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What is a PDU (Power Distribution Unit)?

21 Sep 2015

A PDU is the “professional” version of the common extension lead used to provide electrical power to equipment around the home or office; particularly where the equipment is too far from an electrical socket or where additional outlets are required. This blog explains the many versions of basic pdu’s available for server rack power distribution to equipment mounted in 19” racks. In future blogs I will discuss intelligent pdu’s which can provide information on the power usage of equipment in the rack.

The basic pdu’s described here can be vertical pdu’s, which are mounted down either or both sides at the rear of the rack, or horizontal pdu’s, which are mounted horizontally across the rear of the rack. Using the correct pdu not only provides the required number of power outlets for all the equipment mounted in the rack but also assists in keeping the cables tidy. Because the width of the rack is usually far less than the height, the number of outlets available on horizontal pdu’s is far less than the number possible on vertical pdu’s, which can have up to 24 separate outlets.

How to determine which PDU to use.

The first thing to check is where the pdu itself will be plugged into; what the source of electrical power to the pdu is; very often this is a UPS outlet socket. The pdu comes with a supply lead that can be fitted with a variety of plugs to fit the power source. There are five types most frequently used.

  1. IEC 14. This is a 10 Amp plug which is very common as it is the one used for PC’s, VDU’s and printers. (See picture)
  2. IEC20. This is a 16 Amp plug, slightly larger than the IEC14. It is commonly used for the input to higher power single phase UPS and is also provided as a UPS output socket (IEC19) which the IEC20 plug fits into. High power single phase UPS often offer both IEC13 socket outlets (for the IEC 14 plug) and IEC 19 outlets.
  3. 13 Amp (UK) plugs. Used where there is no UPS or where there is a ring main fed by a remote UPS.
  4. 16 Amp commando
  5. 32 Amp Commando

Commando Plugs and sockets are often used for large UPS outputs.

Having decided on the input plug the type and quantity of outlet sockets on the pdu needs to be considered. Again there is a choice of outlet types to suit the supply required for the rack mounted equipment.

  1. IEC 13 - 10 Amp sockets.
  2. IEC19  - 16A sockets. Used where rack mounted equipment has a larger than normal power requirement; this could be where a UPS is mounted in the rack.
  3. 13 Amp (UK) sockets
  4. Schuko sockets. Schuko sockets are 16 Amp sockets often used on European equipment.

Having decided which type of socket suits the equipment mounted in your rack the final decision is based on how many items need to be powered from the pdu. It is probably a good policy to allow one or two additional outlets if there is a possibility of additional equipment being installed at a later date.

Other considerations

  1. Do you wish the outlets to be fused?

This is not necessary for a rack mount pdu with 13 Amp plugs as they include an internal fuse; a correctly rated fuse can be used. IEC plugs have no facility to fit a fuse but the sockets on the pdu can be specified with fuses.

  1. Is space around the rack tight?

There are often many cables coming from racks and where they are situated in a confined space there can be a danger of cables becoming dislodged. In this case choose sockets with a Click Lock mechanism which means a button has to be pushed to allow removal of the plug.

  1. Do you wish to know the power being consumed by items of equipment?

There are various ways to do this which vary in the amount of information provided and in the cost of equipment and installation. Too much to go into in this blog; but I will issue a future blog giving a range of options to achieve power monitoring.

An IEC Click Lock PDU

An IEC Click Lock PDU

IEC C13 with fused outlets

IEC C13 with fused outlets

Schuko outlets

Schuko outlets

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