Is 208v output likely to cause issues in a Cisco switchbwith PSU rated for 220-240v input?

Question

I’m a net/sysadmin. I had a Cisco switch chassis (4510) with 6000W PSUs go down due to both PSU faulting. Each PSU has one connection each to facility power and an APC UPS outputting 208v. The switch’s logs are uninformative except for a few power errors.

My operating theory is that facility power blipped, causing a flip to UPS. The 6000W PSU is rated for 220-240V input. When the blip happened, I suspect the PSUs undervolted when the UPS could only feed it 208. When I came in to troubleshoot, I could only get the switch to come on without the PSUs faulting by only connecting it to facility power. Once it booted, I connected the UPS again and it has stayed up since.

I’m not an electrician so I could be miles off base, but does think make sense? Any other theories?

What's the part number(s) on your PSU? – Andrew Henle Jan 30 '19 at 16:54 — Andrew Henle, Jan 30 '19 at 16:54
They are Cisco 6000ACV power supplies. – pants_towel Jan 30 '19 at 20:07 — pants_towel, Jan 30 '19 at 20:07

score 3 · Answer 1 · answered Jan 30 '19 at 17:15

Also not an electrician, but lots of data center experience. Short answer, yes: a PSU rated for 220V-240V will be unhappy with 208V. The reason why less to do with absolute (okay, root mean square) voltage and more with phasing. This thread on Ars Technica lays it out pretty clearly:

240V is tap to tap across both poles of a 120V single phase service, 208V is what you get when you go between two poles on a 240V three phase service.

So what's happening is that you have a PSU designed for two pole power: when the voltage is "low" on one wire it's "high" on the other: 180 degrees apart with the root mean square voltage being 240V. 208V is 2 wires out of three phases, 120 degrees apart.

Is 208v output likely to cause issues in a Cisco switchbwith PSU rated for 220-240v input?

1 Answers1