The NFPA 70E Standard “Electrical Safety in the Workplace” recognizes that the quicker a circuit breaker operates (instantaneous trip preferred) the less incident energy (arc flash) an individual maybe exposed who is working on electrical equipment and an arc flash incident occurs.
Circuit breakers that operate instantaneously may reduce their incident energy levels from 40 calories/cm2 (Arc Flash PPE Category 4) or greater to levels that in many cases are 8 calories/cm2 (Arc Flash PPE Category 2) or less. In many cases, depending on available fault current and working distance to exposed energized equipment, the incident energy levels will be less than 1.2 calories or no worse than a second-degree burn. Therefore, we like high speed (or instantaneous) tripping.