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"PDU Tutorial: How to Choose a Power Distribution Unit"


A PDU (Power Distribution Unit, sometimes a "power strip") is a device that allows you to remotely turn on, turn off, and reboot equipment running on AC and/or DC power. PDU technology is absolutely critical to minimize wasted driving out to remote sites. Instead of sending out a technician and burning time, fuel, and wages on the road, you can issue power control commands remotely via a simple LAN connection.

Common uses for the PDU
Power Distribution Units are used in server rooms, data centers, remote telecom sites, and anywhere else that equipment may need to be toggled on or off (including remote reboot) without someone physically being on-site. PDUs are used in telecom, transportation/transit, public utilities, government, military, education, and research environments worldwide.

Several PDU Chassis Options
PDUs come in a variety of form factors, including rack PDU, wall mount, or "0 RU" rack mount.

The advantage of a rack PDU (like the one shown below, and also known as a "rack mount PDU") is that it mounts into your existing equipment racks. This not only gives you a place to install the device, but it also reduces the electrical wiring distance required for connecting to your equipment.

Example back panel of a DC-power PDU (Power Distribution Unit)
The back panel of a DC-powered PDU (Power Distribution Unit)

One disadvantage of the rack PDU model, however, is that it takes up space in your equipment/server racks, where space may be tight. That's where "O RU" PDUs come in. In "0 RU" PDU designs, a long and narrow power strip is attached to the side of an equipment rack, which is ordinarily wasted space. This allows you to add PDU functionality to your site without using up any physical rack space.

If you do choose a rack PDU design, try to find one that is only a single rack-unit tall (referred to as "1 RU" or a "PDU 1U"). A PDU 1U in size minimizes the vertical space in your rack that must be consumed to hold the PDU.

Understand the Value of Redundant-Power-Aware PDUs
When you're evaluating PDUs, consider whether you have devices with multiple redundant power supplies. If you do, look for a PDU that supports "outlet grouping" or a similarly named feature. This will let you toggle all redundant inputs at the same time, rebooting that device (typically a server) with a minimum number of click or keystrokes.

Get the Right PDU for Your Site's Power
There are many different kinds of PDUs out there, so you need to make sure that you choose the right one. Your #1 concern is AC vs. DC power. Although internally they may be quite similar, an AC-powered PDU has very different connectors than a DC-powered PDU. If you don't have the right connectors, you're not going to get much use out of your PDU at all.

Don't Buy Power Distribution Units that Aren't Built to Last
Also, don't skimp on build quality when you're choosing a PDU. Any equipment that deserves remote power control should only be connected to quality power equipment. One good rule of thumb is to look for a powder-coated metal chassis instead of plastic. That's one of the easiest things to look for to get a quick idea of overall build quality. Also, look for a manufacturer that's been around for a while. That lets you leverage a proven design that's been providing reliable service for years.

This PDU is AC-powered
Another PDU example - This is the front-panel of an AC power controller.

Look for Alternatives to a Modular PDU Design
There's plenty to like about modular PDUs. They have the same advantages of all modular equipment: flexibility in both cost and capabilities. Unfortunately, a shelf-and-card approach to PDU design is not perfect. The main problem is that, feature-for-feature, a modular PDU will typically be more expensive than a simple rack-mounted PDU. That's simply because a shelf architecture is loaded with card buses that don't exist in a non-modular design.

But there is a solution if you still want flexible PDU capacity. You simply have to choose a rack-mount PDU that can daisy-chain with additional devices if necessary. This allows you to link several PDU (1U) units together and control them as a single unit. Now you get the benefits of a modular PDU – while avoiding the price tag.

Choose a Managed PDU for High-Uptime Environments
If you want your PDU to be more than an over-hyped surge protector, you need to choose one that has an integrated power management interface. A web interface is the best option for small PDU deployments, because you can access the PDU from any LAN-connected PC on your network. You'll simply type the PDU's IP address into your web browser. From there, you can toggle power on and off.

A web interface is also superior because it travels over reliable LAN transport. Older options include RS-232 serial connections, Telnet, and SSH.

Also, look for a PDU that allows you to switch on, switch off, and remotely reboot equipment on a per-port basis. You shouldn't have to toggle the whole power strip all at once.

Large PDU Deployments Demand an SNMP PDU
For large organizations, there's really no substitute for an SNMP-capable PDU. This kind of power controller can accept SNMP SET messages from an SNMP manager. This allows you to control power as part of your industry-standard SNMP management system. Also, an SNMP PDU can send SNMP TRAP messages back to your manager, providing confirmation that commands have executed successfully. There's nothing worse than sending a power switch command and assuming it executed, when nothing has actually happened.

The Value of Output Current Monitoring
If you want to know how much power each of your devices is monitoring, current monitoring on each PDU output port is ideal. You can view (ideally through the web interface) how much electricity is flowing through each port. Alternatively, you can reduce your costs by monitoring total current passing through the PDU instead of on a per-port basis. This method requires that your manufacturer only includes a single current sensor, rather than one for every PDU output port.

Look for PDUs that also Include Alarm Monitoring
Simple PDUs are available from many different manufacturers, but there are a few that include more functionality than the rest. As an example, the Remote Power Controller 100 is a PDU and RTU (Remote Telemetry Unit) in one box. The PDU functionality allows you to control power, while the RTU capabilities allow you to monitor discrete and analog alarm data from your important equipment.

This combination of remote power control and remote alarm monitoring is especially useful because sites that need power control probably need alarm monitoring, too. If you're trying to minimize wasted time and effort driving out to sites, don't stop at simple power control. You should also increase your remote site visibility with an RTU combo unit. That's a great way to kill two birds with one stone - without spending more than you have to.

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