Quantcast Table 4-1. Relays Generally Used For Motor Protection

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TABLE 4-1
Relays Generally Used For Motor Protection






Type of Protection
Type of Relays



 Phase-Fault Protection.
 Instantaneous overcurrent and


 differential relays.




 Ground-Fault Protection.
 Instantaneous or time overcurrent


 relay.




 Locked-Rotor Protection.
 Time overcurrent relay or a distance


 relay/timer combination.




 Overload Protection.
 Thermal and time overcurrent relays.




 Low-Voltage Protection.
 Instantaneous or time delay


 undervoltage relay.




 Phase-Rotation Protection.
 Undervoltage and phase-sequence


 voltage relays.




 Phase-Unbalance Protection.
 Reverse-phase or phase-balance


 current relay.




 Out-of-Step Protection.
 Directional overcurrent and power


 factor relays.




 Loss-of-Excitation.
 Field and power factor relays.



4.5 FUSES. A fuse is a thermal overcurrent protective device with a circuit-opening fusible
member that is directly heated and severed by the passage of an excessive overcurrent through it.
4.5.1 Purpose. Fuses provide overload and short-circuit protection for electrical apparatus,
cables, and wire. They interrupt abnormal current with minimum system disturbance and
equipment damage. Under overload or fault conditions, a properly applied fuse will open,
extinguish the arc established in the opening process, and maintain open-circuit conditions with
rated-voltage applied across its terminals. To restore service after a fuse has operated, it is
necessary to replace the fusible member or to replace the complete fuse consisting of the fusible
4-21





 


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