A Guide to Rack Mount PDUs for Server Racks

Most server racks have several pieces of IT equipment to power. A typical configuration can include a server, storage device, broadband switch, and routers. In a small server cabinet, you may be able to connect everything to a rack mount UPS if it has enough socket outlets btu the likelihood is that you will need to install a power distribution unit (PDU).

Types of Power Distribution Unit

Power distribution units can be grouped into two types:

Whether to you need a non-intelligent or intelligent PDU there are crucial factors to consider.

Non-Intelligent Rack PDUs

A basic PDU is as the name suggests a power distribution unit with socket outlets and a mains power lead with a plug. The PDU should have a level of power protection in case of overload or short-circuit in the form of a fuse or circuit breaker protection. The PDU may provide for the option of a surge protection circuit to provide additional filtering to the connected loads for electro-magnetic interference (EMI) and spike or transient voltages.

A monitored PDU, is based on a basic PDU and has a built-in digital or LCD meter to show power readings for the total connected loads. Typically, the value shown is the Amps, but some devices may scroll or allow manual scrolling of other values including the voltage supplied via the socket outlets.

When buying a basic or basic monitored PDU, the choices revolve around:

1. Socket Outlets: how many power outlets and what type do you need? Standard IT equipment requires a C13 IEC320 socket outlet or local national socket and plug arrangement. In the UK it is common for a PDU to be supplied with either C13 or BS 1363 UK square pin socket outlets. There be an option for C19 socket outlets for higher power loads.
2. PDU Power Lead and Plug: what type of plug and power cord length do you need? The typical length range for a PDU mains supply lead is 1.8-3m. Longer cable lengths may be available. The PDU cable will be connected to either a C14, C20 or BS 1363 UK Plug or one to suit your country’s mains power supply i.e. Schuko in Europe.
3. Surge Protection: will you need surge protection or are you connecting directly to a UPS system> the PDU may be able with an additional surge protection module. This will provide additional protection to the connected loads but may not be required if the UPS is powered from an uninterruptible power supply.
4. Horizontal or Vertical Formats: do you need to mount the PDU horizontally or vertically? rack mount PDUs can be supplied as horizontal or vertical mounting formats. A horizontal PDU is mounted to either the front or rear rack struts and will have a U-height of 1U. Horizontal mounting limits the width of the rack to 19inches (483mm) and it may be possible for a bespoke PDU to be made to 2U or 3U to offer multiple rows of socket outlets. Otherwise, a vertical arrangement provides a way to provide a greater number of socket outlets up to thirty-six or more and a range of outlet connections (C13 and C19) to suit a mix of loads. Care should be taken with a vertical PDU to ensure it will fit within the overall internal height of the server cabinet. Most vertical PDUs will be mounted on the rear cabinet struts and can be installed in pairs to support A and B power supplies, and cascading installation.
5. Single or Three Phase Supplies: is the mains power supply or UPS outlet connection single (UK 230Vac 50Hz) or three phase (UK 400Vac 50Hz)? most small computer or server room installations will be single phase and require a 1phase PDU. Large installations and loads may require a three-phase (3phase) PDU, but these tend to be of the intelligent type due to the more sophisticated monitoring required. Whilst most PDUs provide AC (alternating current) power, DC (direct current) PDUs and socket outlet arrangements are also available from some PDU manufacturers APC by Schneider, Eaton, Enlogic, Raritan, Server Technology and Structuresource.

When calculating how many socket outlets are needed, it is important to keep in mind the Amperage rating of the types available.

These ratings apply to the outlet and plug. For example, if you have 6 x C13 outlets and a 1 x C14 plug the limiting load size factor is the plug which will not be capable of drawing more than 10A. If you overload the plug, the fuse or breaker protection will activate.

Intelligent Rack PDUs

Intelligent PDUs add level of sophistications to power distribution within IT server racks and include:

• PDU and Outlet Metering: metered PDUs provide power related information locally and remotely, which can include Amps (A), Volts (V), Frequency (Hz), Watts (W), Energy (kWh) and Power (kVA). The power readings can be for individual outlets and the total PDU usage. The information can be used by IT managers for capacity planning, the prevention of circuit overloads, client and cost centre billing (with +/- 1% accuracy) and efficiency calculations including Power Usage Effectiveness (PUE).
• Switched and Outlet Switched: a switching PDU will include metering and add the ability to remotely control (ON/OFF) to the PDU and the outlets. Outlet switched PDUs provide a way for IT and data centre managers to remotely reboot or power down connected loads and allow for cascading power-ups to manage load inrush currents. Switching PDUs add a layer of security to a server room or data centre power plan in terms of controlling unauthorised access to rack level loads and their power connections. Costs are also reduced, as an onsite engineer visit is removed in most instances.
• Remote IP Monitoring: the PDU provides connectivity for remote monitoring and can include HTTP/HTTPS, iPV4 and iPV6, Telnet, SSH, Virtual Serial, SNMP (v1, v2c, v3), JSON-RPC, LDAP, FTP/SFTP and RADIUS for secure login. The monitoring provides a way view the status of the PDU and its individual socket outlets using a browser, monitoring software or a data centre infrastructure management (DCIM) software suite. The PDU may also offer a RESTful API for bespoke communications applications. Dual Ethernet ports can provide communications redundancy and the communications module will typically be a ‘hot-swap’ type.
• Data Centre Environment Monitoring: intelligent PDUs can be supplied with plug-in sensors for temperature, humidity and server cabinet access control. The sensor information can be reported facility monitoring platforms to provide rack-level environmental management information.

The most sophisticated type of intelligent PDU is the switched PDU with outlet metering. This is the most used type of PDU in a data centre environment due to the critical power monitoring and control capabilities it provides.

Automatic Transfer Switches and PDUs

Automatic Transfer Switches (ATS) provide a way to automatically transfer between two power sources (A and B) to protect a critical IT load. An ATS can provide some of the functions of a PDU in terms of monitoring and control and ATS products are available rated at 10/16a and 32A. An ATS differs to a Static Transfer Switch (STS) in that the switching is relay-based with a typical power break of 4-10ms whereas a static switch has a zero-transfer time using make-before-break technology. The 4-10ms break is like that for line interactive UPS and does not typically lead to IT device issues, due to the internal capacitance (energy storage) of the power supply used.

UK Custom Manufactured PDUs

Server Room Environments can also supply custom made power distribution units made in the UK. Custom designed PDUs tend to have a longer lead time but allow the PDU to be fully tailored for client specifications. Typical examples include multi-row horizontal PDUs with individual socket outlet branch breakers, multi-outlet mixes, left and right-hand facing sockets, outlet mix and number and PDU colour finish.

Summary

Power distribution units provide the principal point of connection to the critical power path in IT, computer, server room and data centre racks. Care should be taken to ensure the right power distribution system is installed. Whilst it is important to consider remote PDU monitoring and control, it is also important to ensure the basics are right. These include mapping out each server rack to identify all the IT load connections, their types (C13, C14, BS 1363, and hardwired) and the total power drawn (in Amps per phase, and voltage) by each device. Where an off-the-shelf PDU will not satisfy all the specification requirements, a custom-made PDU can be considered and lead times for these can be fairly quick when working with UK suppliers and manufacturers.