Power distribution unit
||The examples and perspective in this article deal primarily with North America and do not represent a worldwide view of the subject. (December 2010)|
||This article needs additional citations for verification. (March 2009)|
A power distribution unit (PDU) is a device fitted with multiple outputs designed to distribute electric power, especially to racks of computers and networking equipment located within the data center.
The term (PDU) may refer to two major classes of hardware power devices; the first and typically the general unqualified term refers to the category of relatively higher-cost floor-mounted power distribution devices which transform one or more larger capacity raw power feeds into any number of lower capacity distributed power feeds. These floor-mounted PDU devices are typically composed of transformers and circuit breakers and may optionally include monitoring controllers using protocols such as Modbus or SNMP. In a typical data center for example, there would be relatively few of these floor-mounted PDU devices, located along the walls or in central locations for larger spaces. Each floor-mounted PDU would feed a much larger number of racks and rows of racks.
The second class of device is a much smaller and lower cost device which is fitted with multiple appliance outlets designed to distribute electric power within a rack, especially to computers and networking equipment located within a data center. The second type of PDU is sometimes called a Smart-PDU, Rack-based PDU, Intelligent PDU or simply "Power Strip" by various IT professionals.
In the North American 110V/60Hz system, Rack-based PDUs are used for taking the supplied voltage and current and distributing it electrically to more common outlets, for example from 240 V 30 A single phase to multiple 120 V 15 A or 120 V 20 A plugs. They are used in computer data centers, in stage shows, by DJs, and in other electrically intensive applications. Some premium units have features like remote power monitoring and power state control down to the unit or individual plug level. Certain manufacturers also include environmental monitoring via RS485, USB, Dallas 1-Wire or serial ports. The power outlets themselves are typically IEC-C19, IEC-C13 or NEMA 5-20 style outlets.
In the international 230V/50Hz system, Rack-based PDUs simply distribute incoming power, typically from a 230V/16A, 230V/10A or 400V/16A 3-phase==3 x 230V/16A input to multiple individual 230V outlets, which may either rely on the input fusing or be individually protected by smaller 10A or 6A fuses. The outlets are typically either IEC-C13 or one of the country-specific types of power socket. Additional features such as monitoring of power consumption, noise and overvoltage filtering, environment monitoring and remote management are generally the same as in North American Rack-based PDUs.
Rack-based PDU refers to what amounts to a well-constructed power strip suitable for data center use. Two basic varieties distinguished by the type of input power are common: single-phase and three-phase. The output power (i.e. the power to the load device) is almost always single-phase, however. In the case of a three-phase PDU, each of the three phases appears individually on one-third of the included receptacles. Rack-PDUs can be dumb—meaning that they have no instrumentation and are not manageable, or they can be metered—meaning that they are equipped with a display that shows current load on each phase, or they can be switched and metered meaning that some or all of their receptacles can be individually switched on or off remotely and the meter usage as well.
A typical application area could be a mobile measurement station, where several devices can be supplied with power from a single distribution unit.
Some PDUs provide remote access. Common methods include a RS-232 serial connection or a LAN network-controller accessible through Telnet, SSH, SNMP, or a web page. This allows an administrator to access the PDU from a remote terminal and interface with it to turn outlets on or off, to schedule power shutdowns, to control load, etc. This can be helpful if a remote machine has gone into an unresponsive state and will not restart through normal means. An administrator can connect to the PDU the machine is plugged into to power-cycle the machine.
One of the challenges in selecting PDUs for a data-center application is to balance the cost of the rack-PDU in the context of an organization's energy-management goals. Inexpensive rack-PDUs may distribute power effectively, but they offer little if any understanding or control of that energy flow. With energy costs rising over the past several years (circa 2010[update]), IT professionals have begun to take a more comprehensive view of power management with rack-PDUs with more intelligence. The trend is more intelligence and higher-quality rack-PDU devices.
In North America the standard 240-volt circuit (120 volts x2 plus neutral and ground) has two legs at 120 volts. Most recently,[when?] data centers have begun to install single-phase 208-volt connections. Outside North America, the standard data center circuits are 230-volt AC and in higher end telecoms facilities 48V DC from giant UPS installations, neither of which are subdivided into other voltages by PDUs.