Conventional and current-dependent methods for predicting RMS arc currents in building systems

Arcing may be dynamic, explosive, intermittent, or self-extinguishing. In contrast, arcing may be self-sustaining and stable with a fairly constant current magnitude. Self-sustained arcs may release an enormous amount of energy over a fairly long period of time. Although arcing has been known to occur in 208Y/120 V and in single-phase 120 V systems sustained arcing is most commonly associated with 480Y/277 V systems. Short-circuit currents are limited by system impedance; since arcing-fault currents are further limited by arc voltage, the magnitude of the arcing-fault current is often insufficient to immediately trip overcurrent devices. Despite modern advances in system protection, people are critically injured or killed each year when they are in the vicinity of an arcing fault initiated by accidental physical contact or a glow-to-arc transition. The initial phases of an arcing-fault research project involved reviewing the arc physics and reexamining the early arc models, and developing improved arc models. This paper reviews short-circuit calculations and focuses on estimating the arcing-fault short-circuit ratios at several locations for a wide range of building systems. One suggested method of determining the RMS short-circuit ratios is based on a current-dependent arc model.