There are many factors involved in selecting a rack PDU, including data center location, application requirements, IT equipment requirements, available power, space in the cabinet, energy management and efficiency objectives, etc. In this section, we’ll take a look at a number of these considerations individually.

By determining what type of equipment and how many devices are going into your cabinets you’ll be able to define the physical configuration (number and type of outlets), and power capacity (kVA) required of your PDUs. For example, a rack PDU to support 40 x 1U pizza-box servers each with a single power feed would not have the same configuration as one to support three 10U high blade servers with six feeds per server.

Of course, decision criteria for 24/7-manned sites will be different than remote management of lights-out facilities. If you need remote or lights-out management of a facility, then you will probably choose a switched PDU, which requires more security and user access management. Remote applications may also call for SNMP management.

Integration with directory services like LDAP or Microsoft’s Active Directory® may also be important, since it’s an emerging requirement for controlling access to resources rather than requiring a separate access control system. This capability applies to all applications, local or remote, requiring central authentication. For many data center applications, such as federal government and financial institutions, encryption and strong password support is necessary for remote access.

Rack PDUs must supply power to each device plugged into them. You will want to try to prevent any events that can potentially cause the circuit breaker to trip on either the rack PDU or upstream at a panelboard. Outlet sequencing, therefore, is a valuable feature to prevent inrush current from tripping a circuit breaker. This is done by establishing a sequence with appropriate delays for powering on multiple devices. Outlet sequencing not only prevents the tripping of a circuit breaker, but also lets the user specify the order in which services and devices come online or are shut down during power cycling. For example, you will want to power up the database service before the Web servers.

For some applications and equipment, customized alarm thresholds for each outlet or the total PDU may be desirable. For example, switching off outlets powering less critical equipment should the PDU exceed a power draw that threatens to trip a circuit breaker.

In many mission-critical environments, managed devices often have multiple power feeds from different circuits for failover and redundancy. Therefore, the managed device needs to be treated as a single unit regardless of the number of power supplies/plugs and all outlets must be handled simultaneously. This capability is referred to as outlet grouping, a feature where multiple outlets, such as six outlets providing power to a six-power supply blade server, are switched on and off as a single group. Such a capability can also applies to applications, local and remote.

Event-driven power cycling of an outlet/device is required for some applications, particularly for remote or unmanned sites. For example, if a device in a remote location fails to respond and the WAN is not operational, you may be faced with just two options: an expensive, time-wasting visit to the remote site to restart the device, or a rack PDU with the intelligence to trigger a restart of a malfunctioning device, e.g., if the device has not responded after 20 minutes, the PDU recycles power to that device.

Finally, if you wish to maximize power efficiency, rack PDUs can provide valuable data to support those efforts. Look for current, voltage and power factor measurements at the PDU, line, circuit breaker and outlet level. Look for accurate kWh metering at the outlet level, especially if you intend to report on, or charge back to, end-user groups or departments their actual power usage. Metering accuracy can vary significantly. For some rack PDUs, calculations may even be based on assumptions, not actual real-time measurements, which further erodes accuracy. The best PDU systems support billing-grade accuracy of +/- 1%. Advanced PDUs may support environmental sensors and report information such as temperatures at each rack which can be used to adjust HVAC system set points for greatest efficiency with the safety of alerts should hot spots develop.

Rack PDU Functionality

Rack PDUs can vary significantly, not only in operational functions they offer, but also in their monitoring and data collection. Below is an overview of the strengths and weaknesses of the four types/classes of rack PDUs. Clearly, this class definition is not rigid, since features offered by vendors will vary and you will want to select PDUs based on the total fit to your requirement, but this can be a useful guide in your selection.

Basic PDUs

• Strengths: Basic, lowest cost, proven technology, highly reliable.
• Weaknesses: Lack instrumentation and are not manageable on any level.

Metered PDUs

• Strengths: Provide real-time monitoring of PDU current draw. User-defined alarms alert IT staff of potential circuit overloads before they occur.
• Weaknesses: Limited data, e.g., no outlet-level monitoring or critical environmental data; and no outlet switching.

Switched PDUs

• Strengths: Offer some or all of the features of metered PDUs plus remote power on/off capabilities, outlet-level switching and sequential power-up.
• Weaknesses: Must be managed carefully; risk of inadvertent power cycling. May not be appropriate for some environments, such as blade servers. Usually limited data, e.g., no outlet-level monitoring or critical environmental data.

Intelligent PDUs

• Strengths: State-of-the-art devices, they are remotely accessible via Web browser or command line interface (CLI). Models include all the features of switched devices plus outlet-level monitoring, standards-based management, integration with existing directory servers, enhanced security and rich customization. Provide comprehensive data including current, voltage, apparent power, active power, real-time environmental data and in some cases, real-time kilowatt hour (kWh) metering.
• Weaknesses: Higher initial cost relative to basic and metered PDUs due to their greatly enhanced feature set.

Learn more about the advantages of intelligent rack PDUs