Puzzled about Online UPS + GFCI Behavior - Page 1
Forum section: electronics / beginners

author: DrDeke
author_rank: Contributor
timestamp: September 30, 2018, 12:08:28 am
content: The following question came up in conversation with a friend recently:"What if you took an online (double-conversion) UPS, plugged it into a GFCI (RCD)-protected receptacle, turned it on, plugged a GFCI tester into one of the UPS' output receptacles, and pressed the button on the tester? Would the GFCI on the receptacle supplying power to the UPS trip?"After discussing how we think GFCIs, double-conversion UPSes, and GFCI testers work, we both came to the conclusion that nothing would happen when the test button was pressed. Then we went and actually tried it and, to our surprise, the GFCI trips every time.The UPS we are using (a Liebert UPStation GXT1500RT-120) has a block diagram in itsmanual:We're in the United States, so the utility power at the receptacle comes in on three wires, "hot", "neutral", and "ground". The neutral and ground wires are bonded together at the building's service entrance, and the hot wire has 120 V RMS on it.The wall/utility receptacle has a built-in GFCI. My understanding of how a GFCI works is that it measures the difference in current flowing through the hot and neutral lines in the receptacle and, if this difference exceeds a few mA, trips, interrupting power to the circuit.My understanding of how an online/double-conversion UPS works is that it it draws AC utility power, converts that power to DC, and uses the resulting DC power to maintain some batteries in a charged state and to power an inverter. The inverter's output is connected to the output receptacles on the UPS. The output receptacles of the UPS are powered by the inverter during normal operation (when utility power is available) as well as during utility power failures. The only time there would be a direct AC path between the [hot wire of the] UPS input and output receptacles would be if the UPS is in bypass mode. This particular model of UPS has an LED indicator on the front panel which illuminates when the UPS is in bypass mode, and it was not in bypass mode during any of our testing. As far as I understand, all of this is consistent with the block diagram from the UPS' manual.My understanding of how an external (meaning not built into the GFCI device) GFCI tester works is that when the button on the tester is pressed, a resistor is connected between the tester's hot and ground prongs. This causes less current to flow through the tester's neutral prong than is flowing through its hot prong, which should cause the GFCI the tester is plugged into to trip.I don't have a detailed understanding of how an inverter works, and the block diagram is a bit light on details about how the neutral and ground lines are connected (if at all) to the inverter.It seems like the neutral line would have to be connected to the inverter for it to work. If this is the case, and if the neutral and ground lines were bonded to each other inside the UPS, then i can see how current would flow from the inverter's hot line, through the tester, out the tester's ground line, then back to the inverter through the ground/neutral bond inside the UPS. But I don't see how this would result in different amounts of current flowing through the hot and neutral lines in the receptacle that is feeding power to the UPS, and thus I don't understand why the GFCI in that receptacle would trip.If any of you have an idea as to what might be going on here, I will be very interested to hear your explanation!

author: sokoloff
author_rank: Super Contributor
timestamp: September 30, 2018, 12:14:23 am
content: Given that’s the block diagram, it seems clear that the neutral and ground are pass through and the tester that causes some of the current to return on the ground leg will trip the GFCI.(I’d have reasoned the same way you did before testing and prior to looking at the block diagram.)

author: DrDeke
author_rank: Contributor
timestamp: September 30, 2018, 12:21:37 am
content: But since the inverter is the "source" of the current flowing out of its hot wire, in order for any current to be able to flow "out" of the tester's ground wire at all, wouldn't that current need a path back to the inverter? I guess I'm not seeing how that path would involve the hot or neutral wires of the receptacle supplying power to the UPS' rectifier.

author: DrDeke
author_rank: Contributor
timestamp: September 30, 2018, 12:27:16 am
content: Oh! I guess if the neutral wire is connected to the inverter (which it seems like it must be), but the neutral and ground wires are *not* bonded inside the UPS, then the current would flow out the inverter's hot wire, into the tester, out the tester's ground wire, through the UPS on the ground line, into the wall receptacle's ground line, back to the building's service entrance at which point it is bonded to the building neutral, then back down the line to the receptacle, through the receptacle's neutral wire, into the UPS, where it would return to the inverter.If this is correct, then the current coming "toward" the UPS on its supply neutral would offset some of the actual load current returning toward the utility from the UPS, which would definitely cause the GFCI to trip. Perhaps that's what's going on...

author: sokoloff
author_rank: Super Contributor
timestamp: September 30, 2018, 01:01:52 am
content: Yes. That is my assumption.

author: drussell
author_rank: Super Contributor
timestamp: September 30, 2018, 02:23:09 am
content: Quote from: DrDeke on September 30, 2018, 12:08:28 amYes, absolutely it would trip.Any current returning to ground rather than via the neutral will trip the GFCI. A GFCI/RCD is simply a current-imbalance sensor. Anything (more than the threshold of a few mA) that flows "out" the hot that doesn't return via the neutral will cause it to trip.If itdoesn'ttrip, you have a faulty GFCI or a wiring issue somewhere.

author: David Hess
author_rank: Super Contributor
timestamp: September 30, 2018, 04:02:45 am
content: Not all UPSes pass neutral through. There was a change in this requirement a few years ago. They can be identified by measuring the hot and neutral voltages to ground which will be 60 volts (floating) instead of 0 and 120.For the ones that do pass neutral through, what do they do if neutral and hot are reversed?

author: drussell
author_rank: Super Contributor
timestamp: September 30, 2018, 06:01:34 am
content: Quote from: David Hess on September 30, 2018, 04:02:45 amInteresting. I have never seen a UPS that was also some kind of isolation transformer.It would still blow a GFCI though, if you send any current back through the ground since that current is not returning through theinputneutral, even with an isolation transformer.If the UPS is also isolating ground, thenQuoteMost UPSs that I've seen will just show a building wiring fault and refuse to even power on if hot and neutral are reversed.

author: SeanB
author_rank: Super Contributor
timestamp: September 30, 2018, 11:56:57 am
content: The UPS in the diagram is line interactive. It does do dual conversion, but notice the block called dynamic bypass, which is normally going to have the input and output connected directly via a IGBT or relay switch. If power fails, the dynamic bypass will open around 1ms after the first missed mains zero crossing without voltage rising above 20V event, and the inverter stage will then use the stored energy in the HV stage to start supplying the load, and the battery to main bus converter will come out of standby to start charging the main DC bus, or will change mode from acting as charge controller that floats the battery bank to inverter using the battery bank. Simple enough to do, just requires that all diodes in the unit be active switches with appropriate high side drivers.Thus the input and output are connected, and can supply imbalance current to trip a RCD test unit. The only units that do not have any connection are true dual conversion units, which have a transformer between input and battery bank, another transformer between battery bank and the output, and which do not have any bypass capability at all, except by having a huge mechanical switch that disconnects the output from the UPS and transfers it to the incoming raw mains directly, switching both line and neutral ( or all 4 wires in a 3 phase system) together with a break before make contact.

author: 6PTsocket
author_rank: Regular Contributor
timestamp: September 30, 2018, 02:34:02 pm
content: Can somebody please tell me what is the reason for double conversion? Why can't the battery charger, battery and inverter be simply bypassed all the time there is AC available and kick in when power fails? Why would you need inverter AC power at any time there is AC available from the mains? Why is the bypass " dynamic", whatever that means? Thanks.Sent from my SM-G900V using Tapatalk

author: Messtechniker
author_rank: Frequent Contributor
timestamp: September 30, 2018, 03:02:51 pm
content: Facing a similar problem too. I was planning for a small UPS in case the electricity for may gas central heater fails leaving me cold. The supply of gas is ensured in the case of a mains power blackout. They use gas operated gas pressure controllers.So me thinks a small UPS might be nice since my central gas heater draws only 17 W max. The problem is, however, that the flame sensor is of the ionization detection type, thus needing a permanent neutral connection. Now if I insert an UPS, the GFCI is likely to blow - see above.I have asked several professional electricians (one old hand and one with best marks fresh from professional training (Elektromeister)) to recommend a suitable UPS for this scenario. They simply do not have the know how. Another one said "simply try".Well I'm not going to mess around with gas apparatus not knowing what I am doing.As to my question how they install such systems in hospitals, for example, they said. "We get a complete package from the manufacturer and simply install that".. Go figure.So I am still at a loss what to do.

author: DrDeke
author_rank: Contributor
timestamp: September 30, 2018, 05:16:10 pm
content: Quote from: SeanB on September 30, 2018, 11:56:57 amUnless I'm fundamentally misunderstanding something (which is possible), I don't think that's how this UPS works. I have put multimeters on the input and output receptacles of the UPS and watched the input voltage fluctuate over time, while the output voltage remains constant. This suggests to me that the output is being powered by the inverter at all times. The section in the manual describing the dynamic bypass function also seems to suggest that this is how it operates:"The UPStation GXT provides an alternate path for utility power to the connected load, in the unlikely event of a UPS malfunction. Should the UPS have an overload, over temperature, or UPS failure condition, the UPS automatically transfers the connected load to bypass. Bypass operation is indicated by an alarm and an illuminated Bypass LED."Also, for what it's worth, you do hear a mechanical sounding "click" from the unit when putting it into or taking it out of bypass mode.

author: janoc
author_rank: Super Contributor
timestamp: September 30, 2018, 05:31:32 pm
content: Quote from: 6PTsocket on September 30, 2018, 02:34:02 pmIt can - that's called an "offline UPS". However, those are only the cheapest and crappiest units because if you need also power conditioning and seamless switchover when the mains fails, they can't deliver.See e.g. here about the differences:https://www.upspower.co.uk/what-are-the-key-differences-between-online-offline-and-line-interactive-upss/

author: DrDeke
author_rank: Contributor
timestamp: September 30, 2018, 05:41:05 pm
content: Quote from: 6PTsocket on September 30, 2018, 02:34:02 pmIn the marketing text, you usually see double conversion UPSes suggested for use with "critical loads which can tolerate absolutely no interruption to their power supply." It is true that the output of a double conversion UPS should remain completely unchanged during interruption or restoration of input power, whereas other types of UPSes can have brief interruptions / phase discontinuities / etc on their output during such events (as shown on the oscilloscope capture inthisthread).That said, I have never personally run across a load which seemed to suffer from the switching time on a standby or line-interactive UPS. I suppose it's possible that some equipment would be damaged by or run incorrectly during those events. Or maybe it's more that some loads are some combination of very expensive and in need of very high availability, and their owners don't want to take any chances.For me personally, I ended up with my double conversion UPSes for a different reason. Some years ago, I lived in a house which suffered fairly frequent power outages (thus wanting a UPS in the first place), and also suffered from chronically low voltages during the summer because the utility transformer serving that and several nearby houses was undersized/overloaded. Throughout the day as air conditioners turned on and the load on the transformer increased, the voltage as measured at a receptacle in the house would decrease and when it got below the UPS's low-transfer voltage (somewhere around 96 V for the UPSes I was using at the time), the UPS would transfer to battery power, thereby removing several amps of load from the supply circuit. The lighter load would cause the voltage to recover slightly, enough to make the UPS transfer to line power. The resulting increase in load would cause the voltage to dip again, sending the UPS back to battery power. This cycle tended to play out until either the UPS batteries were drained or the UPS itself had failed from the constant, frequent switching.After letting the smoke out of several APC Smart-UPSes in this manner, my roommate and I ended up getting a couple of these double conversion UPSes on eBay and never looked back. The one I've still got, the GXT1500, can run on line power all the way down to 73 V (if loaded to 70% or less - 93 V at full load), while maintaining a steady 120 V output. (The input current to the rectifier would, of course, increase as the input voltage decreases, to maintain the input power.)

author: Bratster
author_rank: Frequent Contributor
timestamp: September 30, 2018, 06:08:55 pm
content: Quote from: Messtechniker on September 30, 2018, 03:02:51 pmNot sure if this is hijacking a thread or not,But why do you think adding a UPS would cause your GFCI to trip?in this thread, the only reason the GFCI was tripping is because he was using a GFCI tester which is supposed to trip it.Sent from my Fi Moto x4 using Tapatalk

author: SeanB
author_rank: Super Contributor
timestamp: September 30, 2018, 06:18:24 pm
content: Double conversion is typical in audio and video use, where you do not want even the briefest dip that can cause lights to flicker. Of course there they are bigger units, that run the entire room. Another place is medical, where the briefest mains failure has a chance of causing machinery malfunction that might be attached to a patient and keeping them alive, thus the double conversion for life critical, and simple medical isolation transformers for those that are merely critical and do not have built in battery backups.

author: Messtechniker
author_rank: Frequent Contributor
timestamp: September 30, 2018, 06:34:59 pm
content: Quote from: Bratster on September 30, 2018, 06:08:55 pmBecause I am expecting the UPS while mains powered to cause a current imbalance on the neutral line.Dependent on the construction of the UPS. I haven't found anyone who would reliably recommenda specific type of UPS which will definitely not trip the GFCI.The other problem may be that the UPS disconnects the neutral line from the central gas heaterthus floating it so that it won' t work. Connection of the central gas heaters neutral line to mains neutralis mandatory for operation of the heater.

author: SeanB
author_rank: Super Contributor
timestamp: September 30, 2018, 06:47:00 pm
content: No UPS I have around causes a trip of the mains, especially those that plug in, as a GFCI is a mandatory fitment here on all socket outlets. There are a few that pass neutral through, and some that break it, but most do not break neutral as they are designed for polarised outlet use only.

author: David Hess
author_rank: Super Contributor
timestamp: September 30, 2018, 07:24:57 pm
content: Quote from: drussell on September 30, 2018, 06:01:34 amAPC mentions ithereand I have run across it several times in standby UPSes. I do not know what online UPSes do but they might disconnect neutral when on battery power only; it is not something I have ever specifically checked.QuoteGround always goes straight through.

author: David Hess
author_rank: Super Contributor
timestamp: September 30, 2018, 07:30:04 pm
content: Quote from: 6PTsocket on September 30, 2018, 02:34:02 pmAn online UPS by its very nature suppresses all line irregularities including surges and dips so it might be thought of as a super power conditioner. If the online UPS has power factor correction, then it also corrects the power factor of the load.QuoteSome online UPSes support this mode of operation as well.QuoteAn online UPS has to be able to bypass in the event of inverter failure or if the load exceeds the capacity of the inverter.

author: Bratster
author_rank: Frequent Contributor
timestamp: September 30, 2018, 09:25:07 pm
content: Quote from: Messtechniker on September 30, 2018, 06:34:59 pmif your heater just has a regular plug on it that goes to the wall outlet I would see no problem plugging that into a UPS and then having the UPS going to the wall outlet. That would not cause a GFCI to trip.the only way I could see it doing that is if the heater has some really weird wiring and there's multiple power paths, but I don't see how that would ever be allowed to happen and have it still be a code compliance safe product.Now the issue of the neutral being broken or not that would depend on the type of UPS.If you get a good quality UPS,it will either not break the neutral, in which case everything is fine or it will break the neutral but then it will make a neutral to ground Bond and again everything will be fine.Sent from my Fi Moto x4 using Tapatalk

author: 6PTsocket
author_rank: Regular Contributor
timestamp: October 01, 2018, 02:48:12 am
content: Quote from: DrDeke on September 30, 2018, 05:41:05 pmThanks for rhe explanation. I have no UPS so I know nothing about them. When the power fails I just fire up my 7000 watt standby generator that can carry everything but the central air. It is not seemles. It is more like 10 minutes.LOL.Sent from my SM-G900V using Tapatalk

author: Jeroen3
author_rank: Super Contributor
timestamp: October 01, 2018, 09:48:53 am
content: Quote from: Messtechniker on September 30, 2018, 06:34:59 pmIf you have a high leakage load, it might trip the GFCI when it's on battery due to the high frequency components in the sine approximation.However, unless the UPS has galvanic isolation, it will trip the GFCI/RCD. Even on battery.

author: Bratster
author_rank: Frequent Contributor
timestamp: October 01, 2018, 05:22:10 pm
content: Quote from: Jeroen3 on October 01, 2018, 09:48:53 amHadn't thought about that, I was thinking if there is any sort of leakage it (the heater) would trip the GFCI by itself, but hadn't considered that not getting a perfect sine wave from the UPS would cause it.Although presumably if the UPS is creating sine wave that means it's on battery power and the mains should be off so the GFCI shouldn't trip.But I could see it tripping when the power is just going out or just coming on.Perhaps the solution is a sine wave UPS to keep the heater happy but the output of the UPS is not connected to the heater itself, it would connect to a small transfer switch.so the heater would be on the output of the transfer switch, with mains straight from the outlet on one input and put the UPS on the other and when the main supply drops out the UPS kicks in and the transfer switch drops off, selecting UPS as the input source.Perhaps this conversation should get moved to its own thread so we can better help the heater OP instead of polluting this thread?Not sure what proper etiquette would be.Sent from my Fi Moto x4 using Tapatalk

author: Jeroen3
author_rank: Super Contributor
timestamp: October 01, 2018, 05:38:37 pm
content: The mains not being present doesn't inhibit the operation of most GFCI/RCD switches.

author: Bratster
author_rank: Frequent Contributor
timestamp: October 01, 2018, 06:03:26 pm
content: Quote from: Jeroen3 on October 01, 2018, 05:38:37 pmHow so?if there is no Mains, then there is no power for the circuitry to detect a ground fault as well as being no power to actuate the trigger mechanism to trip it.Sent from my Fi Moto x4 using Tapatalk

author: DrDeke
author_rank: Contributor
timestamp: October 02, 2018, 01:27:01 am
content: Quote from: Bratster on October 01, 2018, 05:22:10 pmI'm not sure what general etiquette here is either, but as the OP of the UPS+GFCI question, I am also pretty interested in the heater person's question and do not mind reading about it in the thread.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 02, 2018, 06:09:42 am
content: Quote from: Bratster on October 01, 2018, 06:03:26 pmThere is not active circuitry in a A or AC type GFCI/RCD. Yes, some expensive variants will have digital indicators. (Eaton dRCM)F or B type GFCI/RCD will have active circuitry since they are also sensitive to DC or high frequency currents.But cheap GFCI/RCD will use the fault current to trip. Some rare parts do use mains to energise the trip coil.This means an earth fault after the UPS, when the UPS does not connect neutral and earth, can trip them.You can even trip an RCD on your desk with an AA battery. Try it.

author: Bratster
author_rank: Frequent Contributor
timestamp: October 02, 2018, 06:17:46 am
content: Quote from: Jeroen3 on October 02, 2018, 06:09:42 amThat's all news to me, I will definitely be checking out some of the GFCI Outlets we use here in the US.Curiosity has been piqued, thank you.Sent from my Fi Moto x4 using Tapatalk

author: Messtechniker
author_rank: Frequent Contributor
timestamp: October 02, 2018, 06:49:48 am
content: After carefully reading the ideas presented in this thread,I have come up with the following circuit for UPSsing acentral gas heater requiring a mandatory neutral connection for itsionization flame detector/sensor to operate.Note: this is just an idea. Do not implement it withoutadvice from a professional electrician!