Why does SMPS trip the RCD? - Page 1
Forum section: electronics / general

author: jdev99
author_rank: Regular Contributor
timestamp: November 03, 2024, 10:32:47 pm
content: I have a Qidi Q1 Pro 3D printer with a heated chamber, which actually worked very well, up till tonight.I was starting a new print job tonight, and wanted to heat up the chamber, when the printer caused the RCD (earth leakage device) of my house to trip.I connected the printer to an isolation transformer and notice that the voltage between Live and earth was 120V and between Neutral and earth was also 120V.The mains supply voltage (in the UK) is 240V.Is this "normal" to have 120V AC split between Live and Neutral to earth?What can be a possible cause for the RCD to trip in this case?I did contact the Qidi support for answers also.The SMPS is a 350w, 24V model: CZL-350D-24. (Czchenglian)

author: tooki
author_rank: Super Contributor
timestamp: November 03, 2024, 11:17:53 pm
content: Quote from: jdev99 on November 03, 2024, 10:32:47 pmWhen running on an isolation transformer, yes, that is not surprising.Take a look at the schematic of a typical mains input line filter:https://www.we-online.com/en/components/applicationguide/appguide_mains_filterSee the two caps at the right, one from line to earth, the other from neutral to earth? With no earth present, as when using the isolation transformer, those form a capacitive voltage divider. So 1/2 of mains voltage is what you’d expect to find.Unfortunately I don’t really have any suggestions on what might have caused the RCD trip. I had one randomly a few weeks ago in my kitchen, but no idea what from, and it hasn’t recurred…

author: jdev99
author_rank: Regular Contributor
timestamp: November 04, 2024, 12:49:25 am
content: Tried to be clever by using the isolation transformer, to avoid tripping the RCD again, just to confuse myself with the 120V split. Should have known, thank you for the clarification. The minute I saw that schematic you linked it was 100% clear. 🤦

author: coppercone2
author_rank: Super Contributor
timestamp: November 04, 2024, 02:34:35 am
content: small power supplies have ghastly inrush currents sometimes. I have a 12A 12V switch on a supply that is 1.5A output maximum (~150W) and it literally caused sparks to fly out of the rocker switch on turn on, it like a 15 or 20A rocker. I saw sparks shoot out the front through the seam. it makes me think the inrush is giganticI never saw behavior that bad before lol

author: tooki
author_rank: Super Contributor
timestamp: November 04, 2024, 10:55:55 am
content: Quote from: coppercone2 on November 04, 2024, 02:34:35 amYep, charging the input cap of a switching power supply can pull a lot of current, especially if by random chance contact is first made when the AC waveform is near its peak.But what does that have to do with an RCD? That’s not an overcurrent device.

author: SteveThackery
author_rank: Super Contributor
timestamp: November 04, 2024, 01:49:51 pm
content: Quote from: tooki on November 04, 2024, 10:55:55 amExactly. I wonder if the OP used the wrong term and meant the circuit breaker. No current to ground = no RCD trip.

author: coppercone2
author_rank: Super Contributor
timestamp: November 04, 2024, 02:25:59 pm
content: I think it get problem from EMI

author: AVGresponding
author_rank: Super Contributor
timestamp: November 04, 2024, 05:45:33 pm
content: Could be a failing "Y-class" cap; a lot of cheaper gear has caps that aren't properly X- or Y-class rated, and may be failing leaky

author: Siwastaja
author_rank: Super Contributor
timestamp: November 04, 2024, 05:54:41 pm
content: Quote from: AVGresponding on November 04, 2024, 05:45:33 pmA lotmight be exaggeration; note it is an actual and serious criminal offense to purposely import and sell such devices which might end up in jailtime; in reality of course rarely that, just large product recall.I have read about such cases and recalls but don't remember personally ever seeing an example. Even semi-shoddy supermarket chains selling random China crap try to do minimum amount of testing to ensure they don't kill their users, which a wrong type of safety-critical Y capacitor does.If you self-imported mains-powered random China crap from Aliexpress/Ebay then you of course are responsible for killing yourself with it. But I wouldn't say "a lot of cheaper gear". I mean, it is similar to saying "a lot of budget cars have no brakes": the situation is notthatbad IMHO.

author: Brumby
author_rank: Unknown
timestamp: November 05, 2024, 04:21:14 am
content: Quote from: AVGresponding on November 04, 2024, 05:45:33 pm^ ^ ^ This was my thinking. ^ ^ ^Since these typically run in pairs (phase to earth and neutral to earth) if one holds good and the other fails leaky ... there's your imbalance.

author: AVGresponding
author_rank: Super Contributor
timestamp: November 05, 2024, 06:26:31 am
content: Quote from: Siwastaja on November 04, 2024, 05:54:41 pmIf you don't have any to explore yourself, just watch a few DiodeGoneWild or Big Clive vids. Yes, it's cheap crap from ebay or AliX, and not from a bricks and mortar store, but there's plenty of it out there

author: SteveThackery
author_rank: Super Contributor
timestamp: November 05, 2024, 10:02:34 am
content: Quote from: Brumby on November 05, 2024, 04:21:14 amDo we know if the OP's power supply was earthed? It doesn't seem clear from his posts.

author: jdev99
author_rank: Regular Contributor
timestamp: November 23, 2024, 09:45:49 am
content: Yes, PSU is earthed and to answer a previous comment, no mistake , it was the RCD (80A, 30mA) that tripped.Since it is a very new 3D printer, I doubt that it is failing capacitors.It only happened that one time, that is the strange/interesting thing.

author: tom66
author_rank: Super Contributor
timestamp: November 23, 2024, 12:06:34 pm
content: It's possible some residual moisture in the power supply caused momentary leakage. I've encountered that when equipment in my cold garage is brought into the warm house as condensation forms; I've tripped an RCD before, but after drying out the kit, it worked fine. You only need 30mA to trip an RCD, or about 10kohm leakage.

author: AVGresponding
author_rank: Super Contributor
timestamp: November 23, 2024, 02:51:29 pm
content: Worth pointing out also that 30mA is the current limit by which the RCDmust trip(for a nominally 30mA unit, there are other ratings); typically they trip quite a bit lower than that, here's the IET general specification:https://electrical.theiet.org/wiring-matters/years/2019/77-september-2019/which-rcd-type/EDIT: Edited for clarity

author: themadhippy
author_rank: Super Contributor
timestamp: November 23, 2024, 05:02:21 pm
content: Quoteand once you add in the other items that maybe leaking the odd ma or 2 it don't take much to get get into trip current terrotery