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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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