Power distribution unit

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An American Power Conversion 10-outlet rackmount PDU without built-in surge protection, connected to an APC Smart UPS 2200 (bottom unit on right)

A Power distribution unit (Commonly abbreviated to PDU) is a device that distributes electric power.

Large industrial units are used for taking high voltage and amperage and reducing it to more common and useful levels, for example from 220V 30A single phase to multiple 110V 15A or 110V 20A plugs. They are used in computer data centers, stage shows, by DJs, and in other electrically intensive applications. Some have features like remote monitoring or control down to the individual plug level (see: remote power boot switch below).

A 240 volt circuit (120 volts x2 plus neutral and ground) has two legs at 120 volts. A distro switches and provides circuit protection for the taps off of the phases/legs. A PDU allows for a solidly bonded neutral, and a safe grounding system.

PDU also refers to what amounts to a well-constructed power strip suitable for datacenter 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 always single-phase, however. In the case of a three-phase PDU, each of the three phases appears individually on every third receptacle. 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 -- meaning that some or all of their receptacles can be individually switched on or off remotely.

One of the challenges in selecting PDUs for a datacenter application is to balance the relatively high cost of a switched PDU versus the relatively low cost of a dumb PDU, but then managing the risk that the phases may become unbalanced, or devices may be unexpectedly plugged in, possibly tripping a circuit breaker.

[edit] Remote Control

Some PDUs provide a means of remote access. Common methods are a RS-232 serial connection or a computer network controller that can be accessed though TELNET, SSH, SNMP or a Web page . This allows an administrator to access the PDU from a remote terminal and interface with the it to turn on or off outlets, schedule power shutdowns, control load, etc. This can be helpful if a remote machine has gone into an unresponsive state and cannot be restarted though normal means. A administrator can connect to the PDU the machine is plugged into and Power cycle the machine.