UPS Technical Glossary

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 is measured in VA.

Availability

A measure of the system uptime compared to its downtime expressed as a percentage.

Automatic Bypass

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

Automatic Voltage Stabiliser (AVS)

A mains power supply voltage stabilization device. They are also known as an Automatic Voltage Regulator (AVR) or Voltage Regulator (VR).

Autonomy (Runtime or Backup Time)

The amount of time, they are 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 several 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 several battery strings.

Battery String

It comprises several battery blocks arranged in series to achieve a set Vdc and Ah rating.

Blackouts

A power cut or mains power supply failure is also called an outage.

Boost charge

A high charge voltage is applied to a battery set.

Booster-Converter

A transformerless assembly within a UPS is 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.

Brownouts

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

Building Management System (BMS)

This is not to be confused with a Battery Management System (BMS). A centralized alarm monitoring system typically provides either a visual indication board or a computer monitoring screen, both of which will
indicate the power status.

Bunding

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

Bypass

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

Capacitance

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

Chiller

A device used to refrigerate large volumes of water continuously.

Circuit-Breaker

A protection device against high surge currents. The breaker will trip and cut power to the current in a high surge.

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 is generated by disturbances between the supply lines and the earth (phase-to-earth or neutral-to-earth).

Constant Voltage Transformer (CVT)

A type of ferro-resonant transformer.

CRAC

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 cooling
medium, usually piping.

CRAH

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 organization to function and which must be kept operational during a mains power supply
failure.

Critical Systems

Electrical equipment essential to the operation of the organization and 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 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.

Dehumidification

The process of removing moisture from the air.

Direct Current (DC)

Electric current consists of the electrons that flow in a single direction only.

Discrimination

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.

Distortion

A variation in waveform from an actual wave shape.

Distortion Power Factor

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

Double Conversion

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

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)

When a live component with mains power grounds to earth.

Electrical Noise

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

Electro- Magnetic Compatibility (EMC)

The extent to which an electronic device will tolerate and generate electromagnetic 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 electromagnetic disturbance.

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

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

Emergency Response Time

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

Essential Systems

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

Ferro-resonance

A resonant interaction between capacitors and saturable iron-core inductors 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 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 is used to convert kinetic energy into a standby supply of DC power for a UPS 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. It 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 so 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.

Generator

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

Harmonics

Harmonics are multiple waveforms of the fundamental frequency. The fundamental frequency or first harmonic within Europe is 50Hz, and in the 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 that reduces the generated harmonic distortion, providing a high power factor to its AC source.

Hertz (Hz)

The number of complete cycles per second of a waveform. The standard 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 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.

HVAC

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

ICC

Short-circuit current.

Input

Nominal current.

Incomer

The cable carrying the main power supply into a building is from the electrical 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 specified for protection against 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, with eight being the highest protection.

Insulated Gate Bipolar Transistor (IGBT)

A high power switching device used in rectifiers and inverters.

In-rush

For example, the initial surge in current draws loads to charge capacitive circuits.

Inverter

Circuitry that converts DC energy to an AC output.

JBUS

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 is defined as one Watts 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.

Load

The electrical equipment is powered by a device.

Load Shedding

I am switching off non-essential equipment to increase the total runtime of the remaining system being powered by a finite power source.

Maintenance Bypass

A bypass supply 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.

MODBUS

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

Typically 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

During load shedding, electrical equipment can be dropped during a mains power supply failure.

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, a break-free switchover will also be delivered back to mains power. This type of
system uses a double conversion system and is fitted with an automatic bypass. The PowerPure RT is an Online UPS.

Opto-Isolators

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

Outages

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

Over-Voltage

Voltage higher than specified in a circuit.

Parallel-Capacity System

A parallel UPS system where the total electrical demand is met by operating several 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, equally share the load between them. If one UPS fails, the other is capable of supporting the full load.

Parallel Systems Joiner (PSJ)

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

Phase

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) is at 50Hz in 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 mains power is cleaned, saga and spikes removed, and the supply is smoothed before onward transmission to electrical equipment.

Power Continuity Plan

A strategy that addresses the different methods whereby electrical equipment is prioritized and given protection against mains power failure accordingly.

Power Factor (PF)

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

Power Supply Unit (PSU)

The device is 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 is created in an inverter by a switching action that 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 still needs to be cleaned.

Reactive Power

Wasted energy returned 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.

Rectifier

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

Rectifier/ Charger

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

Regulation

Control specification of an AC/DC output voltage.

Resilience

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

Ripple

An undesirable AC disturbance is superimposed onto the DC waveform powering a device.

RoHS

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

Root Mean Square (RMS)

Taking one-half of an AC 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.

RS-232

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.

RS-442

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

RS-485

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

Runtime

The amount of time 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 average 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 that 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.

Sine-wave

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

Single-Phase

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

Slew Rate

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

Spikes

A very brief significant voltage increase on the standard AC supply.

Static Stability

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

Static Switch

A solid-state high-speed switch can transfer 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 the usage of battery power within a UPS.

Surges

Concise duration substantial increase of the mains power voltage.

Switched Mode Power Supply (SMPS)

A type of power supply unit (PSU) is most commonly found in computers, telecommunications, and electronic devices.

Switching Time

They are 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.

Three-Phase

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

Thyristor

An electrical component is a semiconductor 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 includes 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.

Transformer

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

Transients

A very brief but severe voltage increase within the standard mains electrical supply. It is damaging to sensitive electronic equipment.

Transient Voltage Surge Suppressor (TVSS)

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

Triplens (Triple-N)

A multiple of the third harmonic in a waveform.

Under-Voltage

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. A UPS can also clean the power supply of sags, spikes, and similar aberrations. See Power Sonic UPS Systems.

UPS Group Synchroniser (UGS)

A device is used when two groups of parallel UPS need to synchronize their outputs.

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 is used to make or break other circuits.

Watts (W or kW or MW)

A measure of power that flows to electrical equipment.

Wave-form

A graphical depiction on an oscilloscope shows a wave’s shape.

WEEE

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