Redundant power for single power supply

Someone recently asked me how to setup redundant power for a rack of machines without relying on all the equipment to have dual power supplies.

The answer is called an Automatic Transfer Switch. Depending on the sensitivity of your equipment to momentary losses of power, you can choose between an Open Transition Transfer Switch and a Closed Transition Transfer Switch.

  • Open Transition Transfer Switch – This switch always makes the connection to the secondary power supply a split second after the first connection dies.
  • Closed Transition Transfer Switch – This switch will make the connection to the secondary power supply before the power from the first connection dies if possible, ie. the first connection is browning out, etc. In this case the switch briefly connects both power sources with an overlap time under 100 ms. This way there is never a complete loss of power.

Transfer Switches are not unreasonably priced considering the prices of an additional power supply for some servers but they do not solve single point of failure problems- If your single power supply fries, having a second feed won’t help it 😉

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