Active Load Share
A current sharing scheme that adjusts the output voltage of individual rectifiers so that all rectifiers in a system produce the same output current.
Active Voltage Control
Adjusting the rectifier output voltages to maintain a constant system voltage (measured at the output or battery) independent of load fluctuations during normal operation.
Altitude
The maximum height above sea level at which a UPS will operate within specifications at its full output power (typically 4000 ft). Due to lower air density, the allowed power decreases at higher altitudes.
Ambient Temperature
Environmental temperature where a UPS is operating or stored. Unless stated otherwise, the UPS will function within its specifications in the operating temperature region. Storage temperature is the allowed ambient temperature for handling and storing the UPS, while preserving its operational characteristics.
Ampere-Hour (AH)
A figure indicating battery capacity, generally defined in hours discharge time. Battery capacity depends on temperature, aging, number and depth of discharge cycles, and frequency/quality of preventive maintenance.
ASAI (Average Service Availability Index)
Electric Utility Reliability Index that represents the ratio of total customer minutes that service was available to total customer minutes demanded in a time period.
ATS (Automatic Transfer Switch)
See Transfer Switch.
Audible Alarm
An audible signal enunciated upon mains power outage or UPS failure or warning. Intermittent tone signals may indicate various announcements. An alarm silence button enables disabling the audible signal.
Audible noise
Noise emitted by the UPS generally measured in dBA units, at a distance of 1or 1.5-meter from the UPS. Most UPS systems are less than 55dBA which is relatively quiet. High power systems (60 KVA and above) may present noise around 65dBA, which is comparable to the level of traffic on a busy road.
Auto Dial
A feature enabling the UPS to automatically report alarm and status via a modem to a remote location.
Automatic Battery test
A preventive test and alarm aimed to reveal weak or damaged batteries. This test is generally performed at user selectable-programmed intervals.
Automatic Restart
A function that enables, upon mains power restoration, an automatic restart of a UPS that has been safely turned off due to the batteries being discharged during a power outage.
Automatic Retransfer
An automatic transfer from bypass to inverter in an on-line system.
Autonomy Time
Battery back-up time. The time a UPS energizes or supports the load from the batteries. Autonomy time is strongly influenced by the percentage of actual load in watts related to the nominal Watt rating of the UPS. Thus a 1KW UPS that supplies 5 minutes when fully rated, will generally supply more than 15 minutes with 500W load.
Auto-start
See Automatic Restart.
Availability
The degree to which a system or component is operational and accessible when required for use. Availability = 100 x MTBF/ (MTBF + MTTR) where MTBF is the mean time between failures and MTTR is the meantime between repairs. Example: For a MTTR of 24hours and a MTBF of 100,000 H. The availability is 99.976 %. This means that the UPS is operational during 8,757.9 hours per year and is down during a time totaling 2.1 hours per year.
AVR (Automatic Voltage Regulator)
An electronic circuit that regulates output voltage in order to keep it within allowed band limits.
Backfeed Protection
Protection aimed to avoid power backfeed to the utility grid through the Bypass from the output, in order to prevent possibility of hazardous voltage risks on the distribution board, when the utility mains power is disconnected. UL standard requires this protection to operate even if two transfer switch components fail simultaneously.
Balanced Load
Equal loading on each phase at the output of a three-phase UPS. Unbalanced loading can damage UPS components.
Battery Backup
A battery or a set of batteries in a UPS system whose 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, at standard temperature, and at a specified discharge rate.
Battery Charger
Functional UPS module that converts the utility mains AC voltage to DC voltage for charging batteries in order to restore the charge that was withdrawn during mains outage. Generally a system's rectifier also fulfills the charging function.
Battery circuit breaker
A circuit breaker connected to the battery, enabling manual battery isolation for maintenance purposes, and battery protection in case of DC buss short circuit.
Battery current limit
A function that limits battery-charging current. VRLA batteries in a float charging regime do not generally require current limiting, this function is therefore not always available.
Battery end Voltage
The lowest battery voltage for system operation. Typically, the UPS will shut down when the battery voltage drops to preset end voltage set point. Certain systems adjust this set point automatically according to the actual discharge current. The typical end voltage rating is 1.75V/cell, or 10.5V for a 12V battery.
Blackout
AC Power Failure.
Blade UPS
Uninterruptible power supply designed to feed Blade Computers, mainly found in data centers. It has generally a hybrid topology working normally from conditioned mains thus enabling higher efficiencies compared to double conversion systems.
Boost Charging
A charging method used to restore batteries to their full capacity after a deep discharge or after a long storage duration. Boost charging is performed by applying a relatively high voltage (i.e 2.4V/cell for VRLA batteries) for a limited time.
Brownout
Undervoltage.
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.
Bypass
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 safely and without power interruption.
Clean Power
Electrical power that has been conditioned and/or regulated to remove electrical noise from the output power.
Current
The flow of electricity.
Current sharing
A function or electronic circuit that causes equal current sharing between parallel-connected UPS systems.
DC UPS
An electronic device connected between the Utility Power (or alternate AC source) and the electric consumer comprised of a DC power supply (rectifier) and battery. The DC UPS is intended to provide uninterrupted conditioned DC voltage, within strict regulation limits against any utility power disturbances including outages.
Delta Conversion UPS
Hybrid type UPS offered by APC Corporation. It uses an AC/DC converter between the AC input and the DC battery buss. The converter is able to add or subtract energy from the mains in order to stabilize output voltage and correct the input power factor. An output converter is connected between UPS output and battery which acts as an ac/dc rectifier to charge the batteries, and as a DC/AC inverter upon mains outage.
Delta Connection
A method of connecting a three-phase source or load in series for a closed circuit (3-wire, plus ground).
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.
Dual Source UPS
Also called dual input. A UPS with separate bypass input enabling AC input redundancy by feeding the rectifier and the bypass from different sources. UPS output will automatically synchronize with the bypass input source.
Efficiency
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.
EMI (Electro Magnetic Interference)
Electromagnetic disturbance generated mainly by switching circuits and devices that interrupt, obstruct, or degrade the limits of performance of electric equipment.
End Voltage
See Battery End voltage.
EPO (Emergency Power Off)
A functional module in the UPS enabling manual UPS turn off at emergency situation, like fire or safety hazards, while disabling all output voltages. Can be damaging to UPS components as it is a “hard off”.
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.
Ferroresonant UPS
A UPS system which implements output voltage stabilization, by means of a special output transformer, utilizing the constant voltage characteristic of the transformers’ saturated iron core. To eliminate the current drawn from the inverter by the magnetizing transformer, a capacitor is added to form a parallel resonant LC filter tuned to UPS frequency. The ferroresonant method provides output isolation and harmonic filtering. The shortcomings are: relatively low efficiency, increased weight and volume.
FET (Field Effect Transistor)
A transistor in which the current between the power terminals is controlled through the gate terminal by means of an electric field, drawing very low drive power. It is used as a high frequency switching element in low power UPS Inverters.
Filtering
A method of removing noise from the output of a UPS as a means to prevent "dirty power" from reaching connected equipment.
Float Charging
Constant voltage charging method. See also Float voltage.
Float Current
Current drawn by a charged battery in float charging regime.
Float Voltage
DC Voltage applied to the battery by the charger (or rectifier) in a constant voltage charging regime. To extend battery lifetime, the float voltage should be adjusted (compensated) with temperature according to battery manufacturer’s recommendation.
Flooded batteries
Wet electrolyte batteries. Checking and adding water through vent caps is required to periodically maintain these batteries. These batteries have proven reliability of extended (15-20 years of operation). They require large floor space and ventilation systems which generally limit location to dedicated battery rooms.
Frequency (hertz)
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 or 60 Hertz.
Full Load
The greatest load that a circuit is designed to carry under specific conditions. Any additional load is considered an overload.
Graceful Shut Down
Software function that shuts down computers/hardware fed by the UPS after power mains outage a few minutes before the UPS turns off due to battery discharge. Graceful shut down resembles manual computer turn off, no data is lost and the computer reboots.
Hardware Default Voltage
The rectifier output fail-safe operating voltage used if the rectifier microprocessor fails.
Harmonic Distortion
The presence of harmonics that changes the AC voltage waveform from a simple sinusoidal to a 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.
High Rupturing Capacity Fuse (HRC)
A precisely rated fuse that will operate under high fault current conditions, without self-destructing.
Hot-Swappable Batteries
A feature which enables the user to change UPS batteries without powering down the connected load.
Hot-Swappable Power Modules
A feature which enables the user to change UPS power modules without powering down the connected load.
IGBT (Insulated Gate Bipolar Transistor)
A transistor which combines FET input and bipolar Transistor output, forming a switching device with very low drive power, high blocking voltage, and low conduction losses. IGBT is the most popular power switching element in UPS inverters.
Input current distortion
The distortion of the AC input current due to harmonics. Some standards limit the maximum allowed input current distortions, and most systems today limit THDI distortion to less than 10%.
Input voltage distortion
The distortion of the AC input voltage due to harmonics.
Inrush current
The initial current at the input of the UPS upon turning on (see rectifier walk-in). Also the initial current a customer draws from the UPS upon turning on the customer’s load. The inrush current is generally caused either due to charging of the DC filter capacitors, or because of the sub-cycle magnetizing current of a transformer located in the turned on equipment.
Interharmonics
Waveforms with frequencies, which are not integers of the fundamental source frequency.
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.
Inverter
Functional UPS module that inverts the DC battery voltage to 50Hz or 60Hz AC voltage.
Isolation
Refers to the allowed maximum kilo-volts between input and ground, output and ground, and input to output in UPS systems with an isolation transformer.
Isolation transformer
A transformer connected at the input of the UPS to isolate the battery from the utility grid, or at the output of a UPS to isolate the load from the utility grid.
Lagging Power Factor
Power factor of an inductive type circuit in which the current lags behind the source voltage.
Lead Acid Battery
A rechargeable battery common to UPS systems. They are composed of lead plates suspended in electrolyte solution of sulfuric acid and water.
Line-interactive
A UPS containing an off-line inverter that must transfer on during a blackout but provides faster transfer times than an off-line UPS. Power conditioning and surge suppression are provided to protect the connected load.
Low Voltage Disconnect (LVD)
A circuit or device that disconnects the load from the batteries when the battery voltage falls below a preset minimum low voltage value. The LVD reconnects the load to the batteries when the battery voltage rises above a preset value.
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.
Maximum System Current
The maximum current that can be supplied by a power system (excluding batteries) under all conditions, normally 120% of rated system current.
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, MA, first published in 1896.
Nema (National Electric Manufacturers Association)
The leading trade association in the US representing electric industry manufacturer’s of products used in the generation, transmission, distribution, control, and end-use of electricity.
Noise
Random, sporadic or multi-frequency electrical signals that become part of a transmission making the signal or information more difficult to identify.
Noise level
Acoustical noise measured in dBA units (decibals), generally at a distance of 3-3.5 feet from the UPS. Examples of noise levels: quite room 45dBA, conversation 55dBA, car @ 50mph 65dBA, vacuum cleaner 70dBA, diesel truck 90dBA, and threshold of discomfort 120dBA.
Nominal Battery Voltage
The product of nominal cell voltage multiplied by number of cells connected in series. The no load circuit voltage of a charged single battery cell is defined as the nominal cell voltage and equals 2 volts for lead acid battery and 1.25volts for Nickel-Cadmium battery (generally abbreviated as 1.2V). Thus, the nominal voltage of a 6 cell lead acid battery equals 12volts.
Nominal System Voltage
The DC output voltage generally used to describe a type of system, usually 24 volt or 48 volt.
Nominal Value
A designated value that has been accepted for the sake of convenience. For instance, nominal voltages are values assigned to circuits so that the voltages of the circuits can be conveniently discussed as 120 VAC nominal units, or 230 VAC nominal units.
Non-Linear Load
Load comprised of switching components, such as diodes, rectifiers, thyristors. switching or pulse modulating systems or circuits. Non-linear loads generate current and voltage harmonics with integral multiple frequencies of fundamental source frequency.
Normal UPS Operation
Operational state is considered normal when utility power is available and within ratings, the batteries are charged, the load is within rating, and it is fed from the preferred (default) source (typically mains power).
N + 1 Redundancy
A redundancy method based on one module more than needed to support the required performance. Example: three parallel systems, each rated at 10KVA, form a 2+1 redundant system for a 20KVA system. The failure of a single UPS will not affect the system’s operational performance.
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 power 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.
Outlet
Any point on a wiring system where current is taken to supply electrical power for a load.
Output current limit
The maximum current which the inverter is able to supply at a rated voltage.
Output Enable Delay
The delay between the start of the primary side switching and the start of the output current walk in. There are two parts: a fixed hardware delay controlled by the secondary side control circuits and an adjustable delay controlled by the microprocessor. During this output, voltage is at the minimum of approximately 40 V.
Output harmonics
The Harmonics at the output voltage of the UPS. These Harmonics are measured by means of output voltage THDV figure.
Output Waveform
Generally, the output waveform of the inverter. It can be either sinusoidal or rectangular. Some manufacturers call the rectangular waveform by such names as semi-sinusoidal or quasi sinusoidal. Rectangular waveform may cause undesirable operation with certain loads. The manufacturer should therefore be consulted before application.
Overload
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.
Overvoltage Shutdown (OVSD)
A protection method that will shut down any rectifier module with an output voltage exceeding a preset maximum value.
Parallel Redundancy
A combination of more than two systems operating in parallel that includes at least one redundant (extra) unit which is not required to provide the total expected load. A failure of a single UPS does not affect system performance, thus increasing dramatically load power availability.
Power Factor (PF)
The ratio of total real power (W), to the total apparent power in volt-amperes (VA) or W/VA.
Power Failure (Power Outage)
Any degradation of AC power below the minimum rated value, generally lasting more than 10 milliseconds, which may cause the loss of service to a customer or facility.
Power Interruption
Power disappearance for a short period, typically lasting a few seconds to several minutes).
Power Source
Energy source feeding the UPS, typically utility power or stand by generator.
Primary Power (Mains)
External AC power supply from the utility grid or other AC power source.
PWM (Pulse width modulation)
Modulation technique in which the duration of pulses is varied in accordance with some characteristic of the modulating signal. The technique is implemented in UPS systems in PFC circuits, rectifiers and inverters.
Rated Rectifier Current
The maximum output current of a rectifier at 58V for a 48 V (nominal) rectifier, or 29 V for a 24 V (nominal) rectifier.
Rated System Current
The sum of the rated rectifier currents in the power system.
Raw Power
Electrical power that may or may not contain unwanted electrical signals. Rectifier: functional UPS module that converts the utility mains input voltage to DC voltage. In off-line systems the rectifier is used for battery charging. In most on-line systems the main rectifier is used to charge the batteries and feed the load through the inverter.
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.
Redundancy
A method based on using one or more additional backup modules which enable normal system performance even in case of system failures (see N + 1 above).
REPO (Remote Emergency Power off)
EPO function activated by remote signal (generally by means of external dry contacts). The EPO should be associated with disconnection of all energy sources to the UPS, including batteries.
RMS (Root Mean Square)
RMS value, also called effective value, equals the square root of the mean value of the squared amplitudes of all harmonics in a waveform divided by square root of 2 (1.414). Current RMS value equals the equivalent direct current that would produce the same power dissipation in a given resistor. The RMS values of current or voltage are generally measured with a True RMS meter.
Scalable UPS
A UPS that allows for expandability. For example, one that enables a UPS to accommodate a larger load by purchasing additional power modules or adding another UPS system(s) in parallel with an existing system(s).
SCR (Silicon Controlled Rectifier)
A rectifier that has a gate terminal, in addition to anode and cathode that controls current flow from anode to cathode. The SCR is generally the switching element in the static transfer switch and is used frequently as the rectifying element in high power UPS rectifiers.
Series Redundancy
A configuration of two systems with one UPS feeding the bypass of a second UPS. If the first unit fails it drops back to bypass transferring the load to the second unit. The second UPS is a backup unit operating normally in hot standby mode.
Sine Wave
The sinusoidal wave form exhibited by alternating current.
Slope Discrimination Method
A scheme that causes the overvoltage shutdown set point to fall with increasing load.
SNMP (simple network management protocol)
A transmission Internet Protocol (TCP/IP) used to manage and control IP gateways and the networks to which they are attached.
Stand-by UPS
An off- line or line interactive system that normally supplies the load from the utility mains.
Start-On-Battery
Enables user to power up a UPS in the absence of utility power.
Start Up Delay
The interval between power on and the start of current walk in. It is the sum of the primary enable delay and the fixed and adjustable portions of the output enable delay.
Step Wave (modified sine wave)
Enhanced version of square wave that provides adequate input for more sensitive loads but still not as high of quality as a sine wave.
Static Switch
A transfer switch that uses semiconductors/ SCR’s as the power switching elements.
Sub-harmonics
Interharmonic waveforms having frequencies lower than the fundamental.
Surge (Spike)
Sharp high voltage increase lasting up to1milisecond.
Surge Arrester
An electronic device or component used to protect electric circuits from spikes and over voltages. The surge arrester is generally connected in parallel to the input of the protected circuit. It limits the maximum voltage by absorbing excess energy.
Synchronization (phase lock or sync.)
Operation that causes an AC waveform to be at the same frequency and in phase with another waveform. Generally refers to synchronizing the output of the inverter to the utility mains in order to allow smooth transfer of load from mains to inverter and vice versa.
Temperature Sensor
A sensor that is used to produce a variable electrical output representing the temperature of a component, typically a battery.
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.
THD (Total Harmonic distortions)
RMS value of all harmonics in a waveform (excluding fundamental) divided by RMS value of fundamental. THDV refers to voltage waveform. THDI refers to current waveform.
Topology
A term referring to the type of the UPS. Typically the topology is either off line (standby), or line interactive (on-line).
Transfer Switch
Functional UPS module which transfers the load from the output of the inverter, to the bypass (fed by utility mains or alternate source) and vice versa. The transfer from inverter in an online type system is generally performed automatically in case of overload or whenever the output voltage deviates from a preset window. The automatic transfer switch will generally re-transfer automatically to the Inverter (see retransfer)
Transformer
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.
Trickle Charge
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
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.
Unbalanced Load
Conditions in which at least two phases at the output of a three-phase UPS have different loads (magnitude or PF).
Undervoltage
A voltage below the rated RMS value, generally for duration of a few seconds. Shorter duration undervoltages are considered sags.
UPS (Uninterruptible Power Supply)
An electronic device connected between the utility power and end user comprising generally of filters, rectifier, batteries, DC/AC inverter, transfer switch and associated circuits. The UPS is intended to provide clean undisturbed and stabilized AC voltage within strict amplitude and frequency limits to protect the consumer from any utility power disturbances and irregularities.
UPS Topology/Types
Passive Standby (off -line).
Active Standby (line interactive).
Double Conversion UPS (on-line).
User-Replaceable Batteries
User replaceable batteries allow the user 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 that 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 20 amps and 120 V has a VA rating of 2400 or 2.4kVA.
VRLA Batteries (Valve Regulated Lead Acid batteries)
Sealed batteries with internal pressure, which causes the hydrogen and oxygen generated during charging to be turned back into water thus decreasing dramatically the amount of gassing through the valves and the need for water replenishing. The Electrolyte is immobilized using Gel or AGM construction. VRLA batteries can be installed in vented manned locations, are classified as non-hazardous, non spill-able, meet the requirements of air transportation (IATA), and practically do not require maintenance.
Walk-In-Time
The time that the rectifier takes to reach rated output current after the start up delay. The slope is fixed so that a lower output current will have a shorter walk in period.
Walk-In
The process of gradually ramping up the rectifier output voltage (and current) at start up to prevent a large input current surge.
Watt (W)
The unit of measure for true power. Watts = VA x Power Factor.
