UPS Technical Glossary

UPS technical glossary

A technical glossary 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 or 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 per hour
Automatic Bypass The current drawn by a load at a given supply voltage measured in VA
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 (Runtime or Backup Time) The amount of time (minutes or hours) that a battery set or other power source will support the load.
Availability (A) A ratio of system uptime compared to its downtime expressed as a percentage


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
Blackouts A term used to describe a mains power supply failure, also referred to as an Outage
Boost charge A high charge voltage applied to a battery set
Booster-converter An assembly used within transformerless UPS 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, and vice-versa
Brownouts Long duration decreases below nominal (the normal mains supply voltage), which can last for many cycles or longer
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 Spares 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 it cannot recover
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 interrupting 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


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


Ferroresonance A term applied to resonant interactions between capacitators and saturable iron-core inductors. During resonance, the inductive reactance increases to match the capacitative reactance and current flow is limited by the system resistance. The phenomenon is used, for example, within ferroresonant transformers of which Constant Voltage Transformers (CVTs) are a configuration
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 device that converts Kinetic energy generated from combustion into electrical energy to provide an ac or dc power source to a load


Harmonic A variation of a mains power supply sinewave above the fundamental (50 or 60Hz)
Harmonic Distortion or Total Harmonic Distortion (THDi) 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 Short-circuit current
Incomer The cable carrying the mains power supply 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, to charge capacitive circuits
Insulated Gate Bipolar Transistor (IGBT) A high power sw3itching device used in inverters and rectifiers
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 (one master and several slaves) 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 the 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 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 a system 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 creates a hierarchical structure (one master and several slaves) 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
Non-essential Loads Loads that can be dropped during a mains power supply failure because they are not critical or essential to the operation of the organisation
Non-linear Loads A load in which the relationship between voltage and current fluctuates based on an alternating load impedance
Normal Mode Noise A form of high-frequency electrical noise, that occurs between phase and neutral


Opto-Isolators A Device that uses a short optical transmission path to transfer a signal between elements of a circuit while keeping them electrically isolated
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° 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 systems 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 waveform lags the voltage waveform, or leading where the current waveform leads 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
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 into 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 operational level
Ripple An ac element superimposed onto the dc waveform powering a device
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 20kb/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 ten 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 in order to synchronise to the 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 storing electrical charge which can be used to provide applications such as 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 supply
Surges Short duration increases in voltage above the mains power supply nominal, which generally last for several cycles
Switch Mode Power Supply (SMPS) A type of PSU with a 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 phases, each 120° 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 The time it takes to transfer a load between ac supply sources. Also known as switching time
Transformer A wound component consisting of windings around a core, with iron sheet laminated that can be used to change voltage levels and provide Galvanic isolation
Transient Voltage Surge Suppressor (TVSS) A device using MOVs, SAD/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 a 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 period irrespective of the state of the mains power supply 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 or Apparent Power and the current drawn by a load at a given supply 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