Sags - When the line voltages drop to 80 to 85 percent below normal for short periods of time. Possible causes are heavy equipment being turned on, large electrical motors being started, and the switching of power mains (internal or utility). A power sag can have effects similar to those of a power surge, such as memory loss, data errors, flickering lights and equipment shutoff.

SCADA - Supervisory Control And Data Acquisition. Remote monitoring and control of various equipment and loads. 

SCSI - Small Computer Standard Interface.

Separately Derived Source - A source of AC power which has its neutral wire locally derived from the ground wire. In an AC power distribution system, wiring regulations dictate that the neutral is connected to ground only once, and that is at the service distribution panel. The point of power utilization may be at a considerable distance from the neutral grounding point, allowing for the introduction of Common Mode noise. The most effective way to eliminate common mode noise is to connect the neutral directly to ground at the point of utilization, which would violate wiring regulations. It is allowable under the regulations to connect a lead from an isolated power source to ground at any point. Therefore, an isolation transformer at the point of utilization may be used to create an isolated power source which can then have its neutral lead connected to ground, creating a separately derived source. The isolation transformer may be a separate device or it may be built into another unit, such as a UPS.

Series Operation - Master-slave configuration in which two or more isolated converters are connected to obtain a higher output voltage level (converter inputs connected in parallel) or wider input voltage range (converter inputs connected in series) than that obtainable from one module.

Series Redundant - UPS configuration whereby one UPS feeds a second UPS, both with a bypass circuit. The load is connected to the second UPS. If the source fails, the first UPS uses its batteries. When they are exhausted, the second UPS batteries take over. If either UPS were to have a failure, the other continues operation. 

Shielding - A method of blocking electromagnetic interference to protect sensitive devices. In an inductor this is placed in the form of a thin metal sheet, a winding, or the core itself can act as a shield. 

Short Circuit - When two wires become connected, usually by accident, resulting in a system malfunction. In a data circuit, the malfunction may be a loss of signal or information. In an AC power circuit, a short may result in large, uncontrolled current flow which might cause overheating of wiring or cause overcurrent protective devices such as fuses or circuit breakers to operate. 

Sinewave - The sinusoidal wave form exhibited by alternating current.

Single-Phase Power (1Ř) - Power that is provided by a single source which normally includes one hot lead and a grounded return line (neutral).

Single-Point Ground - Tying the power neutral ground and safety ground together at the same point, thus avoiding a differential ground potential between points in a system.

Sinusoidal - Adjective to describe a function which follows a sinewave.

SNMP - Simple Network Management Protocol. A request-response protocol that collects management information from network devices and provides a way to set and monitor configuration parameters. This system is an open system which has been adopted by many users and equipment manufacturers. A device managed by SNMP must have a MIB and an Agent.

SNMP Agent - Software that monitors and manages a specific network device. It maintains that device's MIB and responds to requests from the NMS. In a UPS or PDU with SNMP, the agent can reside in the equipment, or in a SNMP adapter, or in a host computer connected to the equipment.

Specific Gravity (SG) - The ratio of the weight of a given volume of substance (such as electrolyte) to that of an equal volume of another substance (such as water) used as a reference. Used as a measure of the charge state of a wet cell battery.

Spike - A high voltage that occurs when there is a sudden, rapid voltage peak of up to 6,000 volts. These spikes are usually the result of nearby lightning strikes, but there can be other causes as well. The effects on vulnerable electronic systems can include loss of data and damaged circuit cards.

Square Wave - Output waveform generated by very basic, low-cost UPS. Functions adequately for less sensitive loads, but may not provide acceptable quality input for some types of electronic equipment.

Standby Power Supply - See Off-Line UPS.

Static regulation - The ability to hold the controlled level to specification with no variation in other parameters, i.e. static voltage regulation.

Status LEDs - Light Emitting Diodes (LEDs) that show the status of the UPS when they light up or turn off.

Step Wave - (Modified Sine wave) Enhanced version of square wave that provides adequate input for some more sensitive loads, but still not as high quality as a sine wave.

Surge - A transient wave of current, voltage or power in an electric circuit, which usually has a high rate of change outside of normal tolerances. Overvoltage transient surges can seriously damage equipment. Any surge can cause erratic operation, loss of data or damage to loads. Under this condition, computer systems may experience memory loss, data errors, flickering lights and equipment shutoff.

Surge Arrestor - A device or circuit designed to limit the surge to acceptable levels. The joule rating of a surge protection device is the amount of energy that it can absorb before it becomes damaged. In comparing surge protection performance, the Joule rating of a surge suppressor is less important than the let-through voltage rating. This reflects the fact that surge suppressors may protect equipment by deflecting surges as well as absorbing them. There is no standard for measuring the joule rating of surge suppressors which has resulted in exaggerated claims.

Switchmode - A power conversion technique that involves breaking the input power into pulses at a high frequency by switching it on and off and recombining these pulses at the output stage. Using this technique, an unregulated input voltage can be converted to one or more regulated output voltages at relatively high efficiencies. This type of power supply design is used by most modern electronic devices, especially computers. Switchmode supplies are economical, but draw non-linear current with a high crest factor and numerous harmonics.

Synchronization - The process of bringing two sources of AC power into alignment such that there is no phase difference between the sine waves and they are at the same frequency. In a UPS, one of the sources of AC power is usually the utility power which cannot be changed. Therefore, synchronization is accomplished by adjusting the phase of the UPS inverter. 

Technician - Refers to an electronic technician qualified to maintain and repair electronic equipment. Not necessarily qualified to install electrical wiring. Compare to Electrician.

Temperature Derating - Reducing the overall capacity of a power supply to account for an elevated temperature ambient. See Derating.

Terminal Block - An insulating base equipped with terminals for connecting secondary and control wiring. Used on hardwired equipment, such as a UPS, when input plugs and output receptacles are either impractical or unavailable.

Terminal - A connector for attaching a conductor to an electrical apparatus.

Thermal Protection - A device or circuit designed to protect the equipment from damaging temperatures. Some UPS and PDUs are capable of doing this for their connected loads.

Three-Phase Power (3Ř) - 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.

Toroidal Inductor - An inductor constructed by placing a winding(s) on a core that has a donut shaped surface. Toroidal cores are available in many magnetic core materials within the four basic types: Ferrite, Powdered iron, Alloy and High Flux, and Tape Wound. Characteristics of toroidal inductors include: self shielding (closed magnetic path), efficient energy transfer, high coupling between windings and early saturation.

Total Harmonic Distortion (THD) - The square root of the sum of the squares of the RMS harmonic voltages or currents divided by the RMS fundamental voltage or current. Typically a supply sine wave is considered acceptable when THD levels are 5% or less. See harmonics, harmonic distortion and distortion. Can also be calculated in the same way for only even harmonics or odd harmonics.

Transducer - A device that senses one form of energy and converts it to another, i.e., temperature to voltage (for monitoring).

Transfer Switch - A switch which will transfer current from one circuit path to another without interrupting the flow of the current. A switch used to transfer a load between a UPS and its bypass source.

Transfer Time - The amount of time it takes to sense a power failure and to switch the load to inverter power. Applicable to standby or offline UPS units. True online UPS do not have an interruption in the power to the load.

Transformer - (T) A device that raises or lowers the voltage of an alternating current electrical source.

Transient - 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. Transients take place when there is a rapid voltage peak of up to 20,000 volts with a duration of 10 microseconds to 100 microseconds. They are commonly caused by arcing faults and static discharge. In addition, major power system switching disturbances initiated by the utilities to correct line problems may happen several times a day. Effects can include memory loss, data error, data loss and component stress.

Transient Response Time - The time from a step change in load, voltage or other parameters until the power supply output recovers to nominal specification.

Transient Suppression - See Surge Suppressor.

Transverse Mode Noise - Normal mode. An undesirable voltage which appears from line to line of a power line.

Transverse Mode Noise Rejection (TMNR) - The ability of an electronic device, like a UPS, to block transverse mode noise between input and output.

Trickle Charging - With the trickle charging process, the battery receives a constant voltage feeding a low current. Constant use of this method dries the electrolyte and corrodes the plate, reducing potential battery service life by up to 50 percent.

Two-Phase Power (2Ř) - 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.

Turns Ratio - The ratio of the primary voltage (or turns) to the secondary voltage (or turns) in a transformer.

TÜV - A European safety agency that sets standards for product safety. See CE, UL, ETL, CSA and VDE.

TVSS - Acronym for Transient Voltage Surge Suppressor. Another term for surge suppressor taken from the UL designation for this type of product. See Surge Suppressor.

xU - x Rack Units. Each Rack Unit is 1.75 inches in height. Width depends on the rack width, i.e. 19 or 23-inch width. Most rack equipment panel heights are in full rack units, not fractional, i.e. 3U.

UL - Underwriter’s Laboratories, Inc. A US independent safety agency that sets standards for product safety. See ETL, CSA, CE, VDE and TÜV.

UL Listed - A UL Listed product has met to applicable UL Standard for that class of equipment. Listed products are typically end products and not components.

UL Recognized - A UL Recognized device has met to applicable UL Standard for that class of device. Recognized components are typically parts that go into making the finished product.

Undervoltage - By definition, it is voltage at 90% or less of the normal level for more than one minute.  Sometimes it is called a brownout.

UPS - Uninterruptible Power Supply or System or Source. A system designed to automatically provide power, without delay or transients, when the normal power supply is incapable of supplying acceptable power. Some UPS also filter and/or regulate utility power.

UPS Topology - Overall term describing the internal circuitry of a UPS. There are three basic UPS topologies: standby (off-line), line-interactive, and online.

USAF - United States Air Force.

US Army - United States Army.

USCG - United States Coast Guard.

USMC - United States Marine Corp.

USN - United States Navy.

USNI - United States Naval Institute.

User-Replaceable Batteries - User replaceable batteries allow the user to easily exchange UPS batteries, once the unit has been turned off.