| Uninterruptible Power Supply (UPS) Glossary Comprehensive power UPS glossary including relevant power quality terms |
Actual power: (Real Power): See Watts.
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.
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 by the UPS upon battery outage and 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.
The 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 UPS 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 of the UPS, 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 UPS batteries, in order to restore the charge that was withdrawn during
mains outage. Generally, the UPS 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 UPS operation. Generally, the UPS will
shut down when battery voltage drops to end voltage set point. Certain UPS systems adjust
this set point automatically according to the actual discharge current. Typical end voltage
rating for UPS batteries is 1.75V/cell , or 10.5V for a 12V battery.
Black Start (Cold Start): The ability to turn a UPS on from batteries, without mains.
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.
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.
dBA (Decibels Adjusted ): Noise power calculated in dB adjusted to human hearing, by a
Filter. The filter modulates the noise figure by weighting factor corresponding to the
sensitivity of human ear to various frequencies. 0 dBA is equal to 3.16 Pico-Watts.
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.
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 UPS settings.
Dip: See Sag.
Double Conversion: Configuration 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. 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: UPS 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 UPS 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 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 94% for
an Online Dual conversion system.
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.
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 UPS cabinet. In certain cases, the external
battery is connected in addition to the internal battery for Backup time expansion.
Fault Clearing: The ability of a UPS (from inverter or Bypass) to isolate a short-circuited
load branch, by tripping its circuit breaker or fuse.
Ferroresonant UPS: A UPS 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.
Free maintenance battery: See VRLA battery.
Gel Battery: VRLA batteries with electrolyte, which is immobilized by using a gelling agent
like fumed Silica.
Graceful Shut Down: Software function that shuts-down computers fed by the UPS, after
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
normally upon mains restoration.
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.
Hot standby operation: A standby mode of UPS operation generally implemented in Series
redundant systems. The UPS is constantly fully operational and running without load. It
starts therefore 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.
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 0.5
Second. 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 UPS 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 DC to AC 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).
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 in UPS spec, 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 UPS, 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: UPS 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).
Mains (Utility): See Primary Power.
Maintenance Free Battery: See VRLA batteries.
Manual By-Pass (MBP): A manually operated switch, which enables to transfer the load
from the inverter to the utility mains. The MBP is used to energize the load in case of UPS
failure, or for repair or maintenance purpose of the UPS system.
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 UPS failures,
generally expressed in hours. MTBF figure is the reciprocal of the failure rate figure (λ).
Example: UPS MTBF of 100,000 hours means that a UPS 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 UPS 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 of a UPS, 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: UPS 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 UPS, Inverter if On-Line UPS).
Non-Linear Load: UPS 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 UPS): A UPS, 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 UPS): 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 UPS 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 in an On-Line UPS: 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 UPS systems operating in parallel,
that includes at least one Redundant (extra) UPS 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 UPS Operation: Operation of two or more UPS 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 UPS.
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 power.
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.
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 rectifier 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 A single UPS 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 UPS 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 sqrt of
2 (1.414). It is the value of the equivalent direct current that would produce the same power
dissipation in a given resistor. The RMS value of current or voltage is generally measured
with a “ True RMS ” meter.
RS 232: See EIA 232
RS 422: See EIA 422
RS 485: See EIA 485
Rotary UPS: An electromechanical UPS that generally consists of a motor-generator and
additional hardware (Like Flywheel, Capacitor banks, Fuel or Battery) to enable it to operate
from energy storage system in case of mains outage.
Run-Time: See Autonomy time.
Sag (dip): A short voltage reduction (generally lasting 100uSec. to few Seconds).
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.
2003 USA Average SAIFI was 3.24 interruptions. (Source: Electric Department city of
Wadsworth).
Scalability: Ability to increase the output power of the UPS in users premises, by additional
modules or UPS systems, in parallel with the existing system.
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 (Series UPS Operation): A configuration of two On-Line UPS systems,
with only one system (The main UPS) supplying load current via its transfer switch. The
second UPS is a backup system operating normally in Hot Standby mode. The Backup UPS
output is connected to the alternate (Bypass) input of the Main UPS transfer switch. Should
the Main UPS fail, its transfer switch will connect the load automatically to the output of the
Backup UPS.
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 UPS connected between the utility grid and the load.
Stand-by UPS: a UPS that normally supplies the load from the utility mains. It contains a
battery charger and an inverter. Off line UPS and Line Interactive UPS are stand by systems
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 UPS term that refers 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 UPS is generally performed automatically in case of
overload or whenever the output voltage of the UPS deviates from a preset window. The
Automatic Transfer Switch will generally retransfer 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 equivalent to IEC 60950. UL requires UPS systems 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 mains undervoltage, see Transient or Sag).
UPS (Uninterruptible Power System): 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 UPS, Other UPS types include DC UPS and Rotary UPS.
UPS Battery: A battery feeding the inverter in the UPS. 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).
UPS 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: UPS Standard IEC 62040-3 defines three standardized UPS systems:
A. Passive Standby UPS (Generally called Off -Line UPS ).
B. Active Standby (Generally called Line Interactive UPS ).
C. Double Conversion UPS (Generally called ON-Line UPS).
VA (Volt Ampere): Apparent power, defined as product of RMS Voltage by RMS Current
VFD Class UPS: A UPS where the output voltage is Dependant on input Voltage and
Frequency (IEC 62040-3). VFD class generally refers to Passive Standby (Off-Line) UPS
systems. These systems 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 UPS where the output voltage is Independent from input Voltage and
Frequency (IEC 62040-3). VFI class generally refers to Double Conversion ON-Line
systems.
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 UPS 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 UPS systems. These systems are able to solve first five Power Line Disturbances
(see term).
Generally, performance during transfer operation does not conform to class 1 specification.
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.
For more detailed description of major terms please see UPS basics as well as other articles
on this site.
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