Electrical Engineers and Master Electricians (EEAME) Portal

Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

the current required to magnetize the different parts of a magnetic circuit. It is calculated as the ratio of the total magnetomotive force (F) and the number of turns (N). More or less in transformers, and AC synchronous and induction machines, the magnetizing current is the current through the magnetizing inductance. Denoted by Im,it is calculated as the ratio of the induced EMF across the magnetizing inductance to its magnetizing reactance Xm.

motor current signature

analysis the use of the currents of an electric machine to provide diagnostic or other information on the health of the machine, coupling, or load.

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

(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.

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.

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.

subtransient current

the fault current that flows during the subtransient period when the generator and motor apparent impedances are their respective subtransient impedances.

symmetrical fault current

the total current flowing to a fault less the DC offset current. In many cases, fault current calculations are expressed in terms of symmetrical amps.

time overcurrent (TOC) relay

an over-current relay that has intentional, selectable, time delay. The time delay is chosen so that the relay will operate more slowly than downstream relays or fuses, and more quickly than upstream relays or transformer fuses. Relay and fuse curves are generally displayed on time-current curves.

time-current characteristic curve

(1) a relay time-current curve is a curve showing the time versus current characteristic of a time overcurrent (TOC) relay.

(2) a fuse time-current curve shows the melting and clearing times of a given fuse or family of fuses.

(3) a coordination time-current curve shows the relationship of the operating and clearing characteristics of coordinating devices (TOC relays and fuses) on a power system.

time-current curve

See time-current characteristic curve

transient current

the fault current that flows during the transient period when the machine apparent impedance is the transient impedance.

Flexible Alternating Current Transmission System

A static equipment used for the AC transmission of electrical energy. It is meant to enhance controllability and increase power transfer capability. It is generally a power electronics based device.

See FACTS