AC Ripple

An unwelcome alternating AC source that causes a variation of the DC voltage. AC ripples should never reach the battery because they can damage the poles and cause internal heating.

Active Power.

The power is consumed in an alternating current (AC) circuit and measured in Watts.

Alternating Current.

This describes the flow of electrons in an electrical current that switches directions back and forth at regular intervals. This is the sinewave waveform.


The measurement of the current or rate of electron flow in an electrical circuit.

Ampere-hour (Ah).

The measurement of how many amperes a battery can deliver in an hour.

Apparent Power.

Measured in VA, this is the current drawn by a load at a specific supply voltage.

Automatic Battery Test.

A combination of simulated and actual discharges to test an online battery system for weaknesses. An alarm is generated if the battery falls below a predetermined level during the scheduled test.

Automatic Bypass.

A circuit to transfer the load from and to a bypass supply in a UPS. The bypass can be static-switch-based or relay.

Automatic Mains Failure (AMF).

When a standby power generator has an AMF fitted, the AMF detects main power supply failure. As a result, the UPS automatically starts up the generator to replace the main electrical supply.

Automatic Voltage Stabiliser (AVS).

This device, also known as an AVR (automatic voltage regulator) or VR (voltage regulator), stabilises the voltage to a load.


The amount of time a battery backup or power source can support the critical load during an outage. The autonomy, also known as backup or discharge time, will depend on the size of the load.

Availability (A).

The measurement of the yearly downtime due to repairs and system failure. It is a formula using the MTBF (mean time before failure) and MTTR (mean time to repair). The formula to calculate availability is A = MTBF/(MTBF + MTTR).


Battery Block.

Individual and connected battery cells within a self-contained battery.

Battery Cell.

A circuit consisting of positive and negative electrodes or plates as well as an electrolyte and a separator, all within a battery block.

Battery Set.

A string of batteries or various strings together.

Battery String.

The number of battery blocks connected in series. Together, they will reach a determined VDC and Ah rating. The number of batteries in a string should provide the appropriate Ah capacity and voltage (V).


A complete electrical power loss due to a mains failure and sometimes called an outage.

Blade Server Friendly.

A UPS system with the ability to support the leading power factor required in high processing density blade servers in modern transformerless systems.

Boost Charge.

A battery set that receives a high charge voltage.

Booster Converter.

A system using an inverter to set a DC supply from a rectifier or battery set within a transformerless UPS.

Break-Before-Make (BBM) Bypass.

This refers to the break the bypass introduces when a load is transferred to or from the output of a UPS to the bypass supply.


The period of time when the power supply is continuously at a low voltage.



A circuit (capacitive) can store electrical energy as a charge.

Capacity System

UPS modules are connected in a parallel system. Their combined capacity is sufficient for total load support; however, there is no redundancy as the failure of one will overload the others.

Central Power Supply System (CPSS)

Like a UPS, a CPSS is a standby power system to supply backup power for light, security, and medical devices.

Centralised Parallel Architecture (CPA)

The UPS module supplies the load via a central static switch (CSS) which represents a single point of failure.

Circuit Breaker

A device to prevent power surges from damaging equipment due to excessive current flow.

Clamping Level

The level at which a transient protection device clamps the voltage during a spike to a lower level.

Coefficient of Capacity; Coefficient of redundancy

An N+n parallel redundant system where N is the number of modules required for a critical load and n is the number of redundant UPS modules.

Common Mode Noise

Disturbances between earth and supply lines that cause high-frequency electrical noise.

Constant Voltage Transformer (CVT)

A kind of ferroresonant transformer that uses a resonant circuit and non-linear magnetic properties to provide a stable output.

Crash Kits

Additional kits for rapid response in case of a system failure.

Crest Factor

The ratio between the maximum value and root-mean-square value of alternating current.

Critical Loads

The vital networks and systems a business or organisation must maintain at all times during an outage or mains power supply.

Current (A)

The number of Amps flowing in a circuit.

Current Limit

The maximum current that can be drawn from a UPS output or electrical circuit.


Decentralised Parallel Architecture (DPA)

Together, the UPS modules in a DPA directly feed the critical load. One module is the primary source, with connected UPS modules being “slaves”. The slaves take over the master function if it develops a fault or is shut down for maintenance.

Deep Discharge

A battery voltage of direct current (VDC) drops to a level that is not safe for operating and won’t recover from.


A drop or sag in the transient voltage.

Direct Current (DC)

Electrons in a current only flow in a single direction.


During a short circuit or overload, a device around a Power Continuity Plan will disconnect without interrupting the operation and protecting it from potential damage.

Displacement Power Factor

The correlation between W (real power) and VA (Apparent Power) at the fundamental frequency.


A device to monitor abnormal events in the UPS. The pair of contacts are normally closed (NC) or normally open (NO). When closed, the signal can flow for remote monitoring and detection.

Dynamic Stability

The capacity to deliver a stable output voltage waveform during a load-step change

Double Conversion

The online UPS converts mains AC power into DC. This is necessary to condition it, charge the battery and invert the power to clean AC. Double conversion allows the clean AC to power the critical load. In the case of a power failure or overvoltage, the battery provides the load without interruptions.

Double Conversion Efficiency

The comparison of the output power and load measured in kW. Efficiency is maximised when the UPS runs near to full load.


Earth Fault (Ground)

The live component of the mains power supply is connected to an earth connection.

Electric Noise

Sinewave used on certain modes may cause high-frequency noise.

Electromagnetic Compatibility (EMC)

The amount of electromagnetic interference an electronic device can handle and generate.

Electromagnetic Induction

When voltage or an electrical potential difference across a conductor is produced due to a changing magnetic field.

Electromagnetic Interference (EMI)

Electromagnetic disturbance that causes electrical noise.

Emergency Power Off (EPO)

A UPS with a signal contact that causes the UPS to shut down completely.

Emergency Response Time

The amount of time it takes to initiate an emergency plan or for an engineer to arrive.

Extended Warranty Agreement (EWA)

An extended service plan to suit the needs and budget of individual customers to maximise the total cost of ownership (TOC). It also includes response time options.


Ferroresonant Transformer

A transformer that regulates voltage using non-linear resonance.

Fixed Cellular Terminal (FCT)

A device with a built-in mobile phone capacity that can receive and route calls.

Float Charging

A system for UPS batteries that optimises battery life.

Flywheels (DC)

A battery alternative or device that lowers the initial discharge during short interruptions by converting kinetic energy to DC power.

Frequency Converter

A device that converts the input frequency to an alternative output frequency.

Fuel Cell

As well as heat and water, this device uses hydrogen as a fuel to create AC and DC supplies.


Certain UPSs have a future proof design so that when load demand increases, additional hot-swap modules can be added without interrupting the power supply.


Galvanic Isolation

If a transforming coupling only connects two circuits, they can be galvanically isolated. However, this is not typical with a standard transformer-based UPS design.

Gas Discharge Tube (GDT)

Gas containing electrodes inside a temperature-resistant case switches to a low impedance when a spike or transient voltage occurs.

Gas Turbine

The kinetic energy generated from combustion is converted into electrical energy for an AC or DC power supply.


A device using gas or diesel to provide backup power in the event of an outage. If the automatic transfer switch detects a power outage, the generator starts up, and the UPS immediately transfers power to the circuits.



A difference in the mains power supply sinewave over the basic 50 or 60Hz.

Harmonic Distortion

Also known as total harmonic distortion, this is when the mains power supply sinewave is distorted.

Harmonic Filter

A module to minimise disturbances of harmonic distortion and provide clean AC power.

Hertz (Hz)

The number of complete waveform cycles per second, with standard mains frequency being 50 or 60Hz.


A UPS module that can be removed from the system or added to it without interrupting the power supply to the critical load.



A short circuit current.


The nominal current.


The cable that takes the mains power from the closet substation and point of common coupling to a building.


An electromotive force is generated in an inductive circuit. This occurs when the magnetic flux is varied.

Ingress Protection (IP) Rating

A number that specifies the physical protection of electronic equipment against liquids (e.g., water) and solid objects (e.g., dust). “I” is the protection rating against solids and “P” against liquids. For example, IP56 means the equipment has protection agains dust (“5”) and water drops(“6”).

Inrush Current

The initial high input current that electrical equipment or a power supply draws when switched on.

Insulated Gate Bipolar Transistor (IGBT)

A device for inverters and rectifiers to facilitate high power switching.


The part of the UPS that converts DC power to an AC output.


Separation of the output and input supplies in a UPS device.



A protocol used within communications for creating a hierarchical structure from an individual RS-232.

Joule (J)

In the International System of Units, this is one Watt per second.


Line Interactive UPS

A hybrid system that adds voltage regulation features to a bypass line in order to increase performance.


All the electrical devices connected to a power source. In terms of a UPS, this is the power required by all connected electrical devices.

Load Bank Test

A simulated load that tests for preventative UPS maintenance, including the battery. Load bank testing identifies and detects weaknesses before they cause an online malfunction.

Load Shedding

Reducing the total load on a UPS to maximise battery autonomy.


Maintenance Bypass

Also known as a bypass panel, this type of bypass is used when maintenance is required to ensure constant power to critical systems.

Make-Before-Break (MBB) Bypass

A bypass that creates a contact between the UPS outlet and the bypass supply before a load transfer.

Mean Time Before Failure (MTBF)

An indication of the reliability of a device between failures and average operational length.

Mean Time to Repair (MTTR)

The average time it takes to repair a system entirely.

Metal Oxide Varistor (MOV)

A device that can absorb extremely high surges without suffering damage.


protocol used within communications for creating a hierarchical structure from an individual RS-232, much like a JBUS.


A system where individual modules can run in parallel to scale capacity or build in redundancy. A modular UPS it typically a rack system with a battery and hot-swappable modules.


One module contains all the hardware and software to run a UPS and can be connected to more when expansion is needed.


N-n Redundancy

The configuration of modules to increase capacity or build in redundancy. “N” is the number of required modules, whereas “n” is the redundant units.

Noise Level

Regarding a UPS, this is the level of decibels (dB or dBA) an electrical circuit makes.


Off-line System

Raw power from the mains is supplied to the critical loads, and the load is transferred to the UPS when the mains fails, or there is a power surge.

On-line System

Conditioned power supplies the critical load, and an inverter continues to power devices with DC power from a battery in the case of power failure.


A device that transfers a signal between phase and neutral via a short optical transmission.


A failure in the mains power supply, also known as a blackout or total mains power failure.


An unusually high spike or surge in voltage.


Parallel Systems Joiner (PSJ)

A device that connects two separate groups of parallel UPSs.

Parallel-capacity System

The total load demand, without redundancy, is met by using various UPS in parallel.

Parallel Redundancy

Two or more UPSs with an N+X setup can share the load. One UPS will support the load if the other fails.


This can be single or three-phase. Single-phase is one sinewave at the fundamental frequency, and three-phase has three waveforms at a 120° separation.

Phase Power Factor

The correlation between real power (W) and apparent power (VA) at the fundamental frequency.

Point of Common Coupling (PCC)

The point where the cable supplying mains power meets the electric distribution network.

Power Conditioner

A device used to filter, stabilise, and regulate the mains voltage to a load.

Power Continuity Plan

A plan that outlines the details of how a system will maintain power in case of a mains power failure.

Power Density

To calculate a UPS power density, divide the output in Watts by the total floor area in square meters (W/m2).

Power Factor (pF)

The difference between the Watts used and the volts x Amps in a circuit.

Power Management Display

The front panel of a UPS that allows the user to control the UPS. It usually includes LEDs to indicate status and an LCD to show comprehensive information.

Power Supply Units (PSU)

Typically found in new electronics, a PSU is a device that converts waveform into different DC levels needed to supply internal circuits.


A kind of field bus generally used in control and automation.

Pulse Width Modulation (PWM)

When an inverter switches, a composite waveform reaching a sinewave is created.


Radio Frequency Interference (RFI)

Radio waves that are strong enough for a circuit to absorb them and lead to a malfunction.

Reactive Power

The power energy that flows back and forth in a circuit. This is measured in kVAR (kilovolt-ampere reactive) or MVAR.

Real Power

The number of Watts dispersed by a load.


A converter of AC supply to an AC supply.


Like a rectifier, the DC power is used to power an inverter and charge batteries.

Recovery Time

The time it takes to restore regulated voltage after switching loads onto a standby generator.


The number of modules connected is more than what is needed for the load. Therefore, if one module fails, redundant modules continue to provide an uninterruptable power supply.

Redundant Critical Circuits

A UPS system where one or various modules will power the entire load if a redundant critical circuit fails.


To set and control the AC/DC output voltage.

Remote Signalling Panel

Dry relay contacts are used to provide UPS information when the user is up to 100m away from their system.


The capability of a UPS system to deliver continuous conditioned power to the critical load even if there has been a module failure.


Small-capacity modules are used in parallel to provide redundancy. Additionally, it is relatively easy and cost-effective to scale capacity.


An AC element is superimposed onto a DC waveform that is powering a device.

Ripple Free Battery Charging

Protection against AC ripples resulting in longer battery life.


Restriction of Hazardous Substances.

Root Mean Square (RMS)

The square root of the mean values of all squares of current and voltage.

Rotary UPS

A type of UPS that transforms kinetic energy into electrical energy to supply a load.


A standard interface for two compatible devices to communicate at up to 20Kh/s and over 15m.


A communication interface for up to 10 receivers at a rate of 10Mb/s and up to 100m.


A communication interface similar to the transmission rates of the RS-422 but for up to 32 drivers and receivers.



The ability to increase or decrease a power capacity depending on the changing demands of a load. Scalable UPS systems use UPS units in parallel — these are typically modular systems in a UPS rack.

Series Redundancy

The redundant power of one UPS is used to supply the bypass of another UPS.

Silicon Avalanche Diode (SAD)

A fast-acting device that responds to high-energy surges or transient and then lowers the initial surge current.

Simple Network Management Protocol (SNMP)

A global protocol for standard communications to monitor devices connected on a TCP/IP network.


The waveform that quality generators, inverters, and UPSs produce.

Six Nines Availability

A formula to calculate the operational availability of a system as a proportion of the total time needed. The formula is A (availability)=MTBF/(MTBF + MTTR).

Slew Rate

The rate that a UPS can change the output frequency.


High levels of voltage disturbances that are superimposed onto the usual AC supply for a short time.

Static Stability

When a device is able to provide a steady output voltage waveform when load conditions are stable.

Static Switch

A device that switches a load between two AC power sources at high speeds.

Static UPS

Solid-state electronic components in a UPS, and no moving parts.

Super Capacitors

A device that sorts electrical charges to supply applications with short-term power instead of a battery. Typically used in small UPS solutions.


Stort-term voltage increases over the usual mains supply, generally lasting for several cycles.

Switched Mode Power Supply (SMPS)

This is most often found in computers and telecommunication devices. It is a type of PSU with a non-linear current draw.

Switching Time

Also known as transfer time, this is the total time it takes for a load to be transferred between AC supply sources.



A device with a semi-conductor and often within a rectifier or static switch.

Total Cost of Ownership (TCO)

The total cost of a UPS, including initial hardware costs, running, maintenance and repairs.

Total Harmonic Distortion (THDi)

The total of all harmonics produced in a system compared to the usual sinewave.

Total Power Factor/True Power Factor

The total power factor includes distortion power factors and displacement power factors.


A component to adjust voltage levels and provide galvanic isolation.

Transformerless Design

A type of design that enables the implementation of smaller, lighter and more efficient UPSs at lower running costs and minimal noise disruption.

Transient Voltage Surge Suppressor (TVSS)

A device with either MOVs or GDTs to reduce the force of a transient or spike.


Voltage disturbances at high energy levels and short bursts affecting the regular power supply.


(Triple-N). A multiple of the third harmonic in waveform.



An extended dip in voltage lower than the regulated voltage.

Uninterruptible Power Supply (UPS)

A backup power supply system to provide continuous power to a load regardless of the mains power supply. A UPS can keep mission-critical applications functioning or allow for a graceful shutdown of computers and electronic equipment to protect sensitive devices and data.

UPS Group Synchroniser (UGS)

A device used with two sets of parallel UPSs to synchronise outputs.


A universal serial bus port used to connect UPS modules to a computer. This allows user to control the uninterruptable power supply remotely using software.


Value-Regulated Lead-acid (VRLA) Battery

A common type of lead-acid battery for UPS modules.

Volt (V, kV, or MV)

The measure of pressure or electrical force and can also be expressed as VAC or Vdc.

Volt Ampere (VA, kVA, or MVA)

The amount of apparent power and current drawn by a load at a certain voltage or supply.

Volt-Free Contacts (VFCs)

Two contacts are either normally open or normally closed. If the contacts are closed, the current can pass through the circuit.

Voltage and Frequency Independent (VFI)

A UPS output is not dependent on fluctuations in the mains power supply (power voltage) or changes in frequency. Usually, an on-line double-conversion uninterruptible power supply has VFI capability.


Watts (W, kW, or MW)

A measure of the rate of energy transfer or electrical power drawn by a load.


An image reflecting the shape of a wave.


The directive 2002/96/EC to prevent waste electrical and electronic equipment.

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