Current Transformer, 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.

Carbon Usage Effectiveness, 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.

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.

Data Center infrastructure Efficiency, 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.

A control technique which prevents oscillation or unnecessary cycling of a controlled variable. In data center cooling, it typically applies to the action of the CRAC or CRAH relative to the set point. A certain amount of dead band around the set point prevents unnecessary cycling of the compressor or chilled water valve.

The temperature at which air reaches water vapor saturation. Dew point 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 dew point.

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.

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.

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.

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 nonlinear 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.

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.

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.

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.

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.

An ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers) standard established to ensure stable air temperatures. The standard is 9 degrees F per hour.

Power Distribution Unit, 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 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.

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.

A type of heat exchanger commonly used in water to water systems. It is a series of plates held in a frame through which exist 2 paths for water which are adjacent but separate. Plate and frame heat exchanges are simple to size via the addition of additional components and maintenance is straight forward via disassembly.

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.

Potential Transformer. 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.

Upsite Coefficient of Effectiveness, a data center efficiency metric developed by the Uptime Institute synonymous with UCE. (See also CoE)

Uninterruptible Power Supply, 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.

Volt-Ampere, 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.

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.

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.

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.

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.

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.

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).

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).