This glossary is designed to help you understand common terms used in the field of Uninterruptible Power Supplies (UPS).
Active Power
The actual power consumed by a load, measured in watts.
Alternating Current (AC)
An electric current that periodically reverses direction and has a sinusoidal waveform.
Ampere (A)
The unit of measurement for electric current.
Ampere-Hour (Ah)
A unit of electrical charge equal to the charge delivered by a current of one ampere flowing for one hour.
Apparent Power
The total electrical power of a load at a given voltage, measured in volt-amperes (VA).
Availability
The percentage of time a system is operational.
Automatic Bypass
A circuit within a UPS (or bypass panel) that provides a path for bypass power, using either relays or static switches.
Automatic Voltage Stabilizer (AVS)
An automatic voltage stabilizer (AVS), also known as an automatic voltage regulator (AVR) or voltage regulator (VR).
Autonomy Time (also known as Runtime or Backup Time)
The amount of time a battery set or backup power source can support a load, measured in minutes or hours.
Battery Block
An independent unit consisting of multiple interconnected battery cells.
Battery Cell
The smallest energy storage unit within a battery block.
Battery Set
A group of one or more battery strings.
Battery String
A series of battery blocks connected together to achieve a specified Vdc and Ah rating.
Blackout
A complete loss of power, also known as a power failure or outage.
Boost Charge
A higher-than-normal charging voltage applied to a battery set.
Boost Converter
A transformerless device in a UPS that raises the DC voltage from the rectifier or battery set to the level required by the inverter.
Break-Before-Make (BBM) Bypass
A bypass that introduces an interruption when transferring the load from the UPS output to the bypass source or vice versa.
Brownout
A reduction in normal mains voltage, lasting from a brief period to an extended period.
Building Management System (BMS)
A centralized alarm monitoring system that provides visual or computer-based indication of power status.
Bund Wall
A containment barrier designed to prevent liquid spills, such as oil or diesel from backup generators, from spreading.
Bypass
An alternate power path that ensures continuity of power in the event of a failure in the primary path.
Capacitance
The ability of a circuit to store an electrical charge; a circuit with capacitance is called a capacitive circuit.
Central Power Supply System (CPSS)
A backup power system for emergency lighting, security, and medical applications, operating similarly to a UPS.
Chilled Water System
A precision cooling system commonly used in small to large IT environments. It circulates chilled water from a chiller to computer room air handlers (CRAHs) to remove heat.
Chiller
A device that continuously cools a large volume of water.
Circuit Breaker
A protective device that interrupts a circuit to prevent damage from overcurrent, typically by tripping during a surge.
Clamping Level
The maximum voltage allowed by a surge or transient protection device before clamping occurs.
Common Mode Noise
High-frequency electrical noise resulting from interference between power lines and ground (either phase-to-ground or neutral-to-ground).
Constant Voltage Transformer (CVT)
A ferroresonant transformer.
CRAC
Computer Room Air Conditioner. It uses an independent refrigeration cycle to remove heat from a computer room, typically using a cooling medium such as chilled water or refrigerant.
Electrical Systems Glossary
CRAH
Computer Room Air Handler (CRAH). This equipment is typically installed in data centers or computer rooms and uses circulating chilled water to effectively dissipate heat.
Crash Kits
Spare components stored on-site to enable a quick emergency response in the event of a system failure. (Rapid Response Kits / Failure Rapid Recovery Kits)
Crest Factor
The mathematical ratio of the peak value to the RMS value of an AC waveform.
Critical Loads
Electrical systems that directly impact an organization’s operational capabilities and must remain functional during a mains power outage.
Critical Systems
Essential electrical equipment that is vital to an organization’s operations, cannot be allowed to fail, and must remain operational during any mains power outage.
Current (A)
The amount of electrical flow through a circuit, measured in Amperes.
Current Limit
The limitation on the amount of electrical current that can be drawn from any point in an electrical circuit or UPS output.
Dehumidification
The process of extracting moisture from the air.
Direct Current (DC)
Electrical current that flows in only one direction.
Discrimination
A protective device used for short circuit or overload disconnection that protects these devices from damage without interrupting the operation of other equipment. (Selective Circuit Breaking / Selective Tripping)
Displacement Power Factor
The ratio of active power (W) to apparent power (VA) at the fundamental frequency.
Distortion
A deviation of a waveform from its true form.
Distortion Power Factor
The power factor caused by harmonics generated by non-linear loads.
Double Conversion
A method used by online UPSs. Mains input is fed to a rectifier/charger which converts AC power to DC power. The DC power charges the batteries and feeds the inverter, which converts the DC power back to AC power, providing clean power to the electrical equipment.
Dry-Contact
Also known as a Volt Free Contact.
Dynamic Stability
The ability of a device to respond to load step changes while providing a stable output voltage waveform.
Earth Fault / Ground
Occurs when a live part comes into contact with earth.
Electrical Noise
High-frequency interference on a sine wave, which can be either Common Mode or Differential Mode.
Electromagnetic Compatibility (EMC)
The degree to which electronic equipment can tolerate and generate electromagnetic interference (EMI).
Electro-Magnetic Induction
The production of a potential difference (or voltage) across a conductor in a changing magnetic flux.
Electromagnetic Interference (EMI)
A type of electrical noise that causes electromagnetic interference.
Emergency Power Off / Emergency Shut Down (EPO/ESD)
A signal contact on a UPS that triggers a complete shutdown of the UPS system.
Emergency Response Time
The speed at which an engineer will arrive on-site, as stipulated in the maintenance plan.
Essential Systems
Equipment that must remain operational but can tolerate a power interruption of 15 to 30 seconds.
Ferro-resonance
The resonant interaction between a capacitor and a saturable-core inductor to limit current flow, the principle used in Constant Voltage Transformers (CVT).
Ferro-resonant Transformer
A type of voltage regulating transformer that employs the principle of ferro-resonance.
Fixed Cellular Terminal (FCT)
A communication device that is capable of receiving and routing telephone calls via an integrated mobile telephone.
Float Charging
A term that describes the process of charging a battery bank at a constant voltage level.
Flywheels (DC)
Devices that convert kinetic energy into a backup DC power source for a UPS system, and can replace battery banks or minimize initial discharge during short outages.
Frequency Converter
An electrical device designed to change an input frequency to a different output frequency.
Fuel Cell
A device that generates AC or DC power using a gas such as hydrogen or a fuel such as methanol, a more environmentally friendly alternative to batteries or generators.
Galvanic Isolation
Separating the input and output power supplies of a device, ensuring safety and preventing interference.
Wireless Power Transfer:
A method of transferring energy through a field rather than through a direct electrical connection.
Gas Discharge Tube (GDT):
A protective component featuring electrodes within a gas-filled, insulated ceramic or glass housing resistant to high temperatures. When exposed to voltage spikes or transients, it transitions to a low impedance state.
Gas Turbine:
A mechanism that converts the kinetic energy from combustion into electrical energy, used as an AC backup power source.
Generator:
A device converting combustion energy into electrical power, providing AC or DC output.
Harmonics:
Harmonics are multiples of the fundamental frequency waveform.
Harmonic Distortion or Total Harmonic Distortion (THDi):
The deviation of a power supply’s sine wave from its ideal sinusoidal waveform, relative to the fundamental frequency.
Harmonic Filter:
A device used to reduce harmonic distortion and improve the power factor of an AC power source.
Hertz (Hz): The unit of measurement for the number of waveform cycles per second. The standard mains frequency is 50 Hertz in Europe and 60 Hertz in North America.
Cold Aisle/Hot Aisle Architecture:
A rack layout where cabinets are arranged with fronts facing fronts and backs facing backs, separating cooling air in “cold” aisles from hot exhaust air in “hot” aisles. This separation improves cooling efficiency and supports higher power densities in computing environments.
HVAC:
An acronym for Heating, Ventilation, and Air Conditioning. Sometimes “R” is added to represent Refrigeration (HVACR).
ICC:
Short-circuit current.
Iin:
Rated Input Current.
Incoming Line:
The cable that brings mains power from the substation to the building, also known as the Point of Common Coupling (PCC).
Inductance:
The property of an inductive circuit that generates an electromotive force (EMF) in response to changes in magnetic flux.
Ingress Protection (IP) Rating:
IP numbers specify the degree of protection provided by an enclosure for electrical equipment against solids and liquids. The first digit indicates protection against solid objects (1 being the lowest and 6 being the highest), and the second digit indicates protection against liquids (8 being the highest).
Insulated Gate Bipolar Transistor (IGBT):
A high-power switching device used in rectifiers and inverters.
Inrush Current:
The initial surge of current drawn by a load, such as when charging a capacitive circuit.
Inverter:
A circuit that converts DC energy into AC output.
JBUS:
A communication protocol that establishes a hierarchical structure (one master and multiple slaves) from a single RS-232 communication link, similar to MODBUS.
Joule (J):
A unit of energy, defined as one watt per second.
Linear Load:
A load with a constant voltage-current relationship, based on a relatively stable load impedance.
Line Interactive Uninterruptible Power Supply (UPS):
A type of UPS that regulates voltage sags and corrects surges and spikes using an automatic voltage regulator connected to the mains power supply.
Load:
The electrical equipment that is powered by a device.
Load Shedding:
The process of shutting down non-essential equipment to extend the runtime of remaining systems powered by a limited power source.
Maintenance Bypass:
A bypass power source used to power electrical equipment during maintenance, which can be internal or external to the equipment. Also known as a bypass panel or wrap-around bypass.
Make-Before-Break (MBB) Bypass:
A bypass that connects the secondary power source (bypass power) before disconnecting the primary power source (UPS output) to ensure uninterrupted power to the load.
Mean Time Between Failures (MTBF):
A reliability metric that represents the average operating time between failures.
Mean Time To Repair (MTTR)
A quantifiable metric that measures the average time required to restore a system to full functionality after a failure.
Metal Oxide Varistor (MOV)
A protective device designed to absorb extremely high surge currents without sustaining damage.
MODBUS (MODBUS)
A communication protocol that establishes a hierarchical framework (one master and multiple slaves) utilizing a single RS-232 communication link, similar to JBUS.
Noise Level
Typically quantified in decibels (dB) or (dBA). Noise (Noise Level) also refers to high-frequency electrical signals in a circuit that may interfere with electrical operations.
Non-Essential Load
Electrical equipment that can be temporarily disconnected during a mains power outage, especially in load shedding scenarios.
Non-Linear Load
A load where the relationship between voltage and current is not linearly proportional.
Normal Mode Noise
A type of high-frequency electrical noise that occurs between the phase and neutral conductors.
Off-Line UPS
Also known as a standby UPS, this system allows the unconditioned mains power to bypass the UPS while charging the battery. In the event of a mains power failure, the battery power takes over the load.
On-Line UPS
A UPS that is always connected to the mains power, ensuring an uninterrupted power supply during mains power failures. It features a double conversion system and automatic bypass capabilities, such as the PowerPure RT model.
Opto-Isolator
A device that transfers signals between circuit elements via a short optical path, providing electrical isolation.
Outage
A common term for a mains power failure, also known as a blackout or power cut. Some define an outage as a voltage drop below 80V, as most electrical equipment cannot operate at this level.
Overvoltage
A condition where the voltage exceeds the specified limits within a circuit.
Parallel Capacity System
A UPS configuration where multiple UPS units operate in parallel to meet the total power demand without redundancy, also known as parallel capacity.
Parallel Redundancy
A UPS setup where the total power demand is met by two or more UPS units in an N+1 configuration, sharing the load equally. If one UPS fails, the remaining UPS units can support the entire load.
Parallel System Joiner (PSJ)
A synchronizing switch used to connect two independent sets of parallel UPS systems.
Phase
In Europe, single-phase power is 50Hz, 230V AC. Three-phase power consists of three sine waves that are 120 degrees out of phase with each other, with a voltage of 415V between phases. Therefore, users can select either 230V or 415V for the mains input.
Phase Power Factor (PPF)
The ratio of active power (W) to apparent power (VA).
Point of Common Coupling (PCC)
The connection point where the mains power enters a building and connects to the distribution network.
Power Conditioning
The process of improving the quality of mains power by eliminating surges and spikes, thereby providing a stable power supply to electrical equipment.
Power Continuity Plan
A strategic approach that details methods for prioritizing and protecting electrical equipment during mains power failures.
Power Factor (PF)
The ratio between the actual power consumed (watts) in a circuit and the product of voltage and current. Power factor is classified as leading or lagging relative to voltage.
Power Supply Unit (PSU)
A component in modern electronic devices that converts an AC waveform into the necessary DC levels to power the internal circuitry.
Pulse Width Modulation (PWM)
A technique used in inverters to generate an approximate sine wave by varying the width of pulses.
Radio Frequency Interference (RFI)
A type of electrical noise that is generated by all electronic circuits and is referred to as RFI. This interference can originate from external sources or be generated internally by the circuit itself.
RFI filter, installed to protect sensitive equipment from interference.
Raw current:
Refers to utility power without any filtering.
Reactive power:
Energy that is wasted and returned to the AC power source.
Active power represents the actual power (watts) consumed by the electrical equipment.
Recovery time:
The time required for the voltage to stabilize after a backup generator takes over the load.
Rectifier:
A component that converts AC power to DC power.
Rectifier/Charger, a transformer-based component inside a UPS that converts AC power to DC power, which then powers the inverter and charges the battery pack.
Regulation specifies the control parameters for AC/DC output voltage.
Resilience: The ability of a power protection system to adapt to potential hazards by resisting or modifying them. The goal is to maintain an acceptable level of operation.
Ripple: Undesirable AC component interference superimposed on a device’s DC power supply waveform.
RoHS: Abbreviation for Restriction of Hazardous Substances Directive 2002/95/EC.
Root Mean Square (RMS): The square root of the average of the squares of the instantaneous values of alternating current and voltage over one cycle.
Rotary UPS, an uninterruptible power supply that uses the kinetic energy of a flywheel to convert it into electrical energy to power equipment.
RS-232 A standard interface that facilitates synchronous and asynchronous communication between compatible devices (one driver and one receiver) at rates up to 20 kb/s over distances up to 15 meters.
RS-442: Similar to RS-232 but supports a maximum transmission rate of up to 100Kb/s – 10Mb/s, allowing a single driver to connect to up to 10 receivers over distances up to 1000 meters.
RS-485 Similar to RS-442 but can accommodate up to 32 drivers and receivers.
Runtime: The duration that the battery connected to the UPS can maintain the operation of the connected electrical equipment after a mains power failure. (Also known as autonomy time)
Sags and dips refer to a drop in power below normal utility levels, lasting for several cycles.
Sensitive Systems: Electrical equipment that is not critical but requires a clean shutdown.
Series Redundancy: A method in which the output of one UPS module is used to power the bypass of another UPS module. When one UPS fails, the second UPS automatically takes over the load.
Silicon Avalanche Diode (SAD) A device that acts as a buffer to reduce the initial surge current during severe surges and transients.
Simple Network Management Protocol (SNMP), a communication protocol that supports monitoring and controlling hardware on a network with TCP/IP connectivity.
Sine Wave: The graphical representation is the waveform produced by alternating current (AC).
Single-Phase: A single-phase power supply consists of a single sine wave at a base frequency, such as the 230V AC utility supply in Europe.
Slew Rate: The rate at which a UPS synchronizes its output frequency to another AC power source, such as a generator.
Static Stability: A term describing the ability of a device to provide a stable electrical output voltage under steady load conditions.
Static Switch: A high-speed solid-state switch capable of switching loads between AC power sources.
Static UPS, an uninterruptible power supply that utilizes solid-state electronic components.
Supercapacitor: An electronic component capable of storing electrical charge, improving the utilization of battery power in a UPS.
Surge: A sudden, brief increase in mains voltage.
Switch Mode Power Supply (SMPS), a common type of power supply unit (PSU) commonly found in computers, telecommunications, and electronic equipment.
Switchover Time: Also known as transfer time, refers to the delay incurred when switching a load from one AC power source to another.
Thermal Runaway: An adverse condition in which a battery overheats severely, usually due to the battery generating heat while fully charged and being unable to dissipate it to the surrounding environment.
Three-Phase: A three-phase power supply consists of three AC voltages that are equally spaced in time.
Three-Phase Power
A 400V/415V power system achieved by overlapping three independent phases, each offset by 120°.
Thyristor
A semiconductor device used for switching current.
Total Harmonic Distortion (THDi/THD)
The cumulative harmonic distortion present in a system compared to a standard sine wave.
Total Power Factor / True Power Factor / Active Power Factor (often used interchangeably)
The total power factor, considering both the distortion factor and displacement factor.
Transfer Time
Also known as switching time, this is the time it takes for a load to switch from one AC power source to another.
Transformer
An electrical device used for voltage conversion, consisting of coils wound around an iron core with insulated laminations, also providing electrical isolation.
Transient (Transient Overvoltage)
A brief, high-energy voltage surge in a standard power supply that can be harmful to sensitive electronic equipment.
Transient Voltage Surge Suppressor (TVSS)
An electrical device integrated into a circuit to suppress transient voltage spikes.
Triplen Harmonics / Triple-N Harmonics
Harmonics that are integer multiples of the third harmonic frequency (e.g., 3rd, 6th, 9th).
Undervoltage
A voltage that is below its rated value for an extended period.
Uninterrupted Cooling
Continuous heat dissipation in a data center or IT room during a device or power failure, preventing thermal damage and data loss.
Uninterruptible Power Supply (UPS) / Backup Power (UPS)
An Uninterruptible Power Supply (UPS), also known as a backup power supply, maintains power to connected equipment for a defined period. It also protects equipment from power sags, spikes, and other anomalies. See Keheng UPS Systems.
UPS Group Synchronizer (UGS)
A device that ensures synchronization between two independent, parallel UPS groups.
Valve-Regulated Lead-Acid (VRLA) Battery
A lead-acid battery with a regulating valve.
Volt (V or kV or MV)
The base unit of electrical potential (voltage), where one volt drives one ampere through one ohm of resistance.
Volt-Ampere (VA or MVA)
The unit of measurement for apparent power.
Voltage-Free Contact (VFCc)
A contact used to open or close other circuits without carrying voltage.
Watt (W or kW or MW)
A unit of power measurement indicating the rate at which energy flows to or from an electrical device.
Waveform / Waveform Diagram
A visual representation of a waveform’s shape, typically displayed on an oscilloscope.
Waste Electrical and Electronic Equipment (WEEE)
Waste electrical and electronic equipment, directive 2002/96/EC.