AC Failover via relay - Page 1
Forum section: electronics / projects

author: waoj
author_rank: Contributor
timestamp: January 18, 2024, 09:55:34 pm
content: I have a computer that I want to keep running as much as possibleIt has a single AC input however. I have in the past used a DPDT relay with the server on the common terminals, and the coil connected to the normally closed side. Then I had two power cords, one on the coil I called the primary input, and the one on the other side of the DPDT was the standby input. I'd keep both plugged in to two outlets on the same strip, and then if I need to move a plug I can do it one at a time.It worked great. I still have a couple of these in use right now to act as automatic bypasses for single-phase UPSes. I splurged and bought Schneider branded relays for extra cred.I never had the guts to try putting one leg on a different phase of power though. In homes we have two phases 180 deg apart. At work, we have three, 120 deg apart.What would happen to a switchmode computer power supply if I used that apparatus to "walk it" from one phase to another?I'd already be pretty nervous to have opposite phases a millimeter or two apart inside the relay... arc flash?Is there such a thing as a high-speed long travel relay? meant to break arcs for instance?I could try it but I thought I'd let you all talk me out of it and tell me what a terrible and unsafe idea it is instead and how lucky I am to be alive.If you really want to give yourselves a heart attack, look what I actually built, and used, in a live data-center. The nema 5-20 outlets there are wired for 208v (no neutral) and we wanted to change out the circuit breakers without taking things offline. We wore safety glasses, and quadruple checked we were about to switch on the right breakers each time as a pair of workers. It worked exactly as planned.

author: Andy Chee
author_rank: Super Contributor
timestamp: January 18, 2024, 10:11:28 pm
content: Putting aside the safety hazards for a moment.....Quote from: waoj on January 18, 2024, 09:55:34 pmWhy would you want to switch phases when all phases are operational?On the other hand, if you're talking about power failure in one phase and automatic changeover, then if you lose power, there will be no power to cause an arc flash!

author: waoj
author_rank: Contributor
timestamp: January 18, 2024, 10:15:44 pm
content: Quote from: Andy Chee on January 18, 2024, 10:11:28 pmBalancing load. re-configuring the room furniture. construction.Quote from: Andy Chee on January 18, 2024, 10:11:28 pmYes, but when primary power comes back on....So far I've only used a relay like this where both inputs were nominally at the same potential. And if one went to 0v it was because it was disconnected, not shorted or earthed.

author: Andy Chee
author_rank: Super Contributor
timestamp: January 19, 2024, 12:24:25 am
content: Quote from: waoj on January 18, 2024, 10:15:44 pmIf you have 3 phases, say, 50A per phase, lose one of those phases, and switch the 50A load from the dead phase to a live phase, that live phase may potentially have 100A on it, which will no doubt trip the breaker on that phase. If that triggers another relay and connects your dead 100A load to the remaining active phase, that remaining phase will now have 150A on it, which will definitely trip that breaker!In other words, it seems a pointless exercise all round.(*and yes, I know 50A + 50A + 50A does NOT add to 150A. It just seemed easier to explain without the vector math).

author: David Hess
author_rank: Super Contributor
timestamp: January 19, 2024, 01:32:21 am
content: The relay is break-before-make so the phase of the inputs does not matter. They will never be connected together and the relay's voltage rating is sufficient to isolate them.Now what the power supply thinks of having its input switched in phase by 180 degrees when the relay switches is a different matter, however an off-line switching power supply should not care. The big transformer of an off-line linear power supply would object if it saw two positive peaks in a row.

author: waoj
author_rank: Contributor
timestamp: January 19, 2024, 05:15:03 am
content: When I said Balancing load, I did not mean balancing load at the moment of an outage if one phase fails. I meant twice a year if you looked at loads and wanted to balance, you could in theory use a relay to relocate a load from one otoanother phase without rebooting it.Quote from: David Hess on January 19, 2024, 01:32:21 amDid you mean to say off-line twice? electronic computer PSUs probably do have transformers in them, but big transformer, do you mean like an HVAC 24vac transformer?

author: Miyuki
author_rank: Frequent Contributor
timestamp: January 19, 2024, 08:03:50 am
content: To switch between phases you need either two relays and delay when they are switching or use a big contactor capable of disconnecting full current with interphase voltage.I have switches for an entire house, switching between a 3-phase distribution and a 1-phase inverter. It uses 3 contactors with 22 contact arrangements, every time 2 contacts are connected in series. It works fine, but I have a 230V system so it can reach 460V AC between phases in the worst case.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 19, 2024, 12:57:31 pm
content: For a switching supply that can operate from either L-N or L-L voltage, use two bridge rectifiers to combine the AC inputs. (No output capacitor is necessary since the PSU already expects a pulsing voltage.) Then add relays on the AC inputs to ensure isolation when disconnected.

author: David Hess
author_rank: Super Contributor
timestamp: January 19, 2024, 03:59:01 pm
content: Quote from: waoj on January 19, 2024, 05:15:03 amYes, I meant off-line twice. "Off-line" refers to a device which is directly connected to the AC power line.An off-line linear power supply has a big 60 Hz transformer. If the two AC inputs are 180 degrees out of phase and switched with the relay, then there is a possibility of applying two AC half cycles of the same polarity to the transformer will will cause saturation and excessive magnetizing current. The transformer and power supply will not be damaged, but the excessive current could blow a fuse or trip a breaker. You might have heard this happen when a transformer is first turned on and occasionally makes a big hum. This would be a worse case instance of that.An off-line switching power supply has a high voltage rectifier followed by either a capacitor bank or power factor correction stage. It will not have any problems with two AC half cycles of the same polarity because under normal conditions that is all that it sees after the rectifier.

author: Algoma
author_rank: Frequent Contributor
timestamp: January 19, 2024, 04:36:56 pm
content: Most server switchmode power supplies filter then convert the incoming AC to DC through a bridge rectifier, feeding the bulk capacitor. It really only cares that there is enough incoming voltage to keep that capacitor topped up enough to keep operating, provided there is no real transient spikes in voltage, or to too long of a voltage dropout, it will just keep operating as normal. A phase switch would only affect the filtering stage for a cycle, and tirgger any UPSs to breifly click over to battery for a moment due to bad power.Worst case is transient delays in the switch over, causes an inrush current to make up for the switching delay. Most UPS units will switch to battery long before the voltage drops completely out on the incoming line. Just keep enough online UPSs that they take up the load while things switch over.

author: Randy222
author_rank: Frequent Contributor
timestamp: January 19, 2024, 05:58:05 pm
content: Quote from: waoj on January 18, 2024, 09:55:34 pmUS homes are 1phase 240v 60Hz center-tap.There are two "poles" in distribution panel (aka breaker box) with the N bar being the center tap and also tied to earth.When one pole is +v, the other pole is -v, pole-to-pole is 240v, pole to N 120v. Every 1/60th sec there's no voltage anywhere at the same time.

author: SeanB
author_rank: Super Contributor
timestamp: January 19, 2024, 07:04:36 pm
content: Here is a product that does this, available off the shelf, and with certification as well.https://bgs.cc/backups-failsafe-ups-power-switcher/

author: sparkydog
author_rank: Regular Contributor
timestamp: January 19, 2024, 08:11:29 pm
content: It almost sounds like you ought to just plug things in via a UPS. When you want to switch phases, just unplug the upstream, twirl the cord for a few second, take a sip of your favorite hot beverage, and plug it back in wherever. (Some of those steps are optional. 🙂) Point is, you don't need to worry about fast switching and the UPS definitely isn't going to care when mains comes back on a different phase after being absent for a second or two. Plus you get better power-loss protection and power stability.If you also need to be able to swap out the UPS or are paranoid about it failing and want a backup, Sean's gadget might do the trick. I imagine as long as the UPS doesn'talsodie at the same time, it's safe to unplug the not-UPS side from one of those at any time. It should also be safe to have the UPS and not-UPS branches plugged into different phases because an online UPS is effectively its own phase, anyway (and probably doesn't like being back-fed mains on its output, regardless).Basically, the combination lets you plug and unplug things willy-nilly without needing to worry about doing so quickly, and you'll only lose power in the event of concurrent interruptions, which would be pretty unlucky since you'd only have a single point-of-failure for a few seconds.

author: waoj
author_rank: Contributor
timestamp: January 22, 2024, 01:17:03 am
content: Quote from: sparkydog on January 19, 2024, 08:11:29 pmI'd agree, but there's not physical rack space to add one. The whole room is on a 480v UPS that feeds local step-down transformers. So I'm not worried about a utility outage. The data-center is going to play stupid cabling games from time to time, and I want to survive those without an outage. Being able to untangle even the power cable for instance.Quote from: sparkydog on January 19, 2024, 08:11:29 pmThat's actually a good thought experiment. Follow me :-)If you originally assemble this on an a phase outlet, A, Neutral, Ground, then the output of the ups will power on, in sync with A.When you move the UPS input cord to a B phase outlet (B, Neutral, Ground) it will still be 60HZ, but the timing will not match.Right before you plug in the ups to the wall, the output is still in sync with A. So if/when it switches off battery, the output wave will jump a 3rd of a cycle. Or the ups will have to do some elaborate shift to 59 or 61hz for 20 seconds to get in sync, which I've never seen one do to avoid the jump of the wave form.Can that jump cause damage or disruption to the load?If not, then my relay example would be way simpler and smaller than a UPS wouldn't it? I could ziptie a relay to the back of the server. I can NOT glue a UPS to the back of the server.Quote from: sparkydog on January 19, 2024, 08:11:29 pmThat gadget is a relay. right? which can be set to be slower right? The manual doesn't talk about phases at all. but it talks about a 200ms power outage. that seems like a long time to me for a computer pulling 400 watts of power or moreQuote from: sparkydog on January 19, 2024, 08:11:29 pmAn ONLINE, double-conversion UPS.... That's a big detail. Yes you're right its output would be its own phase. Though I would hope they try to stay in sync with the utility to reduce weirdness. A lot of UPSes (most) are not double-conversion. They use a relay, so when the utility is okay, they don't waste so much energy, and they last longer.The UPSes I have at home are all "standby", not "online", and they draw about 10 watts each which is really outstanding. They're just trickle charging the batteries with that. And when the utility goes out they snap into motion fast enough the load doesn't go down. A double-conversion ("online") ups would be burning a hundred or more watts all day every day, converting the entire load to DC and back. And they're much more expensive. It would even be cheaper to buy a whole different server chassis with multiple psu modules, than buy a double-conversion UPS and the additional rack space for it. I'm working with 208v btw, for this server, which greatly limits the available models of UPS.I've built relay-failover outlets before for maybe $50 all in for 120v. I just haven't done it for 208v LLG (a-phase, b-phase, ground)

author: johansen
author_rank: Super Contributor
timestamp: January 22, 2024, 01:22:43 am
content: i've watched a sine wave inverter pull in to the grid frequency in approximately 1 second. i bet even the cheap MSW inverters do that, to avoid the transformer saturation problem which can blow up not just fuses, but the relays in the UPS.

author: BradC
author_rank: Super Contributor
timestamp: January 22, 2024, 01:31:02 am
content: Quote from: waoj on January 22, 2024, 01:17:03 amThe bog standard APC SmartUPS has been doing that for 30 years. It slews the output phase until it's in sync before it brings the line back in.

author: waoj
author_rank: Contributor
timestamp: January 22, 2024, 03:36:27 am
content: Quote from: BradC on January 22, 2024, 01:31:02 amI've watched the log output, thumbed through the menus, and read APC manuals for at least ten years now. probably closer to twenty and saw not one mention of this. Would you point me in the direction of something that supports that observation?

author: BradC
author_rank: Super Contributor
timestamp: January 22, 2024, 05:03:38 am
content: Quote from: waoj on January 22, 2024, 03:36:27 amCan't really give you more than that. I have a pile of APC documents that were available on a Russian ftp site 12 years ago. Best I can do is give you a quote from the SmartUPS 3G technical specification :QuoteI've seen pretty significant phase slews while testing and repairing. A quick test with a dual trace CRO might be enlightening.Edit: Better quote from a later manual version (first version was 10/11/94. This one is from 31/3/99)