Glossary of UPS System Technical Terms

This glossary of technical terms is designed to help you understand the frequently used terms within the uninterruptible power supply (UPS) industry.

Active Power

A measure of actual power (Watts) drawn by an electrical load.

Alternating Current (AC)

An electrical current whose electrons flow briefly in one direction to a peak before dropping back to zero and then flowing in an alternative direction
before repeating itself. The waveform created is a sine-wave.

Amp or Amperes (A)

A measure of the flow of electrical current.

Ampere-hour (Ah)

The number of Amps that a battery set can deliver in one hour.

Apparent Power

The electrical current drawn by a load at a given supply voltage measured in VA.


A measure of system up time compared to its downtime expressed as a percentage.

Automatic Bypass

An electrical circuit incorporated within a UPS (or bypass panel) creating a power path from and to a bypass supply, which may be relay or static

Automatic Voltage Stabiliser (AVS)

A mains power supply voltage stabilisation device. Also known as an Automatic Voltage Regulator (AVR) or Voltage Regulator (VR).

Autonomy (Runtime or Backup Time)

The amount of time, stated in either minutes or hours, that a battery set or other power source will support the load.

Battery Block

A self-contained battery consisting of a number of individual and connected battery cells.

Battery Cell

A small storage device for electricity within a battery block.

Battery Set

Comprises of a battery string or a number of battery strings.

Battery String

Comprises of a number of battery blocks arranged in series to achieve a set Vdc and Ah rating.


A power cut or mains power supply failure, also referred to as an outage.

Boost charge

A high charge voltage applied to a battery set.


A transformerless assembly within a UPS used to step up the DC supply from a rectifier or battery set to the level required by an inverter.

Break-Before-Make (BBM) Bypass

A bypass that introduces a break when transferring a load from the output of a UPS to the bypass supply, or bypass supply to a UPS.


A drop in the normal mains power supply voltage, which can last from a brief period of a few cycles to days.

Building Management System (BMS)

Not to be confused with Battery Management System (BMS). A centralised alarm monitoring system normally providing either a visual indication board, or a computer monitoring screen, both of which will
indicate the power status.


A method of containing liquid spillage from a system, for example, oil or diesel from a standby generator.


A secondary power path providing a power supply in case the primary one fails.


The ability of a circuit to store electrical energy as a charge. This is known as a capacitive circuit.

Central Power Supply System (CPSS)

A type of standby power system used within emergency lighting, security and medical applications, the operation of which is similar to that of a UPS.

Chilled Water System

A type of precision cooling system widely used in mid-sized to large IT environments. Cold water is pumped from a Chiller to Computer Room Air
Handlers (CRAH) designed to remove heat from the IT environment.


A device used to continuously refrigerate large volumes of water.


A protection device against high surge currents. In the event of a high surge, the breaker will trip and cut power to the current.

Clamping level

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

Common Mode Noise

High-frequency electrical noise generated by disturbances between the supply lines and earth (phase-to-earth or neutral-to-earth).

Constant Voltage Transformer (CVT)

A type of ferro-resonant transformer.


The abbreviation for Computer Room Air Conditioning unit. A device that uses a self-contained refrigeration cycle to remove heat from the Computer Room and send it away from the Data Centre through some kind of cooling
medium, usually piping.


The abbreviation for Computer Room Air Handling unit. A device usually installed in a Data Centre or Computer Room that uses circulating chilled water to remove heat.

Crash Kits

Spares stored on site to enable a fast emergency response to a system failure.

Crest Factor

The mathematical ratio of the peak to the root mean square value of an AC waveform.

Critical Loads

Electrical systems which directly affect the ability of an organisation to function and which must be kept operational during a mains power supply

Critical Systems

Electrical equipment essential to the operation of the organisation and which cannot be allowed to fail, and must be kept running during any
interruption of the mains power supply.

Current (A)

The ‘volume’ of electricity flowing in a circuit which is expressed in Amps.

Current Limit

The restriction of the amount of current that can be drawn from any point within an electrical circuit or UPS output.


The process of removing moisture from air.

Direct Current (DC)

Electric current, the electrons of which flow in a single direction only.


A short circuit or overload disconnection protection device that protects other devices from being damaged, and does not interrupt their operation.

Displacement Power Factor

The ratio of Real Power (W) to Apparent Power (VA) at the fundamental frequency.


A variation in wave form from a true wave shape.

Distortion Power Factor

The power factor produced by the harmonics from non-linear loads.

Double Conversion

A system employed by an on-line UPS which feeds mains electricity into a rectifier/charger, and converts AC to DC. The DC then charges the batteries and also feeds the DC into an inverter which then converts the DC back to AC. This clean power then feeds the electrical equipment.


See Volt Free Contact.

Dynamic Stability

The ability of a device to respond to a load-step change and deliver a stable output voltage wave form.

Earth Fault (Ground)

When a live component with mains power grounds to earth.

Electrical Noise

High frequency disturbance on a sine-wave which may be Common Mode or Normal Mode.

Electro- Magnetic Compatibility (EMC)

The extent to which an electronic device will tolerate and generate Electro-Magnetic Interference (EMI).

Electro- Magnetic Induction

The production of an electrical potential difference (or voltage) across a conductor, situated in a changing magnetic flux.

Electro- Magnetic Interference (EMI)

A type of electrical noise that causes an electro-magnetic disturbance.

Emergency Power Off (EPO) / Emergency Shutdown (ESD)

A signal contact on a UPS which will initiate a total UPS shutdown.

Emergency Response Time

The speed of response specified within a maintenance plan for an engineer to attend site.

Essential Systems

Equipment which will need to be kept operational, but can withstand a 15 – 30 second power interruption.


A resonant interaction between capacitors and saturable iron-core inductors which limits current flow. This principle is employed within Constant Voltage Transformers (CVTs).

Ferro-resonant Transformer

A voltage regulating transformer using the principle of ferro-resonance.

Fixed Cellular Terminal (FCT)

A communications device that can receive and route telephone calls through a built-in mobile phone.

Float Charging

A term used when charging a battery set at a steady voltage level.

Flywheels (DC)

A device used to convert kinetic energy into a standby supply of DC power for a UPS either in place of a battery set or to reduce the initial discharge during momentary interruptions.

Frequency Converter

An electrical device for changing the input frequency to a different output frequency.

Fuel Cell

A device that uses gases such as hydrogen, or other fuels such as methanol, to generate an AC or DC electrical supply. Can be used as a greener alternative to battery power or generators.

Galvanic Isolation

A separation of the input and output supplies to a device in such a way that energy flows through a field rather than through electrical connections.

Gas Discharge Tube (GDT)

A protection device consisting of an arrangement of electrodes in a gas within an insulating, temperature-resistant ceramic or glass case which
switches to a low-impedance when subjected to a spike or transient voltage.

Gas Turbine

A device that converts kinetic energy generated by combustion into electrical energy to provide an AC standby power source.


A device that uses combustion to generate electrical energy and to provide an AC or DC power source.


Harmonics are multiple wave-forms of the basic frequency. The basic frequency or first harmonic within Europe is 50Hz USA 60Hz.

Harmonic Distortion or Total Harmonic Distortion (THDi)

Distortion of a mains power supply sine-wave from its fundamental frequency and wave shape.

Harmonic Filter

A device which reduces the generated harmonic distortion, enabling a high power factor to be provided to its AC source.

Hertz (Hz)

The number of complete cycles per second of a waveform. Normal mains frequency in Europe is 50Hz and in North America 60Hz.

Hot Aisle / Cold Aisle Architecture

A rack enclosed architecture in which parallel aisles of enclosures are arranged with rack fronts always facing rack fronts and rack rears always facing rack rears. This ensures separation between cooling air in the ‘cold’ aisle entering the front of the racks and warm exhaust air leaving the back of the racks in the ‘hot’ aisle. Separation of cooling air and exhaust air results in higher cooling system efficiencies and the ability to cool higher power densities in the computing environment.


An abbreviation for Heating, Ventilation and Air Conditioning. Sometimes an ‘R’ is shown at the end to represent Refrigeration (HVACR).


Short-circuit current.


Nominal current.


The cable carrying the mains power supply into a building from the electrical substation and Point of Common Coupling (PCC).


The generation of an electro-motive force in an inductive circuit by varying the magnetic flux through it.

Ingress Protection (IP Rating)

An IP number is often specified when referring to protection against both solids and liquids offered by enclosures around electronic equipment. The
first number refers to solid objects, one being the lowest and six the highest. The second refers to liquids, eight being the highest protection.

Insulated Gate Bipolar Transistor (IGBT)

A high power switching device used in rectifiers and inverters.


The initial surge in current drawn loads, for example, to charge capacitive circuits.


Circuitry that converts DC energy to an AC output.


A communications protocol that creates a hierarchical structure (one master and several slaves) from a single RS-232 communication link,
similar to MODBUS.

Joule (J)

A measurement of energy defined as one Watt per second.

Linear Loads

A load in which the relationship between voltage and current is constant, based on a relatively constant load impedance.

Line-Interactive UPS

This is an off-line UPS with an Automatic Voltage Regulator on the mains supply. This will adjust drops in the mains voltage back to the correct
voltage and also modify electrical surges and spikes.


The electrical equipment powered from a device.

Load Shedding

Switching off non-essential equipment in order to increase the total runtime of the remaining system being powered by a finite power source.

Maintenance Bypass

A bypass supply which is used to power the electrical equipment during maintenance, which can be either internal or external to the equipment. Also known as a bypass panel or wrap-around bypass.

Make-Before-Break (MBB) Bypass

A bypass that makes contact between the primary (UPS output) and secondary (bypass supply) power sources before transferring the load.

Mean Time Between Failure (MTBF)

A measure of reliability and the average length of operational time between failures. This can be based on monitoring a field population, or calculated for a system based on the known MTBF values of its components to a defined process and standard.

Mean Time to Repair (MTTR)

A measure of the average time taken to bring back a system to full operation following a failure.

Metal Oxide Varistor (MOV)

A device capable of absorbing very high surge currents without damage to itself.


A communications protocol that creates a hierarchical structure (one master and several slaves) from a single RS-232 communication link, similar to JBUS.

Noise Level

Normally measured in decibels (dB) or (dBA). Noise may also be referred to as electrical noise in a circuit. This is a high frequency disturbance which
can disrupt electrical circuits.

Non-Essential Loads

Electrical equipment that can be dropped during a mains power supply failure during load shedding.

Non-Linear Loads

A load in which the relationship between voltage and current fluctuates based on alternating load impedance.

Normal Mode Noise

A form of high frequency electrical noise, that occurs between phase and neutral.

Off-Line UPS

Also called a Standby UPS. A system where the raw mains power bypasses the UPS but allows the batteries to be charged. On mains failure the battery power takes up the load.

On-Line UPS

A UPS which is permanently connected to the mains, and will provide a
break-free supply if the mains fails. On resumption of mains power it will also deliver a break-free switchover back to mains power. This type of
system uses a double conversion system and is also fitted with an automatic bypass. The PowerPure RT is an On-line UPS.


A device that uses a short optical transmission path to transfer a signal between elements of a circuit while keeping them electrically isolated.


An American term used to describe a mains power failure, also referred to as a Blackout or power cut. Some people consider a voltage drop of below 80V as an outage, since most electrical equipment will not operate below this level.


Voltage higher than specified in a circuit.

Parallel-Capacity System

A parallel UPS system where the total electrical demand is met by operating a number of UPS in parallel, without redundancy.

Parallel Redundancy

A type of UPS system where the total electrical demand is met by two or more UPS, in an N+1 configuration, sharing the load between them
equally. If one UPS fails the other is capable of supporting the full load.

Parallel Systems Joiner (PSJ)

A synchronising switch joining together two independent groups of parallel UPS.


Within Europe, single-phase consists of an AC sine-wave of 50Hz – 230V. Three-phase is divided into three equal sine-waves with a phase to phase
voltage of 415V. This gives the customer a choice of 230V or 415V mains input.

Phase Power Factor

The ratio of Real Power (W) to Apparent Power (VA) at 50Hz within Europe and 60Hz in North America.

Point of Common Coupling (PCC)

The point where mains power enters the building is connected to the electricity distribution network.

Power Conditioning

The cleaning of mains power, removing saga and spikes and smoothing the supply before onward transmission to electrical equipment.

Power Continuity Plan

A strategy which addresses the different methods whereby electrical equipment is prioritised and given protection against mains power failure

Power Factor (PF)

The difference between the actual energy consumed (Watts), and the Volts x Amps in a circuit. Power factor may be either lagging or leading the voltage.

Power Supply Unit (PSU)

The device within a piece of modern electronic hardware that converts an AC waveform into the various levels of DC required to power internal circuits.

Pulse Width Modulation (PWM)

A simulated sine-wave created in an inverter by a switching action which varies with time.

Radio Frequency Interference (RFI)

All electronic circuits generate an electrical noise known as RFI. These disturbances can be generated externally or can be created by a UPS.
RFI filters, when fitted, will protect sensitive equipment.

Raw Current

Mains power which has not been cleaned.

Reactive Power

Wasted energy returned back to the incoming AC power supply.

Real Power

A measure of the actual power (Watts) drawn by electrical equipment.

Recovery Time

The time taken for the voltage to stabilize after a standby generator takes up the load.


A component part that converts an AC supply into a DC supply.

Rectifier/ Charger

An assembly used within transformer-based UPS which converts an AC supply into a DC supply. This then powers the inverter and charges the battery set.


Control specification of an AC/DC output voltage.


The capacity of a power protection system potentially exposed to hazards to adapt, by resisting or changing in order to reach and maintain an acceptable operational level.


An undesirable AC disturbance superimposed onto the DC wave-form powering a device.


Abbreviation for Restriction of Hazardous Substances, Directive 2002/95/EC.

Root Mean Square (RMS)

Taking one half of an AC current cycle, RMS is the square root of the average values of all the squares of current and voltage.

Rotary UPS

An Uninterruptible Power Supply that converts kinetic energy from a flywheel into electrical energy to power electrical equipment.


A standard interface for synchronous and asynchronous communications up to 20 kb/s between two compatible devices (one driver and one receiver) over distances of up to 15m.


Similar to RS-232 but to a maximum transmission rate of 100Kb/s – 10Mb/s, up to ten receivers from a single driver and a distance of up to 1000m.


Similar to RS-442 but with up to 32 drivers and receivers.


The amount of time that batteries connected to a UPS will support the electrical equipment attached to it after a mains failure. (Also referred to as Autonomy)

Sags and Dips

Decreases in power, below normal mains power, lasting several cycles.

Sensitive Systems

Electrical equipment that is not critical or essential but requires a clean shutdown.

Series- Redundancy

A method whereby the output of one UPS module is used to supply the bypass of another. If one UPS fails the other automatically takes up the load.

Silicon Avalanche Diode (SAD)

A device which acts as a buffer reducing the initial surge currents of severe surges and transients.

Simple Network Management Protocol (SNMP)

A communications protocol that allows hardware, on a network, with a TCP/IP connection, to be monitored and controlled.


A graphic depiction, viewed on an oscilloscope, of the wave-form produced by an Alternating Current (AC).


A single-phase supply consists of a single sine-wave at a fundamental frequency. In Europe this refers to a mains electrical supply of 230V AC.

Slew Rate

The speed at which a UPS can synchronise output frequency with another AC power source, such as a generator.


A very brief large voltage increase on the normal AC supply.

Static Stability

A term used to describe the delivery of a stable electrical output voltage from a device under stable load conditions.

Static Switch

A solid state high-speed switch capable of transferring a load between AC power sources.

Static UPS

An Uninterruptible Power Supply incorporating solid state electronics.

Super Capacitors

Electronic components capable of storing an electrical charge which can supplement usage of battery power within a UPS.


Very short duration substantial increase of the mains power voltage.

Switched Mode Power Supply (SMPS)

A type of Power Supply Unit (PSU), most commonly found within computer, telecommunications and electronic devices.

Switching Time

Also known as Transfer Time. The delay between the load transference from one AC electrical supply to another.

Thermal Runaway

An adverse condition whereby batteries dramatically overheat. Often caused by cells generating heat as they become fully charged and cannot be sufficiently dissipated into the ambient temperature.


A 400V/415V mains supply achieved by overlapping three single-phases, each 120° out of phase with each other.


An electrical component which is a semi-conductor switching device.

Total Harmonic Distortion (THDi)

The total harmonic distortion introduced into a system compared to a normal sine-wave.

Total Power Factor / True Power Factor

The total power factor including both distortion and displacement power factors.

Transfer Time

Also known as Switching Time. The delay between the load transference from one AC electrical supply to another.


An electrical component used for changing voltages. It consists of wire wrapped around a core with insulated iron plates. Also used to achieve
Galvanic Isolation.


A very brief but severe voltage increase within the normal mains electrical supply. Damaging to sensitive electronic equipment.

Transient Voltage Surge Suppressor (TVSS)

An electrical component introduced into a circuit to attenuate brief voltage surges.

Triplens (Triple-N)

A multiple of the third harmonic in a waveform.


An extended drop in the specified voltage.

Uninterruptible Cooling

The process of continued heat removal from the IT Room or Data Centre during equipment failure or power failure to preclude thermal damage and
data loss due to equipment overheating.

Uninterruptible Power Supply (UPS)

A source of standby power capable of maintaining the electrical supply to connected electrical equipment for a specified period of time. A UPS
can also be used as a device for cleaning the power supply of sags, spikes and similar aberrations. See Power Sonic UPS Systems.

UPS Group Synchroniser (UGS)

A device used when two separate groups of parallel UPS need to have their outputs synchronised.

Valve-Regulated Lead Acid (VRLA) Battery

A type of Lead-Acid battery.

Volt (V or kV or MV)

A base unit of electrical force. One Volt will send one Ampere of electrical current through a resistance of one Ohm.

Volt Ampere (VA or MVA)

A unit or units of Apparent Power.

Volt-Free Contacts (VFCc)

A pair of contacts used to make or break other circuits.

Watts (W or kW or MW)

A measure of power which flows to electrical equipment.


A graphical depiction, viewed on an oscilloscope, showing the shape of a wave.


Waste Electrical and Electronic Equipment, Directive 2002/96/EC

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