| Uninterruptible Power Supply (UPS) Glossary Comprehensive power UPS glossary |
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.
For more detailed description of major terms please see UPS basics as well as other articles on
this site.
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