How to test if a UPS is isolated? (continuity on live circuits and RCD question) - Page 1
Forum section: electronics / beginners

author: Malvineous
author_rank: Contributor
timestamp: March 13, 2018, 01:35:04 pm
content: Hi all,I have an old dual conversion UPS (meaning the inverter runs all the time, even when powered by the mains) and I'd like to work out whether the output is electrically isolated from the input. I suspect it isn't to prevent issues interconnecting equipment powered by other devices, but I'd like to know for sure.I was wondering whether I could do this using the continuity test on my multimeter (an EEVBlog BM-235.) The complication is that when the unit is off, all the relays disconnect everything so I can't get a reliable reading. I need to test it when the inverter is actually running off mains power.I thought the continuity tester should probably work even when using it on live AC wires, as the multimeter is electrically isolated by virtue of running off batteries, however on page 10 of the manual it says "CAUTION: Using resistance and continuity function in a live circuit will produce false results and may damage the instrument."I am not quite sure what this means. Does it mean that if you were to perform a continuity test between mains active and neutral then the meter would see 240VAC and break, but if you performed a continuity test between input live and output live then you should be ok, because you will either see the same voltage at both ends (a difference of zero volts for the meter) or there will be no circuit/continuity? The live connections on the input and output are likely out of phase but this implies they are isolated from each other so the multimeter would see no continuity or voltage, right? And the neutral would be at 0V at both ends so again it would be ok to test this way?I thought about using the voltmeter function but I also want to know whether the UPS bonds neutral to earth on the output. Perhaps this is done by checking the voltage between output live and both input neutral and input earth? Both should read zero if they're isolated, or 240VAC if they aren't. Correct?I eventually would like to use the UPS to power a number of devices and I am wondering whether I should include an RCD on the output for safety as the UPS only has circuit breakers on it. My understanding is that if the neutral and earth are bonded at the UPS then the RCD is a good idea and will work, however if the neutral and earth are isolated then it could be safer not to bond them and not to use an RCD, as a live-to-earth short will present no immediate danger (and thus any RCD present would not trip.)Very interested in any advice on continuity testing live circuits, other ways to test if UPS outputs are isolated from the input, and whether it's safe or unsafe to leave things isolated or bond neutral and earth and use RCDs. Thanks!

author: oldway
author_rank: Super Contributor
timestamp: March 14, 2018, 10:57:51 am
content: Welcom to the forumNobody answered your question because it is nonsense.You should read more informations about how an UPS works.Load is switched on main ac power or on the inverter.Only one phase is switched , the other (or neutral) of both main ac supply and inverter are always connected together.Thus output power circuits of an UPS is never "isolated" from the mains input.Another important thing: when you ask a question on a technical forum, doe not forgot to give all the needed technical informations, as the brand and model of UPS, where whe can download the manual you speak about, .....NB: about RCD: as an UPS generally feed a critical load which can't be interrupted, you can't use an RCD with an UPS.....but this is to be discussed .....

author: llkiwi2006
author_rank: Regular Contributor
timestamp: March 14, 2018, 11:12:19 am
content: Quote from: oldway on March 14, 2018, 10:57:51 amBut in his question he said he has a "dual conversion UPS" which I suppose is one of those online UPS that convert AC to DC (connected to battery) and the back into AC to feed the load. So in theory it could be isolated.Now to check if it's isolated you can first when it's powered off and disconnected check if the output is electrically connected to the input (using ohms mode). Then when powered on check voltage between output and earth. Ideally you would use a multimeter with a low-z mode (bm-235 should have that) so capacitive leakages (e.g. through class y caps) won't screw with your reading. If you are on a budget you could probably do something like connect a neon lamp between ups output and earth, but I definitely don't recommended doing that.Also ohm mode on a multimeter won't work where there's something driving a voltage difference across the two points you are trying to measure, since the multimeter works by putting a voltage between the two points and measuring the current, so if there is an external voltage it will screw with the multimeter (and potentially damage it), and produce incorrect readings since there is actually a different voltage across the two points than what the multimeter thinks it has put there.EDIT: I've done measurements with a 10k resistor jammed into the multimeter jacks between + and - to make a poor man's low-z mode. But I did that knowing there is no live voltage between the two points, and was just curious about how much current the class y caps were leaking. Leaving metal with live voltages sticking out of your multimeter is definitely a bad idea, so don't do that unless you know what you are doing.

author: BradC
author_rank: Super Contributor
timestamp: March 14, 2018, 12:23:33 pm
content: Quote from: oldway on March 14, 2018, 10:57:51 amBzzzzt. Wrong. Thanks for playing.Maybe in the *one* UPS you've examined that *might* be the case, but there is a whole variation in UPS configurations out there and your blanket generalization applies to a relatively small proportion of them (if any).Now, grab any one of the actual circuit diagrams of any APC Smart or BackUPS floating around the net and have a look.You will find that in most common UPS configurations both active and neutral are switched. If you cared to probe you'd find that indeed in passthrough mode the neutral is generally at earth potential, but you'd also find that when on the inverter the active and neutral are both way above earth potential. There is often (like I've *never seen it*) no tie between neutral and earth on the UPS side of the incoming supply disconnect relay, and indeed poking a meter between them on the output sees lots of fun voltages.Just picking a random, I've attached a clipping the input from an APC BackUPS 500 (APC are notoriously precious about their pdfs).

author: Cliff Matthews
author_rank: Unknown
timestamp: March 14, 2018, 02:25:50 pm
content: Yup, it would appear the Back-ups 900/1250/2000 also lift the neutral (in this case Relay 4)

author: oldway
author_rank: Super Contributor
timestamp: March 15, 2018, 11:43:23 am
content: @ Cliff Matthews and llkiwi2006: configuration of the UPS is well specified: it isan old dual conversion UPS (meaning the inverter runs all the time, even when powered by the mains), that'swat whe name ON LINE UPS.APC BackUPS 500 and Back-ups 900/1250/2000 are NOT on line UPS.I worked with two major manufacturers of on Line industrial UPS and I never seen any isolated UPS.There is even a good reason for that: when transfering the load from the main to the inverter, there is sometime a short time of paralleling between inverter and mains to reduce the transients.EDIT; this happen also during transfer from inverter to main to clear a load fuse in the case of a short circuit ...It should also be too expensive to switch also the neutral and this give no advantage at allAs I wrote, if Malvineous should have given informations about the brand and model of his UPS, whe would not lost our time to discuss this problem.But I maintain that I never seen an on line UPS with output isolated of the main power supply.....This question is ever non sense because the UPS can switch the load on mains if the inverter fail.It is then obvioulsly not isolated.If you want to isolate your output from main, you have to use an isolation transformer to feed your UPS.

author: David Hess
author_rank: Super Contributor
timestamp: March 15, 2018, 12:17:46 pm
content: Quote from: oldway on March 14, 2018, 10:57:51 amAt least in the US, this requirement has changed at least once.http://www.apc.com/us/en/faqs/FA156549/I checked two different online UPSes; the Powerware 9120 has a continuous neutral connection when operating on battery power and the Liebert GXT2U has a floating output when operating on battery power. I know the Powerware is newer than 2000 and suspect the Liebert is older so their operation is not consistent with the confusing note from APC linked above.As far as testing, I measured the AC voltage between the output neutral and input ground with a load attached. The X and Y capacitors in the load formed an AC voltage divider yielding the behavior APC described.

author: Cliff Matthews
author_rank: Unknown
timestamp: March 15, 2018, 01:30:04 pm
content: @ Oldway - I think on older UPS's, we could say there's 3 types (4 if you count old style ride-thru/diesel or otherwise types).1) Truly off-line - Where the inverter is off, output transfo is off, and only sense circuitry is tracking (typical under 500VA?)2) Interactive - Where inverter is on and phase-tracking, running a larger (less responsive?) output transfo(and thus, can do boost function when/if required)3) Truly On-line - Where the inverter carries the load 100% of the time (I'd think this, the most inefficient in older designs).I have a Liebert PowerSure Interactive 1000 I've never cracked open, along with about 10 other APC, etc. models (I kind of collected these dead battery MOSFET wonder's in my IT days..), but I bet this and the APC i listed would also fit into type 2.I'm not up on newer unit's, but I notice more classes listed on energystar.gov's site (some pages I'll read, time permitting):https://www.energystar.gov/products/office_equipment/uninterruptible_power_supplies

author: oldway
author_rank: Super Contributor
timestamp: March 15, 2018, 02:18:25 pm
content: All ON line UPSs have a dynamic bypass that automatically connects the mains voltage to the output if there is a failure or overcurrent of the inverter. ..... So the question of an isolation of the output of the UPS does not make any sense.The output can never be considered as isolated from the network ...... there is ALWAYS a serious risk of electrocution or electric shock ....The only solution is to diconnect the UPS from ac mains voltage and use it only as a simple battery-powered inverter.

author: Jeroen3
author_rank: Super Contributor
timestamp: March 15, 2018, 02:36:14 pm
content: To see if it's isolated, put it in bypass and megger the inputs to the output.If it is, >1M due to big isolation transformer inside. If it isn't, <1 Ohm.

author: Malvineous
author_rank: Contributor
timestamp: March 18, 2018, 08:31:57 am
content: Quote from: oldway on March 15, 2018, 11:43:23 amMy apologies, I didn't think it was relevant as I was more interested in how to perform the tests myself rather than having people research my specific unit for me. The one I am using is an APC SmartUPS SURT5000XLI but I plan to use others in the future which I haven't bought yet, so I don't know what those models will be. This is why I'd like to understand how to perform the tests myself.Quote from: oldway on March 15, 2018, 02:18:25 pmYes, this UPS has a bypass mode that connects the input mains directly to the output when it detects a failure, and when operating in this mode, the input certainly is not isolated from the output. But when the UPS is operating normally, you say it is still not isolated, but how can I check to confirm this? If there's one thing my life experience has taught me so far, it's that you should never assume and always check for yourself.Quote from: oldway on March 15, 2018, 02:18:25 pmThis is actually what my intention is, with solar cells feeding the batteries and the UPS running as a simple battery-powered inverter. The issue is that I don't know how I should deal with the safety earth in this situation. Do I need to install an RCD or not? If the UPS bonds output neutral with output earth then yes, and RCD would be necessary. But if the UPS does not bond neutral and earth then an RCD would not work even if one was installed. So then the question is, should you bond neutral and earth yourself and use an RCD, or leave them unbonded and not worry about an RCD?

author: Zero999
author_rank: Super Contributor
timestamp: March 18, 2018, 11:50:15 am
content: Quote from: Malvineous on March 18, 2018, 08:31:57 amIt can't be considered to be safely isolated. It's possible only the phase conductor is switched, with the neutral connected all the time and even if both conductors are switched, there's a huge risk it could go wrong and not be isolated.There's a list of stringent rules and criteria a device must meet, before it can be considered to be isolated from the mains and there's no way it will meet them.

author: oldway
author_rank: Super Contributor
timestamp: March 18, 2018, 03:36:28 pm
content: If you use the device as ups, follow the manufacturer's instructions regarding installation and safety.http://www.apc.com/salestools/EALN-79CR5M/EALN-79CR5M_R3_EN.pdfhttp://www.apc.com/salestools/ASTE-6Z8LUC/ASTE-6Z8LUC_R11_EN.pdfAs far as rcd protection is concerned, a customer asked me this question while I was working for a major industrial UPS manufacturer.Indeed, according to the standards of low-voltage installations in Belgium, protection by 30mA rcd is mandatory.However, exceptions are provided, and critical load feeding is one of those exceptions.In addition, a UPS generates many harmonics and therefore requires a heavy input filter.During transients of the input voltage, this filter can cause a peak current to earth sometimes greater than 30 mA and thus cause unwanted triggering of the rcd .... This is not compatible with the function of a UPS .Similarly, it is not recommended to install an output rcd from the UPS depending on whether it is supplying a critical load that can not be unexpectedly interrupted.So, unless local regulations and laws dictate, no rcd with the UPS.On the other hand, if you use the UPS as a simple UPS running on battery without connection to the power grid, the rules applying to UPS no longer apply.It will first be necessary to check if such operation is possible.Other problems arise: Can the battery be grounded?Can the ac output be grounded too?One may wonder if it is interesting to put the output ac to the ground.In a low-voltage installation, the neutral of the high-voltage transformer is always grounded for obvious safety reasons.Indeed, in case of transformer insulation fault, the high voltage could end up on the low voltage network if there was no secondary grounding.But in this case, such a risk does not exist.Therefore, I advise not to ground the output of the inverter.