When any of the 3 Phases L1, L2 or L3 is lost in the system, the condition is termed as Single Phasing or Loss of Phase.
The EOCR operates on current sensing principle and has in built Current Transformers (2 or 3) which immediately detect losss of any Phase and trips the motor.
Other relays which operate on Voltage Sensing Principle are not so accurate and therefore less reliable.
Also extended periods of motor running without any one Phase leads to total failure.