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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.

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Power Usage Effectiveness (PUE)

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

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Power Factor (PF)

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

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Power Distribution Unit (PDU)

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

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Potential Transformer (PT)

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

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Plenum

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

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Plate and Frame

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

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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.

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Packet Switching

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

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Packet

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

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Oversubscription

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

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