The following is a glossary of terms common to the power industry. Please select a letter:


Alternating Current electrical power supplied by a utility company or from an AC generator.
AC Utility
The electric power furnished by an electric power utility company.
A software program that acts as a focal point for data collection and configuration of a specific network entity (hardware or software). SNMP agents provide data to management stations regarding the operation and configuration of devices on a network.
Alternating Current (AC)
Current, which changes (or alternates) direction at regular intervals. Since the current flows in one direction for the same amount of time that it flows in the opposite direction, the average value of the current flow is zero.
Ampere (Amp or A)
The unit of measure for current. One ampere is the amount of electricity per second that flows through a conductor such as a wire.
Automatic Transfer Switch (ATS)
An Automatic Transfer Switch (ATS) is a switch that automatically reconnects power from its primary source to a standby source.
Active Voltage Control


Battery Backup
A battery or a set of batteries in a UPS system. Its purpose is to provide an alternate source of power if the main source is interrupted.
Battery Capacity
The battery ampere-hour capacity at full charge, standard temperature, and at a specified (usually C10) discharge rate.
The data a cable can carry measured in bits per second (bps).
Battery Charger
A device or a system that provides the electrical power needed to keep the battery backup fully charged.
Battery Current Limit
System voltage control that limits the battery charge current to a preset value.
A total loss of the AC utility (commercial power).
A reduction in the voltage of the AC utility without complete loss of power.
Buck and Boost
A proprietary voltage regulation process used when an overvoltage or undervoltage situation occurs in the UPS. Undervoltage is boosted to make the voltage greater, and overvoltage is bucked to reduce it. The result is less reliance on the UPS battery, extending overall battery life.
Bus Voltage
The actual voltage supplied to the load as measured at the bus bars.
A circuit used to change the path of the electrical power so that it goes around (or bypasses) its normal path. In the UPS, the bypass circuit is used to route the power around the major electronics in the UPS so they can be serviced without power interruption.


Circuit Breaker (CB)
A device for manually opening (breaking) or closing a circuit to interrupt or apply electric power to an electrical apparatus. A circuit breaker can also open a circuit automatically when it senses an overload.
Clean Power
Electrical power that has been conditioned and/or regulated to remove electrical noise from the output power.
A device which changes electrical energy from one form to another, such as from alternating current to direct current.
Amount of electricity that flows through a conductor, such as a wire.


Direct Current
Digital Input
An input that recognizes an open-circuit and short-circuit.
Digital Output
A voltage free relay contact.
Direct Current (DC)
A type of current that never reverses direction. Since the current flows in only one direct on, the average value of the current cannot be zero unless the current has stopped flowing.
A UPS design in which the primary power path consists of a rectifier and inverter. Double-conversion isolates the output power from all input anomalies such as low voltage, surges and frequency variations by converting AC to DC to AC. See Online UPS.
Dry Contact
Isolated contacts through which the end user supplies an external circuit. Dry contact UPSs provide basic communication capabilities such as monitoring and shutdown.


The ratio of the output power from the UPS to the input power from the utility. This shows the percentage of the input power that is available as useful output power. For example, a UPS that is 95% efficient delivers 95% of the utility power it receives to the load. The remaining power takes the form of dissipated heat.
Emergency Shutdown
Used to instantly or quickly shutdown all of the electrical power available to the UPS and the load. An emergency shutdown device is usually used during a crisis to prevent damage to the UPS and the load. Some computer-room installations require a Remote Emergency Power Off (REPO) capability as part of their security/safety system.


Fast Charge
Increasing the Operating Voltage after a battery discharge, following an AC failure, to give a rapid battery recharge.
Fault Tolerance
The ability of a system to continue operating in the event of a fault.
A method of removing noise from the output of a UPS preventing “dirty power ”from reaching connected equipment.
The number of cycles (oscillation positive and negative) completed in one second. Defined as Hertz (Hz). In North America, utility power completes 60 cycles per second, (60 Hertz).
Full Load
The greatest load that a circuit is designed to carry under specific conditions; any additional load is considered an overload.


Graphic User Interface (GUI)
A computer system using graphics images on the screen rather than text to display applications information for the user.
Ground (GND)
A conductor connected between a circuit and the soil.


Describes any equipment connected to its power source by hardware attached directly to terminal blocks or distribution panels.
Harmonic Distortion
The presence of harmonics that change the AC voltage waveform from a simple sinusoidal to complex waveform. Harmonic distortion can be generated by a load and fed back to the AC utility line, causing power problems to other equipment on the same circuit.
Hot-Swappable Batteries
A feature that enables the user to change UPS batteries without powering down the connected load.


Input Line Cord
The covered bundle of wiring connected to the input terminals of the UPS. The end of the cord not connected to the UPS is connected, via an input plug, to an AC utility outlet supplying power to the UPS.
Input Plug
Connected to the end of the input line cord. To be plugged into an AC utility outlet receptacle.
Internal Bypass
UPS circuitry that provides a redundant power path. If there is an internal UPS fault, the connected load will still be supplied with unconditioned utility power.
A machine, device, or system that changes direct- current power into alternating-current power.
The separation (often through the use of an isolation transformer) of one section of a system from undesired electrical influences of other sections.
Isolation Transformer
A multiple-winding transformer with physically separate primary and secondary windings. Although the two windings are physically disconnected, the magnetic field in the windings of the primary creates (induces) electrical power in the secondary winding. In this way the electrical power available at the input can be transferred to the output. An isolation transformer does not transfer unwanted noise and transients from the input circuit to the output windings. This attenuation, or reduction in amplitude, could be as high as one million to one.


Light Emitting Diodes located on the front of the UPS that inform users of various power conditions and UPS operations.
A UPS containing an off-line inverter that must transfer on during a blackout, but provides faster transfer times than an off-line UPS.
Equipment that receives power from a UPS.
Load Bus
The bus to which the load equipment is connected.
Load Segments
Groups of receptacles on the rear panel of a UPS that can be independently controlled.


Manual Bypass Switch (MBS)
A manually operated transfer switch used to bypass the major electronics in the UPS, so the UPS can be serviced without power interruption.
The process of assigning physical entities to logical entities, e.g. when a particular analogue channel (internal or external) is assigned to be the channel used for measuring the bus voltage.


National Electrical Code (NEC)
The code of standards and practices for the U.S. electrical and electronics industry. Developed by the National Fire Protection Association of Quincy, Mass. First published in 1896.
Network Transient Protector
An in and out RJ11 jack for telephone/modem protection (120V models only) or RJ45 for 10Base-T network cable. It isolates connected equipment such as modem and fax machines from “back door ”power surges.
Random, sporadic, or multi-frequency electrical signals that become part of a transmission making the signal or information more difficult to identify.


Off-line UPS
A UPS type which feeds power to the load directly from the utility and then transfers to battery power via an inverter after utility drops below a specified voltage. The delay between utility power loss and inverter startup can be long enough to disrupt the operation of some sensitive loads. Also called a standby UPS.
Online UPS
A UPS in which the inverter is on during normal operating conditions supplying conditioned power to the load through an inverter or converter that constantly controls the AC output of the UPS regardless of the utility line input. In the event of a utility power failure, there is no delay or transfer time to backup power.
Operating Voltage
The voltage a UPS is configured to maintain at the load, after adjustments for temperature compensation, equalize, etc.
Any point on a wiring system where current is taken to supply electrical power for a load.
A condition in which the load wants more from the power source (such as a UPS) than the power source has been designed to supply.


Power Factor (PF)
The ratio of total real power, (W) to the total apparent power in volt-amperes (VA); W/VA.
Power Management Software
Software provides monitoring and shutdown for a UPS and its connected load.
Power System
A rack module, single-rack, or several parallel-connected racks, providing DC power to a single DC bus.


Rackmount UPS
UPS that can be mounted in a rack along with servers, hubs, and other devices.
A contact device installed at an outlet designed to accept a single plug. Receptacles on the rear of a UPS accept plugs from supported system equipment such as computers or monitors.
A module fitted to UPS that converts AC input power to regulated DC output power.
Rectifier Bus
The bus to which the outputs of the rectifiers are connected.
Rectifier Voltage
The voltage to which the rectifiers are set. This is assumed to be the same for each rectifier and does not include current share adjustments.
Duplication of elements in a system or installation to enhance the reliability or continuity of operation.
A method of limiting voltage to a narrow range.
Radio Frequency Interference
Also called serial ports; a method of communicating digital information in which the data bits are transmitted sequentially over one line.


Scalable UPS
A UPS that allows for expandability; for example, enables a UPS to accommodate a larger load by purchasing additional power modules.
Simple Network Management Protocol (SNMP)
A request-response protocol that collects management information from network devices and provides a way to set and monitor configuration parameters.
Sine Wave
The sinusoidal wave form exhibited by alternating current.
Single-Phase Power (1Ø)
Power that is provided by a single source which normally includes one hot lead and a grounded return line (neutral).
Simple Network Management Protocol
Standby Power System
See Off-Line UPS (listed above) for more information.
Status LEDs
Light Emitting Diodes (LEDs) that show the status of the UPS when they light up or turn off.
A transient (or momentary) wave of current, potential, or power in an electric circuit.
System Voltage
The nominal voltage of a UPS, equal to the nominal voltage of the rectifier modules. 48V or 24V.


Temperature Compensation
Adjustment of the rectifier output voltage to provide the optimum charging voltage for the battery.
A connector for attaching a conductor to an electrical apparatus.
Three-Phase Power (3Ø)
Power that is provided by a single source with three outputs with a phase difference of 120° between any two of the three voltages and currents.
Transfer Switch
A switch that will transfer current from one circuit path to another without interrupting the flow of the current.
Transformer (T)
A device that raises or lowers the voltage of an alternating current electrical source.
The fast radical change in a smooth sine wave that occurs in both voltage and current waveforms during the transition from one steady-state operating condition to another.
Two-Phase Power
Power which is provided by a single source with two outputs which may be 180 degrees out of phase or 120 degrees out of phase.


Underwriters Laboratories Inc., the leading independent, not-for-profit product safety testing and certification organization in the United States.
Uninterruptible Power System (UPS)
A system designed to automatically provide power, without delay or transients, when the normal power supply is incapable of supplying acceptable power. Some UPSs also filter and/or regulate utility power.
UPS Topology
Overall term describing the internal circuitry of a UPS, e.g., standby (off-line), line-interactive, and online.
User-Replaceable Batteries
Batteries that the user can work with to easily exchange UPS batteries, once the unit has been turned off.


Volt (V)
The unit of measure for voltage. Voltage is the electrical pressure which forces the current to flow in a conductor such as a wire.
Volt-Ampere (VA)
Voltage (V) multiplied by the current (ampere); apparent power. For instance, a device rated at 10 amps and 120V has a VA rating of 1200 or 1.2kVA.


Watt (W)
The unit of measure for true power.