Every medium voltage (MV) circuit breaker (CB) has a maximum interrupting rating at a specified X/R value. A CB may catastrophically fail when called upon to operate if the specified X/R is exceeded. This is due to the resulting asymmetrical DC and total current components which may exceed the CB capabilities. This paper explains the crucial recent changes in IEEE C37.010-2016 and IEC/IEEE 62271-37-013-2015. The X/R value, relay time, and the CB contact opening time play a major role in determining the value of the DC component that is stacked on top of the symmetrical AC current wave. Building upon these new standards, we will clarify and cover the CB selection process to ensure proper CB application.