Beginner guide

Actual power (Real Power): See Watts.

AGM Batteries (Absorbed–Electrolyte Glass Matt): VRLA battery, in which the entire electrolyte is absorbed in a thin
fiberglass sponge like separator.

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).
The allowed power decreases at higher altitudes.

Ampere-Hour (AH): A figure indicating battery capacity.
The AH capacity is a function of discharge time, decreasing at short backup times.

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.   

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.

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.

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converts the utility mains AC voltage to DC voltage for charging UPS batteries, in order to restore the charge that was withdrawn during mains outage.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.

Black Start (Cold Start): The ability to turn a UPS on from batteries, without mains.  

Blackout : Complete loss of power. Some literature considers a voltage drop below about 80V to be a blackout as well since most
equipment will not operate below these levels.
Bypass:  Functional UPS module that connects the load of an On-Line UPS directly to mains in case of overload or UPS failure.

Cold Standby operation: A sleep or idle mode operation of the UPS (generally normal mode in Off-Line systems). 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 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.

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.  

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.

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.

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.

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.

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.  

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.

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.

Internal Battery:  A battery located inside the UPS cabinet.

Interference (high-frequency): High-frequency parasitic current that is either conducted (electrostatic origin) or radiated (electromagnetic origin) by a device.

Input current distortion: The distortion of the AC input current due to harmonics.The distortions are measured by means of input current THD figure at the AC input of the UPS.

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.

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. 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.

Isolation: Generally refers in UPS spec, to the allowed maximum KV 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.

Lead Acid Battery: A rechargeable battery common in UPS systems, it is composed of lead plates suspended in electrolyte solution of sulfuric acid water.

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.

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.

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.

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 45-60; vacuum cleaner (3m): 70: busy road 80; 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 Nicad 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 harmonics: The Harmonics at the output voltage of the UPS. These Harmonics are measured by means of output voltage THDV figure.

Output Waveform: Generally, the output waveform of the 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 Bus 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.

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 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.

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.

Remote Control: The ability to turn a UPS on or off, monitor its behavior or to change it’s setting from remote location.   

REPO (Remote Emergency Power off) – EPO function: activated by remote signal  (generally by means of external dry contacts). The EPO should generally be associated with disconnection of all energy sources to the UPS.

RS232C (Recommended standard RS232C): Standard defining the communication circuits between devices for synchronous and asynchronous transmissions on the following types of lines: two-wire, four-wire, point-to-point, telephone lines and local links with short cables. Though the standard covers only transmissions over distances up to 15 meters, it is often possible to ensure correct transmission over greater distances using high-quality shielded cable in a reasonably satisfactory electrical environment. Most terminals and devices on the market can implement this transmission standard.
RS485 (Recommended standard RS485): This standard is similar to RS422A except that the number of possible links is greater and up to 32 senders may be interconnected to as many receivers. This system is particularly designed for local-area networks.

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 or Brownout : Decrease in voltage levels which can last for periods ranging from fractions of a second to hours. Can be caused by heavy equipment coming on line such as shop tools, elevators, compressors etc. Also occurs when utility companies deliberately do this to cope with peak load times.

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.   

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.

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.

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.

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.  

THD (Total Harmonic distortions):  RMS value of all harmonics in a waveform (excluding fundamental) divided by RMS value of fundamental. THDV refers to Voltage waveform. THDI refers to Current waveform.

Topology: A 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.

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 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

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
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 spillable, 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.