Industry Dictionary - Data Center Glossary

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  • Access Floor

    A system consisting of completely removable and interchangeable floor panels that are supported on adjustable pedestals or stringers (or both) to allow access to the area beneath.

  • Access Line

    A telecommunications circuit provided by a service provider at the demarcation point.

  • Access Provider (AP)

    The operator of any facility that is used to convey telecommunications signals to and from a customer premises.

  • Air Conditioning Airflow Efficiency (ACAE)

    The amount of heat removed per standard cubic foot of airflow per minute.

  • Air Mixing

    The unintended mixing of cold and hot air.

  • Airside Economizer

    A device consisting of fans, ducting and a control which utilizes outside air directly to cool the data center when environmental conditions allow. Air is typically filtered, brought into existing distribution system and then exhausted back to the atmosphere. Economizers can save data center operators substantial operating costs. According to, economization has the potential to reduce the annual cost of a data center’s energy consumption by more than 60 percent. Use of cooler external environmental temperatures to preserve hardware is an important component in sustainable green computing practices in general. Unfortunately, economizers are only useful for data centers located in cooler climates.

  • Aisle

    The open space between rows of racks. Open airflow designs dictates racks should be arranged with consistent orientation of front and back to create ‘cold’ and ‘hot’a isles; but aisle containment designs and alternative cooling methods may dictate alternate configurations.

  • Alternate Entrance

    A supplementary entrance facility into a building using a different routing to provide diversity of service and for assurance of service continuity.

  • Alternating Current AC

    The designation given to power that is delivered in the form of a sinusoidal wave form. AC won out over DC as the preferred method of delivering and using power in the industrial age due to the ease of voltage transformation using static devices (transformers).

  • Amp (A)

    A unit of electrical current.

  • Blade Server

    A server architecture that houses multiple server modules (“blades”) in a single chassis. It is widely used in datacenters to save space and improve system management. Either self-standing or rack mounted, the chassis provides the power supply, and each blade has its own CPU, memory and hard disk. Redundant power supplies may be an option. Blade servers generally provide their own management systems and may include a network or storage switch.

  • Blanking Panel

    A metal or plastic strip mounted in unused U spaces in a rack that restricts recirculation airflow, also called blanking or filler plates.

  • Branch Circuit Monitoring (BCM)

    A monitoring system used to record and monitor an individual electrical circuit. Typical parameters which are monitored include amperage, voltage, power factor, apparent power (volt amps), real power (watts) and energy usage ( watt-hours). The branch circuit is typically defined to be a the circuit fed by a single breaker or 3 phase set of breakers in a multi-breaker panel.

  • British Thermal Unit (BTU)

    A unit of energy. 1kWh = 3412btu. Cooling equipment capacity is commonly specified in btu/hr.

  • Building Management

    Synonymous with BAS, AMS and other computer-based tools used to manage data center facilities assets such as UPS, Generators, Chiller Plant, HVAC, etc.

  • Bypass Airflow

    Conditioned air that does not reach computer equipment. Some bypass air is inevitable and without containment, some bypass air is prudent. Unintended bypass air can occur by escaping through cable cut-outs, holes under cabinets, misplaced perforated tiles or holes in the computer room perimeter walls.

  • CapEx

    Capital Expense, the cost of purchasing capital equipment.

  • Carbon Footprint

    A measurement of the volume of Carbon Dioxide generated by business operations, units are commonly metric tons.

  • Carbon Usage Effectiveness (CUE)

    A metric defined by the Green Grid, which is a measure of data center sustainability in terms of data center specific carbon emissions. CUE is calculated by dividing the total CO2 emissions caused by total data center energy by the energy consumption of the IT computing equipment. An alternative way to calculate CUE is by multiplying the data center’s annual PUE by the Carbon Emissions Factor for the region as determined by the EPA. The units of CUE are kilograms of carbon dioxide per kilowatt-hour.

  • Chiller

    A unit consisting of a compressor, a condensing section and an expansion section. The condensing and expansion sections nearly always have water or glycol as the heat transfer agent to the rest of the system; primary water/glycol on the condensing side and secondary water on the expansion side.

  • Close-Coupled Cooling

    Cooling technology that is installed adjacent to server racks, minimizing the path that air must flow from the cooling unit through the IT equipment and back to the cooling unit.

  • CMDB

    Configuration Management DB. It is the database that contains all relevant about the systems, applications, and configuration.

  • Coefficent of Effectiveness (CoE)

    Uptime Institute metric based on the Nash-Sutcliffe model efficiency coefficient.

  • Coefficent of Performance (CoP)

    Used to rate the effectiveness of heat pumps or cooling units. It is the ratio of the load on a cooling unit and the energy that it uses.

  • Cold Aisle

    An aisle where rack fronts face into the aisle. Chilled airflow is directed into this aisle so that it can then enter the fronts of the racks in a highly efficient manner.

  • Cold Aisle Containment (CAC)

    Cold Aisle Containment system that directs cooled air from air conditioning equipment to the inlet side of racks in a highly efficient manner.

  • Cold Spot

    An area where ambient air temperature is below desired levels. Typically caused by ineffective airflow management necessitating a temperature set point lower than that which would be required with proper airflow management.

  • Cold Supply Infiltration Index (CSI)

    Quantifies the amount of hot air mixing with cold inlet air prior to entering the rack.

  • Computational Fluid Dynamics (CFD)

    A numerical analysis technique commonly used in the analysis of airflow in data centers.

  • Computer Room Air Conditioner (CRAC)

    Pronounced crack, which uses refrigerant and a compressor. Cooling of the air in the data center is accomplished by airflow over the evaporation coils where the refrigerant is being “directly expanded”+A19.

  • Computer Room Air Handler (CRAH)

    Pronounced craw or cray, which uses chilled water passing through a heat exchanger to cool air flowing over the heat exchanger.

  • Container

    Data center container is a self contained module often produced inside a custom built shipping container, that includes a series of rack-mounted servers along, its own lighting, air conditioning/de-humidification and Uninterruptable Power Supply (UPS).

  • Containment

    Using some type of physical barrier between a hot or cold aisle, for example, long plastic curtains or rigid plastic. Keeping warm exhaust air away from the intake of the server racks is a crucial part of making any data center more efficient.

  • Cooling Tower

    A device which cools water via the direct evaporation of some of the water. Water is pumped into the top of the cooling tower and allowed to run down over the fill, typically pads or strips into a sump at the bottom of the cooling tower. Air is drawn in from the sides over the fill by fans in the top of the tower, evaporating some of the water which cools the remaining water. The temperature of the water in the sump is controlled by varying the speed of the fans. The water in the sump is then used to cool the condensing section of a chiller or to cool the secondary loop directly via a heat exchanger (see water side economizer).

  • Critical Load

    Computer equipment whose uptime is critical, typically supported by a UPS.

  • Cubic Feet per Minute (CFM)

    A unit of flow rate, commonly used to specify airflow.

  • Current Transformer (CT)

    A device used to transform electrical current from one level to another with a specific ratio. For example, a 5000:5 current transformer transforms current on the primary side to current on the secondary side with a ratio of 1000:1. CTs are typically used to transform large currents to much smaller currents so that standard metering equipment can be used on a variety of circuits by measuring the secondary current rather than the large primary current.

  • Cut Out

    An open area in a raised floor that allows airflow or cable feeds.

  • Data Center infrastructure Efficiency (DCiE)

    A metric developed by the Green Grid, data center infrastructure efficiency is an efficiency measure that is calculated by dividing the IT equipment power consumption by the power consumption of the entire data center. This measure is the inverse of PUE.

  • DCIM

    Data Center Infrastructure Management. as the integration of information technology(IT) and facility management disciplines to centralize monitoring, management and intelligent capacity planning of a data center’۪s critical systems. Achieved through the implementation of specialized software, hardware and sensors, DCIM will enable a common, real-time monitoring and management platform for all interdependent systems across IT and facility infrastructures.

  • DCRS

    Data Center Ready Solution

  • Delta T

    The difference in temperature across a device. Examples include the temperature difference between the inlet and outlet of piece of IT equipment or between the inlet and outlet of a cooling unit (CRAC or CRAH). Delta T, airflow and thermal dissipation are related: thermal dissipation = airflow x delta T x specific heat of air.

  • Dewpoint

    The temperature at which air reaches water vapor saturation. Dewpoint is constant for a specific amount of water in a specific amount of air while relative humidity varies with temperature. The latest ASHRAE spec for data center environmental conditions includes an upper limit for humidity based on dewpoint.

  • Direct Current (DC)

    A non-time varying method of delivering power. While slightly more efficient then AC if utilized between the DC portion of the UPS and the power supplies in IT equipment, it has not won wide acceptance in modern data centers.

  • Distribution Frame

    A structure with terminations for connecting the cabling of a facility in such a manner that interconnection or cross-connections may be readily made.

  • Dry Cooler

    A liquid-to-air heat exchanger that is a radiator over which air is blown via fans. Typically used as the heat rejection device for water or glycol cooled condensers, may also be used as the heat rejection device for liquid cooled coils in an AHU under proper environmental conditions.

  • Dry-Bulb Temperature

    The temperature of the air measured using a dry-bulb thermometer such that evaporative cooling has no effect. Typically taken in conjunction with a wet-bulb reading which does include the evaporative cooling effect in order to determine relative humidity.

  • Duct Bank

    An arrangement of conduits, used to transport electrical, telecommunications cabling or data cabling inside or outside of a data center facility arranged in an array.

  • Economization

    A way of utilizing the local environment around the data center to aid in cooling of the IT load by natural means rather than use more energy. There are two common types of economization, Airside Economization and Waterside Economization; sometimes referred to as LINK. Economization is a great benefit to those who can take advantage of it due to the large energy saving opportunity.

  • Electrostatic Discharge (ESD)

    More commonly ‘static discharge’.

  • Entrance Facility (EF)

    An entrance to a building for both public and private network service cables (including wireless) including the entrance point of the building and continuing to the entrance room or space. (569)

  • Entrance Point

    The point of emergence for telecommunications cabling through an exterior wall, a floor, or from a conduit.

  • Entrance Room

    (do not use acronym) – use Entrance Facility to avoid confusion with Equipment Room

  • Equipment Distribution Area (EDA)

    The computer room space occupied by equipment racks or cabinets [TIA 942]

  • Equipment Room (ER)

    An environmentally controlled centralized space for telecommunications equipment that usually houses Distributor B or Distributor C.

  • Equivalent Full Cabinets (EFC)

    The number of full cabinets that would exist if all the equipment in the data center were concentrated in full cabinets.

  • External Network Interface

    Interface between the computer room cabling and external cabling coming from a telecommunications carrier or campus cabling.

  • Firewall

    The primary method for keeping a computer secure from intruders. A firewall allows or blocks traffic into and out of a private network or the user’s computer. Firewalls are widely used to give users secure access to the Internet as well as to separate a company’s public Web server from its internal network. Firewalls are also used to keep internal network segments secure; for example, the accounting network might be vulnerable to snooping from within the enterprise.

  • Flywheel UPS

    A flywheel is a heavy shaft-mounted rotating disc that absorbs and stores twisting or spinning motion and then releases it as rotational kinetic energy to provide motion to a stationary, or nearly stationary object.

  • Gateway

    A network element interconnecting two otherwise incompatible networks, network nodes, subnetworks or devices. A gateway is often associated with both a router, which knows where to direct a given packet of data that arrives at the gateway, and a switch, which furnishes the actual path in and out of the gateway for a given packet.

  • GbE

    Gigabit Ethernet

  • Harmonic distortion

    Distortion in the line voltage waveform. Any cyclical waveform can be described as the sum of sine waves of various magnitudes which are integer multiples of the root frequency (60 Hz, 120 Hz, 180 Hz, etc.). Harmonics are often the result of the non-linear loading of the power distribution system due to the nature of solid state power supplies. Harmonics are detrimental to the efficiency and capacity of power delivery equipment and rotating equipment due to increased eddy current losses and non-fundamental frequency torques.

  • Heat Exchanger

    A device used to transfer heat energy from one medium to another. Common uses of heat exchangers are water to air heat exchangers in air handling units, plate and frame heat exchangers in economizers, etc.

  • Horizontal Distribution Area (H.D.A.)

    Horizontal Distribution Area, a space in a computer room where a horizontal cross-connect is located as defined by TIA 942 specifications.

  • Hot Aisle

    An aisle where rack backs face into the aisle. Heated exhaust air from the equipment in the racks enters this aisle and is then directed to the CRAC return vents.

  • Hot Aisle Containment (HAC)

    A system that directs heated air from the outlet side of racks to air conditioning equipment return ducts in a highly efficient manner.

  • Hot Spot

    An area, typically related to a rack or set of racks, where ambient air temperature is above acceptable levels. Typically caused by poor airflow management (insufficient cool air supply or an excess of recirculation).

  • HVAC

    Heating, Ventilation and Air Conditioning system, the set of components used to condition interior air including heating and cooling equipment as well as ducting and related airflow devices.

  • Hybrid Cable

    An assembly of two or more cables, of the same or different types or categories, covered by one overall sheath.

  • In-Row Cooling

    Cooling technology installed between racks in a row that draws warm air from the hot aisle and delivers cool air to the cold aisle, minimizing the path of the air (see close coupled cooling).

  • Inlet Air

    The air entering the referenced equipment. For air conditioning equipment this is the heated air returning to be cooled, also called return air. For racks and servers this is the cooled air entering the equipment.

  • Internet Protocol (IP)

    A communications technology using the internet for communications.

  • ITIL

    IT Infrastructure Library. Most widely adopted framework for IT Service Management. It is a practical approach to identification, planning, delivery and support of IT services to business. Version 3 approaches the requirements by functional area.

  • Kilovolt (kV)

    One thousand volts.

  • Kilovolt Amperes (kVa)

    One thousand watt hours (see Wh). kWh is a common unit of electrical energy.

  • KVM

    Keyboard, Video, Mouse, an interface technology that enables users to access multiple servers remotely from one or more KVM sites. More obscurely, can also mean Kernel-base Virtual Machine: a version of the Java Virtual Machine for small devices with limited memory.

  • Latent Cooling

    The process of condensing water out of air, then evaporating the water later. Energy is given up by the water during condensation. If the water is then later evaporated (such as from a drip tray), the amount of energy used to evaporate the water is the same as the amount of energy given up by the water vapor to the cooling equipment when it was condensed. Cooling is occurring later in time, hence the name “latent cooling”. In a system where condensed water is pumped or drained away, the cooling that may occur from evaporation does not cool the environment where the condensation took place so the cooling capacity spent condensing the water vapor is wasted energy.

  • Latent Cooling Capacity

    Cooling capacity related to wet bulb temperature and objects that produce condensation.

  • Liquid Cooling

    A general term used to refer to cooling technology that uses a liquid to evacuate heat. In data centers, the two prevalent forms of heat evacuation are liquid (chilled water) and refrigerant (DX).

  • Load

    The demand placed on a system, typically used to describe the electrical demand on the electrical supply system or the cooling demand on the cooling system. Units are power such as kW, BTU/hr, Tons, etc.

  • Main Distribution Area (MDA)

    The space in a data center where the main cross-connect is located as defined by TIA 942 specifications.

  • Minimum Efficiency Reporting Value (Merv)

    For air filtration measured in particulate size.

  • Minimum Point of Presence/Engry (MPoP)

    Either the closest practicable point to where the carrier facilities cross the property line or the closest practicable point to where the cabling enters a multi unit building or buildings.

  • MoR

    Middle of Row

  • MTDC

    Multi Tenant Data Center

  • MTTR

    Mean Time to Repair

  • N+1

    Need plus one, a redundancy concept where capacity is configured to include used capacity plus one additional device to enable continued operations with the failure of one system in the configuration.

  • NAS

    Network Attached Storage

  • Network Equipment-Building Systems (NEBS)

    Design guidelines applied to telecommunications equipment.

  • Network Switch

    A network switch or switching hub is a computer networking device that connects network segments or network devices. The term commonly refers to a multi-port network bridge that processes and routes data at the data link layer (layer 2) of the OSI model. Switches that additionally process data at the network layer (layer 3) and above are often referred to as layer-3 switches or multilayer switches.

  • Nominal Cooling Capacity

    The total cooling capacity of air conditioning equipment, includes both latent cooling and sensible cooling capacities.

  • OpEx

    Operating Expense, the ongoing expenses related to operating the data center.

  • Overcooling

    A situation where air is cooled below optimum levels. Typically used in reference to rack inlet temperatures.

  • Oversubscription

    In a communications system in which multiple users share a common resource, oversubscription refers to the ratio of the allocated bandwidth per user to the guaranteed bandwidth per user. Underlying the oversubscription model is the fact that statistically few users will attempt to utilize their allocated bandwidth simultaneously. Calculation and management of oversubscription ratios is common in the CATV industry, Telecommunications, and Local Area Networking.

  • Packet

    A group of binary digits switched as a whole – for instance, a file transfer over a packet switched network would require many steps. These steps are: 1) the data file would be broken down into smaller “packets” of information 2) each packet of information is assigned a code that enables it to be sent to the correct location and, once at that location, for the network to reassemble the packets of information into their original form.

  • Packet Switched Network (PSN)

    A digital data transmission network that uses packet switching technology.

  • Packet Switching

    Refers to protocols in which messages are divided into packets before they are sent. Each packet is then transmitted individually and can even follow different routes to its destination. Once all the packets forming a message arrive at the destination, they are recompiled into the original message.

  • Phase

    A term that describes the relationship between multiple time-varying waveforms which have a constant frequency but differ in their position relative to time. It is also used to refer to the number of sinusoidal voltages that make up the power delivery to a device. Most common are three-phase and single-phase. Single-phase consists of 2 conductors between which a sinusoidal voltage is present. Three-phase is a set of 3 or 4 conductors. In the case of 3 conductors, a sinusoidal voltage of a constant magnitude and frequency but differing relationship with respect to time exists between any 2 conductors. In a 4 wire system, the same voltage as in the 3 wire case exists between any of the three “hot” conductors and in addition, between any of the three “hot” conductors and fourth neutral conductor there exists a voltage that is smaller by a factor of the square root of three than the voltage between any of the “hot” conductors. An example of this is a 208/120 three-phase system. 208 volts exists between any of the three “hot” conductors and 120 volts exists between any of the “hot” conductors and the neutral conductor.

  • Plate and Frame

    A type of heat exchanger commonly used in water-to-water systems. It is a series of plates held in a frame through which exists 2 paths for water which are adjacent but separate.

  • Plenum

    A compartment or chamber to which one or more air ducts are connected and that forms part of the air distribution system.

  • Pole

    A row of power receptacles with power supplied from a PDU.

  • Pole Position

    A power receptacle on a pole.

  • Potential Transformer (PT)

    A device used to transform electrical potential (voltage) from one level to another with a specific ratio. For example, a 480:120 potential transformer transforms voltage on the primary side to voltage on the secondary side with a ratio of 4:1. PTs are typically used to transform large voltages to much smaller voltages so that standard metering equipment can be used on a variety of circuits by measuring the secondary voltage rather than the large primary voltage.

  • Power Distribution Unit (PDU)

    This typically refers to one of two pieces of equipment in the power delivery chain. One is the combination transformer/breaker panel that is often used between a UPS supplying voltage higher than that used by the IT equipment and the cabinets. The other is the smaller “power strip” like device that is used inside the rack to distribute power to the IT equipment.

  • Power Factor (PF)

    Represents the portion of the apparent power that is real power. The source of power factor is non-resistive components (inductors and capacitors) in the load on an AC power system. These components draw current that is 90 degrees out of phase with the voltage across them resulting in zero real power being delivered. While the power delivery system must carry this current (as well as all the current which does result in real power being delivered), it does no useful work. Power companies often charge a penalty for loads which have a power factor that is significantly far away from 1.0 since the size of the equipment that must be in place is dependent upon the total current delivered, but normal billing is based on real power delivered.

  • Power Usage Effectiveness (PUE)

    A metric defined by the Green Grid, which is a measure of data center efficiency calculated by dividing the total data center energy consumption by the energy consumption of the IT computing equipment. This measure is the inverse of DCiE.

  • Pressure Differential

    The difference in pressure between two locations in the data center. Air flows from higher pressure areas to lower pressure areas. Often times, the pressure differential between the under-floor plenum and the above-floor space is controlled by varying the speed of the fans supplying air to the under-floor plenum. This allows the addition of vented floor tiles to occur without affecting the air delivered to existing vented floor tiles so that additional IT load may be placed on the floor without disturbing the tuning of the existing floor.

  • Primary Loop

    Refers to the water loop which cools the condenser side of a chiller. This loop is cooled by dry coolers or cooling towers.

  • Rack Unit (RU)

    Vertical mounting space of 1.75 in (44.45 mm) for cabinets or racks compliant with IEC 60297 or CEA-310-E.

  • Raised Floor

    Metal flooring on stanchions that creates a plenum for airflow and cabling, synonymous with RMF.

  • Raised Metal Floor (RMF)

    An alternate term for the more commonly used term ‘raised floor’.

  • Recirculation

    Air which exits IT equipment and then re-enters either the same IT equipment or another piece of IT equipment without being cooled. Typically caused by poor control of airflow due to missing blanking panels, gaps in rows, insufficient air supply, etc.

  • Recirculation Air Handler (RAH)

    A device that circulates air but does not cool the air.

  • Return Air

    The heated air returning to air conditioning equipment.

  • Rh

    Relative Humidity. The amount of water vapor present in air expressed as a percentage of the amount needed for saturation at the same temperature.

  • ROI

    Return on Investment, a measure of the money that an entity earns as a percentage of the total value of its assets that are invested.

  • Rooftop Unit (RTU)

    An air handler designed for outdoor use mounted on a rooftop.

  • Router

    A network device that forwards packets from one network to another. Based on internal routing tables, routers read each incoming packet and decide how to forward it. The destination address in the packets determines which line (interface) outgoing packets are directed to. In large-scale enterprise routers, the current traffic load, congestion, line costs and other factors determine which line to forward to.

  • RPP

    Remote Power Panel. Electrical power distribution panel.

  • Secondary Loop

    Refers to the water which is used to cool the heat exchangers in AHUs and is cooled via the expansion unit in a chiller.

  • Sensible Cooling

    The action of lowering the dry bulb temperature of air without condensation taking place.

  • Server

    A computer or computer program that manages access to a centralized resource or service in a network.

  • Set Point

    In a control system, this is the value against which the variable that is being controlled is compared. Temperature and humidity set points are common in the cooling system for a data center.

  • SFP+

    Small Form-factor Pluggable (SFP) Plus. High-density/high-bandwidth interconnect system used between servers and switches in data centers.

  • Short Cycling

    Chilled airflow returning to cooling units without passing through IT equipment, also referred to as bypass.

  • Slab

    Refers to either the ceiling or floor that is above a finished ceiling and/or below the raised floor. Often used to measure actual available height of an unfinished data center space and referred to as slab to slab height. Eg. If the raised floor is 3 feet, the finished ceiling is 2 feet and the the finished space is 10 feet; then the slab to slab height is 15 feet.

  • Static Transfer Switch (STS)

    A solid state device which transfers the feed of power from one source to another source such as from the incoming utility to a generator.

  • Storage Area Network (SAN)

    A network of storage disks. In large enterprises, a SAN connects multiple servers to a centralized pool of disk storage. Compared to managing hundreds of servers, each with their own disks, SANs improve system administration. By treating all the company’s storage as a single resource, disk maintenance and routine backups are easier to schedule and control. In some SANs, the disks themselves can copy data to other disks for backup without any processing overhead at the host computers.

  • Storage Array

    A disk storage system which contains multiple disk drives. It is differentiated from a disk enclosure, in that an array has cache memory and advanced functionality, like RAID and virtualization. Attached to SAN for multiple devices to access and store data.

  • Sub-Floor

    The open area underneath a raised computer floor, also called a sub-floor plenum.

  • Supply Air

    The cooled airflow emitted from air conditioning equipment.

  • Suspended Ceiling

    A ceiling that creates an area or space between the ceiling material and the structure above.

  • Telecommunications Room (TR)

    An enclosed architectural space designed to contain telecommunications equipment, cable terminations, or cross-connect cabling. (TIA 942).

  • Thermistor

    A type of resistor with resistance varying according to its temperature.

  • ToR

    Top of Rack

  • Triton Coefficient of Effectiveness (TCoE)

    A data center efficiency metric developed by the Uptime Institute synonymous with UCE. (see also CoE).

  • Uninterruptible Power Supply (UPS)

    A device placed in series with the supply of power from the utility which contains energy storage such that the supply of power from the UPS is continuous even when the utility supply is removed. While battery-based energy storage is the most common, flywheel-based energy storage is gaining in popularity due to the reduced maintenance cost. UPS is used to hold the load until either the generator turns on to handle the load or the utility power is restored. Battery based UPS systems have a finite time that they can handle the load, usually less than 10 minutes. Flywheel-based energy storage is usually less than 5 minutes depending on the design.

  • Upsite Coefficient of Effectiveness (UCE)

    A data center efficiency metric developed by the Uptime Institute synonymous with UCE. (see also CoE).

  • Utility Tunnel

    An enclosed passageway, usually placed between buildings, for the distribution of utility services.

  • Very Early Smoke Detection Apparatus (VESDA)

    A system that samples the air on a continuing basis and can detect fire at the pre-combustion stage. Pronounced VEZ-DUH.

  • Volt (V)

    A unit of electrical potential.

  • Volt-Amp (VA)

    A unit of apparent power. In AC circuits, the magnitude of the voltage across a circuit times the current through the circuit is the apparent power. Including a representation of the angle between the two waveforms in the form of power factor (see PF) yields the real power.

  • Water Usage Effectiveness (WUE)

    A sustainability metric defined by The Green Grid, which is a measure of the water used on-site for data center operations including humidification and on-site evaporation for cooling or energy production. WUE is calculated by dividing ‘annual water usage’ by the ‘energy consumption of the IT computing equipment’ The units of WUE are liters/kilowatt-hour (L/kWh).

  • Water-side economizers

    Use cold air to cool an exterior water tower. The chilled water from the tower is then used in the air conditioners inside the data center instead of mechanically-chilled water, reducing energy costs. Water-side economizers often operate during nighttime to take advantage of cooler ambient temperatures. Economizers can save data center operators substantial operating costs. According to, economization has the potential to reduce the annual cost of a data center’s energy consumption by more than 60 percent. Use of cooler external environmental temperatures to preserve hardware is an important component in sustainable green computing practices in general. Unfortunately, economizers are only useful for data centers located in cooler climates.

  • Watt (W)

    A unit of power, commonly used in electrical discussion, watts are the product of potential (volts, see V) and current (amps, see A). If the current and voltage are AC, the relationship between watts, volts and amps includes power factor (see PF), watts = volts x amps x PF.

  • Watt per Square Foot (WPSF)

    A unit of power density. In a data center this is a bulk term that refers to the total load in a particular space divided by the total area of that space. This is a design parameter for total capacity of the cooling and power systems and is used in conjunction with point load (the amount of load in a small space such as a rack). In higher density data centers most operators refer to Kilo Watts per cabinet as measurement of capacity.

  • Watts per Cabinet

    Usually based in Kilo Watts (KW) per cabinet is a unit of power density with in cabinet in the data center. This is a design parameter for total capacity for power load within a cabinet and the ability of the cooling plant to cool this load. At times the capabilities of the power installed within a cabinet may exceed the ability to cool the load which caused the KW per cabinet to be derated.

  • Wet-Bulb Temperature

    The temperature of the air measured using a wet-bulb thermometer, that is, the temperature to which a wet surface can be cooled by evaporation. This temperature is affected by both the dry bulb temperature and the dew point of the air. Dryer air has a lower wet bulb temperature. This is a design constraint when utilizing cooling towers or evaporative pads in the cooling process.

  • Work Cell

    The area of a rack and the related area immediately in front of and behind the rack. If a rack is 2 feet wide and 4 feet deep. Standard aisles are 4 feet wide front and back, so half of that space is workspace for a given rack. This results in a standard work cell of 32 square feet 2′ W x 4′ D x 2′ Front x 2′ Back. Actual work cell size varies with data center design.

  • Zone Distribution Area (ZDA)

    A space in a data center where an equipment outlet or a consolidation point is located. (TIA 942)

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