The circuit breakers on the motor starter or the fuses have tripped. What might be the cause?

Circuit breakers and fuses trip for numerous reasons: Undervoltage, incorrectly designed circuit breaker or fuses, too low a setting for the motor-protective circuit breaker, asymmetric three-phase current, short circuit on the motor, incorrect wiring or faulty electrical connections, worn or blocked pump, defective capacitor for single-phase motors, motor-protective circuit breaker exposed to a higher ambient temperature than the motor.

However, the pump may also be undersized for the application in question, and may be operating in excess of its nominal flow rate. In this case, try to partially close the shut-off valve on the pressure side in order to throttle the flow rate until the motor current returns to within the acceptable limits.