A datacenter's redundant PDUs were recently upgraded. Each server rack is connected to the datacenter's PDUs. While testing power redundancy, one of the datacenter PDUs was turned off, and a server…

A defective rack-mounted PDU would cause intermittent failures or no output from that specific unit, but does not explain a complete power loss when only one datacenter PDU is deliberately switched off during testing.

A UPS is the critical component that maintains continuous power during PDU failures or maintenance testing by using battery reserves to bridge the gap. Without the UPS connected in the power chain, shutting down a PDU immediately removes that power path with no battery failover, causing the server rack to lose power entirely despite the presence of redundant PDUs.

Bad UPS batteries would prevent the UPS from sustaining power during an outage, but the scenario describes the UPS as not connected at all, which is a more fundamental missing configuration rather than degraded batteries.

Both rack PDUs being on the same circuit is a valid cause of total power loss when that circuit is cut, but this explanation does not align with the recently upgraded redundant PDU infrastructure described in the scenario.