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Ask the Engineer - Do you have any rack-mount UPS recommendations?

Posted by Robert Faulkner on Oct 4, 2010 1:40:00 PM

On occasion, an installation consists of only a few cabinets and it is desirable to place individual UPS units within each cabinet. Although Server Tech does not provide UPS products and remains manufacturer agnostic, there are a couple things that should be considered from the power distribution point of view.

Of course, the primary purpose of the UPS is to continue operation of attached equipment during mains power loss. Sizing is thus the primary selection criteria. Most of the time, being that the UPS is a source, the rating will be in volt-amps (VA). For instance, a 5kVA UPS is good for roughly 24A at 208V output. Next, you need to consider the hold time of the battery and decide if expansion batteries are needed.

Redundancy is another consideration. If you have multiple-power-supply equipment stacked in your cabinets, it should be considered whether the protection is against a power supply failure, or a UPS failure. Some installations simply have Y-cables providing power from a single power outlet to both power supplies of a server. Some have the two power supplies plugged into different circuits of the UPS, and some have the two power supplies plugged into separate UPS's. The Y-cable really provides no redundancy being that if one of the power supplies shorts, the outlet protection fuse or breaker will fault resulting in the other power supply going down as well. The split circuits configuration allows better redundancy such that the fault of one supply will not bring down the other unless something causes a fault at the input of the UPS. The separate UPS provides the best redundancy in that it protects even if the entire input circuit faults. It is not uncommon to see a UPS in one rack providing the "A" feed power to a coupling of two racks; and a UPS in the second rack providing the "B" feed power. It is critical that one realize that the total power loading in the two racks combined must not exceed the capacity of either one of the UPS units.

Next is the outlet consideration. Most rack-mount UPS's have a variety of outlets available; however, there is often not enough. In that case, a rack-mount PDU (or CDU) can be used to provide the necessary outlet count as well as simpler cable routing for the attached equipment. Locking outlet receptacles are recommended on the UPS for this case.

Intelligent CDU's providing local LED displays of amperage at the input, branch, or phase level as well as in-network/remote access for monitoring and management can also add value to the installation. Although many UPS's provide power monitoring capability options, look at the incremental cost difference when determining whether to add the monitoring into the CDU or UPS. Also consider CDU features such as remote reboot, graceful operating system shutdown, smart load shedding, and environmental monitoring when determining whether to use intelligence in the CDU vs. intelligence in the UPS.

Topics: rack mount power distribution units

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