The growing need for a highly reliable electrical supply for critical applications has spawned the development of local distribution systems with the extended use of power electronics. The advantages are numerous. The downside is that the higher interactions within the system can easily lead the system to operate outside its limits due to instabilities. For three-phase systems, high power factor rectifiers usually present constant power flow to the system by their regulated nature or directly reflecting its regulated loads. They are known to cause these unwanted interactions. To address this, a new stability criterion is proposed to predict and assess the stability conditions at ac interfaces feeding PWM rectifiers, enabling the safe operation of the power system as well as the comprehensive system-oriented design of these converters from a stability standpoint. A key advantage of the proposed stability criterion is its unmatched simplicity and practicality.