You are cabling a new 2U application server that has two hot-swappable power supplies. The rack is in a Tier III colocation facility that offers two completely independent UPS systems:
UPS-A - powered by Utility Company #1
UPS-B - powered by Utility Company #2
Every rack has an A-side PDU wired only to UPS-A and a B-side PDU wired only to UPS-B. The customer's service-level agreement demands that the server remain online even if one utility provider suffers a total outage. Which cabling practice BEST satisfies this requirement?
Attach the first power supply to the A-side PDU and leave the second power supply disconnected for future maintenance use.
Connect one power supply to a receptacle on the A-side PDU (UPS-A) and the other power supply to a receptacle on the B-side PDU (UPS-B).
Plug both power supplies into two different outlets on the A-side PDU to balance the load across phases.
Feed a rack-mount static transfer switch from the B-side PDU only and connect both server power supplies to the switch.
For maximum availability the two power supplies must follow different electrical paths all the way back to separate utility feeds. Plugging one PSU into the PDU that is fed by UPS-A and the other into the PDU fed by UPS-B creates two independent (2N) power paths. If Utility #1, UPS-A, or PDU-A fails, the server continues to draw power through Utility #2, UPS-B, and PDU-B, meeting the SLA. The other options leave both PSUs on the same power chain or leave one supply unused; a single failure in that chain would still drop the server, so they do not meet the requirement.
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