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

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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


 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.

 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


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