Relays Generally Used For Motor Protection
Type of Protection
Type of Relays
Instantaneous overcurrent and
Instantaneous or time overcurrent
Time overcurrent relay or a distance
Thermal and time overcurrent relays.
Instantaneous or time delay
Undervoltage and phase-sequence
Reverse-phase or phase-balance
Directional overcurrent and power
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