Accidentally grounded live wire from UPS line - Page 1
Forum section: electronics / general

author: cmosfxx
author_rank: Newbie
timestamp: July 31, 2021, 11:18:25 am
content: Hello everyone,So... yeah, for the first time in my life I did something really stupid allow me to say.I was trying to take some measurements when I accidentallyshorted live wire to groundfor a split second by touching a metallic object between these two wires coming straight from the UPS line (APC SMART SMC)UPS was NOT in battery mode at that time.The fuse in the circuit breaker for that room (25A 240v) popped, nothing else actually tripped. (there is a 10A A/C unit on the same line)UPS switched to battery mode and all equipment kept working like normal.Now I'm wondering,1. Why the RCD didn't trip. Is the breaker really faster than the RCD on short circuit situations?2. Why the overload breaker on the UPS didn't trip ether3. There is no short circuit protection on APC UPS? Shouldn't the UPS switch off to protect its self?4. Is the UPS ok now or there is a hidden damage somewhere?I have already contacted APC about the incident and they told me if the UPS is working fine and there are no errors then it's fine but you know, something tells me it isn't right.Any thoughts would be appreciated.

author: thm_w
author_rank: Super Contributor
timestamp: August 03, 2021, 09:41:36 pm
content: To verify, you shorted the output of the UPS from live to earth?If you want proper RCD protection you'll need to put one on the output of the UPS right? Because if its running off of the battery your supply RCD is not connected at all.The overload breaker on the UPS would be a very slow thermal overload and would take a while to trip.

author: james_s
author_rank: Super Contributor
timestamp: August 03, 2021, 09:52:33 pm
content: I see no reason for a GFCI (RCD) on the output of a UPS, are you going to use your computer while in the bath? Outside next to the pool? It wouldn't make sense to plug electric gardening tools or work lights into a UPS.The circuit breaker for the room may indeed be faster than the RCD in the case of a large overload like that.

author: thm_w
author_rank: Super Contributor
timestamp: August 03, 2021, 11:55:13 pm
content: Quote from: james_s on August 03, 2021, 09:52:33 pmConsidering the whole reason for this thread was OP was playing with live wiring, the RCD could have some value.

author: james_s
author_rank: Super Contributor
timestamp: August 03, 2021, 11:57:31 pm
content: Well, there is that. You can't protect a person from every conceivable edge case though, IMHO it's better to just not play with live wiring.

author: Red Squirrel
author_rank: Super Contributor
timestamp: August 04, 2021, 12:21:12 am
content: Maybe the breaker just reacted faster than the RCD since it was a dead short?Since UPS was not on battery it would just be passing AC straight through, unless it's an online UPS, which I doubt as those are not cheap or very common as far as small consumer units go.

author: cmosfxx
author_rank: Newbie
timestamp: August 04, 2021, 07:47:16 am
content: Quote from: thm_w on August 03, 2021, 09:41:36 pmYes, I accidentally shorted the output side of the UPS - Live wire to ground.A metallic object dropped on a meanwell PSU straight on the live wire screw and touched the metallic (grounded) case. (these PSUs have no protection on the screws)I've been told you can't install a RCD on UPS ouput since there is no real neutral on battery mode.Quote from: Red Squirrel on August 04, 2021, 12:21:12 amI understand now that a circuit breaker (MCB) is faster than RCD on dead shorts (live to ground)There is a pop-out style of circuit breaker on the UPS but I think these are trip-free (trips at much higher loads than rated) so it didn't pop.The SMC series is indeed a line-interactive type of UPS.Do you guys think the UPS is fine?

author: thm_w
author_rank: Super Contributor
timestamp: August 04, 2021, 08:20:46 pm
content: If its working its probably fine, you can do a full charge and discharge test with a decent load to be sure.QuoteThe RCD is detecting an imbalance between L and N, so it doesn't really matter as long as the supply is coming from L and N.Some discussion here:https://www.iceinspace.com.au/forum/showthread.php?t=82649

author: Red Squirrel
author_rank: Super Contributor
timestamp: August 04, 2021, 08:52:53 pm
content: UPS is probably fine, though the internal wiring is probably not rated for the amount of amps that were involved during the short. The relays MAY be damaged, but honestly I would think it's fine. I'd give it a good test, unplug it with loads to make sure it works, plug it back in charge/discharge it etc.

author: tom66
author_rank: Super Contributor
timestamp: August 04, 2021, 08:59:11 pm
content: 1. Why the RCD didn't trip. Is the breaker really faster than the RCD on short circuit situations?Yes - the RCD has a time-to-trip to ignore transients, of around 10-30ms depending on the type/age/installation. Whereas a circuit breaker usually has no electronics and the short-circuit trip is set by the magnetic field in the solenoid coil, so a short circuit will generally trip it 'instantly' (a few milliseconds plus arcing time)2. Why the overload breaker on the UPS didn't trip etherProbably the breaker on the UPS is a thermal overload breaker only, so it needs to get hot which takes a while.3. There is no short circuit protection on APC UPS? Shouldn't the UPS switch off to protect its self?If it has electronic short circuit or overload protection, it likely applies to the battery output only. If it's anything like older UPSes I used to repair, it was purely based on the battery voltage dropping below a low threshold, as the batteries were usually too weak to support anything more than say 800W load.4. Is the UPS ok now or there is a hidden damage somewhere?It's probably fine if you can't find any obvious defects.

author: ajb
author_rank: Super Contributor
timestamp: August 07, 2021, 06:49:34 pm
content: Quote from: thm_w on August 04, 2021, 08:20:46 pmThe benefit of an RCD/GFI on the output depends on the construction/topology of the UPS. If the output of the inverter is isolated from the equipment ground (except maybe via some suppression caps) then there is no real path for current to escape from the output, because there's no way to complete a circuit from the line output except via the neutral output. If the UPS is a type that has a direct connection between input and output when not in battery mode then an output RCD will still be effective in that mode. If the UPS is an 'online' type that always runs the load from the inverter then the output RCD will never be effective. This is all regardless of any ground connection AT ALL to or through the UPS. If the UPS somehow has its output referenced to ground then an RCD could be effective on the output--I don't know for sure, but I doubt that's a common configuration among cheap consumer UPS systems because doing so would imply certain assumptions about the way the supply or load are wired that are probably too risky for household situations.I assume that building-scale UPS/backup generator systems integrated at the service or panel level use a similar neutral/earth bonding system as utility power does. Those schemes vary with locality, but such UPS systems would be exactly the same as utility power in terms of RCD protection.It's really important to note that if you plug an RCD into the output of a UPS, just because it trips when you press the test button DOES NOT mean that it's providing any protection! The test button just connects a resistor from the output side of the differential current transformer to the input side, which causes the sort of current imbalance the RCD is designed to detect and protect against. It's done this way, rather than connecting the trip test load from line to ground, because it allows the test button to work even if the circuit has no equipment ground* or the line and neutral conductors are swapped, and it ensures that the test button won't trip an upstream RCD instead of the one it's built in to. The only way to test that an output RCD is doing anything useful is to use an external plug-in tester, which DOES route the test trip current to ground, since that's the only way it can cause a current imbalance in the line and neutral conductors, and to test it while the ups is running from the battery.* At least in the US, installing an RCD is the only legal way to fit grounding receptacles on a circuit that doesn't have an equipment grounding conductor (so long as those receptacles are labeled "no equipment ground" as well), so this is an expected configuration.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 07, 2021, 09:02:21 pm
content: I read on Facebook a week or two back about someone who had a UPS that would give him a shock if he unplugged it to test it and touched the prongs in the process. Turns out that UPS didn't isolate the neutral and the load had a ground fault H-G, which means that there was a path for voltage to N-G on the input.

author: cmosfxx
author_rank: Newbie
timestamp: August 08, 2021, 01:18:24 pm
content: Quote from: tom66 on August 04, 2021, 08:59:11 pmI can't seem to find any infos about overload/short protection. If it's like an older model I have, the fuse on the back is just an overload (thermal?) Fuse on input hot side only.Now there is a big problem. The 230v versions, like the one I've got, are missing a critical feature. The reversed polarity warning.Guess what? All this time I had reversed polarity on the input of the UPS.So the input neutral was actually the hot.This is not a problem untill you really need the fuse to play its role.And that's why the onboard fuse never tripped.The UPS is working fine so I'm thinking maybe the relay handled all the power throught it for that time.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 08, 2021, 01:47:22 pm
content: Quote from: cmosfxx on August 08, 2021, 01:18:24 pmUS 208V and 240V are balanced meaning there is no reverse polarity. Therefore, there was originally no need for a reverse polarity indicator and they didn't want to redesign it just to sell overseas.

author: cmosfxx
author_rank: Newbie
timestamp: August 08, 2021, 07:08:30 pm
content: Quote from: NiHaoMike on August 08, 2021, 01:47:22 pmOoh that makes sense now, thanks for the info.

author: james_s
author_rank: Super Contributor
timestamp: August 09, 2021, 05:29:30 am
content: Even if that weren't the case, it isn't the responsibility of the UPS to check the polarity, that's up to the property owner to take care of.

author: cmosfxx
author_rank: Newbie
timestamp: August 09, 2021, 07:13:55 am
content: Quote from: james_s on August 09, 2021, 05:29:30 amIt is if you sell the same unit to europe. Schuko plug has no markings and nothing really prevents you from plugging the cord the "wrong" way.

author: james_s
author_rank: Super Contributor
timestamp: August 09, 2021, 07:12:14 pm
content: Quote from: cmosfxx on August 09, 2021, 07:13:55 amWhat stops you from plugging any other equipment in the wrong way if the UPS is not there? Does everything have a polarity indicator? I'm kind of surprised that the Schuko plug has not been updated by now to at least have a polarity marking on it, a notch could be added easily enough to make polarized plugs while still allowing the old non-polarized plugs to be inserted.

author: tom66
author_rank: Super Contributor
timestamp: August 09, 2021, 09:12:19 pm
content: Quote from: james_s on August 09, 2021, 07:12:14 pmUnearthed European appliances near-universally use double-pole switches for this reason.It's a bit of a loophole for IEC connectors. In the UK and in some other European countries, our plugs ensure polarisation so most IEC connectors use single-pole switches.