Single-phasing is one of the major causes of motor failure in the industry. The electric motor market is changing fast due to minimum energy performance standards being adopted around the world. In a large number of cases, particularly when the original old motors are retrofitted with motors of higher efficiency classes, the respective overload protection devices are not properly tuned/configured or upgraded/replaced. In this paper, an overview of the motor protective devices and the main results of an experimental study on the behavior of five three-phase 7.5-kW, 400-V, 50-Hz, four-pole motors, namely, four squirrel-cage induction motors including IE1, IE2/EPAct, IE3/National Electrical Manufacturers Association (NEMA) Premium, and IE4/Super Premium efficiency classes and one line-start permanent-magnet of IE4 class, under single-phasing supply at no-load, locked-rotor, and part-load operation, are presented. On the basis of the obtained results, it can be concluded that, in general, modern commercial thermal overload relays or thermal-magnetic circuit-breakers are able to protect line-operated motors against single-phasing operation, tripping within a safe time window, as long as they have a differential mode mechanism, their trip class is properly selected, and the current setting correctly adjusted as a function of the motor efficiency class, technology, service factor, load, rated power, and rated current.