Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

See Underwriters Laboratory
UL classes
a classification system established by Underwriters Laboratory (UL) for the purpose of defining certain operating characteristics of low voltage fuses. UL classes include G, J, L, CC, T, K, R, and H.
ultra-high frequency (UHF)
electromagnetic spectrum with frequencies between 300 MHz and 3000 MHz or wavelengths between 10 cm and 100 cm. Also called as decimetric waves.
an imaging modality that uses reflected high-frequency sound energy to image the interface between materials with different acoustic impedances.
a term referring to wavelengths shorter than 400 nm, but longer than 30 nm. The region 400–300 nm is the near ultraviolet, 300–200 is the middle ultraviolet; and 200–30 nm is the far ultraviolet or vacuum.
ultraviolet laser
laser producing its output in the ultraviolet region of the spectrum.
unbalanced line
refers to a signal carrying line where one of the conductors is connected to ground. Contrast with balanced line.
unbalanced magnetic pull
a phenomenon in electric machines arising from the rotor not being symmetrical with respect to the stator or the axis of the rotor and stator not being coincident. Results in a higher pulling force on the side with the smaller airgap, resulting in additional bearing stresses.
unbalanced operation
in an n-phase system (n> 1), a condition in which the phase voltages (currents) are either

1. not equal-amplitude sinusoids or

2. have phase angles displaced by a value other than that specified for balanced operation.

The term "unbalanced" is also used to describe a machine that has unsymmetrical phase windings.
a condition of operating a synchronous machine, in which the current to the DC field winding is insufficient to establish the required magnetic flux in the air-gap. As a result, the machine requires reactive power from the AC system. An underexcited synchronous motor operates at a lagging power factor, as it appears as an inductive load to the AC system. An under-excited synchronous generator operates with a leading power factor, since it must deliver power to a leading (capacitive) system.
underfrequency relay
a protection device that curtails loads in an area that is deficient in generation. Lower generation compared to load demands give rise to lower frequency and a frequency threshold can be used by the relay to initiate load shedding in order to balance generation and demand.
underground distribution
a class of electric power distribution work, typically used in densely-populated urban business districts, in which conductors are carried in conduits under streets between manholes and submersible distribution transformers are mounted in underground vaults.
underground residential distribution
practices involved in the underground distribution of electric power to residential subdivisions through direct-buried cables and pad mound transformers.
a voltage that is less than nominal for a time greater than 1 minute.
undervoltage relay
a protective relay that operates on low voltage or loss of voltage.
Underwriters Laboratory
an insurance industry testing agency that establishes standards for and conducts testing of electrical equipment.
unforced system
a dynamic system where all of the external sources of excitation are identically zero.
ungrounded system
an electrical distribution system in which there is no intentional connection between a current-carrying conductor and ground.
uninterrupted power supply (UPS)
(1) a power supply designed to charge an energy storage medium, while providing conditioned output power, during the presence of input power and to continue providing output power for a limited time when the input to the supply is removed. These power supplies are typically used in critical applications to prevent shut-down of these systems during power failures, power surges, or brownouts.

(2) a device that provides protection for critical loads against power outages, overvoltages, undervoltages, transients, and harmonic disturbances. A typical UPS is a rectifier supplied battery bank for energy storage, and a PWM inverter-filter system to convert a DC voltage to a sinusoidal AC output. UPS systems can be on-line, as shown in the figure, where the UPS inverter powers the load continuously, or off-line where the load is connected directly to the utility under normal
operation and emergency power is provided by the UPS.
unsymmetrical fault
a fault on a three-phase power line in which the fault current is not equal in all three phases, e.g., a single-line-to-ground, double-line-toground or line-to-line fault.