C-6 Dell PowerEdge 4200 Systems User’s GuideSurge ProtectorsSurge protectors are available in a variety of types andusually provide a level of protection commensurate withthe cost of the device. Surge protectors prevent voltagespikes, such as those caused during an electrical storm,from entering a system through the AC power source.Surge protectors, however, do not offer protection againstbrownouts, which occur when the voltage drops morethan 20 percent below the normal AC line voltage.Line ConditionersLine conditioners go beyond the overvoltage protectionof surge protectors. Line conditioners keep a computer’sAC power source voltage at a fairly constant level and,therefore, can handle brownouts. Because of this addedprotection, line conditioners cost more than surge protec-tors—up to several hundred dollars. However, thesedevices cannot protect against a complete loss of power.Uninterruptible Power SupplyAn uninterruptible power supply (UPS) offers the mostcomplete protection against variations in power becauseit uses battery power to keep the system running whenAC power is lost. AC power, while available, charges thebattery; when AC power is lost, the battery can providepower to the system for a limited amount of time—from15 minutes to an hour or so—depending on the UPSsystem.UPS systems range in price from a few hundred dollars toseveral thousand dollars, with the more expensive unitsallowing you to run larger systems for a longer period oftime when AC power is lost. UPS systems that provideonly 5 minutes of battery power let you conduct anorderly shutdown of the system, but are not intended toprovide continued operation. Surge protectors should beused with all UPS systems, and the UPS system shouldbe Underwriters Laboratories (UL) safety-approved.If you are using the optional redundant power suppliesinstalled in your PowerEdge 4200 system, connect thetwo power supplies to different circuits, if possible.You do not need to back up both power supplies in a sys-tem; you only need to back up one. This allows thesystem to log AC source failures while remaining opera-tional through the other power supply and the UPS. Thisalso protects a UPS failure from being a single point offailure for the system.
