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U-bend test - A cable test in which the insulation is tested for resistance to corona and ozone.

UART (universal asynchronous receiver-transmitter) - A device that converts outgoing parallel data from your computer to serial transmission and converts incoming serial data to parallel for reception.

UDP - Universal datagram protocol.

UF - A UL Underground Feeder cable type. Thermoplastic underground feeder or branch circuit cable.

UHF - Ultrahigh frequency, the band extending from 300 to 3,000 MHz as designated by the Federal Communications Commission.

UL - Underwriters Laboratories, Inc. A U.S. independent testing laboratory that also publishes standards for most products in the NEC.

UL Approved - Tested and approved by the Underwriters Laboratories, which was established by the National Board of Fire Underwriters to test equipment that may affect insurance risks of fire and safety. Most phone systems are tested and approved. Most of the testing focuses on the power supply feeding the phone system. The power supply plugs into the AC wall outlet, takes 120 volt AC and converts it to low-voltage DC power that the phone system typically runs on. If the power supply tests OK, then that is usually sufficient UL testing. It is the power supply - and what happens to the commercial AC power that feeds into the power supply - that determines the potential fire hazard a phone system poses. In addition to the UL approval, the other major fire concern is the use of proper wire in new building construction, with special emphasis on Teflon-covered cable in plenum ceilings.

UL Certified (Verified or Classified) - A product that has been tested to a manufacturer or industry standard (i.e., electrical performance).

UL listed - A product that has been tested and found to comply with applicable standards. Listing also involves regular follow-up to ensure continued compliance.

UL Recognized - A product that contains components that are UL Listed.

UL Style - A subset of UL Type AWM (appliance wiring material) consisting of thousands of different styles. Many UL styles are single-conductor hook-up wire. A unique four or five digit number, e.g., UL 1015, identifies each style. Styles identify an additional subset of features from the referenced standards in order to allow customers to quickly identify the type of wire needed.

Ultrasonic cleaning - Immersion cleaning aided by ultrasonic waves that cause microagitation.

Ultrasonic detector - A device that detects ultrasonic noise such as that produced by corona or leaking gas.

Ultraviolet - Radiant energy within the wavelength range 10 to 380 nanometers. It is invisible and can be filtered out by glass.

Unbalanced line - A transmission line in which voltages on the two conductors are unequal with respect to ground (e.g., coaxial cable).

Unbalanced-to-ground - Describing a two-wire circuit, where the impedance-to-ground on one wire is measurably different from that on the other, compare with BALANCED LINE.

Unconnectorized - A term used to describe bare-ended cable, i.e., cable without factory-attached connectors. See also CONNECTORIZED.

Unidirectional conductor - See CONCENTRIC-LAY CONDUCTOR.

Unidirectional stranding - A term denoting that in a stranded conductor all layers have the same direction of lay.

Unilay - More than one layer of helically-laid wires with the direction of lay and length of lay the same for all layers. See CONCENTRIC-LAY CONDUCTOR.

UPS (uninterruptible power system) - An online power system that generates load voltage.

Upstream - On a ring network, the direction opposite to that of data flow. Also, the direction on the cable from the modem to the headend.

USE - A UL cable type. Underground service entrance cable, XLP or rubber-insulated, CSPE or XLP jacketed.

USOC (universal service order code) - An old Bell System term to identify a particular service or equipment offered under tariff.

UTP - Unshielded twisted pair. Two wires, usually twisted around each other to help cancel out induced noise in adjacent circuits. An unshielded twisted-pair cable usually contains four pairs in a single cable jacket.