Glossary of Data Center Terms





# - A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z




2N: A redundancy model that ensures every component has a backup such that the data center has no single point of failure.


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A: Ampere (A, amp) A unit of measurement of electric current. One ampere is equal to the current produced by one volt flowing through a resistance of one ohm.


AaS (As a Service): An operational model where the information technology (IT) service provider delivers an information technology service to a business. 


AC: Alternating Current, 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).


AC/DC: AC stands for alternating current, and DC stands for direct current.


ACAE: Air Conditioning Airflow Efficiency, the amount of heat removed per standard cubic foot of airflow per minute.


Access Point: A hardware device that acts as a communication hub for wireless device users to connect to a wired local area network (LAN).


AHU: Air Handling Unit. Air-Sampling: Sampling smoke detector, sometimes called a VESDA system, is capable of detecting microscopic particles of smoke. Air Mixing: The unintended mixing of cold and hot air.


AI (Artificial Intelligence): The science of making machines that can think and act on their own, like humans. Companies can create autonomous data centers that can optimize and perform generic data engineering tasks without human intervention.


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.


Aisle: The open space between rows of racks. Best-practice dictates racks should be arranged with consistent orientation of front and back to create ‘cold’ and ‘hot’ aisles.


Anti-Static: Anti-static refers to an environment – whether within a product or a physical space – that features components allowing for the reduction of electrical static or that prevent its build-up. Anti-static agents are the actual compounds used in a variety of products (such as ESD wrist straps) to make either the products or the wearer less likely to conduct potentially damaging static electricity.


Arc Flash: An arcing fault is the flow of current through the air between phase conductors or phase and neutral or ground. An arcing fault can release tremendous amounts of concentrated radiant energy at the point of the arcing in a small fraction of a second resulting in extremely high temperatures, a tremendous pressure blast, and shrapnel hurling at high velocity.


ASHRAE: American Society of Heating, Refrigerating and Air-Conditioning Engineers is an international technical society organized to advance the arts and sciences of air management.


ATS: Automatic Transfer Switch, a switch that automatically transfers electrical loads to alternate or emergency standby power sources.


Available Space: The total amount of space that is currently being marketed as available for lease in a given time period. It includes any space that is available, regardless of whether the space is vacant, occupied, available for sublease, or available at a future date.


Auto Transfer Switch: A device that automatically transfers power from one source to another.


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Battery: Two or more cells connected together electrically. Cells may be connected in series or parallel, or both, to provide the required operating voltage and current levels.


Big Data: Datasets whose size is beyond the ability of typical database software tools to capture, store, manage and analyze.


Biometric Access: A technique used in computer and facility security to authenticate an operator using physical characteristics, i.e., facial recognition, fingerprint scans, retinal scans, iris recognition and hand geometry.


Blade Server: A computer optimized to use minimal physical space and power without compromising functionality. A standard data center rack can hold 42 1-U servers vs. 128 blade servers.


Blanking Panel: Blanking panels make data center cooling systems run more efficiently by blocking the transfer of hot and cold air in empty rack space. Blanking panels provide low-cost protection against overheating along with noticeable savings on energy costs. Most models are easy to install and fit standard cabinet openings.


Bonding: The permanent joining of metallic parts to form an electrically conductive path that will ensure electrical continuity and the capacity to conduct any current to be present in a safe manner.


Branch Circuit: A branch circuit is defined as, the circuit conductors between the final overcurrent device protecting the circuit and the outlet(s).  


BTU: British Thermal Unit. BTUs are typically used to rate air conditioners (and some heaters). One BTU is defined as the energy required to increase the temperature of one pound of water by one degree Fahrenheit. To convert fromBTUs to KWs: use 1 watt = 3.4129BTUs.


Build-To-Suit: A term describing a particular property, developed specifically for a certain tenant to occupy, with structural features, systems, or improvement work designed specifically for the needs of that tenant. A build-to-suit can be leased or owned by the tenant. In a leased build-to-suit, a tenant will usually have a long term lease on the space.


Burn-In: A test in which a system or component is made to run for an extended period of time to detect problems.


Bypass Airflow: Conditioned air that does not reach computer equipment. Unintended bypass airflow can occur by escaping through cable cut-outs, holes under cabinets, misplaced perforated tiles or holes in the computer room perimeter walls. 


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Cabinet: Device for holding IT equipment, also called a rack.


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


CADE: Corporate Average Data Center Efficiency.


Cage: An Enclosure to subdivide colocation space within a data center. Most data center cages provide full visibility to what is inside.


Calibrated Vectored Cooling (CVC): An air-cooling technology developed by IBM for server systems with high component density. CVC optimizes the path of air flow through the equipment, increasing the efficiency with which heat is removed.


CapEx: Capital Expenditure, the cost of purchasing capital equipment. Carrier Neutral: A data center facility that permits clients to contract with any telecommunication provider they choose.


Carrier Hotel: Similar to a colocation, carrier hotels are primary internet exchange points for an area. Network providers share space in carrier hotels which house data center infrastructure and are built out with fiber optic connections and telecom equipment.


Category 6 Cable: Commonly referred to as Cat-6, is a cable standard for Gigabit Ethernet and other network protocols that is backward compatible with the Category 5/5e and Category 3 cable standards. Cat-6 features more stringent specifications for crosstalk and system noise. The cable standard provides performance of up to 250 MHz and is suitable for 10BASE-T / 100BASE-TX and 1000BASE-T / 1000BASE-TX (Gigabit Ethernet). It is expected to suit the 10GBASE-T (10Gigabit Ethernet) standard, although with limitations on length if unshielded Cat 6 cable is used. Category 6 Cable can be identified by the printing on the side of the cable sheath.


CCTV: Circuit television, a private television system, typically used for security purposes, in which the signal is transmitted to a limited number of receivers.


Ceiling to Deck Height: The space between a suspended or false ceiling and the structural surface above used as a distribution system that provides a pathway for cables serving the work area outlets from above.


CFD: Computational Fluid Dynamics, a computational analysis model predicting airflow and air temperature in a computing environment using a computer and software. Commonly used in the analysis of airflow in data centers for optimal air conditioning design.


CFM: Cubic Feet per Minute, a unit of flow rate, commonly used to specify airflow.


Chilled Water System: A type of precision cooling system widely used in mid-sized to large IT environments. A chilled water system uses water as a cooling medium. Cold water is pumped from a chiller to computer room air handlers designed to cool the space. A chilled water air conditioner can be thought of as similar to a car radiator with a fan, with hot air being cooled by being blown through a cool radiator. In a chilled water system cooling an IT facility, the chilled water may be provided as a utility in the building, or special dedicated water chillers may be installed.


Chiller: A unit consisting of a compressor, a condensing section and an expansion section used to chill water to cool and dehumidify air in a data center. 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. A chiller uses the refrigeration cycle to produce large volumes of chilled water distributed to Computer Room Air Handlers (CRAH) units designed to remove heat from the IT environment.


Chiller Feeds: A type of precision cooling system widely used in mid-sized to large IT environments. A chilled water system uses water as a cooling medium. Cold water is pumped from a chiller to computer room air handlers designed to cool the space. A chilled water air conditioner can be thought of as similar to a car radiator with a fan, with hot air being cooled by being blown through a cool radiator. In a chilled water system cooling an IT facility, the chilled water may be provided as a utility in the building, or special dedicated water chillers may be installed.


Closed-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.


Cloud Computing: A general term for anything that involves delivering hosted services over the Internet. These services are broadly divided into three categories: Infrastructure-as-a-Service (IaaS), Platform-as-a-Service (PaaS) and Software-as-a-Service (SaaS).


Cloud Storage API: An application program interface that connects a locally-based application to a cloud-based storage system, so that a user can send data to it and access and work with data stored in it. To the application, the cloud storage system is just another target device, like tape or disk-based storage.


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 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.


Colocation (Colo): The practice of locating critical technology assets, like computers, routers and data storage systems, in an off-site data center which specializes in managing the critical power and cooling infrastructure required to support computer operations.


Commissioning: A quality assurance-based process used to uncover deficiencies in design or installation via field verification. A formal review and integration of all project expectations during planning, design, construction, and occupancy phases by inspection and functional performance testing, oversight, and record documentation.


Concurrently Maintainable: A design standard that provides a data center the ability to perform planned and unplanned emergency maintenance without disrupting the computer hardware operation.


Condensate Piping: Water that results as a by-product of dehumidification. Condensate is usually pumped out of the IT room or data center (via a condensate pipe) into the building drainage system. Since maintaining humidity is a desired goal of a computer room air conditioning system, dehumidification is typically not a desired function. However, dehumidification and the resultant production of condensate commonly occur as a result of suboptimal design.


Condenser: A device or unit used to condense vapor into liquid with access to outside air for heat dissipation. The media (water or refrigerant) is run through a coil. A fan or set of fans draws air through the coil creating a heat transfer between the media and the air. The heat is ejected from the system into the outside air.


Containment: Using either long curtains or rigid plastic to maintain a physical barrier between a hot and cold aisle. Keeping warm exhaust air away from the intake of the server racks is a crucial part of making any data center more efficient.


Content Delivery Network (CDN): A geographically distributed network of caching proxy servers deployed in multiple data centers that work together to accelerate the delivery of content to devices connected to the internet.


Converged Infrastructure: An approach to managing a modular data center that relies on a specific vendor and the vendor's partners to provide pre-configured bundles of hardware and software.


Cooling Tower: A heat rejection method that transfers heat energy from a data center or IT room to the outside atmosphere via the evaporation of water. In a cooling tower, water is sprayed onto a high surface-area packing material as large volumes of air are drawn across through the structure. The net effect of this process is that a small portion of the water circulated through the cooling tower evaporates into the outside atmosphere. The remaining water (now cooler) is collected at the bottom of the cooling tower.


CPU (Central Processing Unit): A hardware device in a computer that executes all the instructions from the software.


CR (Computer Room): A secure environment for equipment and cabling directly related to the critical load.


CRAC: CRAC stands for “computer room air conditioning.” CRAC can refer to any system that controls air temperature, humidity levels and the distribution of air within data center equipment.


CRAH: CRAH stands for “computer room air handler”. CRAH uses chilled water passing through a heat exchanger to cool air flowing over the heat exchanger.


Critical Load: Computer equipment whose uptime is critical, typically supported by a UPS. Equipment that must have an uninterrupted power input to prevent damage or loss to a facility or to itself, or to prevent danger of injury to operating personnel. It does not include any ancillary load for cooling, lighting, common areas or other equipment.


Critical Cooling Load: The usable cooling capacity at the data center floor. It does not include any ancillary load for lighting, common areas or other equipment.


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


CW: Chilled Water.


CWR: The abbreviation for chilled water return, the term used for all piping intended to return chilled water from room air handlers to the chiller.


CWS: The abbreviation for chilled water supply, the term used for all piping intended to deliver chilled water from the chiller to the room air handlers.


Cybersecurity: The practice of protecting systems, networks and programs from digital attacks.


Cybersecurity Systems: Protects networks, devices and data from unauthorized access or criminal use. This helps ensure confidentiality, integrity and availability of information.


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D/H: Dehumidifying/Humidifying.


Data Center: A facility used to house computer systems and associated components, such as telecommunications and storage systems. It generally includes redundant or backup power supplies, redundant data communications connections, environmental controls (e.g., air conditioning, fire suppression) and security devices. Also includes colocation, a subset of data centers.


Data Center Administrator:  Monitors systems, installs equipment and cabling, and participates in change processes and everyday procedures that support information technology.


Data Center Bridging (DCB): A collection of open standards-based enhancements to ethernet that extend networking and management functions to the data center.


Data Center Shell: A building that has been pre-qualified for power and telecom access, with or without any other improvements, and amenable to data center development and use. May become a single-tenant property or colocation.


Data Hall: The secure area of the data center housing technical infrastructure. These areas are typically filled with server cabinets and a cooling system (many use the term “data center” when referring to a data hall).


Data Integrity: The assurance that digital information is uncorrupted and can only be accessed or modified by those authorized to do so.


Data Center Networking (DCN): The process of interconnecting all data center resources together.


DC: Direct Current, 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.


DCaaS (Data Center as a Service): The provision of off-site physical data center facilities and infrastructure to clients.


DCIE (Data Center Infrastructure Efficiency): An efficiency measure calculated by dividing the IT equipment power consumption by the power consumption of the entire data center. This measure is the inverse of PUE.


Dedicated Hosting: The provider operates and or rents server capacity to single customers.  Server space is not shared by multiple customers. Typically the customer maintains full control over the server excluding maintenance.  


Dehumidification: Remove moisture from air by drawing it over a refrigerated coil with a fan. Since the saturation vapor pressure of water decreases with decreasing temperature, the water in the air condenses, and drips into a collecting area or is pumped to a drain. The air is then reheated by the warmer side of the refrigeration coil. Dehumidification is typically not desired in a data center but occurs anyway when the return air is low enough in temperature so that the dew point is reached as the air passes through the air conditioner.


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.


Direct Space: Space that is being offered for lease directly from the landlord or owner of a building, as opposed to space being offered in a building by another tenant (or broker of a tenant) trying to sublet a space that has already been leased.


Dirty Power: An abnormality in the power quality being delivered to a system. These abnormalities can include low power factor, voltage variations, frequency variations, surges and sags.


Disaster Recovery: The process of resuming normal operations following a disaster by regaining access to data, hardware, networking equipment, software, power and connectivity.


Discharge Rate: The rate, in Amperes or Watts, at which current is delivered by a battery.


Distribution: The way power is routed to various current-using sites or devices. Outside the building, distribution refers to the process of routing power from the power plant to the users. Inside the building, distribution is the process of using feeders and circuits to provide power to devices.


Distribution Cell: The cellular floor sections from which cables emerge into work areas.


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.


Distribution Panel: A wiring board that provides a patch panel function, and mounts either in a rack or on a wall.


Double Interlock Pre-Action Fire Sprinkler: Air or nitrogen is used to pressurize the dry pipe fire sprinkler system. The purpose is to first monitor piping for leaks and second to hold water from system piping in the event of inadvertent detector operation.


Downtime: A period of time, or a percentage of a time span, that a system is unavailable or offline. This is usually a result of the system failing to function because of an unplanned event, or because of routine maintenance.


Down Flow: An air conditioner or air handler that discharge air in a downward direction mainly used on raised floors.


Draw Cap: A contractually determined limit on how much power customers can draw/use within their data center cages.


DRaaS (Disaster Recovery as a Service): Provides continuous data protection from your primary environment to your target site, extending the life of legacy assets and maximizing investments.


Dropout: Loss of equipment operation due to noise, sag or interruption. A discrete voltage loss or a voltage sag (complete or partial) for a very short period of time (milliseconds).


Dropout Voltage: The voltage at which a device fails to operate.


Ducting (Data Center Cooling): An approach to air management that uses a series of metal or plastic pipes to carry heated or cooled air from one place to another.


DX: An abbreviation for direct expansion. This refers to the use of refrigerant directly expanded into evaporation coils in the supply air stream of an air conditioning unit.


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Economizer: A mechanical device used to reduce energy consumption. Economizers recycle energy produced within a system or leverage environmental temperature differences to achieve efficiency improvements. An additional cooling coil installed into glycol-cooled air conditioning unit to provide free cooling in cold climates. The economizer coil contains cold glycol circulating directly from the fluid cooler when atmospheric conditions allow.


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.


Edge Computing: A distributed computing paradigm that brings computations and data storage closer to the sources of data. Expected to improve response times and save bandwidth.


Edge Data Center: Smaller data center facilities located close to the populations they serve that deliver cloud computing resources and cached content to end users. They typically connect to a larger central data center or multiple data centers.


EFC: Equivalent Full Cabinets, the number of full cabinets that would exist if all the equipment in the data center were concentrated in full cabinets.


Electronic Data Interchange (EDI): Following specific standards, EDI is a way for businesses to securely transfer important data, such as invoices and purchase orders, via the Internet.


Electrical Service: The wiring that connects the electric utility's cables in the street to the building. Specifically, electrical service is the wiring from the street, through the meter and up to the panelboard, but no farther. Electrical service can be provided directly from the utility company's transformer or though service laterals. Besides referring to the physical wiring, the term electrical service also refers in an abstract sense to the provision of electricity to a building.


Emergency Power System: When regular power systems fail, a system that may include generators and other apparatus will provide backup power and lighting during an outage.


Enterprise Data Center: A fully company-owned data center used to process internal data and host mission-critical applications.


EPO: Emergency Power Off. A single disconnecting means for all power required within a NEC 645 rated space.


ESD: Electrostatic Discharge, more commonly ‘static discharge’.


ESD Wrist Strap: An ESD (electrostatic discharge) wrist strap is worn while working with or around computer systems to ground the technician and to prevent static electricity buildup within the body. Also referred to as an anti-static strap or a ground bracelet, ESD wrist straps are typically made from stretchy fabric containing conductive fibers. The design of the strap allows high-voltage charges to leak through but prevents damaging electric static from releasing on sensitive computer equipment.


Evaporative Cooling (EC): A strategy for cooling air that takes advantage of the drop in temperature that occurs when water that's exposed to moving air begins to vaporize and change to gas. Existing Inventory: The square footage of buildings that have received a certificate of occupancy and are able to be occupied by tenants. It does not include space in buildings that are either planned, under construction or under renovation.


E-Waste: Any refuse created from discarded electronic devices and components as well as substances involved in the manufacturing process or while being used.


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Fault Current: The current that flows as a result of a short-circuit condition.


Feeder Conductor: The conductors between the load side of the service equipment and the line side of the final branch circuit overcurrent device are feeder conductors.  


Fire Rated: Having an official rating or designation as determined by the NFPA (i.e., one-hour rated firewall).


Fire Stop: The act of sealing all penetrations into a fire rated area to maintain said fire rating.


Firewall: A continuous barrier used to prevent fire spreading from one fire zone or area to another. One or more security mechanisms (hardware and/or software) designed to prevent, detect, suppress, and/or contain unauthorized access to a network.


Floor Drains: A fixture that provides an opening in a floor that drains water into a plumbing system.


Floor Puller: A Floor Puller is a tool that uses suction cups to attach to and lift sections of flooring. Floor pullers are used in many industries to remove the panels of raised floors in data centers.


Floor Tile: Part of a raised-floor system. The floor tile is a removable component that is commonly 2ft x2ft. Floor tiles are made of hollow metal or filled with concrete or wood.


Flywheel: A rotating mechanical device that is used to store rotational energy. Serves the same purpose as a battery system.


Fluid Cooler: A system consisting of coils and fans to used transfer heat energy from a flowing glycol stream to the outside atmosphere.


Free Cooling: A practice where the outside atmosphere is used to directly provide cooling via air free cooling or water-free cooling. Water uses an additional cooling coil containing cold glycol circulating directly from the fluid cooler when atmospheric conditions allow. Air introduces cold outside air directly into the interior when atmospheric conditions allow.


Frequency: The number of occurrences of a repeating event per unit of time, represented in hertz (Hz).


Fuel Cell: An electrochemical cell that derives its energy from combustible substances such as hydrogen, methane, propane, methanol, diesel fuel or gasoline.


Full Service Rental Rate: Rental rates that include all operating expenses such as utilities, electricity, janitorial services, taxes and insurance.


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Generator: A device that converts mechanical energy to electrical energy for use in an external circuit.


Generator Electrical Feeds: Wires that feed from the electrical generator into the building structure.


Glycol: Ethylene glycol and water used as a heat removal medium. The glycol mixture is resistant to freezing in cold climates.


Glycol Cooled System: A type of precision cooling system widely used in critical environments of all sizes. In a glycol system the air conditioner absorbs heat from the air and removes it from the room in the form of heated liquid water/glycol solution. The heated liquid flows via pumps and piping system to an outdoor radiator and fan where the heat is expelled.


GPH: Gallons per Hour.


GPM: Gallons per Minute, a unit of flow rate.


Green Data Center: Data Centers being built considering energy efficiency, environmental impact, and sustainability.


Ground: A conducting connection, whether intentional or accidental by which an electric circuit, or equipment, is connected to the earth or some conducting body that serves in place of the earth.


Ground Fault: An undesired current path between ground and an electrical potential.


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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.


Hall-POD: Module or a room, segmented area of a datacenter.


Halon: Any of several compounds consisting of one or two carbon atoms combined with bromine and one or more other halogens. Halons are gases and are used as fire-extinguishing agents. They are between three and ten times more destructive to the ozone layer than CFCs are.


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.


HEPA (High Efficiency Particle Accumulation): HEPA is a type of vacuum air filter that catches a very high percentage of all particles that could damage or destroy sensitive data center equipment. These filters are able to trap extremely small particles missed by other types of filters.


HDG: Hot Dipped Galvanized. Horizontal Pathways: Horizontal Cabling connects telecommunications rooms to individual outlets on the floor.


Hosting: The service of running servers on behalf of another party, allowing those organizations to focus on managing their applications, instead of hardware and operating system administration. There are various levels of service and various kinds of hosting offered (e.g. dedicated, shared, virtual, etc.).


Hot Air Return: The path or entry point for hot air to return to the mechanical unit to be cooled.


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 (Hot Aisle Containment): A system that directs heated air from the outlet side of the racks to the air conditioning equipment return ducts in a highly efficient manner.


Hot Aisle/Cold Aisle: An organized layout in which parallel aisles of equipment enclosures are arranged with rack fronts always facing rack fronts and rack rears always 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.


Hot Gas Line: A refrigerant line used to connect a compressor to a condensing coil in an air conditioning system.


HPDC: High-Performance Data Center, a data center with above average kW loading, typically greater than 10kW/rack.


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).


Humidification: Humidity is introduced to help control electrostatic discharge ESD. Too much humidity wastes energy, too little (less than 20%) and there is an increased risk of ESD.


Humidifier: The device used to provide humidification. The moisture is usually added to the air stream exiting the air conditioner or air handler.


HVAC: Heating, Ventilating and Air Conditioning--the central air conditioning system used to regulate temperature and humidity indoors while optimizing conditions and efficiency.


Hybrid Cloud: Combining public and private clouds together, allowing for workloads to be processed on public cloud infrastructure, while others are run in private clouds.  


Hyperscale Data Center: A facility which houses critical compute and network infrastructure that provides scalability and high-speed processing for large volumes of data. These allow big companies like Amazon, Google, and Microsoft to deliver key services to customers worldwide.


Hybrid IT: Combines in-house data centers with private and/or public clouds. Companies that run applications in multiple environments need to ensure their various systems are able to work together efficiently.


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IEEE: Institute of Electrical and Electronics Engineers. Pronounced "eye-triple-E", this non-profit U.S. engineering organization develops, promotes, and reviews standards within the electronics, computer, and electric power industries.


In-Row Cooling: An in row cooling unit is designed to increase cooling efficiency by maximizing the dynamics of airflow inside a computer room. A unit is positioned between a server cabinet to provide cooling to the cold isle and also handles return air from the hot isle.


Inergen: Inergen is an environmentally green three-dimensional fire suppression agent replacing Halon 1301. Inergen is a blend of three naturally occurring gases--Nitrogen, Argon, and Carbon Dioxide. An Inergen system lowers the oxygen content of the protected area to a point sufficient to sustain human life, but insufficient to support combustion.


Infrared Scanning: Thermographic testing is used to locate defects and potential failures in electrical equipment by scanning for abnormal hot spots and heating patterns.


Infrastructure as a Service (IaaS): Also known as cloud infrastructure services. Delivers computer infrastructure, typically a platform virtualization environment, as a service. Rather than purchasing servers, software, data center space or network equipment, clients instead buy those resources as a fully outsourced service.


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.


Inrush Current: The initial surge of current experienced before the load resistance of impedance increases to its normal operating value.


Intelligent Data Center: A data center that is optimized and automated using AI, machine learning and IoT devices to improve efficiency, security, etc.


Intelligent Power Management: A combination of hardware and software that optimizes the distribution and use of electrical power in computer systems and data centers. While the installation of IPM involves up-front cost and ongoing maintenance, the technology can save money in the long term as a result of reduced electric bills, reduced downtime and prolonged hardware life.


Interconnection System: A connection between two electrical systems that allows the transfer of electric energy in either direction.


Intermediate Distribution Frame (IDF): A wall-mounted or free-standing rack used to manage and interconnect a telecommunications cable between end-user devices and the main distribution frame (MDF).


Internet Service Provider: A company that provides access to the Internet. Inverter: A device that converts the battery DC output to AC.


IP Address: A unique string of numbers separated by periods that identifies each computer using the Internet Protocol to communicate over a network.


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Junction Box (J Box): A box with a blank cover that provides space to connect and branch enclosed conductors.


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kW: Kilowatts, one thousand watts (see W).


kWh: Kilowatt-Hour, one thousand watt hours (see Wh). kWh is a common unit of electrical energy.


kVA: Kilovolt Amperes = voltage x current (amperage) (see VA).


KVM (Keyboard Video Mouse): An interface technology that enables users to access multiple servers remotely from one or more KVM sites.


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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: The fraction of total capacity an air conditioner or air handler uses to condense liquid water from the air stream being cooled. Latent cooling capacity does not contribute to data center equipment cooling.

Latent Heat of Evaporation: A term describing the amount of latent heat transferred during a liquid / vapor phase change for a particular substance.


Lead Acid Battery: The assembly of one or more cells with an electrolyte, based on dilute sulfuric acid and water, a positive electrode of lead dioxide, and negative electrodes of lead. The positive plate is comprised of lead dioxide and the negative of finely divided lead, both react with the sulfuric acid electrolyte to form lead sulfate on discharge. The reaction is reversed on recharge. Batteries are constructed with lead grids to support the active material and individual cells are connected to produce a battery in a plastic or glass case.


Leak Detection: Use of electronic detectors to monitor and alarm in the presence of moisture.


Leasing Activity: The volume of square footage that is committed to, and signed under, a lease obligation for a specific building or market in a given period of time. It includes direct leases, subleases and renewals of existing leases. It also includes any pre-leasing activity in planned, under construction, or under renovation buildings.


Leasing Space: All the space that has a financial lease obligation, including: all leased space, regardless of whether the space is currently occupied by a tenant. Leased space also includes space being offered for sublease.


Lightning Arrester: This device provides protection for equipment from high voltage surges caused by lightning. Connected from line to ground potential, the device has a very high resistance to current flow at normal voltages but when a very high voltage surge hits it, it becomes a very low resistance, passing damaging surges and current to ground.


Lights Out Server Room: A Lights out server room functions as a center to house various servers. “Lights out,” refers to the fact that lights are kept off within the room. Additionally, the room is kept locked and is devoid of IT personnel. Control of the servers is handled externally, either from another room in the complex or from a remote location. A lights out server room is considered to provide a high level of security.


Line Noise: Distortions superimposed on the power waveform that may cause electromagnetic interference.


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


Liquid Line: A refrigerant pipe carrying liquid refrigerant connected the output side of the condensing coil and to the input side of the expansion valve.


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. The total impedance of all the items in an output circuit measured in units such as kW, BTU/hr, Tons, etc.


Load Balancing: A methodology to distribute workload across multiple computers or a computer cluster, network links, center processing units, disk drives, or other resources, to achieve optimal resource utilization, maximize throughput, minimize response time, and avoid overload.


Load Fault: A malfunction, like a tree touching a power line, that causes the lead to demand abnormally high amounts of current from the source.


Load Shedding: A way to distribute demand for electrical power across multiple power sources.


Load Bank: A device that creates an electrical load. Used for testing of electrical and mechanical systems, typically during commissioning.


Low Voltage System: Low voltage is an electrical engineering term that broadly identifies safety considerations of an electricity supply system based on the voltage used. While different definitions exist for the exact voltage range covered by "low voltage", the most commonly used ones include "mains voltage". "Low voltage" is characterized by carrying a substantial risk of electric shock, but only a minor risk of electric arcs through air.


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Machine Learning (ML): Allows data center operations to effortlessly track equipment performance and make data-driven decisions about equipment replacement by developing and studying statistical algorithms that can learn from data and perform tasks without explicit instructions.


MAH: Makeup Air Handler, an air handler that conditions and delivers outside air into an occupied space.


Main Distribution Frame (MDF): A panel or set of panels where all communication cables from different parts of the building are both terminated and connected.


Make-Up Air: Outside air introduced into the interior. Make-up air is referred to by building codes primarily to ensure the space is fit for human occupancy.


Managed Hosting: An IT provisioning model in which a service provider leases dedicated servers and associated hardware to a single client.  The equipment is at the hosting provider's facility and managed there by the service provider.  


Managed Services: A managed services provider manages and assumes responsibility for assets or objects on behalf of a customer.   


Mantrap: A small room/area in between two sets of doors to ensure restricted and managed access to secure environments. These allow only one person to pass through at a time by using some form of a security scan, reducing unauthorized access.


Manual Transfer Switch (MTS): A manually actuated switch that transfers electrical loads to an alternate or emergency-standby power source.


Maximum Temperature Rate of Change: An ASHRAE standard established to ensure stable air temperatures. The standard is 9 degrees F per hour.


Mass Market Hosting:  Provide services such as web hosting and domain registration.  These players build their own infrastructure capacity instead of renting it out.  


MBP/MBC: Maintenance Bypass Panel/Maintenance Bypass Cabinet


MCB: Main Circuit Breaker MDF: Used in telecommunications, an MDF is commonly a steel rack that allows for the connection of networks within a plant or facility to outside cables and other equipment. Accessible from two sides, main distribution frames feature termination blocks arranged horizontally at the front of rack shelves. The MDF makes assigning facilities more flexible with lower cost and higher capacity than patch panels.


Mean Time Between Failures (MTBF): Is the mean (average) time between failures of a system, and is often attributed to the useful life of the device, not including 'infant mortality' or 'end of life' if the device is not repairable. Calculations of MTBF assume that a system is fixed, after each failure, and then returned to service immediately after failure.


Mean Time to Repair (MTTR): Is the maximum time to recovery after an equipment failure which can be easily measured and a supplier held accountable. Memory Overcommit: A hypervisor feature that allows a virtual machine (VM) to use more memory space than the physical host has available.


MERV: Minimum Efficiency Reporting Value, ASHRAE 52.2, for air filtration measured in particulate size. Metered Power: A way of measuring power consumption that ensures the customer only pays for the amount of power used over a designated period of time.


Micro Data Center: Essentially an edge data center pushed to the extreme, e.g., a small office closet.


Modular, Non-Scalable UPS: Based on incriminated growth based on size rating, can modularly increase power capacity.


Multiprocessing: The utilization of two or more central processing units (CPUs) in a single computer system.


Multi-Tenant: Buildings that house more than one tenant at a given time. Usually, multi-tenant buildings were designed and built to accommodate many different floor plans and designs for different tenant needs.


MW: Mega Watt, a measure of power equal to one million watts. Often used to describe the size of data centers in terms of power capacity.


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N+1 Redundancy: A measure of redundancy where additional “spare” components are available in the event of a failure in one of the main critical components. N is the “Needed” components and +1 represents the spare component that is on hand for emergency use. N+1 safeguards against lost productivity if one component fails.


NEBS: Network Equipment-Building System design guidelines applied to telecommunications equipment.


NEC: National Electrical Code


Net Absorption: The net change in occupied space over a given period of time. Unless otherwise noted Net Absorption includes direct and sublease space.


Net Rental Rate: A rental rate that excludes certain expenses that a tenant could incur in occupying office space. Such expenses are expected to be paid directly by the tenant and may include janitorial costs, electricity, utilities, taxes, insurance and other related costs.


Network Cabinets: A cabinet designed to hold a series of controllers, all connected via a telecommunications cable.


Network Cross Connections: A physical network connection bridging a telecommunications vendor with a customer’s computer environment inside a data center.


New Space: Sometimes called first generation space, refers to space that has never been occupied and/or leased by a tenant.


NFPA: National Fire Protection Association


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


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Occupied Space: Space that is physically occupied by a tenant. It does not include leased space that is not currently occupied by a tenant.


OCP: Overcurrent Protection, overcurrent exists when current exceeds the rating of equipment or the ampacity of a conductor.  This can be due to an overload, short circuit, or ground fault.  Overcurrent devices protect conductors and equipment from overcurrent.  


OLA (Operational Level Agreement): A contract defining how various IT groups within a company plan to deliver a service or set of services.


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.


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Packet Acceleration: TCP acceleration is the name of a series of techniques for achieving better throughput on an Internet connection than standard TCP achieves, without modifying the end applications. It is an alternative or a supplement to TCP tuning.


Parallel Generator System: A system that is capable of providing large amounts of stand-by power or emergency power by incorporating multiple generators in parallel via a series of switching and controls.


Parasitic Load: The power and cooling load for all ancillary equipment and common area operation.


Pedestal: The vertical component of the raised-floor, which supports the flooring system.


PDU: Power Distribution Unit typically refers to either to the transformer/breaker panel that is used between a UPS supplying voltage higher than that used by the IT equipment and the cabinets. The other is the smaller rack based “power strip” device that is used inside the rack to distribute power to the IT equipment.


PH: 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.


Platform as a Service (PaaS): A way to rent hardware, operating systems, storage and network capacity over the Internet. The service delivery model allows the customer to rent virtualized servers and associated services for running existing applications or developing and testing new ones.


Plenum: Plenum refers to the area where air circulates in systems running heating and air conditioning units. This space is commonly found between two ceilings or between a raised floor and the actual floor, and it often serves as a place where computer and telephone network cables are run.


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. CTs 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: Electric power is defined as the rate at which electrical energy is transferred by an electric circuit. 


Power Cycling: The process of turning hardware off and then turning it on again. In the data center, technicians use power cycling to test the durability and reliability of network components.


Power Density: Electrical power used in a space divided by the area of the space.


Power Factor: The ratio of energy consumed (watts) versus the product of input voltage (volts) times input current (amps). In other words, power factor is the percentage of energy used compared to the energy flowing through the wires.


Powered Shell: A data center facility that has it’s exterior completed and is connected to power and connectivity, but has an unfinished interior, allowing for flexibility and adaptability for users’ needs.


Prefabrication: Manufacturing process generally taking place at a specialized facility, in which various materials are joing to form a component part of a final installation.  


Preleased Space: The amount of space in a building that has been leased prior to its construction completion date, or certificate of occupancy date.


Premium Colocation: Tier III+ data centers that generally qualify as extremely desirable and command the highest rents or sale prices compared to other buildings in the same market. They are generally the most attractive and eagerly sought by firms willing to pay a premium for quality.


Pre-Action: An Action involved in a dry system prior to actually flooding the pipe with water and potentially discharging into the protected space.


Pre-Action Fire Sprinkler: A fire sprinkler system that employs the basic concept of a dry pipe system in that water is not normally contained within the pipes. Water is held from piping by an electrically operated valve, known as a pre-action valve. Valve operation is controlled by independent flame, heat, or smoke detection. Two separate events must are required to initiate sprinkler discharge. First, the detection system must identify a developing fire and then open the pre-action valve. This allows water to flow into system piping, which effectively creates a wet pipe sprinkler system. Second, individual sprinkler heads must release to permit water flow onto the fire.


Precision Air Conditioning: A term describing air conditioning or air handling systems specifically designed to cool information technology equipment in a data center or server room. Precision air conditioning systems typically maintain temperature (+/- 1F) and humidity (+/-4%). These systems provide high airflow rates (170+ CFM/kW or 4.8+ Lps/kW), are designed for continuous 24/7/365 usage, and provide high levels of air filtration.


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.


Preventative Maintenance: Regularly-performed maintenance on a piece of equipment to lessen the likelihood of it failing. 


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.


Private Cloud (Internal): A proprietary computing architecture that provides hosted services to a limited number of people behind a firewall.


PUE: PUE measures the efficiency of power usage in a data center. Within a typical data center, power will be used to run the computer equipment as well as for cooling, lighting and other functions. Developed by the Green Grid, PUE shows the ratio of power used to run computer systems vs. overall power usage.


Public Cloud: Cloud infrastructure is available to the general public and is owned by a large provider of cloud services.  


Pump Package: A pump or pumps and enclosure or skid used in coordination with a cooling system to circulate condenser water or glycol on applicable systems.


Pumps: A pump and enclosure used to circulate condenser water or glycol on applicable systems. Pump packages are specified based on desired flow rate and piping losses for each application.


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Rack: Device for holding IT equipment, also called a cabinet.


RAH: Recirculation Air Handler, a device that circulates air but does not cool the air.


RAT (Return Air Temperature): The air returning to air conditioning equipment.


Raised Floor: A system of pedestals, stringers, and floor tiles that create an elevated floor environment. The under-floor is accessible and can be used as a distribution path for air conditioning, HVAC piping, data, and electrical systems.


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.


Redundancy: The duplication of critical infrastructure support systems with the intention of backing up the primary systems and protecting against system down-time due to failure. Redundancy: A backup system or component.


Refrigerant: The working fluid used in the refrigeration cycle is known as the refrigerant. Modern systems primarily use fluorinated hydrocarbons that are nonflammable, non-corrosive, nontoxic, and non-explosive. Refrigerants are commonly referred to by their ASHRAE numerical designation. The most commonly used refrigerant in the IT environment is R-22. Environmental concerns of ozone depletion may lead to legislation increasing or requiring the use of alternate refrigerants like R-134a.


Relative Humidity: The amount of water vapor contained in air relative to the maximum amount the air is capable of holding. Expressed in %.


Reliability: Reliability is classically defined as the probability that some item will perform satisfactorily for a specified period of time under a stated set of conditions.


Remote Data Backup: The process of backing up data created by remote and branch offices (ROBOs) and storing it securely. Businesses with ROBOs require backup and recovery solutions that can support the company's data protection policies and business service levels.


Remote Replication: The process of copying production data to a device at a remote location for data protection or disaster recovery purposes.


Return Air: The heated air returning to air conditioning equipment.


RFI: Radio Frequency Interference.


Rh: Relative Humidity.


Rope Detector: Leak detection instrument used to detect the presence of moisture along its length.


Row-Based Cooling: Row-Based Cooling solutions provide cooling directly to multiple adjacent cabinets. These solutions are typically deployed in locations that do not have traditional computer room air conditioning. They may also be implemented to augment existing cooling, especially in areas where cabinets are densely populated. Options range from medium-density (5 to 7kW per cabinet) to high-density (20+ kW per cabinet).


RPM (Revolutions Per Minute): Used for measuring fan speeds/flywheel rotation.


RPP: Remote Power Panel.


RTU: Rooftop Unit, an air handler designed for outdoor use mounted on a rooftop.


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S+S: System plus system.


SAN: Storage Area Network.


Scalability: The ability for the required infrastructure to be enlarged or handle increased capacity input without efficiency or reliability being compromised.


SCFM: Standard Cubic Feet per Minute, the volumetric flow rate of a gas corrected to standardized conditions of temperature, pressure and relative humidity.


Screen Scraping: A data collection method used to gather information shown on a display to use for another purpose.


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.


SDDC (Software-Defined Data Center): A data storage facility where networking, storage, CPU and security are virtualized and delivered as a service.


Seebeck Effect: A phenomenon in which a temperature difference between two dissimilar electrical conductors or semiconductors produces a voltage difference between the two substances.


Sensible Cooling Capacity: The amount of heat energy the air conditioner can be expected to remove from an environment with computer or server equipment. Depending on the operating conditions, this may be less than the air conditioner rating because some of the air conditioner cooling capacity may be used up dehumidifying the air. The resultant condensation of the humidity onto the air conditioner coils represents a loss of cooling capacity. Ideally, the air returning to the air conditioner is a high enough temperature so that the dew point is not reached during cooling, in which case no dehumidification occurs and the full air conditioner rating is used to cool the heat load.


Sensible Heat: Sensible heat is defined as the heat energy that causes a change in temperature of a substance but does not contribute to a change in state (for example, steam to liquid water) for the substance. The only type of heat energy produced by computers and IT type equipment.


Sensible Heat Ratio: The ratio between an air conditioner's sensible heat removal capacity and its total heat removal capacity. In a data center environment, higher sensible heat ratios contribute to lower operating costs and more effective equipment cooling. Ideally, this ratio is 1, meaning the entire air conditioner capacity is available to cool the load. When this number is less than 1, it means that undesirable dehumidification is occurring in the air conditioner.


Server Cabinets: A cabinet designed to hold a network device that combines hardware and software to provide and manage shared services and resources on the network.


Server Room: A location specifically designed to house a high concentration of information technology equipment.


Service Provider (SP): An organization that provides cellular or local exchange telecom services, cloud data center networking services, or other internet-related services


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.


Shared Hosting: Customers share server capacity.  


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


Single Phase: Single-phase electric power refers to the distribution of electric power using a system in which a single leg or phase is taken from a three-phase source either by connection between a phase and neutral or by connecting the load between two phases.


Single-Tenant: Buildings that are occupied, or intended to be occupied by a single tenant. 


Smart Data Center: A data center environment where intelligent, remote, self-service capabilities deliver an elevated level of fluidity to enrich real-time visibility, access and control of critical data center capabilities and assets.


Spare/Space: Terms used to identify unused electrical circuits in a panel. A spare indicates a breaker is present but unused. A space indicates there is an opening for a breaker but no breaker is present.


Spike: A sudden marked jump in voltage, which can damage electronics and corrupt or destroy data.


Spike/Surge Protector: An appliance designed to protect electrical devices from voltage spikes. Surge protectors attempt to regulate the voltage supplied to an electric device by either blocking or by shorting to ground voltages above a safe threshold.


Spot Detector: Leak detection instrument designed to detect moisture that accumulates directly under the detector.


SSID: Service Set Identifier.


Standard Temperature and Pressure (STP): Standard temperature and pressure, abbreviated STP, refers to nominal conditions in the atmosphere at sea level.


Standby Generator: The generator is the main component in a back-up electrical system that operates automatically. Within seconds of a utility outage an automatic transfer switch senses the power loss, commands the generator to start, and then transfers the electrical load to the generator. The standby generator begins supplying power to the power distribution system. After utility power returns, the automatic transfer switch transfers the electrical load back to the utility and signals the standby generator to shut off. It then returns to standby mode where it awaits the next outage. To ensure a proper response to an outage, a standby generator runs weekly self-tests. Most units run on diesel, natural gas.


Static Transfer Switch (STS): A static transfer switch selects between two or more sources of power and provides the best available power to the critical load.


Step Down Transformers: A transformer that reduces voltage - an electrical device by which alternating current of one voltage is changed to another voltage.


Stringer: The horizontal component of the raised-floor, which supports the floor panels. The stringers form a bridge between two adjacent pedestals. Flooring systems may also be stringer less.


Structured Data: Data that has a defined length and format, such as number and dates, and is usually stored in a database.


STS: Static Transfer Switch, 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.


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


Suite: Hard-walled rooms within a data hall used for customers who require more data space. These typically offer more square footage than cages and come with dedicated security features.


Supply Air: The cooled airflow emitted from air conditioning equipment.


Surge: A surge is a prolonged over-voltage condition. Surges can damage electronics and corrupt or destroy data.


Switchgear: Electrical Distribution hat breaks out power from high to medium to low voltage.


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


Thermoelectric Cooling: Thermoelectric cooling is a way to remove thermal energy from a medium, device or component by applying a voltage of constant polarity to a junction between dissimilar electrical conductors or semiconductors.


THHN: THHN is a type of wire primarily used in electrical installations in conduit and is not rated for use in a plenum space outside of conduit do to the PVC jacket.


Three-Phase: Three-phase refers to an electric power system having at least three conductors carrying voltage waveforms that are 2ð/3 radians (120°, 1/3 of a cycle) offset in time. Electric utilities generate three-phase power and transmit it to load centers where it may be consumed at Three Phase or Single Phase.


Tier I: A Tier 1 data center is a basic server room implementing the general guidelines for computer system installations. This first level runs within a 99.671 percent availability through one non-redundant distribution path with non-redundant capacity components.


Tier II: A Tier 2 data center Includes all requirements of Tier 1, plus a guarantee of 99.741 percent availability with redundant site infrastructure capacity components.


Tier III: In addition to fulfilling requirements of Tiers 1 and 2, Tier 3 data centers provide dual-powered IT equipment to receive data from multiple independent distribution paths with an increased availability of 99.982 percent guaranteed.


Tier IV: Tier 4 data centers include the components of the first three Tiers with the addition of independently dual-powered cooling equipment. The site infrastructure is fault-tolerant with distribution capability and the capacity to store electrical power. An availability of 99.995 percent is guaranteed.


Ton: A ton is heat energy equal to 12,000 BTUs and is the amount of heat energy required to melt 2000 pounds (907kg) of ice in one hour. This is an archaic term typically used to specify heat output when expressed in Tons/day, where the use of the more modern term Watts is the simpler and more universal measure that should be used.


Tons: Ton (Cooling) A measurement of heat energy commonly used historically to measure heat loads in data centers and IT rooms in North America. A ton is equal to 12,000 BTUs and is the amount of heat energy required to melt 2000 pounds (907kg) of ice in one hour. This is an archaic term typically used to specify heat output when expressed in Tons/day, where the use of the more modern term Watts is the simpler and more universal measure that should be used.


Topology: Network Design, planning data center operations and enhancement require both physical and logical topologies.


Traditional, Non-Scalable UPS: Based on a particular size rating. It is fixed and cannot be increased or decreased.


Transformer: An electro-magnetic device used to change the voltage in an alternating current electrical circuit.


Transient Voltage Surge Suppressor: A device used to reduce voltage surges. Products may be wired in series or in parallel with the AC electrical conductors Transition Operating System: An operating system (commonly abbreviated to either OS or O/S) is an interface between hardware and user; it is responsible for the management and coordination of activities and the sharing of the limited resources of the computer. The operating system acts as a host for applications that are run on the machine. As a host, one of the purposes of an operating system is to handle the details of the operation of the hardware. This relieves application programs from having to manage these details and makes it easier to write applications. Almost all computers, including handheld computers, desktop computers, supercomputers, and even video game consoles, use an operating system of some type. Some of the oldest models may however use an embedded operating system that may be contained on a compact disk or other data storage device.


Turning Vane: An air management device installed in many floor stands to assist in redirecting the flow of cooling air from vertical to horizontal as it exits the computer room air conditioner or air handler.


TVSS: Transient volt surge suppression; shunt to ground over volt spikes to prevent electrical fires and/or system failures.

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U: A unit of space in a rack, equal to 1.75″. The vertical dimension of racks and IT equipment is often specified in “U's” such as 42U.


UL: Underwriters Laboratories, Inc.


Unified Computing System: A unified computing system (UCS) is a data center architecture that integrates computing, networking and storage resources to increase efficiency and enable centralized management. When UCS is sold as a product, hardware and software are designed or configured to work together effectively.


Uninterruptible Cooling: The process of continued heat removal from the IT room or data center during equipment failure or power failure to preclude thermal damage and data loss due to equipment overheating.


Unstructured Data: Data which does not follow a predefined data model or fit into relational databases, such as video, text, email and social media. This makes up the bulk of data.


Up Flow: A term applied to air conditioners and air handlers that discharge air in an upward direction.


UPS: UPS is a battery-powered device that sends power to a computer system when the normal power source (usually the utility main) fails. As an auxiliary power supply, a UPS provides instant or near-instant continuation of electrical current after a power failure. There are two distinct types of UPS, off-line and line-interactive (also called on-line). An off-line UPS remains idle until a power failure occurs, and then switches from utility power to its own power source, almost instantaneously. An on-line UPS continuously powers the protected load from its reserves (usually lead-acid batteries or stored kinetic energy), while simultaneously replenishing the reserves from the AC power. The on-line type of UPS, in addition to providing protection against complete failure of the utility supply, provides protection against all common power problems, and for this reason it is also known as a power conditioner and a line conditioner.


UPS Power: Uninterrupted Power Supply, a critical data center component that cleans incoming “dirty” power from the commercial utility power and provides instantaneous back-up power when a main power source fails.


Usable KVA per Circuit: De-rating of available Kilo-volt-amps to usable amps to prevent circuit overloads and fires.


Usable Kw per Circuit: De-rating of available Kilo-watts to usable amps to prevent circuit overloads and fires.


Usable VA per Circuit: De-rating of available volt-amps to usable amps to prevent circuit overloads and fires.


Utility Power: Power from the local utility company. Utility Storage: A service model in which a provider makes storage capacity available to an individual, an organization or a business unit on a pay-per-use basis. The utility model is sometimes called metered services or storage on demand.


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V: Volt, a unit of electrical potential.


VA: Volt-Amp, 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 yields the real power.


Valve Regulated Sealed Cell (Battery): A battery in which the cells are closed but have a valve which allows the escape of gas if the internal pressure exceeds a predetermined value.


Vapor Barrier: Paint, plastic sheeting, floor, or ceiling material specifically designed to minimize the migration of humidity into or out of an area.


Venting: The controlled or accidental release of gas from a battery cell.


VESDA System (Very Early Smoke Detection Apparatus): Stops fires in a specific area before they can spread to other areas of the data center. These systems detect smoke and release water at a certain temperature to prevent fires from spreading.


VFD: Variable Frequency Drive, a device which supplies AC power of varying frequency, typically used to control the speed of induction motors. In the data center, it is common to vary the speed of fans, pumps, and chillers.


Virtualization: Virtualization is a framework or methodology of dividing the resources of a computer into multiple execution environments, by applying one or more concepts or technologies such as hardware and software partitioning, time-sharing, partial or complete machine simulation, emulation, quality of service, and many others.


VMware Backup: VMware backup is the copying of data on a virtual machine (VM) in a VMware environment to prevent data loss. VMware backup, and virtual server backup in general, is a common challenge for storage and backup administrators.


VoIP: Voice over Internet Protocol.


Voltage: Electrical force or potential, a technical synonym for voltage is emf or "electromotive force." Voltage is the parameter of electricity which causes current to flow when a circuit is completed. Voltage is always presented in an energized line, whether or not the circuit is complete.


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W: Watt, 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.


Water-Cooled System: A water-cooled system uses water instead of air as a condensing medium. Condensation takes place in a refrigerant/water heat exchanger typically located within the air conditioner. The water flows in a continuous loop to an outdoor cooling tower where heat is rejected to the outside atmosphere.


Water Cooling: Water cooling is a method used to lower the temperatures of computer processors, and sometimes other components such as graphics cards, using water rather than air as the cooling medium.


Water Detector: A device used to sense the abnormal presence of liquid water due to a leak or condensation.


Waterside Economizer: A system which uses a source other than a chiller to cool the secondary loop water used by the AHUs. This typically consists of either a dry cooler or cooling tower, piping, valves and in the case of a cooling tower, it also includes a heat exchanger since the secondary loop water is treated completely differently than the primary loop water and is typically much “better” water.


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.


Wg: – Inches of water column, a unit of pressure based on the height of a column of water supported by the pressure differential between the top and bottom of the column. 1 inch wg = .036 psi.


White Box Server: A data center computer that is not manufactured by a well-known, brand-name vendor.


Wholesale Data Center: Sell data center space in large capacities.  These may be built-to-suit space for a single tennant that includes the building owner providing facility management and maintenance.  


Wholesale Turnkey: A datacenter that is designed, supplied, built, or installed fully complete and ready to operate. The term implies that the end user just has to turn a key and start using the product or service.


Work Cell: The area of a rack and the related area immediately in front of and behind the rack. Standard racks are 2 feet wide and 4 feet deep. Standard aisles are 4 feet wide, so half of that space is workspace for a given rack. This results in a standard work cell of 16 square feet. Actual work cell size varies with data center design.


WPSF: Watts per Square Foot, 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).


WUE: Water Usage Effectiveness, 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).


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Zinc Whiskers: The name given to very small structures of zinc that resemble hairs. They have been found on the metal surfaces of raised flooring in data centers. If allowed to grow, they can become airborne and conduct electricity. The danger of zinc whiskers is that they can enter computer systems through air intake vents.


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