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R      [A B C D E F G H I J K L M N O P Q R S T U V W X Y Z]

Rack - A mechanical structure for mounting electrical equipment. Size and mounting patterns are defined by EIA 310.

Rack unit (U) - A common increment of equipment space height as defined by EIA 310. Typically 1 U equals 1.75 inches in height. Racks are sized in whole number of unit, i.e. 45 U rack has 78.75 inches of panel space.

Rackmount - A piece of electronic equipment such as a UPS that can be mounted in a rack along with servers, hubs, and other devices.

Receptacle - 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.

Rectifier - An electronic device that converts AC power to DC power (AC/DC)

Redundancy - Duplication of elements in a system or installation to enhance the reliability or continuity of operation.

Regulation - A method of limiting voltage to a narrow range.

Redundant Operation - Parallel configuration of converters used in distributed power system to increase system reliability. Converters may be used in a N+1 architecture.

Remote Sensing - Using sense leads connected at the output load provides feedback to voltage regulation circuits of a converter. This arrangement is used to compensate for voltage losses from long leads to a load.

REPO - Remote Emergency Power Off. See Emergency Shutdown.

Resonant Converter - Switching converter technology in which a resonant tank circuit operating at very high frequencies is used to transfer energy to the output.

Reverse Voltage Protection - Converter feature that prevents damage to internal components if a reverse voltage is inadvertently applied to the input or output terminals.

RMS - Root Mean Square. The square root of the average value of the squares of all instantaneous values of voltage or current during one half cycle in an AC circuit. For a sine wave, the RMS value is approximately equal to 0.707 times the peak value of the waveform. RMS is also called the effective value.

Rolling Blackout - A condition where power utilities purposely impose blackouts over a portion of their service area to free up capacity so that the remaining service area can continue to be served.  After a time, another portion of the service area has a blackout imposed on it, so that power can be restored in the original area.

Rolling Brownout - A condition where power utilities purposely impose brownouts over a portion of their service area to free up capacity so that the remaining service area can continue to be served.  After a time, another portion of the service area has a brownout imposed on it, so that power can be restored in the original area.

RS-232 - Also called serial ports; a method of communicating digital information in which the data bits are transmitted sequentially over one line.

S      [A B C D E F G H I J K L M N O P Q R S T U V W X Y Z]

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. 

Scalable UPS - A UPS that allows for expandability; for example, enables a UPS to accommodate a larger load by purchasing additional power modules.

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 (1Ø) - 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 UPSs. Functions adequately for less sensitive loads, but may not provide acceptable quality input for some types of electronic equipment.

Standby Power System - 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, potential (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 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.