²).

Incident Energy Analysis. A component of an arc flash risk assessment used to predict the incident energy of an arc flash for a specified set of conditions.