UPS Technical Glossary

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


AC ripple Is the residual periodic variation of the DC voltage within a UPS which has been derived from an unwelcome alternating (AC) source. An AC ripple causes internal heating and deterioration of the UPS battery poles and so should be prevented from reaching it.

Active power A measure of the actual power (Watts) dissipated by a load.

Alternating current An electrical current whose electrons flow periodically in one direction to a maximum level before dropping back to zero and then flowing in an alternative direction before the pattern repeats itself. The waveform is that of a sinewave.

Amp/Amperes (A) A measure of the electrical current flow.

Ampere-hour (Ah) A measure of the number of Amps that a battery set can deliver.

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

Automatic bypass A circuit within a UPS (or bypass panel) to transfer the load from and to a bypass supply, which may be relay or static-switch based.

Automatic mains failure (AMF)  is a useful function for standby power generators. A generator fitted with AMF can detect and respond to a mains failure by automatically starting up and becoming a ‘mains replacement’ for the UPS.

Automatic voltage stabiliser (AVS) A device to stabilise the mains power supply voltage to a load. Also known as the Automatic Voltage Regulator (AVR) or Voltage Regulator (VR).

Autonomy The amount of time (minutes or hours) that a battery set or other power source will support the load. The load amount will dictate the autonomy.

Availability (A) A ratio of system up-time compared to its downtime expressed as a percentage. It provides the probability of a system being operational at any given time during its working life; A = MTBF/(MTBF + MTTR).

MTBF – Mean Time Between Failures, MTTR – Mean Time To Repair


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

Battery cell A simple electrical circuit within a battery block consisting of positive and negative electrodes or plates, an electrolyte and separator.

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.

Blackout A term used to describe a mains power supply failure, also referred to as an outage or a total loss of electrical power.

Boost charge A high charge voltage applied to a battery set.

Booster-converter An assembly used within transformerless UPS to set 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, and vice-versa.

Brownout A long duration of low voltage power from the mains power supply.

Building management system (BMS)  A centralised alarm monitoring system usually providing either a visual status indication board or computer monitoring screen.

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

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


Capacitance The ability of a circuit to store electrical energy as a charge. The circuit 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.

Circuit-breaker A device inhibiting high surge currents over a set stated figure. Under such conditions the breaker will operate and the circuit is isolated.

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

Common mode noise A form of high-frequency electrical noise which results from disturbances between the supply lines and earth (phase-to-earth or neutral-to-earth).

Constant voltage transformer (CVT) A type of ferroresonant transformer.

Crash kits Spare kits held on-site to enable a fast emergency response to a system failure.

Crest factor The mathematical ratio of the peak to RMS value of an AC waveform.

Critical loads Systems which directly affect the ability of an organisation to operate and which must be kept running during a mains power supply failure.

Current (A) The ‘volume’ of electricity flowing in a circuit and expressed as Amps.

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


Deep discharge A battery charge state whereby the battery voltage, (Vdc), has dropped below a safe operating level from which is cannot recover.

Dip a transient voltage decrease – also called a sag.

Direct current (DC) Electric current, the electrons of which are flowing in one direction only.

Discrimination  The protection around a device within a Power Continuity Plan that will disconnect it, if a short-circuit or overload is applied, to prevent it from damaging other devices, and without interruption their operation.

Displacement power factor The ratio of real power (W) to Apparent Power (VA) at the fundamental frequency.

Distortion A variation in waveform from a true wave shape

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

Dry-Contact See Volt-free contact

Dynamic stability The ability of a device to respond to a load-step change and deliver a stable output voltage waveform.

Double conversion Also called Online Double Conversion ensures that the UPS provides instant power should the mains fail. Instantaneous switch over from mains to UPS power is achieved.


Earth fault (Ground) A connection from the live component of a mains power supply to an earth connection

Electric noise High frequency noise on a sinewave which may be Common Mode or Normal Mode

Electro-magnetic compatibility (EMC) The extent to which an electronic or electrical 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) A signal contact on a UPS which will initiate a total UPS shutdown

Emergency response time The speed of response specifies within a maintenance plan for an engineer to attend site


Ferroresonant transformer A voltage regulating transformer using the principle of ferroresonance

Fixed cellular terminal (FCT) A communications device that can receive and route telephone calls through a built-in mobile phone capability

Float charging A method of charging a battery set at a steady voltage level

Flywheels (dc) A device used to convert kinetic energy into a standby supple of dc power for a UPS either in place of a battery set or to reduce the initial discharge during momentary interruptions

Frequency Converter A device for changing the input frequency to a different output frequency

Fuel cell A device that uses hydrogen as a fuel to generate an ac or dc supply in addition to heat and water


Gas discharge tube (GDT) 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 from combustion into electrical energy to provide an ac or dc power source to a load

Generator A generator automatically provides backup power within seconds of a mains power outage. An automatic transfer switch senses the power loss, commands the generator to start and then transfers the electrical load to the generator. Generators are usually fuelled by diesel, natural gas or liquid propane gas.


Harmonic A variation of a mains power supply sinewave above the fundamental (50 or 60Hz)

Harmonic distortion or total harmonic distortion Distortion of a mains power supply sinewave from its fundamental frequency and wave shape.

Harmonic filter A device to reduce the harmonic distortion generated by a device and enable it to provide a high power factor to its ac source.

Hertz (Hz) A measurement of the number of complete cycles per second of a waveform. Normal mains frequency is either 50 or 60 (Hz)


ICC A short circuit current

In Nominal current

Incomer The cable carrying the mains power into a building from the nearest substation and point of common coupling (PCC)

Inductance 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 used when specifying the environmental protection offered by enclosures around electronic equipment. The first refers to the protection against solid objects and the second against liquids.

In-rush The initial surge in current drawn by loads, for example, the charge capacitive circuits

Insulated gate bipolar transistor (IGBT) A high power switching device used in inverters and rectfiers.

Inverter The circuit within a UPS system which converts dc energy to an ac output.

Isolation or 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.


JBUS A communications protocol that creates a hierarchical structure from a single RS-232 communications link, similar to MODBUS.

Joule (J) An energy measurement unit, determined as one Watt per second.


Linear loads The system or systems powered from a device.

Load shedding The reduction of he total load placed on a device. For example, in the case of a UPS, load shedding (when the mains power supply fails) reduces the total load on the UPS to increase the amount of runtime available from the battery set.



Maintenance Bypass A bypass supply which is used to power the load during maintenance and which may be internal or external to the device. It is also known as a bypass panel or wrap-around bypass.

Make-before-break (MBB) bypass A bypass that makes contact between the primary (UPS outlet) 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 failires. 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 to full operation following a failure.

Metal oxide varistor (MOV) A device capable of absorbing very high surge currents without damage to itself

MODBUS A communications protocol that created a hierarchical structure from a single RS-232 communications 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.


Opto-isolators A device that uses a short optical transmission path to transfer a signal between phase and neutral.

Outages An American term used to describe a mains power supply failure, also referred to as a blackout.

Overvoltages Any higher voltage than that agreed as a regulated voltage in a circuit.


Parallel systems joiner (PSJ) A device to join together two independent groups of parallel UPS.

Parallel-capacity system A type of parallel UPS system where the total load demand is met by operating a number of UPS in parallel without redundancy.

Parallel-redundancy A type of parallel UPS system where the total load demand is met by operating two or more UPS in an N+X configuration with all the UPS sharing the load between them equally. If one UPS fails, the other supports the load.

Phase A single-phase supply consists of a single sinewave at the fundamental frequency. A three-phase supply consists of three waveforms each separated by 120 degrees from each other. Phase is also used to refer to the difference between the voltage and current waveforms when used in relation to power factors.

Phase power factor The ratio of real power (W) to apparent power (VA) at the fundamental frequency.

Point of common coupling (PCC) The point where a building incomer is connected to the electricity distribution network

Power conditioner A device to stabilise, regulate and filter the mains power supply voltage to a load which may be electronic or transformer-based (constant voltage transformer)

Power continuity plan A plan outlining the methods by which system that ensure continuity are protected from mains power supply failures and power problems.

Power factor (pF) The difference between the actual energy consumed (Watts), and the volts x Amps in a circuit. Power factor may be lagging, where current waveforms lags the voltage waveform, or leading where the current waveform leads the voltage.

Power supply units (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.

Profibus A type of field bus typically used within control and automation

Pulse width modulation (PWM) The switching action in an inverter which varies with time, and creates a composite waveform approaching a sinewave.


Radio frequency interference (RFI) Radio waves of sufficient intensity to be absorbed by a circuit and to cause a circuit malfunction.

Reactive power A flow of energy (VAr) superimposed back onto the source from which it was drawn

Real power A measure of the actual power (Watts) dissipated by a load

Recovery time The time incurred to obtain a fully regulated voltage after switching loads onto a standby generating set.

Rectifier An assembly that converts an ac supply to a dc supply.

Rectifier/charger An assembly used within transformer-based UPS which converts an ac supply into a dc supply from which to power an inverter and charge a battery set.

Regulation Control of an ac/dc output voltage to within a set specification.

Resilience 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 operation level.

Ripple An ac element superimposed onto the dc waveform powering a device.

RoHs 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 into electrical energy to power a load.

RS-232 A standard interface for synchronous and asynchronous communications up to 20kh/s between two compatible devices (one driver and one receiver) over distances up to 15m.

RS-422 Similar to RS-232 but up to a maximum transmission rate of 10Mb/s-100Kb/s, up to 10 receivers from a single driver and a distance of up to 100m.

RS-485 Similar to RS-422 but with up to 32 drivers and receivers.


Sags or dips Short duration decreases, below the nominal mains power supply voltage, lasting several cycles.

Series-redundancy A method of providing redundant power to a load whereby the output of one UPS module is used to supply the bypass of another. If one UPS fails the other automatically powers the load.

Silicon avalanche diode (SAD) A device capable of responding quickly to a high-energy surge or transient and reducing its initial surge current.

Simple network management protocol (SNMP) A communications protocol that allows hardware with a TCP/IP connection on a network to be monitored and controlled.

Sinewave The waveform naturally produced by a well designed generator, inverter or UPS.

Single-phase A single-phase supply consists of a single sinewave at the fundamental frequency.

Slew rate The rate at which a UPS can adjust its output frequency of, for example, a standby ac source such as a generator.

Spikes Large voltage disturbances superimposed onto the normal ac supply with a short duration.

Static stability The ability of a device to deliver a stable output voltage waveform under stable load conditions.

Static switch A solid state high-speed switching device that can transfer a load between two ac power sources.

Static UPS An uninterruptible power supply using solid state electronics and therefore no moving parts.

Super capacitors A device for sorting electrical charge which can be used to provide applications such as a small UPS, with a momentary supply or power in place or a battery set or to reduce its usage during momentary breaks in mains power.

Surges Short duration increases in voltage above the mains power supply nominal, which generally lasts for several cycles.

Switched mode power supply (SMPS) A type of PSU with non-linear current draw, most commonly found within computer, telecommunications and electronic devices.

Switching time The time it takes to transfer a load between ac supply sources. Also known as transfer time.


Three-Phase A supply consisting of three single phase, each 120 degrees out of phase from one another.

Thyristor A semi-conductor gate device most commonly used within a rectifier or static switch.

Total harmonic distortion (THDi) A measure of all the harmonics induced in a system compared to a normal sinewave.

Total power factor/true power factor The total power factor including both distortion and displacement power factors.

Transfer time The time it takes to transfer a load between ac supply sources. Also known as switching time.

Transformer A wound component consisting of wingdings around a core, with an iron sheet laminated that can be used to change voltage levels and provide galvanic isolation.

Transient voltage surge suppressor (TVSS) A device using MOVs or GDTs to attenuate a transient or spike.

Transients High energy burst voltage disturbances, with a short duration, superimposed onto the normal supply.

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


Undervoltages A voltage below the set regulated voltage for an extended period.

Uninterruptible power supply (UPS) The terminology used to describe a system (either static or rotary) capable of maintaining power to a load for a defined time irrespective of the state of the mains power itself – also known as an uninterruptible power system.

UPS group synchroniser (UGS) A device to synchronise the outputs from two separate groups of parallel UPS.


Valve-regulated lead-acid (VRLA) battery A type of Lead-acid battery commonly used within a UPS.

Volt (v or kV or MV) A measure of electrical force or pressure, which can be expressed as Vac or Vdc.

Volt ampere (VA or kVA or MVA) A measure of apparent power and the current drawn by a load at a given supply or voltage.

Volt-free contacts (VFCs) A pair of contacts that are normally open (NO) or normally closed (NC). When closed they form a circuit through which a current, and therefore signal, can flow for remote detection.


Watts (W or kW or MW) A measure of the real power drawn by a load.

Waveform A graphical representation of the shape of a wave

WEEE Waste electrical and electronic equipment, directive 2002/96/EC.

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