Rack Power Distribution Unit (PDU) Tutorial

A rack PDU ("rack Power Distribution Unit") is a tool for selectively switching AC and/or DC power to your devices. This means that, with a rack power distribution unit and its switching ability you can turn on servers or other equipment from hundreds of miles away via LAN, dial-up, etc. Obviously, you can also power down equipment remotely. You can even reboot your servers if they get "jammed up" or "frozen". In this way, a simple rack PDU can save expensive trips out to distant locations. Why dispatch a technician? This eats up your time, fuel, and wages on the road. It's much better to issue power control commands remotely via a simple LAN, dial-up, cellular, or other connection.

Who can benefit from using rack Power Distribution Units?

Power Distribution Units are used in server rooms and data centers, remote telecom sites, and anywhere else that you need to be toggled control power to equipment without someone physically being on-site. PDUs are used in telecom, transportation/transit, public utilities, government, military, education, and research environments worldwide.

What form factor is best? Rack Power Distribution Unit or something else such as a Wall Mount?

PDUs come in a variety of form factors. Rack PDUs are probably the most popular, but there are other types that mount to a wall. If you're in a commercial/industrial environment, a rack PDU is probably the best choice.

A rack PDU like the one shown below mounts into your existing equipment racks. With a rack PDU, your power distribution unit is secured just as strongly as the rest of your gear. This is absolutely critical, since a lot of key equipment will be powered via the PDU. If it isn't secured, your entire site is at risk.

Example back panel of a DC rack PDU (Power Distribution Unit)
The back panel of a DC-powered rack 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 limited. 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.

Which rack Power Distribution Unit should I choose if I have servers with redundant power supplies?

While any PDU will do an "OK" job with redundant power supplies, there are ways to make managing devices like this much easier. Look for a PDU with enough intelligence to logically group multiple outputs. This means that, if you have to reboot a server with 2 redundant power supplies, you can toggle power to all of those supplies simultaneously, triggering a reboot. If you don't have this feature on your PDU, you'll be stuck manually toggling outputs individually to reboot your servers.

Will a rack Power Distribution Unit handle both AC and DC power at my sites?

The most basic decision you have to make as you evaluate rack PDUs is: "AC or DC?" This decision will affect the internal power tolerances of the PDU and, more importantly for general use, the connectorization you'll use to attach your devices. An AC rack PDU will typically have the standard plugs you see in homes and offices around the world. With a DC PDU, you're more likely to see an industrial connector that accepts bare wiring.

Are most rack PDUs built for industrial/commercial use?

Unfortunately, many PDUs available in the marketplace are not of the best quality. Considering what PDUs do, this can make choosing a low-quality distribution unit an especially costly mistake. Any equipment that deserves remote power control should only be connected to quality power equipment. You should look for a powder-coated metal chassis instead of plastic. That's one of the best 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.

What is the difference between managed and unmanaged rack Power Distribution Units?

An unmanaged PDU is nothing more than an industrial-style power strip, power bar, etc. You won't have any way to control power to individual outputs. This means that you can connect multiple devices to a single power source, but you'll have no way of remotely turning on, turning off, or rebooting your gear. A good managed rack PDU will have a web interface. If you've every configured a network router for your home, you've seen one of these interfaces. A web interface makes it simple for just about anyone to remotely toggle power to your equipment.

What is Simple Network Management Protocol, exactly? Why does it mean in a rack Power Distribution Unit context?

SNMP stands for "Simple Network Management Protocol". Although the word "simple" in the acronym has been the subject of many sarcastic laughs, SNMP is an important feature for a rack PDU. Instead of simply toggling power through the PDUs native interface, SNMP compatibility allows you to integrate your PDUs into your broader monitoring and control system. When PDUs are under your SNMP manager umbrella, commands can be issued even faster - or even automatically. Look for a rack PDU that can send SNMP Trap messages and accept SNMP SETs and SNMP GETs.

I'm concerned about power consumption. Can I monitor current flow using a rack Power Distribution Unit?

Especially in the modern business culture that's increasingly demanding "green" power technologies, monitoring how much power your devices are using is a big deal. With good PDUs, you can monitor overall current flow through the PDU, and sometimes also the flow out of each output. Either one of these options is much better than no current monitoring, since you can pinpoint the equipment that's hogging your power.

How else can a rack Power Distribution Unit limit the number of site trips I have to make?

Simple rack 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 is a PDU and RTU (Remote Telemetry Unit) in one metal 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. Look for opportunities like this to do more with a single chassis, a single unit of rackspace, a single power supply, and a single remote interface.

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.

Convert your Remote Telemetry Unit into a Power Distribution Unit

With a simple add-on box, you can use your RTU as a PDU.