Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

ground-fault circuit interrupter (GFCI)
a device designed to detect ground-fault current above a threshold value (several milliamperes) and then interrupt the source of electrical power by opening a circuit breaker or a set of contacts. GFCIs are designed for personnel protection and are generally available in the form of circuit breakers and receptacles.
hysteresis curve
a graph describing the relationship between the magnetic flux density and the magnetic field intensity in a (usually ferromagnetic) material.
inrush current
the transient current drawn by an electrical apparatus when it is suddenly connected to a power source. The inrush current may be larger in magnitude than the steady-state full-load current. The transient response is short in time and the electrical equipment generally supports the inrush current, provided it does not happen frequently. For a single transient, the thermal limit of the equipment is not reached, but if it is switched on and switched off several times within a short period, the temperature can rise very quickly. In case of transformers, the inrush current is not sinusoidal even if the voltage is due to the hysteresis of the ferromagnetic core.
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.
instantaneous-trip circuit breaker
essentially an inverse-time circuit breaker with the thermal element removed. It will only trip magnetically in response to short-circuit currents. Thus, it is often referred to by other names, such as magnetic circuit breaker and magnetic-only circuit breaker.
intrinsic demagnetization curve
the second quadrant portion of the hysteresis loop generated when intrinsic induction (Bi )is plotted against applied field (H ), which is mathematically related to the normal curve; most often used to determine the effects of demagnetizing (or magnetizing) fields.
inverse-time circuit breaker
a circuit breaker in which the allowed current and time are inversely proportional. It contains a thermal element and a magnetic element in series. The thermal element is designed to trip as a result of heating over time in response to overload currents, while the magnetic element is designed to trip magnetically, with no intentional time delay, in response to short-circuit currents. Also called a thermal-magnetic circuit breaker.
Kirchoff's current law (KCL)
a fundamental law of electricity that states that the sum of the currents entering and exiting a circuit node must be equal to 0.
left-hand circular polarization
the state of an electromagnetic wave in which the electric field vector rotates anticlockwise when viewed in the direction of propagation of the wave.
line-current harmonic

See electromagnetic interference filter
locked-rotor current
the current drawn by an induction motor when the shaft is not moving and rated voltage is applied. The starting current is essentially equal to the locked rotor current and may be as much as eight times the rated current of the machine.
magnetic circuit
the possible flux paths within a system consisting of a source of flux (electromagnets, permanent magnets), permeable flux carrying materials (steel, nickel) and non-flux carrying materials (aluminum, air).
magnetic current density
a fictitious source vector in electromagnetics that quantifies the amount of magnetic charge (also fictitious) crossing some cross-sectional area per unit time. The magnetic current density is often introduced in problems where duality and equivalence concepts are employed. The direction of the magnetic current density is in the direction of magnetic charge motion. SI units are volts per square meter.
magnetization curve

See hysteresis curve
magnetizing current
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.
molded case circuit breaker
a low voltage air circuit breaker that includes thermal and/or magnetic overcurrent sensing which directly trips the breaker. The molded case circuit breaker is nearly always manually closed, opened, and reset.
motor circuit
the three components of an electrical circuit are source, load, and interconnecting circuit conductors. A motor circuit is an electrical circuit designed to deliver power to a motor. It includes the over-current protective devices, controller, disconnect switch, circuit conductors, and the motor itself.
motor circuit protector (MCP)
a listed combination motor controller containing an adjustable instantaneous-trip circuit breaker and coordinated motor overload protection. MCPs can provide short-circuit and bolted ground-fault protection via the circuit breaker magnetic element, overload protection via the overload device, motor control, and disconnecting means all in one assembly.
motor control circuit
a circuit containing devices such as the start/stop switches, mainline coil, main-line sealing contacts, overload contacts, timers and timer contacts, limit switches, antiplugging devices, and anything else used to control devices in the motor circuit.
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.
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