| Uninterruptible Power Supply (UPS) Glossary Comprehensive power UPS glossary |
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 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.
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.
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 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.
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.
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