UPS For Your SAN - Do You Have The Power? Page 3


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How A UPS Is Rated

UPSs are equipped to handle blackouts, brownouts, noise, spikes and surges. Here are the ratings and features you should look for when determining which UPS is right for you and your environment.
  • VA rating.
  • Run time.
  • Joule rating.
  • Surge protection.

VA Rating

A VA rating is the total load (power) that a UPS can support. The higher the rating, the larger the load. To calculate the VA rating, multiply the number of the volts (V) of a hardware device by the number of amps (A). If you find only the wattage, multiply it by 1.35 to get a conservative VA rating. Do this for each device that you want to connect to the UPS, and calculate the sum of the individual VA ratings to get an overall VA rating.

The VA rating of the UPS should exceed the VA rating of your SAN hardware by about 20%. For example, if the VA rating of your SAN hardware is 450, the UPS you select should have a VA of at least 540. If you buy a UPS with a VA that is not high enough, you will not have enough power to run everything during a blackout to perform a proper shutdown.

Run Time

Run time is the length of time a UPS can provide power on its own. VA ratings affect run time: the higher the VA rating, the longer the run time. You must decide whether you want just enough time to save everything and shut down or a longer period of time for production. Keep in mind that most blackouts do not last longer than one hour.

Joule Rating

A joule is the amount of energy delivered by one watt of power in one second or by one million watts of power in one microsecond. This rating indicates the amount of surge a UPS can handle. The higher the rating, the larger the surge a UPS can absorb.

Be wary of this rating. Because there is no standard guideline, joule ratings tend to be inflated. Check for added protection circuitry in UPSs that claim high ratings (more than 800 joules).

Surge Protection

Surge protection is rated according to how well a UPS handles a surge. A higher rating equals better protection. Vendors give different measurement units for this rating.

Finally, how do you determine what size UPS system is needed? Lets see!

UPS Sizing

The UPS or other power protection hardware chosen to accompany a system or SAN hardware must have sufficient capacity to provide adequate protection. An estimate of the Volt-Ampere (VA) load can be made by adding the VA specifications of the individual loads. If the equipment specifications are given in amperes, the VA can be calculated by using one of the following formulas:
  • k = Kilowatt
  • V = Volt
  • A = Ampere
  • VA = Volt Ampere

For 3-phase systems, you should use the following formula:

kVA = V x A x √ 3

And, for single-phase systems, you should use the following formula:

VA = V x A

The currents to be used in the preceding equations are the equipment running/steady-state currents, not the surge current, inrush current or circut-breaker rating.

Finally, it is important to note that by simply adding the VA values, it may result in an erroneous estimate of the total power draw of the system, particularly for unbalanced loading conditions. A more accurate estimate of the required kVA size for the power protection system can be made by balancing the SAN hardware on the three phases, then determining the required system kVA from the phase with the largest load. When sizing a power protection system, it's generally recommended that the specifier add at least another 20 to 50 percent to the total VA rating. This allows for overload protection; and, it also provides additional capacity to expand the system later without the need to buy a larger UPS.

Summary And Conclusions

Youll never really notice UPSs until the power goes out or an electrical disaster strikes. Whenever you are making purchasing decisions on new SAN hardware, make sure you have a UPS to protect your investment.

In other words, selection of a UPS system needs careful thought and analysis. Too many users see the purchase as a 'black box' solution with the main criteria being price rather than performance and suitability for their SAN hardware. Many factors must be considered.

Offline UPS may offer an economical solution provided the applications being run on the SAN hardware are not business critical. To guarantee continuous power to your critical systems in the event of all power problems; however, you need to be sure that you have selected a true online UPS unit which is matched to the size and configuration of the SAN hardware.

Finally, with the preceding in mind, Part II continues the UPS for SAN theme by discussing a categorized and prioritized collection of SAN hardware powering needs and problems, etc. See you there!

About the Author :John Vacca is an information technology consultant and author. Since 1982, John has authored 36 technical books including The Essential Guide To Storage Area Networks, published by Prentice Hall. John was the computer security official for NASA's space station program (Freedom) and the International Space Station Program, from 1988 until his early retirement from NASA in 1995. John can be reached at jvacca@hti.net.

» See All Articles by Columnist John Vacca

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