A 
AC or ac Abbreviation for alternating current.
Alternating current Electrical current which periodically reverses direction, usually several times per second.
Ampere The measurement unit for electrical current.
Automatic transfer switch A switch that automatically transfers electrical loads to alternate or emergency-standby power sources.
B
Black-out A complete loss of power lasting for more than one cycle. A black-out can damage electronics, corrupt or destroy data, or cause a system shutdown. Blackouts can result from any of a number of problems, ranging from Acts of God (hurricanes or other high winds, ice storms, lightning, trees falling on power lines, floods, geomagnetic storms triggering by sunspots and solar flares, etc.) to situations such as cables being cut during excavation, equipment failures at the utility, vandalism, corrosion, etc. Also known as an outage.
Brown-out A prolonged sag, occurring when incoming power is reduced for an extended period. Usually caused when demand is at its peak and the line becomes overloaded.
C
Capacitor Any AC circuit element possessing the property of capacitance (i.e., the ability to store a charge). Normally a capacitor is a dedicated device, designed for the prime purpose of exhibiting the property of capacitance (as opposed to inductive devices, in which inductance is used by the device to produce other results, such as turning a motor shaft).
Critical load Equipment that must have an uninterrupted power input to prevent damage or loss to a facility or to itself, or to prevent danger of injury to operating personnel.
Current The flow of electricity in a circuit. The term current refers to the quantity, volume or intensity (1) of electrical flow, as opposed to voltage, which refers to the force or "pressure" causing the current flow.
Current may be either direct or alternating. Direct current refers to current whose voltage causes it to flow in only one direction. Common direct current sources are batteries.
Alternating current refers to current whose voltage causes it to flow first in one direction, then the other, reversing direction periodically, usually several times a second. A common alternating current source is commercial/household power. This current reverses direction 120 time each second, thus passing through 60 complete cycles each second for a frequency of 60 Hertz.
D
Direct current Electrical current which flows consistently in one direction.
E
EMI/RFI Electromagnetic/Radio Frequency Interference. These high frequency signals are generally low level (<1V) and range from 1MHz up. EMI/RFI filters are generally not suitable for large amplitude surge suppression.
F
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H
Harmonic distortion A measure of the degree to which the impedance of a UPS affects the shape of the output voltage waveform. Distortion is stated as a percentage and may refer to any single harmonic or to the total waveform, in which case it is referred to as "total harmonic distortion" (THD).
I
Inverter The DC to AC power converter driven by the UPS rectifier-charger or battery via the DC bus. The inverter output drives the critical load.
IEC555 A German standard that requires power factor corrected (PFC) loads.
J
K
KVA Abbreviation for kilovolt-amperes. (1000 x volt-amperes)
L
Line disturbance analyzer A tool used in analyzing problems in a facility's incoming power. The line disturbance analyzer is connected at the power input to measure and record incoming power, then left in place for long enough to gather data typical of the site.
M
N
Noise Noise is the result of distortion of the normal line power sine wave by hundreds or thousands of small increases in voltage similar to EMI/RFI, though it encompasses lower frequencies. The amplitude of this type of disturbance is less than a surge but may be as low as EMI/RFI.
Normal line power Commercial electricity supplied by U.S. power utilities is generally delivered as 60 cycle (Hz) alternating current (AC).
O
Overload capacity A UPS's overload capacity is its ability to respond to sudden surges in load current without allowing the output voltage level to decrease.
P
Power conditioning systems A broad class of equipment that includes filters, isolation transformers, and voltage regulators. Generally, these types of equipment offer no protection against power outages.
Power factor corrected (PFC) supply A recently developed type of computer power supply, which exhibits an input power factor equal to one. IEC 555 will force most computers to use a power supply of this type at some point in the future.
Power synthesizer Power synthesizers actually use the incoming utility power as an energy source to create a new sine wave that's free from power disturbances. They can be as much as 99% effective against power disturbances. Types of power synthesizers include magnetic synthesizers (capable of generating a sine wave of the same frequency as the incoming power - 60 Hz), motor generators (which use an electric motor to drive a generator that provides electrical power), and UPSs.
Q
R
S
Sag A momentary decrease from nominal voltage lasting one or more line cycles. Severe conditions may indicate a need for a UPS or voltage regulator. Also known as a temporary undervoltage (TUV).
Sine wave A periodic oscillation. The fundamental waveform from which other waveforms may be generated by combinations of various group of harmonics. The voltage and current waveforms produced from the power company generators (alternators) are basic sine waves.
Surge A surge is a prolonged over-voltage condition. Surges can damage electronics and corrupt or destroy data.
Spike A spike involves a sudden marked jump in voltage, which can damage electronics and corrupt or destroy data.
Spike/surge protector These products are inexpensive solutions that provide minimal protection against surges, but no protection against sags and outages.
Suppressed voltage ratings Several ranges are assigned by UL for grading transient suppression voltages. For instance, a 400 volt rating indicates a maximum peak voltage between 330 and 400 volts. These ratings appear between 330 volts peak and 6000 volts peak.
Swell An increase from nominal voltage lasting one or more line cycles.
T
Transfer time Transfer time can refer to either the speed with which an off-line UPS transfers from utility power to battery power, or to the speed with which an on-line UPS switches from the inverter to utility power in the event of an inverter failure. In either case, the time involved must be shorter than the length of time that the computer's switching power supply has enough energy to maintain adequate output voltage. This hold-up time may range from eight to 16 milliseconds, depending on the point in the power supply's recharging cycle that the power outage occurs, and the amount of energy storage capacitance within the power supply. A transfer time of 4ms is most desirable, however, it should be noted that an oversensitive unit may make unnecessary power transfers.
Transient suppression voltage (let-through voltage) The maximum peak voltage occurring within 100µs after the test wave.
Transient voltage surge suppressor (TVSS) A device used to reduce voltage surges. Products may be wired in series or in parallel with the AC electrical conductors.
U
UL 1449 United Laboratories, Inc.'s Standards for Safety of Transient Voltage Surge Suppressors (TVSS).
UPS Uninterruptible power supplies (sometimes called uninterruptible power systems). A system designed to protect against short-term power outages.
V
Volt The quantitative unit of measurement of electrical voltage.
Voltage A term referring to the electrical force or potential. A technical synonym for voltage is emf or "electromotive force." Voltage is the parameter of electricity which causes current to flow when a circuit is completed. Voltage is always presented in an energized line, whether or not the circuit is complete (i.e., whether or not current flows).
Voltage regulator A device designed to regulate RMS voltage by removing swells and sags (such as an automatic tap-switching transformer or ferroresonant transformer).
W
Watt The quantitative unit of measurement of actual power. Actual power in an AC circuit is the measurement of the effective energy available for doing work, and is normally less than apparent power (volt-amperes) because of power factor considerations. Watts may be measured directly, by means of a wattmeter, or may be calculated by multiplying volt-amperes by the power factor of the equipment.
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