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Commonly used terms in the Electrical industry.
directional overcurrent relay
an over-current relay that operates only for overcurrents flowing in the tripping direction. Direction sensing is typically done with respect to a voltage or current signal, which is not affected by fault location.
instantaneous overcurrent relay
an overcurrent relay that operates with no intentional delay following sensing of a power frequency overcurrent, i.e., a current above its set point.
negative sequence overcurrent relay
a protective relay that senses and operates on negative sequence overcurrent. Typical applications include the sensing of unbalanced faults and the protection of synchronous and induction machines from rotor overheating.
overcurrent relay
a protective relay that operates when fed a current larger than its minimum pick-up value.
residual overcurrent relay
an overcurrent relay that is connected to sense residual current. Residual current is the sum of the three phase currents flowing in a current transformer secondary circuit, and is proportional to the zero sequence current flowing in the primary circuit at that point.