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Commonly used terms in the Electrical industry.
mutual coupling
electromagnetic interaction between the elements in a phased array antenna in which the radiation from one element causes distortion of the current distributions or aperture field distributions of the other elements. Feed network mutual coupling occurs when the amplitude and phase of the excitation at a feed point are altered by the presence of reflected waves in the network.
MVA interrupting rating
the interrupting rating of a device expressed in terms of megavolt-amperes. The conversion between fault volt-amps and fault current in three-phase systems is VA = 1.73 x operating RMS line-line voltage x largest phase fault current expressed in RMS symmetrical amperes. Power circuit breakers can have separate MVA and maximum current interrupting ratings, with adjustments when the circuit breaker is operated below rated voltage.
National Fire Protection Association (NFPA)
sponsor and publisher of the National Electrical Code and other safety standards.
negative-positive-zero (NPO)
an ultra-stable temperature coefficient (@ 30 ppm/
O
C from -55
O
C to 125
O
C) temperature-compensating capacitor.
negative-sequence impedance
the impedance offered by a circuit when negative-sequence currents alone flow through it, expressed in ohms. The impedance is complex, with its real part being the circuit resistance and imaginary part, which is a function of frequency and inductance referenced as negative-sequence reactance, also expressed in ohms.
NEMA type
for induction motors NEMA establishes five types of induction motors (A, B, C, D, and E) that have different torque-speed characteristics to account for various types of loads.
NFPA
See
National Fire Protection Association
no load tap changer
device that provides for changing the tap position on a tapped transformer when the transformer is de-energized. Different taps provide a different turns ratio for the transformer.
non-transposition
refers to a three-phase electric power transmission line whose conductors are not transposed See transposition.
normally open contact
a contact that is open under normal operating conditions and closes when an action is initiated in its controller. For a contact that is part of a relay, the contact remains open when the relay is deenergized and closes when the relay is energized.
normally open, time to close a time delay
relay that is open when the power to its actuator is off. When power is applied to the actuator, the relay remains open for an adjustable time delay, after which it closes. When power is removed from the actuator, the relay opens immediately.
nuclear power plant
a thermal electric power plant in which the heat for steam turbines is produced by nuclear fission.
ODP
See
open drip-proof
oil-paper insulation
an insulation scheme used in transformers and cables in which conductors are insulated with heavy paper impregnated with a dielectric oil.
open drip-proof (ODP)
pertaining to a ventilated machine whose openings are constructed to prevent drops of liquid or solid particles falling on the machines at an angle less than 15. from the vertical from entering the machine either directly or by rolling along a horizontal or inwardly inclined surface of the machine.
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.
open-delta transformer
a connection similar to a delta-delta connection, except that one single-phase transformer is removed. It is used to deliver three-phase power using only two single-phase transformers. The normal capacity of the open-delta transformer is reduced to 57.7% of its delta rating.
optical fiber
a single optical transmission fiber usually comprised of a cylindrical core (5-100 mm diameter) in which the light is guided of higher index of refraction surrounded concentrically by a cladding (125-250 mm diameter) with a lower index of refraction. More properly defined as an optical waveguide. Some optical fibers may have multiple concentric cores and/or claddings.
Optical fibers made be of all glass, all plastic, or a combination of glass core and plastic cladding construction. Optical glass fibers may be silica-or fluoride-based glass.
optical fiber signal distortion
a change in the temporal shape of an optical signal transmitted through an optical fiber caused by a combination of wavelength effects (dispersion) and multimode and polarization effects. The wavelength effects include material dispersion, profile dispersion, and waveguide dispersion. The multi-mode effects cause distortion by the differential time delays between the various modes propagating in a multimode fiber. The polarization effect causes distortion by the differential time delay between the two polarizations of a single mode.
oscillograph
a continuous recording of the waveforms of an electric power line, formerly made with a cathode-ray tube but currently with a digital signal recorder, kept updated to record abnormalities during switching operations and fault conditions.