Definitions of common critical power terms and acronyms.

Critical Power Terminology

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Active power

In a DC circuit or in an AC circuit whose impedance is pure resistance, it is the product of root mean squared voltage and the root mean squared current expressed in watts. Also referred to a True or Real power. 

Apparent power

Expressed in volt-amps, it is the product of the applied voltage and current in an AC circuit. As the power factor is not included in the calculation, it is not the true power (which is expressed in watts).




Testing a UPS at a predetermined power level for a predetermined time period


Building Management System. Also known as Building Automation System (BAS). A computerized system installed in buildings that controls and monitors the building's mechanical and electrical equipment such as ventilation, lighting, power, fire systems, and security systems. This can also refer to a Battery Management System used for controlling batteries. These systems help to prevent adverse battery states and monitor and extend battery life.




By definition, the function of a capacitor in an electrical circuit is to store energy to maintain a supply of power.  With regards to a UPS, they are specifically used to remove fluctuations present in the power source to provide a clean power signal to equipment downstream.

Catcher system

A distributed redundant configuration whereby a UPS is connected to “catch” the load from one or more other UPSs in the event of their failure. 

Chopper circuit

Converts fixed dc input voltage to a variable dc output voltage directly.

Cloud Service Provider

A company that offers network services, infrastructure (such as storage), or business applications in the cloud. The cloud services are hosted in data centers and services are provided to customers over the Internet.

Colocation (colo) data center

A colocation (colo) is a data center facility in which multiple businesses rent space from the data center owner.  Typically, the colo data center owner provides the building, cooling, power, bandwidth and physical security while individual businesses provide their own servers and storage networks.  Individual businesses’ equipment can be partitioned in cages and/or rooms. 


An electrical device for converting electrical energy. In uninterruptible power supplies, converters are used to convert from AC to DC.

Critical Load Cabinet

UPS paralleling switchgear/switchboard.  Paralleling is the operation in which multiple power sources are synchronized and connected to a common bus. 


Carrier Stored Trench Bipolar Transistor.    Utilizes a new chip structure to provide reduced on-state voltage compared to conventional IGBTs




Data Center Infrastructure Management. The synchronization of data center facility management with information technology made available by electronic equipment signaling

De-rating curve

Reducing a UPS module operating characteristics due to a set of circumstances. Typically due to load power factor, altitude, or operating temperature.


A measure of time that a computer, machine, process, operation, is not running, not working, and unavailable.

Double conversion

Refers to the conversion of the incoming AC voltage to DC voltage and then back to a pure AC sine wave.  This process has the effect of eliminating any power fluctuations and enables the UPS to deliver a clean, steady signal to the load, providing the highest quality of continuous conditioned power.



ECO mode

An energy saving operating mode of a double-conversion online UPS.  However, this operating mode is often similar to Standby or Line Interactive models of smaller single phase UPSs. As the incremental energy savings over a highly efficient true online double conversion UPS is small and the risks of being exposed to unprotected utility power are high, customers that understand the high costs of downtime usually don’t operate their uninterruptible power supplies in ECO mode. 

Edge data center

Data centers geographically located close to end users and intended to reduce latency, decrease network congestion, keep mission-critical applications on premises, and/or act as a data-aggregation and content-caching point between a user and a central data center.


The useful power output of an electronic component or system divided by the total electrical power it consumes.  For critical power devices such as UPS, typically expressed as a %.

Energy Star

Energy Star is a government-backed labeling program that helps people and organizations save money and reduce greenhouse gas emissions by identifying factories, office equipment, home appliances and electronics that have superior energy efficiency.




Factory Acceptance Test


Software that resides on a hardware device.  It typically resides in ROM (Read Only Memory), and can be updated by deleting a previous version and replacing with a more current one.


A mechanical device specifically designed to efficiently store potential energy.  The amount of energy stored in a flywheel is proportional to the square of its rotational speed.


Factory Witness Test




Harmonics are distortions of the normal electrical current waveform, generally transmitted by nonlinear loads.

Hyper-scale data center

Large scale; in general, refers to a data center with power capacity of >50 MW; also denotes varying types of data center architecture.




International Fire Code – stationary storage battery systems


Insulated Gate Bipolar Transistor; An electronic switching device with 3 terminals used for controlling power flow through a circuit.


An electronic device or circuitry that changes direct current (DC) to alternating current (AC)

Isolated Redundant system

A UPS configuration, sometimes known as an “N+1” system, where the main UPS normally handles the load and a secondary (isolated) UPS feeds the static bypass of the main UPS. When operating normally, the main UPS module handles the full critical load and the isolated module has no load. Whenever the primary module fails or the load is transferred to static bypass, the isolated module instantaneously sees the full load of the primary module.


For critical power infrastructure, Integrated System Testing of multiple-to-all linked or interfaced equipment; typically conducted onsite during commissioning. 




kilo-Amperes Interrupting Capacity.  Circuit breakers used in power backup systems typically include a kAIC rating, indicating the maximum amount of current that the circuit can be protected against.  If a circuit breaker is installed with a kAIC that is too low for the circuit, it may not sufficiently protect the circuit from damage if overloaded.



Line Interactive UPS

Similar to the Standby UPS the Line Interactive UPS has a transfer switch that opens when the input power fails so that power flows from the battery to the UPS output. The DC battery-to-AC power converter (inverter) however is always connected to the output of the UPS which provides additional filtering and yields reduced switching transients when compared with the Standby UPS topology.

Lithium Ion batteries

Rechargeable batteries whereby lithium ions move from the positive electrode to the negative electrode during charge and back when discharging.


The active electric power being drawn by an electronics or electrical component, device, circuit, or system.

Load bank

A device which develops an electrical load, applies the load to an electrical power source and converts or dissipates the resultant power output of the source. A load bank includes load elements with protection, control, metering, and accessory devices required for operation. Load banks are used for testing power sources such as standby generators and batteries.



Maintenance bypass

Also known as a wraparound bypass, it enables the UPS to be isolated for maintenance or repair without interrupting power to the load.


A proprietary critical power system from Mitsubishi Electric that integrates UPS, batteries, and critical load cabinets into one cohesive package that minimizes total system footprint and eliminates costly conduit and cable runs.


A Multi-Module UPS System operated in a parallel configuration


A serial communications protocol for transmitting information among multiple electronic devices connected to the same network.  Over the years, it has become the industry standard and is commonly utilized for the collection of data points in the power industry. 

Modbus RTU

Modbus Remote Terminal Unit - Modbus protocol for serial line.

Modbus TCP/IP

Modbus Transmission Control Protocol/Internet Protocol - Modbus protocol for Ethernet.


Metal Oxide Semiconductor Field Effect Transistor.  Typically of Silicon construction, MOSFETs offer higher frequency operation and reduced switching losses over IGBTs.


Mean Time Between Failure. The predicted elapsed time between inherent failures of a system during normal operation.


Mean Time To Repair. The average time required to repair a failed component or device.



NiCad Battery

NiCad stands for Nickel Cadmium, a reference to the electrodes on this type of battery being made of nickel oxide hydroxide and metallic cadmium.   NiCad batteries are rechargeable batteries and offer good cycle life and performance. 



Offline UPS

In this type of UPS, the load is directly connected to the incoming AC power supply, subjecting the load to any power fluctuations present in the raw power source. In the event of main power failure, the inverter turns on and power is supplied to the load from the backup power supply (i.e, batteries).  Because the inverter only turns on when the incoming power fails, there is finite transfer time in switching to the backup power source, which may be intolerable to many mission critical applications.

Online UPS

In this type of UPS, the inverter is always on, supplying a consistent and clean supply of power.  When main power supply is present, it provides power to the inverter section while also charging the batteries.  Because the inverter is always on, transfer time to switch to backup power is practically instantaneous and power to the load is uninterrupted.


OSHPD Preapproval of Manufacturer’s Certification is a voluntary program for review and preapproval of seismic design of supports and attachments for nonstructural components to be used in health facilities construction in California.  OSHPD is an acronym for California’s Office of Statewide Health Planning and Development.



Parallel Redundant System

Multiple UPS modules of the same size connected to a common output bus.  Prevents critical load from transferring to a utility source when one of the UPS module fails.  The UPS modules in these systems evenly share the critical load in normal operating situations. When one of the modules is removed from the parallel bus (due to failure or maintenance), the remaining UPS modules immediately accept the load of the “removed” UPS module.


Power Distribution Unit; distributes power to servers and networking equipment within a data center.

Power Factor

The ratio of active power to apparent power in a circuit


 Power Usage Effectiveness, PUE is the inverse of the total efficiency in which a data center operates. This takes in to account he input to the data center. The ideal PUE is 1.0.  




An electromechanical device generally used for improving the voltage profile of transmission.

Real Power

See Active Power. 


An electrical component that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction.


Remote Power Panel.  A power distribution cabinet consisting of breakers and panelboards.


 Remote Status Alarm Panel, The RSAP is a dedicated panel that displays the UPS status and any active alarms. Control centers commonly use RSAPs for a visual UPS status display.




Supervisory Control And Data Acquisition


Silicon carbide, the latest material used in power devices.  The intrinsic characteristics of SiC-based semiconductors also allow them to operate more efficiently, at higher frequency, and at higher temperatures than conventional Si-based semiconductors. The resulting reduction in UPS heat loss and the ability to operate at higher temperatures help customers decrease their cooling costs, which account for approximately 35% of a data center’s electrical-energy consumption.

Single phase power

Power that is delivered over a two wire circuit.  Power flows between the power wire through the load and returns to the neutral wire.


Solenoid Key Panel Mounted – relative to a trapped key interlock system


Solenoid Key Release Unit – relative to a trapped key interlock system


Single Module UPS System


Simple Network Management Protocol. An internet standard protocol for collecting information about and configuring network devices on an internet protocol networks.


Static Transfer Switch.  Power equipment that uses power semiconductors, typically thyristors (commonly known as a silicon-controlled rectifiers (SCRs)), to transfer a load between two sources

Standby UPS

See Offline UPS.

Static Bypass

Circuit integral to the UPS that uses the STS to connect its input to the output so that the load continues to be supported during an internal UPS fault.


In an electric power system, switchgear is a package of disconnect switches, fuses or circuit breakers used to control, protect and isolate electrical equipment. Switchgear de-energizes equipment for maintenance and to clear faults downstream.




Total Cost of Ownership.  Often performed for large UPS purchases this is the sum of the initial capital expense as well as all of the estimated operating expenses that that will be incurred over the expected operating life including preventive maintenance, fan replacement, capacitor replacement, and battery replacement.  

Thermal runaway

Thermal runaway occurs in situations where an increase in battery temperature changes the conditions of the battery in a way that causes a further increase in temperature, often leading to a destructive result. It is a kind of uncontrolled positive feedback.  More info.

Thermographic imaging

Use of infrared radiation detecting cameras to see variations in temperature.  When viewed through a thermal imaging camera, warmer objects stand out against cooler backgrounds.  In power management and quality, thermographic imaging can be used to locate components and lines that are overheating to unsafe levels.

Three phase power

Power whereby three conductors each carry an alternating current of the same frequency and voltage amplitude relative to a common reference but with a phase difference of one third the period.

Trapped key interlock system

A system of locks and keys to ensure proper sequence of operation for safety purposes.  Interlock devices may include an electrical solenoid which holds the key ransomed until an electrical circuit is interrupted; for example, the key will not turn until the power supply for a high-voltage cabinet has been de-energized.

True Power

See Active Power.




UL (Underwriters Laboratories) Standard for Uninterruptible Power Supplies


UL (Underwriters Laboratories) Standard for Safety of Emergency Lighting and Power Equipment


A measure of time that a computer, machine, process, operation, is running, working, and available.



Virtual neutral

 Mitsubishi's proprietary technique for ensuring the UPS operates without electrically floating. This system also provides a means for harmonic mitigation and fault detection. 

VRLA batteries

Valve-Regulated Lead Acid; sometimes referred to a seal lead acid or gel cell.



2-level inverter topology A 2-Level inverter topology refers to the number of steps in the UPS output inverter. The inverter uses the DC-link voltage to create a sine wave by switching the DC voltage in either of two modes, on or off.​

3-level inverter topology

 A 3-Level inverter topology refers to the number of steps in the UPS output inverter. The inverter uses the DC-link voltage to create a sine wave by switching the DC voltage in either of three modes, on, half voltage or off. Taping the middle of the capacitor bus creates the half voltage step.

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