Altitude: The maximum height above Sea Level at which the UPS will operate within
specifications at its full output power (generally 4000 ft. -1280 m). Due to lower air density, the
allowed power decreases at higher altitudes.
Ambient Temperature: Environment temperature where a UPS is operating or kept in storage.
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 for 5, 10 or 20 Hours
discharge time. AH figure should be divided by the discharge time to get the maximal discharge
current. The AH capacity is a function of discharge time, decreasing at short
backup times. Thus, 20H rated 10AH battery, may supply only 3.5AH for 15 minutes or 2.5AH
for 5 minutes. Battery capacity also depends on temperature, aging, number and depth of
discharge cycles, and 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.
2003 USA Average ASAI =0.9994. (source: Electric Department City of Wadsworth).
ATS (Automatic Transfer Switch): See Transfer Switch.
Audible Alarm: An audible signal enunciated upon mains 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 by 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 ( above 60 KVA) may present noise figure of about 65dBA, which is
the level of main road high traffic environment.
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.
The 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, which was turned off, due to batteries discharge during Power outage.
Automatic Retransfer: An Automatic transfer from Bypass to Inverter in an On-Line system.
For Conditions: see Retransfer.
Autonomy time: Battery back-up time. The time a UPS energizes the load from Batteries.
Autonomy time is strongly influenced by the percentage of actual load Watts related to
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)
Example: For MTTR of 24H and MTBF figure of 100,000 H. The availability is 99.976 %. This
means, that the UPS is operational during 8757.9H 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.
Back-up time: See Autonomy time.
Balanced Load: Equal loading on each phase at the output of a three-phase UPS.
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, system's Rectifier fulfills also the charging function.
Battery circuit breaker: A circuit breaker connected to the battery, enabling manual battery
isolation for maintenance purpose, and battery protection in case of DC buss short circuit.
Battery current limit: A function that limits battery-charging current. VRLA batteries in
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. Generally, the UPS
will shut down when battery voltage drops to end voltage set point. Certain systems adjust this set
point automatically according to the actual discharge current. Typical end voltage rating is
1.75V/cell , or 10.5V for a 12V battery.
Battery-Less UPS : Electromechanical Backup system using ride through energy stored in a
flywheel to backup load after power outage until local generator kicks in.
See Electro-mechanical UPS
Black Start (Cold Start): The ability to turn a UPS on from batteries, without mains.
Blade UPS : Uninterruptible power supply aimed to feed Blade Computers, mainly used in
Data Centers. It has generally a hybrid topology, working normally from conditioned mains, thus
enabling higher efficiencies compared to double conversion systems.
BSI (British Standard Institute): BSI is the National Standards body of the UK. It develops
standards and standardization to meet the needs of business and society.
Boost Charging: A charging method used to restore batteries to their full capacity after a
deep discharge or after long storage duration. Boost charging is performed, by applying relatively
high voltage (generally 2.4V/cell for VRLA battery) for a limited time.
By-Pass: Functional UPS module that connects the load of an On-Line UPS directly to mains in
case of overload or UPS failure.
CAIDI: Consumer Average Interruption Duration Index. A reliability index used by Electric
Utilities to measure in hours the average power interruption experienced by single customer.
It represents the mean time that takes the utility to restore service.
2003 USA average CAIDI was 66 minutes per interruption ( source: Electric Department city of
Wadsworth).
CBEMA Curve : See ITI Curve.
CE Marking: Manufacturers declaration that the product complies with the essential
requirements of the relevant European health, safety, and environmental directives.
Cenelec: European Committee of Electrotechnical Standardization.
Charger: See Battery Charger.
Charger Control: Control of charger's output voltage or current . Main controlled parameters:
Temperature compensation of float voltage, Transfer from boost charge regime to float voltage,
Battery current limit, Input current limit (used mainly when working from generator).
Cold Standby operation: A sleep or idle mode operation of the UPS (generally normal mode
in Off-Line systems), aimed to decrease power consumption. The UPS starts operating in
case of power failure or upon receiving an external command and will be able to support load
after the turn on interval.
Cold Start: See Black Start.
Cos Phi: The cosine of the angle between the voltage and current waveforms with the
fundamental source frequency.
Crest factor: The ratio between the Peak and the RMS values of a periodic current waveform.
The Crest Factor of a sinusoidal current waveform is 1.4142.
Current Harmonic Distortions (THDI): See THD.
Current sharing: A function or electronic circuit that causes equal current sharing between
parallel-connected UPS systems.
DCE (Data Center Efficiency) - An efficiency metric, derived by dividing Total IT equipment
power by total Data center power. More information, see PUE, the reciprocal of DCE.
DC UPS: An Electronic device connected between the Utility Power (or alternate AC source)
and electric consumers, comprising 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 input power
factor. An output converter is connected between UPS output and battery. It acts as an ac/dc
rectifier to charge the batteries and as a dc/ac inverter upon mains outage.
Display: LCD or Led display, generally located on the UPS front, providing information about
the UPS status, including voltages, currents frequency etc. The display is also used in certain
systems to program system settings.
Dip: See Sag.
Double Conversion: Topology of On-Line UPS (VFI class per IEC 62040-3). The AC mains
voltage is converted to DC by means of an ac to DC Rectifier (or Charger), The DC voltage is
then converted to conditioned AC by means of the Inverter.
Dry contacts: Isolated relay contacts generally used for alarm or status signalization.
Dual Source UPS: Also called Dual input. UPS with separate Bypass input, enabling AC input
redundancy by feeding the rectifier and the by pass from different sources. UPS output will
automatically synchronize with the bypass input source.
Dynamic Regulation: System's output voltage deviation caused by connecting or
disconnecting rated load. Generally, different values refer to linear and non-linear type loads.
Earth fault: Leakage or short circuit to ground (Chassis) from isolated battery, isolated input
or output power lines, including Neutral. Some standards require activation of earth fault alarm,
if the earth fault current exceeds a rated value.
Earthing system: System for earthing of exposed conductive parts and neutral: IT system:
Neutral Isolated from earth (or high impedance to earth), exposed parts earthed Individually.
TN system: Exposed conductive parts connected to earthed Neutral.
TT System: Exposed conductive parts and Neutral are directly earthed.
ECO mode: An economical mode of operation, which enables certain On-Line UPS systems to
work in an Off-Line mode from the Utility, while the inverter, which is in hot standby operation,
is able to accept load upon mains failure.
Effective value: Effective voltage or current. See RMS.
Efficiency: UPS efficiency is defined as AC output Watts divided by AC Input Watts. The
full load efficiency of Standby or Line Interactive UPS is generally within 95% to 98%, and
within 90% to 95% for an Online Dual conversion system. Full load Efficiency is defined as
the ratio between stated UPS output Watts (stated Volt -Ampere capacity times stated output
Power Factor) divided by input Power in Watts.
For AC to DC ( Output Watts to DC bus) efficiency, see Inverter Efficiency.
EIA (Electronics Industry association): Australian membership based association to assist
development of national Electronic Industry.
EIA 232: Former RS232, now EIA 232 is a Communication Standard for Serial signal
transmission between a computer and peripheral devices. The maximum distance is about 50
feet (16m). Generally, the connection is made between UPS to the computer by means of 9 or
25 pin D-Type connector (often using only three wires) directly, or through modem and
telephone line.
EIA 422: Former RS 422, now EIA 422 is a 4 wire Communication Standard for Serial signal
transmission between a computers and peripheral devices. The maximum distance is 4000 feet
(1200 m). One driver can be connected to up-to 10 receivers. .
EIA 485: Former RS 485, now EIA 485 is a 2-wire Communication Standard for Serial signal,
multi point transmission. The maximum distance is 4000 feet 1200m). The standard specifies
up to 32 drivers and 32 receivers on a single buss.
Electromechanical UPS : ( Also called: Rotary , Batteryless, or Flywheel UPS).
An electromechanical Uninterruptible Power Supply that generally consists of a motor-generator
and additional hardware which enables to supply short time energy by utilizing kinetic energy
stored in a flywheel. It is normally applied in high power installation, by supplying the ride-
through energy from primary power outage until backup generator kicks in . It offers higher
efficiencies due to standby mode of operation and high Reliability due to absence of batteries.
EMC (Electro-Magnetic Compatibility): The ability of equipment to perform its function without
causing or suffering unacceptable degradation due to Electromagnetic Interference to or from
other equipment in the same environment.
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.
ENV 50091 – European Standard, Equivalent to IEC 62040, published by Cenelec. See IEC
62040
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.
External Battery: A battery located outside the electronic cabinet. In certain cases, the
external battery is connected in addition to the internal battery for Backup time expansion.
Fault Clearing: The ability to isolate a short-circuited load branch, by tripping its circuit
breaker or fuse.
Ferroresonant UPS: A Usystem 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 high frequency switching element in low power UPS Inverters.
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 constant
voltage charging regime. The float voltage value at 25 degrees Celsius is generally 2.3V/cell for
Lead-Acid battery and 1.42V/cell for Nickel-Cadmium battery. To extend battery lifetime, the
float voltage should be adjusted (compensated) with temperature according to battery
manufacturers recommendation.
Flooded batteries: Wet electrolyte batteries. Checking and adding water through vent caps
should periodically maintain these batteries. These batteries have proven reliability of extended
20 to 30 years of operation. They require large floor space, and ventilation systems, which
generally limit location to dedicated battery rooms.
Flywheel UPS: Electromechanical Backup system using ride through energy stored in a
flywheel to backup load after power outage until local generator kicks in.
See Electro-mechanical UPS.
Graceful Shut Down: Software function that shuts-down computers fed by the UPS, after mains
outage, a few minutes before it turns off due to battery discharge. Graceful shut
down resembles manual computer turn off, no data is lost and the computer reboots
Grounding: Connecting the UPS chassis to external protective ground.
Harmonics: A sinusoidal component of a periodic wave, having a frequency that is an integral
multiple of the fundamental source frequency.
High Frequency UPS: A term sometimes used for systems which utilize high frequency waveform
in the internal UPS inverter. Inverter switching is performed by high ultrasonic carrier frequency (
above 16 kHz) waveform, pulse width modulated with a sinusoidal system's output frequency
waveform. The carrier frequency is attenuated by Low pass filter to provide a sinusoidal 50 or 60
Hz output waveform. This technique is today standard in almost all Line Interactive and ON-Line
systems.
Hot standby operation: A standby mode of operation generally implemented in Series redundant
systems. The UPS is constantly fully operational and running without load. It starts to supply
current immediately upon load connection.
Hot Swappable modules: Battery modules or redundant modular sub-assemblies, which
can be replaced without turning the UPS off.
Hybrid UPS : Uninterruptible power supply that uses a DC to AC 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 input power factor.
IEC: International Electrotechnical commission of standards. Many countries adopt IEC standards
as their national standards.
IEC 60529 (IP code): See IP Code.
IEC 61000-3-2: Standard- Limits of input current harmonics (equipments with Input Current <=
16A/Phase).
IEC 61000-3-4: Standard- Limits of input current harmonics (equipments with Input Current >
16A/Phase).
IEC 61000-4-11:Standard- Voltage dips, short interruptions and voltage variations immunity tests
(equipments with Input Current <= 16A/Phase).
IEC 61000-4-34:Standard- Voltage dips, short interruptions and voltage variations immunity tests
(equipments with Input Current > 16A/Phase).
IEC 60146-5: Standard - Switches for Uninterruptible Power Systems.
IEC 60950 (former IEC 950): Standard –safety of Information Technology equipment (Applicable
also to UPS systems).
IEC 62040-1-1: Standard -General and safety requirements for UPS systems used in restricted
access locations.
IEC 62040-1-2: Standard -General and safety requirements for UPS systems used in operator
access areas.
IEC 62040-2: Standard- UPS Electromagnetic compatibility (EMC) requirements.
IEC 62040-3: Standard- UPS performance requirements and test methods.
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.
ITI (Information Technology Industry Council). Formerly known as CBEMA : Computer and
Business Equipment Manufacturer Association. ITI is a group of leading high tech companies in
the USA.
ITI Curve (CBEMA Curve): An AC input voltage envelope, published by ITI council, that shows
input voltage limits which typically can be tolerated ( will not cause functional interruption) by
most Information Technology Equipment.
According to ITI curve a computer will not turn off in short (less than 20 ms) power interruptions,
or in case of line voltage sags up to 70% of nominal lasting less than 0.5Second. The curve
describes also the influence of high input voltage on computer operation continuity.
Internal Battery: A battery located inside the UPS cabinet.
Interharmonics: Waveforms with frequencies, which are not integers of the fundamental source
frequency.
Input current distortion: The distortion of the AC input current due to harmonics. The
distortions are measured by means of input current THDI figure at the AC input of the UPS. Some
standards limit the maximum allowed input current distortions. Most systems today limit THDI
distortion to less than 10%.
Input voltage distortion: The distortion of the AC input voltage due to harmonics. The distortions
are measured by means of input voltage THD figure at the AC input of the UPS.
Inrush current: The initial current at the input of the UPS upon turn on (see Rectifier
walk-in). Also the initial current a consumer draws from the UPS upon turning on the consumer.
The inrush current is generally caused either, due to charging of DC filter capacitors, or because of
sub-cycle magnetizing current of a transformer, located in the turned on equipment.
Inverter: Functional UPS module that inverts the DC battery voltage to 50Hz or 60Hz AC voltage.
Inverter Efficiency: Also called AC to DC efficiency: AC Output Watts divided by
Inverter Input Watts, when load is connected to Inverter output. (Inverter input watts
equals DC buss voltage multiplied by the current drawn by the Inverter). Inverter efficiency is used
for calculating battery capacity. Ratings vary from 80% at low DC voltages to 95%, at high DC
buss voltages. Full Load Efficiency is defined as Rated Output Watts (Rated Volt-Ampere times
stated output power factor) divided by DC input Current times Nominal DC input Voltage.
IP Code: Degrees of Enclosure protection evaluated as described in IEC 60529 standard. The first
character of the IP code indicates protection degree against the ingress of solid objects. The
second character indicates protection against ingress of water. For example
IP 20 means: Protection against ingress of solid foreign objects of 12.5mm Diameter and greater,
whereas the second character indicates no protection against water. IP31 protects against ingress
of solid objects greater than 2.5mm Diameter and against vertically falling water drops.
ISO (International Organization for Standardization): ISO is a global International network that
Identifies, develops, and delivers standards required for business, government and society.
ISO 9001: A set of standards for quality management concerned with production and service
policies. ISO 9001 2000 has replaced the old ISO 9001, As well as ISO 9002 and ISO 9003 which
became discontinued.
Isolation: Generally refers to the allowed maximum KV ( Kilo-Volts) between input and ground,
output and ground and input to output in UPS systems with 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.
IT System: See Earthed System.
Lagging Power Factor: Power factor of an Inductive type circuit, in which the current
lags behind the source voltage (negative cos phi)
Lead Acid Battery: A rechargeable battery common in UPS systems, it is composed of
lead plates suspended in electrolyte solution of sulfuric acid and water.
Leading Power factor: Power factor of a capacitive type circuit, in which the current
leads the voltage (positive cos phi).
Line: See primary power.
Line Interactive UPS: A system, which energizes the load from the utility mains
providing conditioned power by filtering and stabilizing mains voltage (VI class per IEC 62040-3).
Upon mains outage the load is energized from batteries via the Inverter.
Linear Load: load comprised of linear (non switching) components, such as, resistors,
capacitors, inductors, motors, lamps, transformers etc.
LVD (Low Voltage Disconnect): A circuit or device that disconnects load from battery, when the
battery discharges below a minimum low voltage level (generally 1.75 V/cell for lead Acid batteries).
MAIFI (Momentary Average Interruption Frequency): Electric Utility Reliability Index which
presents the average number of power interruption per customer lasting less than 2 minutes.
2003 USA average MAIFI =1.86. (source: Electric Department City of Wadsworth).
Maintenance Free Battery: See VRLA batteries.
MBP ( Manual By Pass): A manual switch connected between utility mains and UPS output. The
MBP is used to energize the load for during UPS maintenance, or in case of UPS failure, until
repaired.
Modbus: A communication protocol which enables monitoring and control of distributed devices
in a network from a master supervisory computer.
MTBF (Mean Time between failures): The average expected time between failures, generally
expressed in hours. MTBF figure is the reciprocal of the failure rate figure (λ).
Example: MTBF of 100,000 hours means that a system which works continuously is
expected to fail once in 11.41 years. Total MTBF figure of a site with 200 such systems is 500H
(100,000/200), such site is expected to experience about 18 failures per year (365*24/500).
MTBF figure refers generally to failures that let the UPS down. It is either a value calculated
according to Reliability Standards, or based on actual Field experience.
MTTR (Mean Time To Repair): The time (generally expressed in Hours) from failure incident,
until the system is repaired and restored to normal operation.
MTTR should take in account in addition to actual field repair time, also detection time, travel
time, waiting time etc.
N+1 Redundancy: A redundant method based on one module more than needed to fulfill the
required performance. For instance, three parallel systems, each rated 2KVA, form a 2+1
redundant system for a 4KVA consumer. Failure of a single UPS will not affect systems
operational performance.
Nema (National Electric Manufacturers Association): The Leading trade association in the US
representing electric industry manufacturers of products, used in the generation, transmission
and distribution, control and end-use of electricity.
Nema Standard PE-1: Standard IEC 62040-3 with certain differences. See IEC 62040-3.
Noise level: Acoustical noise measured in dBA units, generally at a distance of 1meter or 1.5
meter from the UPS. Typical noise levels (in dBA): quite room: 40; office 45-55; vacuum cleaner
(3m): 70, disco: 100.
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 2V for lead acid battery and 1.25V for Nickel-Cadmium
battery (generally abbreviated as 1.2V). Thus, the nominal voltage of a 6 cells lead acid battery
equals 12V.
Normal UPS Operation: Operational state is termed Normal, when Utility Power is available and
within ratings, the battery is charged, the load is within rating and it is fed from the preferred
(default) source (Mains Power if Stand-By , Inverter if On-Line system).
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.
OFF-LINE UPS (Passive Standby): A system, which normally energizes the load directly
from the utility mains (see VFD classification by IEC 62040-3). It contains a charger and an Off-
Line Inverter. The Inverter is switched ON upon mains outage to supply the load.
ON-LINE UPS (True on line ): A double conversion system, which energizes the load
continuously from the inverter (see IEC 62040-3, VFI Classification ). The inverter is fed
from mains via a rectifier in normal operation, or from batteries upon mains outage.
Output current limit: The maximum current, which the Inverter is able to supply at rated
voltage.
Output harmonics: The Harmonics at the output voltage of the UPS. These Harmonics are
measured by means of output voltage THDV figure.
Output regulation: The deviation of output voltage from its nominal rating, due to changes in
other parameters, like load, battery voltage, ambient temperature, mains voltage, within their
allowed and rated limits.
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: The maximum load current allowed for a limited time while still keeping rated
conditioned voltage. Typical, Inverter overload is 125% of full load for 10 minutes 150% for 30
seconds.
Overvoltage: RMS voltage increase above the rated value, generally for a duration of a few
seconds (Shorter time, see Transient or Swell).
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.
Parallel Operation: Operation of two or more systems with outputs connected to a common
Load Buss for Redundancy or power enhancement purpose. To enable parallel operation the UPS
systems should have equal output voltages, operate synchronously (same frequency and same
phase), and have load-sharing capabilities.
Passive Standby UPS: See OFF-Line .
PFC (Power Factor Correction): Correction of input Power factor by means of decreasing the
harmonic currents at the input of the UPS. PFC is generally implemented by means of special
filters, multiple pulse rectifiers or electronic functional modules.
Phase angle: Generally relates to the angle between the output phase voltages in a three phase
UPS. Ideally, the phase angle should be 120 degrees. Practically a deviation exists, due to system
and load unbalance..
Phase Imbalance: A condition (or figure) in which the angle between phases in a three-phase
system differs from 120 degrees. Example: UPS output phase imbalance at 50% unbalanced
loads is 120° ± 3°.
Power Factor (W/VA): Actual Power divided by Apparent (or Total power). In new UPS
systems with power factor ( PF) correction, UPS input power factor is greater than 0.95.
UPS Output power factor is currently 0.7 in most cases. New regulations require to increase
servers input power factor above 0.9. UPS Watts and VA should exceed respective load ratings.
More on this see UPS Sizing.
Power Failure (Power Outage): Any degradation of AC power below the minimum rated
value, generally lasting more than 10 mS, which may cause the loss of service to a customer
or facility.
Power Interruption: Power disappearance for a short period (generally seconds to minutes).
Power Inverter: See Inverter.
Power Line Disturbances: The ten most frequent disturbances (IEC 62040-3): 1.Power outage
(>10ms), 2.Voltage fluctuations (<16ms), 3.Voltage transients (4.16ms),
4.under-voltage (continuous), 5.over-voltage (continuous), 6.Lightning effects (sporadic <1ms),
7.Voltage surges(<4ms), 8.Frequency fluctuations (sporadic), 9.Voltage bursts
( periodic), 10.Voltage harmonics(continuous).
Power source: Energy source feeding the UPS. Generally Utility power (See primary power) or
Standby Generator.
Primary Power (Mains): External AC supply from the Utility grid (or other AC power source).
Generally: Single-phase 230V 50Hz, or 120V 60Hz; Dual voltage 120/240V 60Hz; Three Phase
230/400V 50Hz, 120/208V 60Hz.
PUE (Power Usage Effectiveness (PUE) metric): A figure representing Data center efficiency.
PUE is derived by dividing the total data center power by the power consumed by IT equipment.
IT equipment relates to such items as servers, storage systems, networking equipment such as
routers switches. Total power includes additional equipment such as power conditioning, UPS,
cooling, distribution equipment, lightning etc.
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.
Rechargeable battery: Battery which can be recharged (as opposed to one time battery).
See VRLA battery.
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 current limit: Rectifier protection, that limits the maximum current which can be
drawn from the rectifier. Rectifiers' current limit setting takes in account the DC current
drawn by Inverter at full rated load, and in addition the current required to recharge the
batteries.
Rectifier walk in: A gradual increase of output voltage, aimed to limit Inrush currents.
Generally, the walk in time may take up to 5 seconds.
Redundancy: A method based on using one or more extra backup modules, which enable
normal system performance even in case of system failures. For example, redundancy is
achieved by feeding a consumer of 1KVA by means of two 1KVA rated UPS systems connected
in parallel, hence single unit failure does not affect load performance.
Reliability: The probability that equipment will perform satisfactory during a specific time.
Remote Control: The ability to turn a system on or off, monitor its behavior or to change it’s
setting from remote location.
Retransfer: Manual or automatic Retransfer of Load from Bypass to Inverter in an online
system. Generally enabled only when no fault condition exists, Inverter output voltage is in phase
with Bypass voltage and the load is within its nominal rating.
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.
RS 232: See EIA 232
RS 422: See EIA 422
RS 485: See EIA 485
Rotary UPS: Electromechanical Backup system using ride through energy stored in a flywheel to
backup load after power outage until local generator kicks in. See Electromechanical UPS
Run-Time: See Autonomy time.
SAIDI (System Average Interruption Duration Index): Electric Utility Reliability Index, which
represents the average power interruption duration for customers served by the electric utility
during a specific time period.
2003 USA average SAIDI was 65.09 minutes ( source: Electric Department city of Wadsworth).
SAIFI (System Average Interruptions Frequency Index): Electric Utility Reliability Index,
which is defined as the average number of times that customer's power is interrupted during a
specific time.
Scalability: Ability to increase the output power of the UPS in user's premises, by additional
modules or UPS systems, in parallel with existing units.
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 Static Transfer Switch and is used frequently as the rectifying element in high power
UPS Rectifiers.
Series Redundancy: A configuration of two systems, with only one (The main UPS)
supplying load current via its transfer switch. The second is a backup unit operating normally
in Hot Standby mode. Backup's output is connected to the alternate (Bypass) input of the Main
transfer switch. Should the Main UPS fail, its transfer switch will connect the load automatically
to the output of the Backup unit .
Slow Start: See Rectifier Walk-In.
Smart UPS: Generally means Line Interactive type UPS.
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
Spike: See surge.
SSR (Solid State Relay): An electronic module substituting generally a relay with a single
Normally Open AC contact. The SSR is used in UPS Transfer Switches mainly due to its fast
turn-on and extended life expectancy.
Stand-alone UPS: A single system connected between the utility grid and the load.
Stand-by UPS: an Off line or Line Interactive system that normally supplies the load from
the utility mains.
Static Switch: A transfer switch that uses semiconductors (generally Triacs or 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 to 1mSec).
Surge Arrester: An electronic device or component used to protect electric circuits from
spikes and overvoltages. The surge arrester is generally connected in parallel to the input of the
protected circuit. It limits the maximum voltage by absorbing excess energy.
Surge energy rating: A measure of maximal energy in joules a Surge arrester can absorb
without damage.
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.
Swell: An RMS increase in the AC voltage, lasting from half cycle to few seconds.
Temperature compensation: See float voltage.
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.
TN System: See Earthed System.
Topology: A term referring to the type of the UPS . Generally the topology is either Off Line
(Sometimes called standby), Line Interactive or 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 to other alternate source), and vice versa. The
transfer from Inverter in 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)
Transfer Time: Time interval from sensing transfer requirement until the load is transferred to
alternate source.
Transient: A switching or other disturbance of the normal power voltage waveform, lasting
less than half cycle. The transient can be either over-voltage or under-voltage.
TT System: See Earthed System.
U: A dimensional unit used to measure the height of equipment or racks. 1U equals 1.75 inches
(44.45 mm).
UL: (Underwriters Laboratory Inc): US testing and standard publishing authority.
UL 1778: USA standard for Uninterruptible Power Supply equipment.
UL 60950: USA standard equivalent to IEC 60950. UL requires UPS to comply with
UL 1778 and UL 60950-1
Unbalanced Load: Conditions in which at least two phases at the output of a three-phase UPS
have different (magnitude or PF) loads.
Undervoltage: A voltage below the rated RMS value, generally for a duration of a few seconds
(Shorter time undervoltages, see Transient or Sag).
UPS: (Uninterruptible Power Supply): An Electronic device connected between the Utility Power
and electric consumers, comprising generally of filters, Rectifier, Battery, DC/AC Inverter,
Transfer Switch and associated circuits. The UPS is intended to provide clean undisturbed
stabilized AC voltage, within strict amplitude and frequency limits, to protect the consumer
from any Utility Power disturbances and irregularities, including outages for a limited time
dictated by the capacity of the Battery Bank. The term UPS refers generally to AC Static
systems, Other types include DC and Rotary UPS.
UPS Battery: A battery feeding the inverter. In most cases it is a VRLA type battery.
UPS Classification Code: IEC 62040-3 Standard classifies UPS performance by 8-digit code:
The first three digits classify output dependency of input mains in normal operation,
(VFI-Voltage and Frequency Independent, VI – Voltage Independent, VFD-Voltage and
Frequency Dependant - See relevant Terms).
The next two digits refer to output waveform: S-Sinusoidal for linear and non-linear loads
(THDV<8%), X – Non sinusoidal with non-linear loads, Y- Non-sinusoidal. The last three digits
refer to output dynamic performance (including transients during transfer switch operation).
Frequency stability: Deviation of output frequency from nominal value when the UPS is not
synchronized to mains power (at mains outage). The stability is generally +/- 0.5%, with RC
oscillator and +/- 0.05% with crystal oscillator.
UPS TYPES: Standard IEC 62040-3 defines three standardized UPS systems:
A. Passive Standby (Generally called Off -Line ).
B. Active Standby (Generally called Line Interactive ).
C. Double Conversion UPS (Generally called ON-Line ).
VA (Volt Ampere): Apparent power, defined as product of RMS Voltage by RMS Current
VFD Class UPS: A system where the output voltage is Dependant on input Voltage and
Frequency (IEC 62040-3). VFD class generally refers to Passive Standby (Off-Line) systems.
These are able solve the first three Power Line Disturbances (see term). The output from the
Inverter is generally non-sinusoidal.
The Dynamic classification is generally 3 due to power interruption during transfer switch
operation.
VFI Class UPS: A system where the output voltage is Independent from input Voltage and
Frequency (IEC 62040-3). VFI class generally refers to Double Conversion ON-Line types. .
These systems generally solve all 10 Power Line Disturbances (see term).The Output is
generally sinusoidal for linear and non-linear loads, with class 1 (No break) dynamic
performance.
VI Class UPS: A system where the output voltage is Independent on the Input Voltage
(IEC 62040-3), but depends upon the input frequency. VI class generally refers to Line-
interactive type. These units are able to solve first five Power Line Disturbances (see term)..
Voltage Harmonic Distortions (THDV): See THD.
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 (Thus frequently called maintenance free batteries).
Watts: Actual power, equal to the product of fundamental (source frequency) harmonic RMS
voltage, times fundamental harmonic RMS current, times cosine of the phase displacement angle
between the voltage and the current.
