ACCOUNTING FOR DECAYING SHORT‐CIRCUIT CURRENTS IN ARC FLASH CALCULATIONS

This chapter illustrates that ANSI/IEEE methods for calculating the short‐circuit duties on switching devices have limitations with respect to calculations of bolted three‐phase currents for arc flash analysis when decaying components of the short‐circuit currents are involved. It discusses some theoretical aspects of decaying short‐circuit currents and then demonstrates a comparison with ANSI/IEEE empirical calculations versus a new algorithm proposed in this chapter for arc flash calculations. The chapter provides a discussion on leakage reactance, subtransient reactance, transient reactance, synchronous reactance, quadrature‐axis reactances, negative sequence reactance, and zero sequence reactance. The saturation factor is usually applied to transient and synchronous reactances, though all other reactances change with saturation. The chapter explains time constants of synchronous machines and behavior of synchronous machine on terminal short circuit. The crowbar method consists of high‐speed detection of arcing and intentional creation of a three‐phase bolted fault with a high‐speed grounding switch.