An overhead crane’s 3-phase slip ring motor can single phase while operator is hoisting/lifting a load. This has a potential of the load suddenly dropping to the floor crashing the operator or damaging equipment. From the principle of operation of a 3-phase induction motor, if it single phases whilst running, motor keeps running (but won’t restart if it trips until single phase problem is solved) until it trips on protection, eg, single phasing, fuses blowing, phase imbalance, winding temperature high etc. This therefore means that the motor keeps running but doesn’t have enough torque to hold the load. 

(Obviously during normal operation, the brakes are released but fail safe when operation is cancelled or stopped).

How best can such a scenario be avoided or how can it be stopped from happening since overhead cranes are used everyday in our industries.