What’s the Best Way to Achieve Power Redundancy for Servers with Dual Power Supplies?

If you have a server with dual power supplies, you might be wondering about the best way to achieve power redundancy. While a single-circuit, single-UPS configuration is functional and very common, there are many benefits to a dual-circuit, dual-UPS configuration. Let’s take a look at examples of how the two configurations work.

Configuration 1: Common But Not Recommended (One Circuit and One UPS System)

Building Circuit: Single “A” feed (120V, 20A)
UPS System: Single “A” feed UPS (120V, 2200VA)
Server Redundancy: Yes, separate “A” and “B” inputs, both connected to the single “A” feed UPS

1.    Can you survive a spontaneous server power supply failure? Yes, if both power supplies are powering the load 50/50. If either power supply is running past 50%, the system won’t survive a power failure.
2.    Can you survive a spontaneous UPS failure? No. If the UPS fails in such a way that the power turns off, both power supplies will lose power and your server could crash.
3.    Can you operate through a spontaneous building circuit failure that goes uncorrected? No. If there’s only one building circuit and the breaker trips and remains off, the UPS connected to that circuit will run both power supplies until its batteries run out. Then, your UPS software will take over and power off your connected equipment. Nothing will crash or be damaged, but you’ll lose access to the server until power is restored.
4.    But will it work? Yes, this common setup will work. However, it’s not as good as a configuration with full redundancy.
5.    Possible improvement: Instead of plugging the “B” input into the UPS, plug the “B” server cord into a regular wall socket or surge protector. If you have a spontaneous UPS failure, your server will remain powered by the “B” connection.

Configuration 2: The BEST Configuration (Two Circuits and Two UPS Systems)

Building Circuits: Two separate dedicated circuits; “A” feed (120V, 20A) and “B” feed (120V, 20A)
UPS Systems: Two separate UPS systems; “A” UPS (120V, 2200VA) and “B” UPS (120V, 2200VA)
Server Redundancy: Yes, separate “A” and “B” inputs; “A” input connects to “A” UPS powered by circuit “A” and “B” input connects to “B” UPS powered by circuit “B”

1.    Can you survive a spontaneous server power supply failure? Yes, without conditions.
2.    Can you survive a spontaneous UPS failure? Yes, without conditions.
3.    Can you operate through a spontaneous building circuit failure that goes uncorrected? Yes, without conditions.
4.    But will it work? Yes. Using separate building circuits to run your server’s dual power supplies gives you all the benefits of full-on redundancy. This is the best way to go.

Other Considerations
There are two possible modes for dual power supply configurations.
•    Redundant Mode: Both power supplies are sized to power 100% of the load individually. During normal operation, the dual redundant power supplies power the load 50/50. For example, your server pulls 500W total and you have two 750W power supplies. If one 750W power supply fails, the other 750W power supply can provide enough power to keep the 500W server running.

•    Combined Mode: Both power supplies are required for the server to run because the power supplies cannot power the entire load individually. For example, your server pulls 1000W total and you have two 750W power supplies. If one 750W power supply fails, then your entire server will fail because the remaining 750W power supply isn’t powerful enough to keep the 1000W server running. This mode is somewhat rare in smaller servers, but it’s common for very large chassis network switches powered by multiple power supplies.

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