UPS TERMINOLOGY

UPS Terminology

A/C	Alternating Current
A/D	Analog to Digital
Alternating Current	The type of electrical current available from utility companies. The current which alternately flows back and forth along a conductor, in phase with a voltage which varies from positive to negative along a square wave, sine wave, or other waveform.
Ambient Temperature	The temperature of the environment surrounding a product, generally assumed to be room temperature.
Ampacity	The current carrying capacity of conductors.
Amperage (AMP)	The strength of the current as it flows through a circuit which is expressed in Volt - Amperes. Also see voltage.
Ampere	A unit of measure of current flow in a conductor. Ten amperes of current will flow in a conductor with a voltage of 100 volts and a resistance of 10 ohm from one point to another.
Ampere-hour	The product of current flow is amperes times the length of time of current flow in hours.
Anode	One of two power connections on a diode or SCR, (+) positive.
ANSI	American National Standards Institute.
Antimony	An element sometimes used to harden the plate material used in led acid cells.
Arc Sparking	Results when undesired current flows between two points of differing potential. It may be due to leakage through the intermediate insulation, or to a leakage path across it due to contamination.
ASCII	American Standard Code for Information Exchange. A 7-bit binary code rep of letters, numbers and special characters.
Auto-transformer	A transformer used to step voltage up or down. The primary and transformer secondary windings share common turns, and it provides no isolation.
Auxiliary Source	A power source dedicated to providing emergency power to a critical load when commercial power is interrupted.
Availability	The capability of an item, under the combined aspects of its reliability and maintenance to perform its required function at a stated instant in time. When a record is maintained on an operating system, the availability may be computed with the following formula: Availability=Up Time/Up Time+ Down Time.
Average Battery Voltage	The average of all series cells in a single battery string. The average is obtained by adding the total string voltage then dividing by the number of batteries in the string.
Battery	A group of cells connected to deliver more voltage and/or more current than a single cell. A lead-acid battery using an antimony material as the stiffening element in the battery places is called lead-antimony battery. A lead-acid battery using a calcium base paste as the stiffening element in the battery plates is called a lead-calcium battery. Lead-calcium batteries are the most popular for UPS application.
Battery Charge Current Limit	This option limits the amount of battery charging current to a specific value. In multi-module systems with a common battery, this option improves the charging current sharing between the rectifiers.
Battery Disconnect Switch	This DC switch disconnects the battery reservoir from the UPS. Provides protection when either the UPS or batteries need service.
Battery Disconnect Switch	This fused, DC rated, 3-pole, bolted pressure switch includes the required shunt trip and auxiliary switches and is supplied in a NEMA 1 floor (or wall-mounted) enclosure.
Battery Sealed	One containing gelled or captured liquid electrolyte, but having no access for water replenishment.
Battery String	A collection of cells, grouped together to provide higher voltage and/or higher current than a single cell.
Bias, Back	A voltage applied to a semiconductor to hold it in a normally non- conductive state.
Bias, Forward	A voltage applied to a semiconductor to hold it in a normally conductive state.
Blackout	Total loss of commercial power.
Bleeder Resistor	A resistor usually connected across a filter circuit to discharge capacitors when the unit is turned off.
Branch Circuit	One division of a load circuit; current drain during a fault is limited by a fuse or circuit breaker.
Breakdown Voltage	See Isolation.
Bridge	(1) Rectifier circuit incorporating four diodes (full-bridge) or two diodes (half-bridge). (2) Converter or chopper section of switching power supplies incorporating four transistors (full-bridge) or two transistors (half-bridge).
Brownout	(1) A deliberate commercial line voltage reduction necessitated by inadequate generator capacity at a particular time. (2) Condition during peak usage periods when electric utilities reduce their nominal line voltage 10% to 15%.
Brownout Protection	The ability of a UPS to continue operating within specification through the duration of a brownout.
Burden Resistor	A resistor which is connected across the secondary of a current transformer to convert the current signal to a voltage signal for current sensing.
Burn in	(1) The period directly following the very first turn on of a given power supply. It is characterized by a relatively high and declining failure rate. (2) The operation of items prior to their ultimate application intended to stabilize their characteristics and to identify potential failures.
BUS	(1) The system of conductors (wire, cable, copper bars, etc.) used to transport power from the power supply to the load. (2) A communications structure used to control various instruments and subsystems (e.g., IEEE-488 bus).
Busbar	A heavy rigid conductor used for high power feeders.
Bypass	The power circuits of a UPS can be bypassed by a circuit that carries power directly to the load. The circuit is referred to as "they Bypass" or "Bypass Line."
Bypass Breaker	Somewhere in the bypass circuit a switch must close to complete the bypass line. The switch may be: 1. A protective or automatic breaker, 2. A non-protective or non-automatic breaker, 3. A contactor (electric operated switch)
Bypass Phase Offset	The phase difference between the bypass source and the Inverter output.
Bypass Source	An alternate source for the critical bus which can be connected to the load in the event of a UPS failure or for routine maintenance.
Calcium	An element sometimes used to harden the plate material used in lead acid cells.
Capacitor	Two conductors separated by a dielectric material. Applying a voltage across the plates causes current to flow and stores a charge.
Cathode	One of two power connections on a diode or SCR, (-) negative.
Cell	A unit containing a combination of metal plates and an electrolyte solution. When connected to an external circuit a charged cell reacts chemically and delivers an electrical output. A cell of a lead acid battery provides about 2 volts.
Charge Rate	A given amount of battery charge current.
Charge Voltage	The voltage level maintained in a cell or battery in its maximum charge condition.
Choke	An inductor, usually part of a filter network, that impedes the flow of current at certain frequencies to remove undesired AC components from an incoming voltage.
Chopper	(1) An electronic circuit used to convert continuous DC power to pulses of DC power used as input to a PWM inverter. (2) See Inverter.
Circuit Breaker	A device, usually electro-mechanical, which detects excessive power demands in a circuit, and self-interrupts when they occur.
Clear	A term indicating that a fuse is blown or open.
Comm	Commutation
Common-Mode	Noise that component of noise common to the output and return lines with respect to an electrically fixed point, usually chassis ground.
Common Mode Noise	Electrical noise found between all power wires and neutral when measured to ground. The noise will have the same characteristics and be simultaneous.
Common-Mode Rejection	The ability of a circuit or device to protect against common mode electrical noise.
Common-Mode Voltage	Voltage common to both signal input terminals of a circuit. An undesired common-mode voltage is usually developed between the zero signal reference ground and some other ground point.
Communications Interface	The printed circuit board used to connect two systems together so they can communicate. The board will be configured such that its circuitry provides the appropriate match up to each system's electronics.
Commutation	A term used to mean "turning off an SCR". For this to be accomplished, the gate signal must be removed and the current through the SCR must be reduced to zero.
Commutation Notch and Ring	A. When the SCRs in a rectifier/charger (R/C) turn on and off there is a Notch and moment when two phases of the incoming feeder bus are shorted together. Ring The resulting current pulse reacts with the distributed inductance of the feeder to create a negative voltage. The voltage shows up as a small "notch" in the input voltage waveform. As the notch collapses, a short high frequency "ring" occurs. This notch and ring effect occurs once for each SCR in the R/C., B. The notch and ring effect occurs once for each SCR in the R/C. Therefore a full wave three phase (six SCRs) will have six current pulses per cycle. Some UPS inputs (the Exide Electronics Series 3000 for example) will have two R/C circuits (total of twelve SCRs) which operate in a fixed but off-set time relationship thus creating twelve independent pulses in the feeder bus. These R/C are sometimes referred to as "six or twelve pulse" R/C.
Compensating Winding	A primary winding on a transformer that is connected in voltage opposition to an output winding to improve regulation.
Conduit	Round metal pipes through which electrical wiring is carried from one location to the next.
Confidence Level	The percentage of probability that an item will survive for its stated MTBF.
Constant Current	A UPS that regulates current level regardless of changes in load resistance.
Constant Voltage	A UPS that regulates voltage level regardless of changes in load resistance.
Control Panel	The location of the system controls and status indicators required to monitor and modify UPS systems operation to insure satisfactory results.
Convection	The transference of thermal energy in a gas or liquid by currents resulting from unequal temperatures.
Cooling	Removal of heat, which, in a UPS, is generated by transformation, rectification, regulation and filtering. It can be accomplished using radiation, convection, forced air, or liquid means.
Core	The iron form or frame on which a choke or transformer is wound.
Core Saturation	Condition when an inductor or transformer core can no longer increase flux density.
Crest Factor	(1) The current waveform in a non-linear load is sometimes characterized by comparing the peak of the current waveform to the RMS value of the waveform. This comparison is expressed as a ratio Peak/RMS or Peak: RMS (for example, "the Crest Factor is 3:1") (2) This is the actual peak current demand of the load, expressed as a ratio of the demand versus the average Root Mean Square (RMS) current supplied by the UPS. For example, if the peak demand is 24 A and the average current is 8 A, then the crest factor is 24:8, or 24/8, which is 3. A switching power supply can often create the need for a high crest factor because of the power supply's diode circuitry switching on and off. If the UPS cannot accommodate this momentary demand for current, the UPS will shut down.
Critical Bus	The power carrying bus that connects to the critical load.
Critical Load	Those loads which will not continue to function property on power that is unstable or intermittent.
Current	The flow of electrons along a conductor from a point of negative potential (voltage) to positive potential (voltage).
Current Control/Current Limit	The point at which the maximum allowable current from the charger or inverter is reached and the regulator circuits switch from the voltage regulation mode to the current regulation mode.
Current Limiting Circuit	A bounding circuit designed to prevent overload of a constant-voltage power supply. It can take the form of constant, fold back or cycle-by-cycle current limiting. Foldb ack Current Limiting Circuit gradually decreases the output current under overload conditions until some minimum current level is reached under a direct short circuit. Constant Current Limiting Circuit holds output current at some maximum value whenever an overload of any magnitude is experienced. Cycle-by-cycle (or instantaneous) current limiting circuit immediately reduces output current to some minimum level whenever an overload of any magnitude is experienced.
CT/Current Transformer	A current transformer (or CT) is used to step down a large AC current to a small AC current for metering or sensing.
Cycle	The variation of electrical phenomenon, voltage or current usually from some point or points and back again to the original point. Cycle implies repetition of this motion over time. In electrical work a current or voltage cycle repetitious over time (cycles per second) is called a Hertz (Hz) (i.e. normal alternating current in the U.S. is 60 Hz)
DC	Direct Current
DC Link	The common DC bus which connects the output of the charger, the battery, and the input of the inverter together.
Dead Fault Test	A test of the UPS which connects the output wires together causing the UPS to go into a maximum overload response.
Delta	A method of connecting a three-phase source or load in a closed series loop with the output or input connections made to each of the three junctions. There is no common junction.
Delta to Wye, Wye to Wye	The input windings (primary) of a three phase transformer can be connected in a delta or wye configuration, as can the output (secondary). One may say then that a transformer is delta to wye or wye to wye connected when comparing the input configuration to the output.
Delta-Delta	The connections between a delta source and a delta load.
Delta-Wye	The connections between a delta source and a wye load.
De-rating	A reduction of some operating parameter to compensate for a change in one or more other parameters. In power supplies, the output power rating is generally reduced at elevated temperatures.
Differential-Mode Noise	That component of noise measured with respect to output return; it does not include common-mode noise.
Diode	A solid state electrical device whose major characteristic is that it will only conduct power in one direction.
Direct Current	The type of electrical current produced by batteries. It flows in only one direction along a conductor or from a power source such as a battery.
Distribution Transformer	The transformer converting the feeder line voltage to the proper voltage for the distribution panel.
Downstream Protective Device	A fuse or circuit breaker located between a source and load.
Downtime	The period of time during which an item is not in a condition to perform its intended function.
Dry Contact	An alarm relay contact which does not have any voltage on it and can, therefore, be used by other devices.
Dynamic Load	A load that rapidly changes from one level to another. To be properly specified, both the total change and the rate of change must be stated.
ECM, EMI, RFI	Acronyms for various types of electrical interference.
Efficiency	The ratio of output power to input power. It is generally measured at full- load and nominal line conditions. In multiple-output switching power supplies, efficiency can be a function of total out-put power and its division among the outputs. The efficiency of a UPS indicates how much power it consumes from the utility input. On-line UPS products tend to be less efficient than stand-by or off-line products. This is because they provide maximum line filtering and are always providing on-line protection without any transition time to batteries. Stand-by products are the most efficient, but provide the least protection. Their primary use is to provide battery back-up during a power outage. Off-line products offer good protection from power contamination. Some may have lengthy transfer times to battery. Typically, an off-line UPS should transfer to batteries within four to seven milliseconds to avoid dropping the load. Some vendors will specify their product's efficiency under the best possible conditions. Thus, the number alone can be very misleading. The true efficiency of the UPS is determined by completely discharging the batteries and running the load on the UPS while the UPS batteries are being recharged. (See also Stand-By UPS, Off-Line and On-Line Operations.)
Electrolyte	The acid or alkaline solution surrounding the plates of a battery cell.
Electro-Magnetic Interference	Also called radio-frequency interference (RFI), it is unwanted high-frequency energy caused most often by the switching transistors, output rectifiers, and Zener diodes in switching power supplies. EMI can be conducted through the input or output lines or radiated through space.
EMI	Electromagnetic Interference.
Engine Generator	Combination of an internal combustion engine and a generator.
EPO	Emergency Power Off. When activated, all power is removed from the UPS and critical load is dumped. Also referred to as Power Off on the +80 UPS.
Equalize	A brief charge with an elevated voltage to equalize the specific gravity of the cells in a battery string.
Feeders	The main lines delivering power to a distribution system.
Ferroresonance	(1) Resonance resulting when the iron core of an inductor that is part of an LC circuit is saturated, increasing the inductive reactance to the value of the capacitive reactance. (2) The principle used in a simple open-loop (non-feedback) voltage-stabilizing power supply.
Ferroresonant Transformer	A transformer designed so its output has a natural frequency of oscillation.
Filter	A frequency-sensitive network that attenuates unwanted noise and ripple components of a rectified output.
Firmware	Program instructions permanently loaded in the control read-only memory (ROM) of the unit's central processor.
Flashover	Flashing due to high current flowing between two points of different potential. Usually due to insulation breakdown caused y arcing.
Float Charge/Voltage	Float charging is a method of charging a battery by applying a constant voltage to the battery. The float voltage is the charger output voltage required to float charge the battery. The float voltage is calculated by multiplying the number of cells in the battery by the voltage required for each cell.
Floor Loading	A statement of the force, usually in pounds per square foot (lbs/ft2), exerted on a floor when equipment is installed.
Form C Contacts	Form C contacts are a single set of isolated normally open (N.O.) and normally closed (N.C.) contacts with a single common. Contact ratings are 0.5 amps at 125VAC or 1 amp at 28VDC maximum.
Frequency	(1) How often something happens in a given time frame. In electrical work the frequency of oscillation is 60Hz (cycles per second) in the U>S> and Canada and 50Hz in most other parts of the world. (2) Deviation A shift from nominal frequency.
Frequency Changer	Power-conversion equipment that transforms AC electric power from one frequency to another without affecting its other characteristics.
Frequency Tolerance	You may intend to power your UPS from an emergency generator during lengthy utility power outages. Most generators have a much wider output frequency range than supplied by the utility company. You may find some UPS systems cannot synchronize with the generator's frequency if the UPS input requirements are too stringent. This will result in the UPS going into or remaining in a battery backup mode. The UPS would then shut down once the back-up holdover capability is exceeded, dropping the load - even though the generator may be in use for a much longer time. Today's high frequency switching UPS designs tolerate a wider range of input frequencies. Even though these UPS products permit a wide range of input frequencies, the UPS will hold the output frequency. Some UPS products also allow the output frequency tolerance to be tightened to a very narrow tolerance if required. Older UPS technology vendors may state they provide a very tight output frequency tolerance (i.e. 0.5%). If they don't state the input frequency tolerance limits, this will suggest the input tolerance is very narrow. Therefore, the UPS may not work with a generator. If narrow output frequency specifications may impact the UPS' operation with your generator, a UPS that can tolerate a wide input frequency range will provide you with greater flexibility.
Full Wave (Bridge) Rectifier	An assembly of electrical components that form a circuit whose purpose is to convert AC power to DC power (a rectifier). "Full wave" denotes that both the positive and negative half of the AC power is converted.
Full-Wave Rectifier	Rectifier circuit that rectifies both halves of an AC wave.
Gassing	Formation of hydrogen and oxygen gas due to breakdown of the water in an electrolyte solution.
Gate	(1) An electrical device that can be turned on and off by one electrical signal to inhibit or pass another electrical signal. (2) Can refer to a logic element, but usually refers to the signal lead of an SCR. By applying a "gate signal" to this lead, the SCR can be commanded to the conducting state if it is forward biased.
Gate Firing Module (GRM)	A logic board in the charger and static switch legs which sends the actual gating signals to the SCRs.
Gate Turn Off	A solid state device that must be turned on and off by an electrical signal.
Gating Signal	A signal generated by the UPS logic to command the gating of an SCR.
Ground	A connection from a circuit or object to the earth.
Ground Fault	(1) Any undesired current path from a point of differing potential to ground. (2) Can refer to a fault to ground on the inverter output, a utility source, or the battery.
Ground Loop	A feedback problem caused by two or more circuits sharing a common electrical line, usually a common ground line. Voltage gradients in this line caused by one circuit may be capacitively, inductively, or resistively coupled into the other circuits via the common line. With power supplies, this problem can be reduced using single-point grounding.
Half-wave Rectifier	Single-diode rectifier circuit that rectifies only one-half the input AC wave.
Hardwire	The act of wiring two points directly together without using switches, plugs or other means of disconnection.
Harmonics	A periodic waveform can be expressed as a sum of sinewaves which are integral multiples of the repetitive frequency of the waveform. The first harmonic is often referred to as the fundamental and is one times the frequency. The second harmonic is 2X, the third 3X, etc. Harmonics present is a waveform will cause its shape to deviate from a sine wave.
Harmonic Distortion	(1) According to theories of waveform analysis, any cyclical waveform can be described as the summation of a fundamental sine wave and some combination of harmonic waveforms (a square wave can be described as the summation of the fundamental and all the harmonics). When there are harmonic voltages or currents present in a circuit, they will add to or subtract from the fundamental waveform creating a resultant waveform that is a distorted version of the fundamental. This distortion is referred to as harmonic distortion. (2) In UPS terminology, this refers to distortion in the inverter output sinewave which is caused by the presence of unwanted harmonics or distortion caused by the Rectifier feeding back to the source.
Hi-Pot (High Potential Voltage)	Ability of a UPS to withstand a high voltage potential placed either from the input terminals to ground, from any of the output terminals to ground, or between any pair of input and output terminals. This specification is important for safety reasons and is partially dependent on the mechanical design of the UPS.
HVAC	Heating, Ventilating and Air Conditioning.
HZ	Electrical cycle per second.
I/O Cabinet	The In/Out Cabinet
IEEE	Institute of Electrical and Electronic Engineers.
IGBT	Insulated Gate Bipolar Transistor.
Impedance	In AC power circuits the "resistance" to current flow is called impedance because it is a complex characteristic of the circuit that is proportional to frequency (the same circuit can be a different value at different frequencies).
Individual Battery Voltage	The ability to monitor each positive post of a battery whether battery contains one or more cells connected in series.
Inductor	A conductor, usually coiled, which tends to oppose any change in the flow of current through itself.
Input and Output Frequency And Voltage	Many of the larger size 3-10 kVA UPS products on the market today (such as older ferroresonant technologies) require you to specify the input and output frequency and voltage when the order is placed. Should you need to relocate the UPS to another application or country that has a different frequency or need to change the voltage parameters, you would not be able to reuse your existing UPS. This is very costly, since you may have to purchase another UPS for the new application. Recent technology allows user selectable options.
Input Filter	A circuit on the input of the UPS used to reduce or eliminate current distortion due to harmonic currents created by the operation of an R/C Circuit.
Input Voltage Range	The range of line voltages for which the UPS meets its specifications.
Inrush Current	(1) The initial surge current demand before a load resistance or impedance increases to its normal operating value. When power is first applied to a circuit, various elements in the circuit have to charge up. A higher than normal current is drawn for a few cycles. This "inrush current" may be five or six times higher than normal steady state current. (2) A high surge of input current that occurs in switchers and occasionally in linear power supplies upon initial turn on.
Inverter	(1) A device that delivers AC power when energized from a source of DC power. Inverters may be frequency, amplitude, or pulse-width modulated to vary output voltage. (2) The chopper section of a switching power supply. (3) The portion of a UPS that converts DC power to AC power. The term is sometimes used to indicate the entire UPS.
Inverter Efficiency	The efficiency of the inverter section of the UPS from the battery (DC) to the inverter output (AC).
Isolation	The degree of electrical separation between two points. It can be expressed in terms of voltage (breakdown), current (galvanic), or resistance and/or capacitance (impedance). In power supplies, the input-to output isolation is important to maximize.
Isolation Transformers	The windings have no electrical connection and are therefore "isolated" from each other. Power is coupled from one winding to the other via the magnetic field.
KVA Kilo-volt Amperes	1000 Volt-Amperes. A measure of apparent power expressed in volt thousands.
KW Kilowatts	A measurement of real power expressed in thousands.
LCD	Liquid Crystal Display.
Lead-Antimony Battery	A lead-acid battery using an antimony material as the stiffening element in the battery plates.
Lead-Calcium Battery	A lead-acid battery using a calcium base paste as the stiffening element in the battery plates. Lead-calcium batteries are the most popular for UPS applications.
Leakage Current	Current flowing between the output buses and chassis ground due to imperfections in electronic components and designs. It must be tightly controlled to satisfy safety regulations such as UL and VDE.
Lightning Protection	Some UPS products provide limited lightning protection. These products may protect your equipment and the UPS itself from lightning surges. Some may only protect your equipment. Some may protect neither your equipment nor the UPS. Care must be taken to ensure the UPS complies with the ANSI C62.41 Category B. This standard specifies how electrical products are to be tested to ensure surge suppression and Electro-Magnetic Interference compliance.
Line Drop	Reduction in the voltage transmitted over a power line as a result of the resistance of the conductor.
Line Impedance	The apparent resistance in an electrical circuit to the flow of alternating current.
Line Inductance	Any wire carrying current has a permeability that causes the wire to look inductive. If the wire is a transmission line, the inductance can be called line inductance.
Line Interactive	A type of on-line UPS that interacts with the utility to control power flow as opposed to a USP that does a complete conversion of all power from AC to DC and them back to AC. Its advantages are higher efficiency and faster response to load variations.
Line Regulation	The variation of an output voltage due to a change in the input voltage, with all other factors held constant. Line regulation is expressed as the maximum percentage change in output voltage as the input voltage is varied over its specified range.
Line-to-Line Voltage	The potential between lines of a multi-phase feeder.
Line-to-Neutral	In power distribution there is usually one or more hot wires and one neutral wire. Line-to-Neutral implies that whatever is being discussed is between one hot line and the neutral. Line-to-Line implies two things. First, more than one hot line and second, the measurement is being taken between two of the hot lines.
Load	(1) The driven device that uses the power supplied by the source. (2) for voltage-regulated power supplies, the load is the output current.
Load Balancing	Switching the various loads to equalize the power demand on each line of a multi-phase feeder.
Load Dump	A loss of power on the critical buss or critical load.
Load Fault	A malfunction that causes the load to demand abnormally high current from the source.
Load Regulation	Variation of the output voltage due to a change in the output's load from no load to full load, with all other factors held constant. It is expressed as a percent of the nominal DC output voltage. (See Regulation.)
Load Share Loop	A set of series-connected sets of CTs used to balance load current between two or more UPS. May refer to input or output current.
Load Switching	Transferring the load from one source to another.
Local EPO	EPO switch found on the equipment as opposed to a remote location (such as a doorway).
Magnetic Field	The energy field developed in the immediate space around a current carrying wire or a permanent magnet.
Maintenance Bypass Switch	The switch used to close the Maintenance Bypass power path around the UPS.
Major Alarm	An alarm condition under which the UPS system cannot continue to function. A major alarm will cause the module to shut down.
Make Before Break	Refers to the switching of the load on the critical bus between the bypass source and the inverter by first closing a breaker to connect one source to the load, and then opening another breaker to disconnect the other source from the load. Thus, the load experiences no loss of power since one source is connected before the other source is removed.
MBP	Maintenance Bypass Switch
Mean Time Between Failures	A measure of reliability. The reliability interval calculated in accordance with the procedures of MIL-HDBK 217. Demonstrated MTBF is the probable range of true MTBF under test conditions; observed MTBF within a stated confidence interval. Observed MTBF is equal to the total operating time of the equipment divided by the number of relevant failures. Predicted MTBF is that value of MTBF determined by reliability stress analysis methods and is based on the design and the use environment.
Mean Time Between Maintenance	The mean of the time intervals between maintenance actions (preventive, corrective or both).
Mean Time To Repair	The average time required to repair a product. It is a result of both electrical and mechanical design factors. Indicating the time required to return a failed item to service, this is the total corrective maintenance time divided by the total number of corrective maintenance actions during a given period of time.
MG	Motor-Generator
MIB	Module Input Breaker
Minor Alarm	An alarm condition which is annunciated and needs correction, but is not severe enough to cause the module operation to cease. The alarm can be a normal part of the system operation.
Modular	A physically descriptive term used to describe a UPS made up of a number of separate subsections, such as an input module, power module, or filter module. Modular construction tends to lower the MTTR.
Module Battery Breaker	This DC rated circuit breaker includes the required shunt trip and auxiliary switches and it supplied in a NEMA 1 enclosure for wall mounting.
Module Emergency Shutdown Switch	This "pull to shutdown" switch is provided on the module's meter door to turn off the module and open the input, battery and output breakers. If there is redundant capacity on line, then only the individual module will be shut down. If there is not, then this switch will also force a transfer to bypass.
Module Input Breaker	A module input breaker is required and can be supplied by the customer or Exide Electronics. Three different input breaker options are available based on interrupting capacity.
Motor Operator	A motorized device which mounts on the front of a circuit breaker and opens or closes the breaker on command.
Motor-Generator	Combination of a motor and a generator. In effect, the motor-generator is a rotary power-isolation device.
MOV	"Metal Oxide Varistor" - A device which is normally a very high resistance, but whose resistance drops drastically if the voltage across the device exceeds a certain rating. It is used for voltage transient suppression.
MTBF	Mean Time Between Failures.
MTTR	Mean Time to Repair.
Neutral	The junction point of the legs in a wye circuit.
Neutralizer	A chemical solution that neutralizes the acidity or alkalinity of battery electrolyte.
NiCd	Nickel Cadmium, sometimes used in batteries.
Nominal Output Voltage	The intended, ideal voltage of any given output.
Non-Linear Load	When voltage is applied to a non-linear load, the resultant current will not have a linear relationship with the voltage applied.
Off-Line Switcher	A circuit configuration commonly used in PWM switchers in which the input rectifier and filter sections sit directly across the AC input line.
Off-Line UPS	The inverter does not control the flow of power to the load except when the utility fails. The inverter may be turned off completely or turned on but in a standby (no load) mode.
On-Line Inverter	The inverter portion of the UPS controls the flow of power to the load 100% of the time regulating voltage and frequency whether the utility power is available or not.
Operating Temperature	The range of temperatures within which a UPS will perform within specified limits.
Orderly Shutdown	Sequential shutdown of the units comprising a computer to prevent garbling or loss of data, or damage to the system.
Oscillation	Flywheel action in an LC circuit caused by the current generated by the alternate charge and discharge of the capacitor and the expansion and contraction of the magnetic field around the inductor.
Oscillator	An electrical circuit that develops a cyclical waveform for use in the rest of the circuit.
OSHA	Occupational Safety and Health Act.
Over Current Protection	See Current limiting circuit.
Overload	Many UPS products cannot handle momentary or instantaneous overload conditions during switching power supply and hard disk start up phases. As a result, you would have to oversize the UPS to avoid a UPS overload shutdown. This can be costly since you would have to pay for greater power capacity than you will need during the UPS's normal operation. An overload can be defined as the percent (i.e. 200%) of normal rated (RMS) current value that a UPS can maintain for a period of time (i.e. 10 cycles). As an example, a UPS rated at 8 Amps capacity with a 200 per cent overload capability can provide 16 Amps (8 X 200%) for 10 cycles.
Parallel For Capacity	Paralleling the operation of several UPS modules in order to increase the total power output capability.
Parallel For Redundancy	Adding one more UPS module than is needed to support the total load.
Parallel Operation	The ability of power supplies to be connected so that the current from corresponding outputs can be combined into a single load.
Passive Filter	An electrical/electronic filter that has no active components (transistors SCRs, GTO, etc.).
PDM	Power Distribution Module
pF	Power Factor
Phase	The work phase as used in electrical power terminology indicates the time relationship between two cyclical waveforms. If the waveforms reach corresponding points in the time simultaneously (maximum peak, minimum peak, zero crossings) they are said to be "in phase." If corresponding points are reached at different times the waveforms are said to be "out of phase." Voltages can have a phase relationship to other voltages. Currents can have a phase relationship to other currents. Voltage and current can have a phase relationship.
Phase Angle	The out of phase relationship between two waveforms can be expressed in angular degrees instead of time.
Phase Compensation	Switching capacitors into or out of the power distribution network to compensate for load power factor variations.
Phase Control	A form of regulation which controls the output of an SCR circuit by varying the conduction time of the SCR(s).
Phase Detector	Detects the instantaneous difference in electrical degrees between two AC signals and develops an output signal that varies in direct proportion to that difference.
Phase Shift	An operating power system can have circuit components that cause voltage and/or current waveforms to shift in time. The output voltage may be "out of phase" or "phase shifted" when compared to the input voltage.
Phase-On	The gradual rise of the output voltage of the Inverter or Rectifier during the start-up sequence.
Pilot Cell	A cell selected from a given group and used as a test specimen representative of the group.
Potential Transformer
Power Distribution Module	A low power voltage transformer which is used to reduce the voltage on its primary to a lower voltage to be used for voltage sensing.
Power Factor	(1) Power factor is an electronics industry term used to size the UPS for the product it will protect. Power factor indicates the ratio between true power (in watts) and apparent power (Volt-Amps), or W: VA. The majority of today's electronic products requiring protection are computer systems. These systems contain switching power supplies having power factors in the range of 0.6 to 0.7 that are often supported by UPS products. If the UPS is not designed to support a 0.6 - 0.7 power factor load, the UPS must be oversized. (2) The ratio of real power (kW) to apparent power (kVA). It will be "leading" or "lagging," depending on which way the load shifts the circuit current phase with respect to the voltage phase. Inductive loads cause the current to lag, capacitive loads cause the current to lead. Most loads are inductive. (3) The ratio of actual power used in a circuit to the apparent power. Power factor is the measure of the fraction of current in phase with the voltage and contributing to average power.
Power Outage	An interruption of commercial power.
Power Profile	A graph of the power requirements for a composite load during the start-up period, from turn-on until steady-state operation commences.
Power Supply	An electrical circuit that provides electrical power for use by other circuits. Electronic devices that provide DC output voltages when powered by an AC primary source.
Power, Apparent	RMS Volts X RMS amperes.
Power, Real	Watts
PT	Potential Transformer
Pulse Width Modulation	(1) Varying the width of each pulse in a pulse train in proportion to the modulating signal's significant characteristic so the pulse train represents the signal's variation. (2) A circuit used in switching regulated power supplies where the switching frequency is held constant and the width of the power pulse is varied, controlling both line and load changes with minimal dissipation.
Push-Pull Converter	Used in switching power supplies where the main switching circuit uses two transistors operating in push-pull. The main advantage is simplicity of design.
PWM	Pulse Width Modulation
Pumpback Diode	A diode in the Inverter assembly. This diode pumps excess energy back into the DC link during transistor turn off or transfers on and off line.
Ramp into Load	This term describes the gradual rise of current in the inverter when the load is being transferred to the UPS.
Recovery Time	The time period after a transient overshoot or undershoot during which the output quantity decays to within specified limits.
Rectifier	Converts AC power to DC power.
Rectifier/Charger	The subsystem of the UPS which converts AC power from the input bus to DC power which is used to power the inverter and to charge the battery.
Redundancy	(1) The inclusion of extra assemblies or circuits within the UPS, with provision for automatic switch-over from a falling assembly or circuit to its operational counterpart. (2) A redundant UPS system has two or more UPS modules wherein one module may be removed and the remaining module(s) will still be capable of powering the full load. Redundancy-The ability to connect power supplies in parallel so that if one fails the other will provide continual power to the load. This mode is used in systems when UPS failure cannot be tolerated.
Regulator	The part of a UPS that controls the output voltage. In most cases, the regulator acts to stabilize the output voltage at a preset value.
Remote Monitor Panel (RMP)	An annunciator panel which provides remote indication of UPS status and issues an alarm if a failure occurs with the system.
Remote Sensing	A method of moving the point of regulation from the output terminals to the load. Compensates for IR drops in the power distribution bus.
Resistor	A device or material that develops a voltage drop when current passes through it.
Response Time	The amount of time it takes for an output to react to dynamic load change and settle within some tolerance band following the load change.
Reverse Voltage Protection	The ability of a UPS to withstand reverse voltage at the output terminals when hooked up in the reverse polarity.
RFI	Radio Frequency Interference. See EMI.
Ripple	The periodic AC noise component of the UPS output voltage.
RMP	Remote Monitor Panel.
RMS	Root-Mean-Square.
Root-Mean Square	The effective DC value of a waveform. Dissimilar waveforms can only be compared by equating the amount of work they represent. The square root of the sum of the squares (RMS) is a mathematical method for developing the amount of work capability in a waveform.
Rotary Switch	A switch whose opening and closing action is implemented by rotating the operating mechanism. The contacts are carried past each other in a wiping action.
Rotating Field	The electrical field developed in a multi-phase generator. The varying currents through the pairs of stator windings cause the magnetic field to vary as if it was a single rotating field.
Saturable Reactor	A transformer-like device. One winding has a core that saturates when a small amount of current is passed through it. This saturation causes a large change in the inductance of the main winding, thus controlling the current through it.
SCR	"Silicon Controlled Rectifier" - The primary switching device used in UPS systems in the charger and static switch subsystems.
Sealed Batteries	Sometimes referred to as VRLA (Valve Regulated Lead Acid) or maintenance free. Because the battery cases are sealed, the electrolyte levels can not be inspected or replenished. The case is typically opaque, therefore the plates cannot be visually inspected.
Self-Discharge	The decrease of a battery's charge due to its internal leakage current.
Series	One after the other.
Series Regulator	A linear regulator in which the active control element (transistor) is in series with the load.
Service Entry	The junction box containing connection panels and switchgear located at the point where the power lines enter a building.
Shielded Transformer	A transformer constructed so that grounded shields exist in the path of the magnetic field. The shields help suppress electrical noise, especially high frequency transients.
Short-Circuit Protection	See Current limiting circuit.
Shunt	A device used to convert a large DC current to a low DC voltage for monitoring and metering purposes.
Shunt Trip	An electromagnetic trip used in circuit breakers to allow control circuits to open the breaker.
Silicon Controlled Rectifier	A solid-state switch used to control AC current. An SCR can be turned on and off using electronic control signals, and has a turn on time orders of magnitude faster than any mechanical switches or relays.
Simple Network Management Protocol	A network management specification (designed to simplify multivendor device management) defines a method of communication between a network management software application (called a client) and the SNMP code embedded in a device (called an agent). Developed by the Internet Engineering Task Force, it's enjoyed vendor support since it became a standard in May, 1990. Vocabulary is defined in a management information base (MIB) which includes a number of defined objects known as variables. SNMP II, a new standard in the SNMP family, offers additional functionality including security features such as authentication and data encryption.
Sine Wave	A waveform of voltage or current which is shaped like a sinusoid.
Sine Wave Output	Any wave form other than a true sine wave will likely cause electrical interference with telecommunications products. Computer systems may not be affected as much, but recent studies indicate computer LANs are also very susceptible. Sine wave UPS output is more difficult and expensive to generate. Therefore, many vendors offer "sine wave like" products. These are referred to as quasi-sine wave, pseudo-sine wave, step-sine, square wave, or trapezoidal wave form. Caution must be sued when a vendor claims a UPS offers sine wave output at typical load, at no load, or when the AC utility is present. This strongly suggests the unit will not produce a true sine wave during battery operation. This may result in improper load operation or damage to the load. Thus, you would have to oversize the UPS with additional capacity to ensure that type of wave form does not interfere with or damage the load. Be careful when examining UPS specifications if you need to avoid products that produce non-sine wave outputs.
Single Phase	A single phase power system is one that has only one primary waveform.
Single-Line Electrical Diagram	Diagrams indicating routing location, and specifications for a power distribution system, including all its components and loads.
Sinusoid	A smooth curve that describes the mathematical formula Sine. If one looks at the power line voltage with an oscilloscope, the waveform will appear as a sinusoid with a frequency of 60 Hz.
Slew Rate	The rate in cycles per second that the tracking oscillator shifts frequency to track the bypass source.
SNMP	(Simple Network Management Protocol) Format used for network management data. Data is passed between SNMP agents (processes that monitor activity in hubs, routers, bridges, etc.) and the workstation used to oversee the network. Originating in the UNIX community, it has spread to VMS, DOS and other environments.
Snubber	A network containing a resistor, capacitor, and diode used in the switching power supplies to trap high-energy transients and protect sensitive components.
Snubber Circuit	A series of RC network connected across each SCR to protect it against voltage transients which could accidentally gate the SCR or cause physical damage.
Soft Start	Input surge current limiting in a switching power supply where the switching drive is slowly ramped on.
Source Impedance	The apparent resistance of an alternating current source to supply current for instantaneous load changes.
Specific Gravity	A measure of the concentration of acid or alkali to water in battery electrolyte. It is based on the relative densities of the acid or alkali and water.
Spike	Generally refers to a voltage aberration that occurs on the power line. It deviates sharply from the general wave shape. Its amplitude will be high and duration short.
Square Wave	A waveform of voltage or current which has a square or rectangular shape. In UPS systems this is usually the unfiltered output of an SCR step wave or PWR inverter.
Stand-By UPS	A UPS that operates in the standby mode until demand is made. It then switches to an on-line mode. Stand-by products are the most efficient, but provide the least protection. Their primary use is to provide battery back- up during a power outage.
Static Load	A load that remains constant over a given time period. It is usually specified as a percentage of full load.
Static Switch	A solid state switch noted for its high speed. The static switch connects the bypass source to the critical bus in the event of a UPS failure. In most systems, it is designed only for momentary use until the bypass breaker has time to close. In the Powerware 80-150, it carries the load when the unit is on bypass continuously.
Step Wave	A voltage or current waveform which looks like a stair step. It is created by combining multiple square waves in an output circuit. The stair step waveform is shaped to closely resemble a sine wave.
Stiff Current Source	One that can supply relatively large amounts of current with little or no decrease in output voltage. A source that has very low output impedance.
Storage Life (Shelf Life)	The length of time an item can be stored under specific conditions and still meet specified requirements.
Substation	A place where power a lower levels is taken from the high voltage transmission line. Houses step-down transformers and switchgear required for local distribution.
Summary Alarm Contact	This option provides a single form C contact for customer use to indicate an alarm or trip condition has occurred in the module.
Supervisory Contacts	There are 10 form C contacts that indicate the status of various components the module as listed. Utility Power On, UPS On, UPS Off UPS Alarm, UPS FAIL, Output Breaker Closed, Battery Discharge, and additional contacts for single module bypass status.
Switchgear	Breakers, switches, relays, or protective devices used to control power distribution.
SYNC	Synchronization: - Refers to the act of adjusting the UPS frequency to match the phase of another source. Usually refers to syncing the inverter to the bypass.
Synchronous Motor	An AC motor whose speed is exactly proportional to the power input frequency.
THD	Total Harmonic Distortion. (See Harmonics)
Thermal Protection	Protection via a thermally actuated switch that interrupts the operation of a circuit, device, or UPS if the internal temperature exceeds a predetermined value.
Thermistor	A device with relatively high electrical resistance when cold and almost no resistance when at operating temperature. Thermistors are sometimes used to limit inrush current in off-line switchers.
Three Phase	A three phase power system has three primary voltage waveforms spaced equally (in time) out of phase with each other.
Three-Phase of Power	Three separate outputs from a single source. There is a phase difference 120 degrees between any two of the three voltages and currents.
Total Battery String Voltage	The total voltage of all cells in a single string.
Total Harmonic Distortion	The total amount of distortion created by the harmonics present in a circuit. Usually expressed as a percent.
Total Loss of Power	Zero voltage condition lasting more than one Hz (17 milliseconds); occurs in less than 5% of all power loss problems.
Transducer	Any device that senses one form of energy and converts it to another, as sound, force, temperature, or humidity to electrical energy or vice versa.
Transfer	Refers to the act of switching the critical load from the UPS to the bypass source, or from the bypass source to the UPS output.
Transfer Switch	One used to transfer the load between the UPS system and the bypass line.
Transfer Time	The amount of time required to transfer the load from the output of the inverter to the output of the bypass circuit.
Transformer	(1) An electrical device constructed of wire wrapped in even layers around an air or metal core. Several such windings can be wrapped on a common frame. The frame acts as a circuit path for magnetic fields to connect from one winding to another. By controlling the ratio of turns of wire from one winding to the next, one can convert voltage and/or current from one value to another and connect circuits with dissimilar impedance characteristics. (2) A magnetic device that converts AC voltages to AC voltages at any level. An ideal transformer is a lossless device in which no energy is stored and that requires no magnetic current.
Transformer Tap	An extra connection point on a transformer winding that allows one to adjust the input/output turns ratio without actually changing the physical construction of the transformer.
Transient	(1) A high-amplitude, short-duration pulse superimposed on the normal voltage. (2) A temporary and brief change in a given parameter. Typically associated with input voltage or output loading parameters.
Transient Response	For power circuits, the response of a circuit (usually specified in time) to return circuit Voltage to normal, or a specified Voltage range, after a transient Voltage surge is applied to the circuit.
Transient Response Time	The amount of time taken for an output to settle within some tolerance band, normally following a stated change in load.
Transistor	A solid state switch or amplifier. An active device used in electronic circuits to switch current on or off to amplify current.
Transmission Line	The conductors used to carry electrical energy from one location to another.
Trip	If a major alarm condition is detected, the UPS will automatically remove itself from the sync bus and shut itself down. This is known as a "trip".
Turnkey	A project in which the contractor takes full responsibility for making a system operational so that all the purchaser has to do is "turn-the-key."
Turn On	The conduction of a semiconductor to the "ON" state.
UL	Underwriters' Laboratories An independent, not-for-profit organization testing for public safety in the United States. UL recognition is required for equipment used in some applications.
UL and CSA Compliance	All U. S. domestic UPS products must comply with Underwriters Laboratories UL1778 regulations after April 1, 1992. These regulations are written specifically for UPS products and are more stringent than the general UL regulations. UL1778 compliance is necessary for products supporting telecommunications systems. Some UPS products also comply with Canadian Standards Act (CSA) regulations. CSA requirements are similar to UL1778. Any product operated in Canada must comply with CSA standards. Many U.S. vendors charge extra for this option. Some vendors manufacture their products in Canada to comply with CSA regulations but they may not comply wit UL1778 requirements (and vice versa). Products should not be sold or operated in the United States without UL1778 compliance or in Canada without CSA compliance, otherwise they can void a firm's insurance coverage.
Uninterruptible Power System	A device designed to supply power in the event of temporary or permanent loss of ac line power. Often these supplies will operate with either an AC line input or DC battery backup input. Alternate power source that corrects contamination, restores power, and protects the equipment from further power loss. Previously Uninterruptible Power Supply and Uninterruptible Power Source.
UPM	"Uninterruptible Power Module" - A module (one of several) which provides conditioned and uninterruptible power to the sync bus in a UPS system. UPM subsystems include: the Rectifier/Charger, Inverter, and possibly a battery. The static switch/transfer components are not present. A UPM is a UPS component and not a UPS.
UPS	"Uninterruptible Power System" - A system which provides conditioned and uninterruptible power to a critical load. UPS subsystems include: the Rectifier/Charger, Inverter, battery and static switch/transfer components. A stand-along, complete UPS system.
Utility AC	A source derived from commercial power or an auxiliary generator which is used as an alternate source for the critical load.
Volt-Ampere	The product of a Volt and an Ampere: a measure of power equivalent to a watt at 1.0 Power Factor (Apparent Power).
Voltage	The term used for the measure of potential difference between two points in an electrical circuit. Voltage difference creates the electromotive force which causes electrons to flow along a conductor.
Voltage (Volts)	The potential force of an electric current flowing in a circuit. Voltage can be thought of as the amount of water in a reservoir.
Voltage Control	A mode of regulation for the rectifier/charger or the Inverter in which the output voltage is regulated to a constant value.
Volts/Cell	Volts per cell.
Voltage Regulator	A circuit that develops a constant output voltage regardless of input voltage variations.
Voltage Sag	Below normal voltage input lasting approximately 0.12 - 0.53 seconds. Causes: lightning, elevator or air conditioning.
Voltage Surge	Above normal voltage input. Causes: heavy loads stopping
Walk-Up	The gradual rise of the charger output voltage when the charger begins operation.
Walk Into Load	See Ramp into Load.
Watt	The rate of work (1/746 horse power) represented by a current of one Ampere under a pressure of one Volt (True Power).
Waveform	A cyclical voltage or current will take on a repetitive characteristic shape that can be described as having the form of a wave when viewed on a display device.
Wet Cell Batteries	Often referred to as lead/acid or flooded, each case may contain one or more cells. The cases are usually clear making plate inspection easy. This battery case allows easy access for routine maintenance and service or electrolyte levels.
Witness Test	A test conducted by the manufacturer and witnessed by the purchaser to confirm proper operation.
Wye	A three-phase source or load connection, with single common junction and three-phase lines out.
Wye-Delta	The interconnections between a wye source and a delta load.
Wye-Wye	The interconnections between a wye source and a wye load.