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DISCLAIMER: I know little to nothing about electronics. :-(

We are in the US and the bulk of the Data Center is running 110-120v devices and outlets. Our Data Center's large central UPS system is being worked on this weekend. We have been told that while it's unlikely there will be any power issues, if commercial power goes while the UPS is down, devices will go down.

One particular older device has my manager worried. It has dual power supplies that can "automatically" switch between 120v and 240v. Both are presently on 120v.

We have a portable UPS but it is for 240v. What my manager wants me to do is to move the device from its 120v source to the 240v source WHILE IT IS STILL POWERED ON -- meaning that at one point, one power supply will have theoretically switched to 240v while the other is still running 120v.

Again, I know little or nothing - but the very idea made alarm bells go off. However, I couldn't tell him with any certainty that this would end badly.

I understand that it may depend on the hardware, manufacturer, the zodiac sign it was built under or whatever - but what is the general opinion here on what the outcome might be?

Thanks for any response, and I am sorry for my ignorance.

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  • \$\begingroup\$ This all seems very sketchy, so if you manager wants this done, tell him to do it himself. If I were you, with the experience you claim to (not) have, I would stay right away from this. Find someone more experienced to work on it and who can evaluate whether or not it will work the way you want. Failure to dot the i's and cross the t's could result in serious physical harm to you or the systems. The safest bet would be to power down, hook up through the UPS, power up. Again though, without more information, find someone in your immediate vicinity who has the necessary experience. \$\endgroup\$
    – Brendan Simpson
    Commented Feb 24, 2016 at 20:25
  • \$\begingroup\$ Thank you for responding. I have asked around here - we have some EEs doing remodeling in the basement, and my Better Half has had Electronics training (albeit many years ago) but so far it's been like my own reaction: alarm bells but no hard answers. So I thought a wide-ranging community like this one might be helpful. And though I do like the thought of telling managers where to go sometimes, my manager is actually a pretty good guy and a good boss. Besides, mortgage and all that .. . . \$\endgroup\$
    – AnonOfIbid
    Commented Feb 24, 2016 at 21:09
  • \$\begingroup\$ Fair enough. I think without more info, it is best to play it safe. Explain to your manager that you don't feel comfortable doing this for risk of personal harm and equipment damage, and that it is good practice to not try and "hot swap" an entire system that runs on utility AC power. The thought gives me shivers... If he does force you to do it, get a face shield and some welding gloves! \$\endgroup\$
    – Brendan Simpson
    Commented Feb 24, 2016 at 21:49

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How well or poorly this works is mostly subject to how well or poorly the power supplies in the device are engineered, and how well or poorly it actually "hot-swaps" in practice.

In the fluffy bunny sunshine & flowers version, the fact that you have individual AC to DC power supplies, that are independent and which supply, share, and should be able to hot-swap the DC load means "no problemo, dude." It could go that way.

DEPENDING on it going that way is a whole 'nother ball of wax.

The AC side of the devices should never be getting shared or come into contact (at least with typical hot-swappable dual supplies, each having it's own line cord.) I'd want to be VERY sure the grounding was correct on the 240V UPS, though - would not want fighting grounds. Mostly a concern from it being portable, and wondering where it's plugging in, .vs. where your 120 supply is plugging in, and if the 240V extension cord that presumably takes is wired properly. Again, it SHOULD be fine, if everything is wired properly.

More broadly, any time one stresses items which are supposed to prevent failure (ie, hot-swapping supplies) that have not been regularly tested to verify that they do that correctly, you do have some potential that you will find that they don't do that correctly, and you'll crash whatever it is that you so desperately want to keep up and running. Of course, the potential for crashing under testing keeps folks from testing the functions much, if ever - a nice catch-22.

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  • \$\begingroup\$ Many thanks EC. The power supplies are Dell-Branded but who the heck knows the actual OEM - they don't say. I spent some time on the Dell site seeing what I could find but not much truly technical and of course the warranty is expired so I can't engage a human. I haven't seen the UPS yet - when I asked, I was told that it was new and hadn't arrived yet. We have a number of brand-new 240v power cables as well. So the weakest link superficially is Dell. \$\endgroup\$
    – AnonOfIbid
    Commented Feb 24, 2016 at 22:59
  • \$\begingroup\$ BTW, I don't do Fluffy Bunny, in spite of my image :-) I talked again with my manager and we are likely going to take our chances with the Devil we know (simple power outage) rather than the Devil we don't (mayhem and fryage internal to the storage device). \$\endgroup\$
    – AnonOfIbid
    Commented Feb 24, 2016 at 23:02
  • \$\begingroup\$ @AnonOfIbid - huh - If I even noticed your avatar, it must have been very subliminally - at normal post size it looks more like a white rat, (ears being the bunny's eyes, eyes being the bunny's nostrils) though I see the bunny if I look at the larger version on the profile page. Serendipity. As you might infer from mine still being the site auto-generated thing, not something I put a huge degree of stock into. \$\endgroup\$
    – Ecnerwal
    Commented Feb 25, 2016 at 3:30
  • \$\begingroup\$ You're right! Very rattish. Probably should have just left the default, but eh, it can stay. Sometimes I am a rat. \$\endgroup\$
    – AnonOfIbid
    Commented Feb 25, 2016 at 22:47

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