Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

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.
normal demagnetization curve
the second quadrant portion of the hysteresis loop generated when magnetic induction (B)is plotted against applied field (H ), which is mathematically related to the intrinsic curve; used to determine the performance of a magnet in a magnetic circuit.
oil circuit breaker
a power circuit breaker that uses oil as an insulating and arc-clearing medium.
open-circuit test
a transformer test conducted by applying nominal voltage on the low voltage side while keeping the high voltage side open. By measuring the power in, current, and voltage, the magnetizing reactance of the transformer equivalent circuit can be determined.
overcurrent
(1) current in a circuit that exceeds a preset limit.

(2) motor current magnitude in the normal circuit path exceeding the full-load current.
overcurrent protection
(1) the act of protecting electrical and electronic devices or circuits from a dangerous amount of input current.

(2) the effect of a device operative on excessive current.
overcurrent relay
a protective relay that operates when fed a current larger than its minimum pick-up value.
peak let-through current
the maximum value of the available short-circuit current that is let through a current-limiting fuse.
See current limiting fuse
persistent current
a current circulating in a closed structure without applied potential. Examples are the supercurrent in a superconducting magnet and the current in a closed mesoscopic ring in a magnetic field.
pickup current
the specified value that, if exceeded, causes the relay to act on its contact and cause a circuit breaker action. It is the threshold current for system protection, and a magnitude above this is considered a fault or abnormal condition.
power angle curve
a curve shown the relationship between the active power output of a generator and its power angle.
residual current circuit breaker
European term for ground fault interrupter.
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.
right hand circular polarization
the state of an electromagnetic wave in which the electric field vector rotates clockwise when viewed in the direction of propagation of the wave.
sealing current
the current necessary to complete the movement of the armature of a magnetic circuit closing device from the position at which the contacts first touch each other.
security
the ability of the power systems to sustain and survive planned and unplanned events without violating operational constraints.
SF6 circuit breaker
a power circuit breaker where sulfur hexaflouride (SF6)gas is used as an insulating and arc clearing agent.
short circuit
a condition on the power system where energized conductors come in contact (or generate an arc by coming in close proximity) with each other or with ground, allowing (typically large) fault currents to flow.
short circuit admittance
the admittance into an N-port device when the remaining ports are terminated in short circuits. For port 1 of a 2-port device, it is the input admittance into port 1 when port 2 is shorted.
short circuit gain-bandwidth product
a measure of the frequency response capability of an electronic circuit. When applied to bipolar circuits, it is nominally the signal frequency at which the magnitude of the current
gain degrades to one.
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