Server Fault is a question and answer site for system and network administrators. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

Some devices, such as servers and high-end switches have dual PSUs/power inputs, which can then be hooked up to separate UPSes, which are hooked up to separate power circuits (correct so far, right?).

However, many devices, such as standard switches, only have a single power input. This is problematic, because in a scenario where Circuit #1 fails, your servers might still be running off of Circuit #2, but your whole network infrastructure is down because it's also on Circuit #1.

How can power redundancy be set up for devices with only one power input (maybe via a power strip-type device with dual power inputs in itself)? Or would one have to specifically buy devices which are built with that capability in mind?

share|improve this question
up vote 6 down vote accepted

The term you are seemingly looking for is "transfer switch", although it should be installed into the feed of an UPS and not behind it.

As for switches, redundancy typically would be achieved by duplicating the equipment and using techniques like cross-stack-aggregation or MPIO for link failover. Also, switch PSUs turn out to be not as error-prone as the server variants, it is much more likely to encounter something like a fan fault which would not be helped by a second PSU.

share|improve this answer
Just as a note, I agree with you overall, but the OP might think that a transfer switch would fail from Circuit #1 to Circuit #2. Transfer switches are meant to switch between two power sources, and it is doubtful that either Circuit #1 or Circuit #2 in a building would be online if building power fails. The transfer switch would facilitate failing over to the generator backup (in your answer) but just clarifying that it won't provide redundancy for 2 circuits internal to the building (unless they are on individual power somehow). The OP should speak with the building electrician. – TheCleaner Apr 18 '12 at 21:49
@TheCleaner I assumed it would be obvious that a reliable (as in "available at all times") power circuit must be present for it to work. Further clarification is a good thing, though. – the-wabbit Apr 18 '12 at 22:02
"into the feed of the UPS" meaning, between the wall and the UPS? – Bigbio2002 Apr 18 '12 at 22:32
@Bigbio2002 yes, assuming you have reliable power to switch over to (like a generator or any other kind of standby set or a separate utility). The reason for installing it into the feed would be to avoid synchronization issues, voltage drops or spikes having reach your equipment - they would be filtered/evened out by the UPS(es). – the-wabbit Apr 19 '12 at 6:18

You need to be careful of introducing weaker components into the system here.

It's very tempting to throw a UPS into the system, as that will provide mains and backup power via a single PSU, but cheap UPSes can often cause more problems than they solve. If your mains is generally stable, you'll probably have more UPS related outages than actual mains one, particularly if you have a cheap UPS.

You could spend more on a UPS of course, and mitigate that point somewhat. Or you could also buy network gear with multiple PSU options :)

Another option is a transfer switch, such as this APC unit. It accepts two mains inputs and handles the switchover internally. Again, it's introducing a SPOF, but these devices are explicitly designed to do a single, simple task.

share|improve this answer

You already know the answer, you're just overlooking it perhaps.

Plug your single PSU devices into a UPS that can handle brown-outs and whatever time threshold for black-outs you need.

If power goes out on the actual circuit then the UPS supplies power during the outage. If the UPS dies then it simply bypasses it to supply power from the actual circuit.

If you need longer uptime during an outage than a UPS can provide, then you need to look at generators.

That's the best option you've got on single power supply devices, IMO. Even your original post about servers with dual PSUs going into separate UPS' into separate circuits seems overkill, since the likelihood of all 3 in a single line (the PSU, the UPS, and the circuit) all failing at the same time is miniscule.

share|improve this answer
You don't need all three devices in a single line to fail in order to have a power failure, only one of them. – Daniel Lawson Apr 18 '12 at 21:22
Yeah, but as long as the power supply in the switch/router is fine then you only need 1 of the others to work (UPS or circuit) at least for a short period of time. Syneticon is right on how to mitigate risk on switches, but the question was much more vague "How can power redundancy be set up for devices with only one power input" - a transfer switch won't provide immediate failover, the "device" would still lose power during the transfer, hence a UPS. Individual circuits in a room would typically still tie back to a single power pole. Oh wellThe answer is done as far as the OP is concerned. – TheCleaner Apr 18 '12 at 21:45
Hmm, so transfer switches AREN'T instantaneous? – Bigbio2002 Apr 18 '12 at 22:26
No, not quick enough that you won't need a ups. You need an electrician to explain one to you. You also need to realize it isn't moving power between circuits but between sources. – TheCleaner Apr 19 '12 at 1:29

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.