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The BPA option is required for clients who order an Internal Battery Feature (IBF), Water Cooling Unit (WCU), or Balanced Power. The BPA requires two or four power cords. All BPAs are concurrently repairable in the field. 

## Power requirements 

The 8561 operates from 2 or 4 fully redundant power supplies. These redundant power supplies each have their own power cords, or pair of power cords, which allows the system to survive the loss of customer power to either power cord or power cord pair. 

If power is interrupted to one of the power supplies, the other power supply assumes the entire load and the system continues to operate without interruption. Therefore, the power cords for each power supply must be wired to support the entire power load of the system. 

For the most reliable availability, the power cords in the rear of the frame should be powered from different PDUs. All power cords exit through the rear of the frame. The utility current distribution across the phase conductors (phase current balance) depends on the system configuration. Each front end power supply is provided with phase switching redundancy. 

The loss of an input phase is detected and the total input current is switched to the remaining phase pair without any power interruption. Depending on the configuration input power draw, the system can run from several minutes to indefinitely in this condition. Because most single phase losses are transients that recover in seconds, this redundancy provides protection against virtually all single phase outages. 

Power cords for the BPAs are attached to the options that are listed in Table 11-5. 

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The source power cords ratings for BPA systems are listed in Table 11-6. 

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a. Japan (same physical cord as Note b) 

b. US, Canada (same physical cord as Note a) 

c. US, Canada only 

d. US only 

