Symmetrical, total, and peak; Oh my! - Is current all I use to select my circuit breaker?

Abstract

ANSI/IEEE Medium-voltage circuit breaker ratings have changed in recent years, causing a great deal of confusion over how to properly select a breaker for short-circuit duty. This paper reviews basics of an asymmetrical short-circuit current waveform. It illustrates the effect of the X/R ratio and how the relationship between the dc component and the circuit breaker contact parting time determines the interrupting time of the short-circuit current. Five examples are discussed. The first set examines basic power distribution system applications that illustrate the potential issues of mixing older MVA rated switchgear with new circuit breakers rated using the voltage range factor K=1. The second set examines the complexities that occur when a generator with significant X/R ratio is directly supplying distribution switchgear. Examination of these different conditions is intended to aid equipment users in specifying the correct circuit breaker by identifying the critical factors and conditions affecting the performance of the circuit breaker.