What is the protection system of the motor terminal?

There is more than three protections methods for this case.
1-You can use overload with phase failure detection function this device monitor the current at the three phases if there is no current at one phase the device stop the motor.
2-You can use a sensitive phase failure relay with unbalance detection function and adjust the action time to 0.1 second and the rate of unbalance to less than 5% when one phase loss a high current pass through the other two phases that make drop down at voltage and unbalance occur then the phase failure relay stop the motor ( this method useful when the motor is big than 20 HP).
3-You can connect another phase failure relay at the motor’s join box to monitor the three phases that come to the motor this phase failure relay become active after the motor start so we need to delay the activation of this relay about 5 to 8 second so we need to use a timer and diode.
One question you must ask yourself in order to prescribe the proper protection for the motor is that what goes on inside the motor when there is sudden loss of a phase i.e single phasing.
1. Immediately there is a loss of phase, the load becomes unbalance in the winding of the electric motor, to compensate for these the remaining phases pull more line currents;
2. The winding of the motor gets heated up;
3. Magnetic flux distribution becomes unbalanced;
4. Shift in the phase sequence.
Solutions: An overload Relay that is sensitive would determine the varied thermal heat due to high current drawn by the motor.
A phase sequence monitor would detect change in the 3 phase sequence.