UPS Specifications and Considerations
| Selecting the right UPS for a server or server room is not a trivial issue. The greater the number of servers you need to protect, the more complex the task can become. Although most UPS vendors provide online interactive buying guides to help you through the selection process, it's still useful to understand the factors that go into a vendor's recommendations. These include the following:
The following sections discuss each of these factors. Server Capacity and PurposeThe first question you need to answer before you select a UPS is, "How is this server being used?" If you fail to answer this question first, you will not be able to make an informed choice of UPS. Here's why this question is important:
You need to think about these considerations before you purchase a UPS. Note Some UPS vendors offer special models optimized for medical environments. These units meet UL 60601-1 medical electrical equipment standards for minimal current leakage. You should specify this type of UPS if you need to place a UPS in an environment sensitive to current leakage, such as hospitals, air traffic control, and laboratories. Some UPS vendors offer special models designed to handle industrial environments such as those specified in MIL-STD-810. You should specify this type of UPS if the unit will be used on a loading dock or in another rough environment. VoltagesIf you are selecting a UPS for use with a standard x86 or 64-bit server, the UPS you use will be designed for either 120V/60Hz AC (U.S./Canada) or 240V/50Hz AC (Europe/Asia). If your server room has 208V AC power, which is used by telecom equipment and midrange systems, you can select UPS units that accept 208V AC incoming voltage and step it down to 120V AC. Some UPS devices do not permit much, if any, adjustment of input voltage ranges. If your location is subject to a lot of variation in input voltage, a UPS with limited or no ability to adjust the voltage at which the device switches to battery power could result in unnecessary use of the battery. This can lead to premature battery failure or to a UPS not yet being fully recharged when total loss of AC power takes place. UPS models that can be configured to work with a wide range of incoming voltage are recommended if your location has substandard or overstandard voltage. RuntimesThe runtime for a particular combination of UPS and connected hardware is determined by three factors:
The actual wattage your server uses is not the same as the server power supply's wattage rating. The server's power supply (like a PC's power supply) is designed to provide power well in excess of the server's requirements. However, each external device connected to a server also has a wattage rating that must also be taken into account. You can determine the size of UPS you should purchase for your server based on its load or its devices. Some vendors offer both methods. The easiest way to calculate the power used by a server is to use a vendor's UPS selector calculator, assuming it's detailed enough. You should be able to specify the following:
Before displaying recommended configurations, many vendors display the calculated wattage based on your selections. If you know the wattage ratings of your server and attached peripherals, you can use the "configure by load" option (if offered by the UPS vendor) to create a list of recommended UPS configurations. Typically, vendors provide two or more UPS configurations that satisfy the device or load requirements you provide. Tip Include only essential devices (or the wattage loads they require) when you use a UPS selector wizard. Printers are not essential devices: A printer (particularly a laser printer) can run down even a large UPS unit very quickly and should not be connected to the UPS. However, external devices such as hard disk arrays, tape backups, and monitors should be included. When you view the vendor's list of suggested configurations, you might notice that the same UPS device is listed more than once with different runtimes. This indicates that the device is listed with both its standard single-battery configuration and with additional batteries. BatteriesIf you plan to connect more than one server to a UPS, or if you want to add additional components to a server connected to a UPS, choosing a UPS that can be connected to multiple batteries is a good idea. Although you cannot increase a UPS unit's VA rating (which specifies the maximum load it can support) by connecting additional batteries to the UPS, you can connect additional batteries to increase runtime, and doing so permits a UPS to handle a load closer to its maximum while still providing adequate runtime. For example, an APC Smart-UPS XL 750VA (120V version) connected to an Intel SC5250-E server with four internal SATA drives, a 15-inch CRT, and an external tape backup (about 387 watts or 553VA) offers a runtime of 35 minutes when used with its internal battery, according to APC's online UPS selector. However, the same battery backup unit provides a runtime of up to 160 minutes when an external battery pack (SUA24XLBP) is added to the basic configuration. If you opt for UPS units that support multiple batteries, you should try to standardize on products that can use the same additional battery pack. For example, Tripp Lite's BP48V24-2U external battery pack works with a variety of 2,200VA3,000VA Tripp Lite UPS systems. By using different UPS models that use a common additional battery pack, you make handling capacity issues and swapping parts in case of failure far easier than if you don't use interchangeable battery packs. Even if you choose a UPS that does not support multiple batteries, choosing a unit that supports hot-swap batteries makes sense. A UPS with hot-swap battery support enables you to swap a failed battery or one that is not fully charged for another battery without shutting down the UPS or the server. Note that you cannot swap out a battery if the UPS has only one battery and the battery is currently powering the server. Batteries for server-class UPS units are typically based on sealed lead-acid technology. If you need to store a UPS unit for an extended period of time, you should reconnect the unit to AC power and recharge the battery every three to six months to enable the battery to receive and retain a full charge. For additional battery-conditioning tips, consult the UPS manufacturer's instruction manual. AC ConnectorsThe UPS unit you select must have enough AC connectors to support the devices you need to connect to it. For example, if your server has an external tape backup, a broadband modem, and a broadband router, you need a total of five connectors, one each for the server, the monitor, the tape backup, the broadband modem, and the broadband router. Some UPS systems offer two or more individually switchable load banks. If you use the management software provided with the UPS, one bank can be configured to drop less critical loads to extend runtime for more critical loads. For example, you could plug the server and tape backup into one load bank and the other devices into a second load bank. The second load bank could be configured to shut off if necessary to provide longer runtime for the devices in the first load bank. You need to make sure the UPS you select is designed to connect with the electrical outlets available in the area. 120V AC units with up to 1,400VA can plug in to a standard 15-amp outlet. However, units with larger VA ratings must be connected to 20-amp or 30-amp outlets or must be hard-wired, depending on the VA rating of the device. You should be sure to check the specifications for incoming power and outlet type used in your location before you specify a UPS. Safety RatingsYou need to make sure the UPS unit you buy meets the relevant safety ratings for your country or region. You should check the product's specifications or product manual to determine the safety ratings for a particular product. |
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