My first scope is arriving but my home (old) doesn't have mains grounding - Page 1
Forum section: electronics / beginners

author: Fergo
author_rank: Newbie
timestamp: August 08, 2019, 08:50:35 pm
content: Hey everyone!I recently ordered my first scope (DS1054Z) and it's arriving soon. In the meantime, I watched tons of videos about how to proper use the scope and, specially, how to safely use one. After watching Dave's video about not blowing up your scope, which is heavily focused on grounding issues, I was a bit unsure about my situation. I live in an old building where the mains outlets don't have the ground pin.What is the safest way to power the scope and make safe measurements in this situation?I will mostly use the scope to probe low voltage such as uC/TTL/Arduino signals and maybe some 12V signals. From what I saw, I should be fine if the DUT is DC or battery powered, but I will mostly use the DUT connected to my computer with USB (as I do a lot of serial stuff), which can provide some risks as the USB negative is theoretically tied to earth (theoretically because my computer is not grounded either, as the sockets don't have the ground pin).Unfortunately, moving to a more "electrically safe" home is not an option at the moment, and since I live in the 21st floor, neither it is to get proper grounding on my outlets.Thanks in advance. I really appreciate your help.Fergo

author: mindcrime
author_rank: Unknown
timestamp: August 08, 2019, 09:05:02 pm
content: You could get a USB isolator that uses something like opto-isolation. Something like thishttps://www.adafruit.com/product/2107or this:https://www.sealevel.com/product/iso-1-oem-seaiso-embedded-single-port-inline-usb-isolator/could work for you.There are also opto-isolated USB hubs, but they aren't cheap. For example:https://www.sealevel.com/product/hub7i-optically-isolated-7-port-usb-hub/You might also be able to get an electrician to set you up with at least one properly grounded outlet if there's a good earth ground to be had somewhere in the house. Would cost a little bit, but it might be worth it.

author: Fergo
author_rank: Newbie
timestamp: August 08, 2019, 10:00:42 pm
content: Thanks for the quick reply. I'll get one of those USB isolators then.And regarding the scope itself, anything I should be specially aware of since I'm also plugging it on an unearthed socket?The building where I live is actually in the process of renovating all the electrical infrastructure, but that will take some time.Thanks!

author: mindcrime
author_rank: Unknown
timestamp: August 08, 2019, 10:43:15 pm
content: Quote from: Fergo on August 08, 2019, 10:00:42 pmThey do make (at least for US style outlets) little adapters that plug into a 2 pin socket and give you a 3 pin socket, and then there's a green wire (or a grounding tab) you're supposed to connect to an earth ground.In many cases, the center screw that holds the face plate on the outlet is actually an earth ground and you simply connect to that and you're good to go. BUT, there is no guarantee that that center screw is actually properly grounded. So be very careful of making any assumptions regarding any of this. For example, if the center screw *isn't* grounded you may think "I'll just run some wire to this copper water pipe"... now you have to be sure that the copper water pipe is actually grounded, and that the wire you use is rated for the correct voltage and that the resistance of the wire run isn't too high, that the wire doesn't get physically kicked loose, or chewed by a dog, cat, child, etc. etc.Disclaimer: I AM NOT AN ELECTRICIAN and am NOT necessarily recommending you do anything involving outlets and mains voltage. If you do it, I'd recommend consulting an electrician or at least some of the more experienced members here for advice first. There may also be quirks related to the way wiring is done where you live, etc., that I'm not familiar with.Edit: just re-read your post and saw about living on the 21st floor and all. I was thinking you were in a house, just an older one without the earth ground outlets. Yeah, I wouldn't do anything involving the outlets without consulting an electrician who's licensed in your area.

author: Ian.M
author_rank: Super Contributor
timestamp: August 08, 2019, 10:46:23 pm
content: This has been gone over many times before on these forums. Basically, if you use a scope and many other items of mains powered test equipment on ungrounded circuits, there is a significant risk of safety hazards (why nearly all scope manufacturers nowadays tell you*NEVER* 'float your scope' - I wont rehash the horror stories and risks in this post), and a likelihood that their performance will be impaired due to leakage current and noise pickup, and a medium to high risk of equipment damage if you probe or work on any devices under test that are connected to any signal that has a true ground (e.g to any part of your home entertainment system if you have cable, or communal satellite and/or TV antennae.)A *LOT* of the problems can be mitigated byequipotential bonding. Basically you create an equipotential zone where all 'grounds' connect together even that's floating with respect to true ground + the use of a GFCI (RCD in 'europespeak') to trip the mains supply if there is significant leakage current from it. Within the zone, everything works as-if it was grounded but you have to be careful of external signal connections e.g. aerials, cable, network cables and phone lines that go anywhere outside the zone as a hazardous voltage difference may exist between them and equipment 'grounded' to the zone.For a single workbench not near other exposed metalwork, or a wet zone (e.g. near sinks, baths, showers etc.), with an insulating floor or floor covering under and around it (i.e. no bare reinforced concrete floors), that can be as simple as plugging in an extension socket strip consisting of three pin sockets with a common ground into a GFCI protected socket or adapter ad plugging all your equipment into the socket strip. If you need more than one socket strip (not daisychained) each must be on a GFCI protected circuit and you must join their grounds.Its important that *ALL* metalwork that would normally be grounded that there is any risk of coming into contact with near your bench must be tied to your bench ground so if you have pipes, a kitchen sink or 'white goods' appliances near your bench, you *MUST* talk to a locally qualified electrician to find out how an equipotential bonding zone can be set up without breaching local electrical codes, or if you'll need to relocate your bench.Tempting as it is to ground to a metal water pipe, that becomes a severe hazard if done without a full understanding of your local electrical code. e.g. If there is a high resistance joint anywhere below you in the building, (or if a plumber patches in a length of plastic pipe to fix a leak without connecting an adequate bonding wire across the plastic section), a fault in any mains appliances or equipment using the water pipes for ground, or simply with a metal case in contact with a water pipe, could result in all plumbing above the faulty joint becoming live. As you cant control or even know if another tenant has a dodgy fridge or cooker pushed up against a water pipe,*NEVER* trust a water pipe ground in a multi-occupancy dwelling(or anywhere else where *YOU* don't have full responsibility for and control of all plumbing from the water supply company stopcock onwards)!

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 08, 2019, 11:27:52 pm
content: since you going into this business. maybe its time to...not just to protect you, but to protect the IC's from yourself...ymmv.

author: Mr. Scram
author_rank: Super Contributor
timestamp: August 08, 2019, 11:30:32 pm
content: Quote from: Mechatrommer on August 08, 2019, 11:27:52 pmI assume you missed the part about living on the 21st floor.

author: ogden
author_rank: Super Contributor
timestamp: August 08, 2019, 11:41:32 pm
content: Quote from: Mr. Scram on August 08, 2019, 11:30:32 pmSorry, could not resist

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 08, 2019, 11:51:14 pm
content: Quote from: Mr. Scram on August 08, 2019, 11:30:32 pmright. maybe burying the rod in the concrete floor may help? it should be better than nothing, no?

author: Mr. Scram
author_rank: Super Contributor
timestamp: August 08, 2019, 11:57:36 pm
content: Quote from: Mechatrommer on August 08, 2019, 11:51:14 pmYou'd only be moderately dead instead of very dead.

author: ogden
author_rank: Super Contributor
timestamp: August 09, 2019, 12:33:46 am
content: Quote from: Fergo on August 08, 2019, 08:50:35 pmIn such "low voltage works" case get grounded multi-outlet adapter (splitter) where you plug all your equipment including scope and computer. Obviously you have to avoid any works related to mains voltages.

author: Fergo
author_rank: Newbie
timestamp: August 09, 2019, 03:05:21 am
content: Thanks for all the suggestions everyone. They've been very insightful (well, with the exception to that plastic bag ground perhaps).I am aware that my situation is far from ideal. As a hobbyist, I wanna avoid doing stupid things as I don't have much electrical knowledge, despite messing with electronics for quite some time now (don't even know if that makes any sense...). Anyway, as I mentioned, things should get much better next year, when renovations are planned to start.QuoteYou mean to use something like this 2 prong RCD to plug the equipotential power strip to my mains?https://www.amazon.com/OAONAN-Replacement-Interrupter-Protection-Conditioner/dp/B073PRH8H3/ref=sr_1_3?keywords=OAONAN+GFCI+Replacement+Plug+Assembly+2-Prongs&qid=1565318806&s=gateway&sr=8-3I was also giving my situation a little bit more thought and two ideas popped up:1) I have a 120V UPS 2000VA that is not being used for anything atm. What if I disconnect it from the mains so it becomes active and use it to power the scope and eventually everything that I'd be testing, including the PC and it's USB interface?2) If I decide to go with the option of using the scope plugged into the mains and have to do some probing on an Arduino connected to a PC via USB: using my notebook while on battery power instead of my desktop computer should be fine, right? Since the device being tested - including it's power source (the notebook) - are isolated.Again, thank you all for the replies.KR,Fergo.

author: MosherIV
author_rank: Super Contributor
timestamp: August 09, 2019, 06:51:05 am
content: Hi.Since your electrical system does not provide Earthing :None of your equipment will be earthed/grounded no matter what gadget you use/try (only creating an Earth/Ground will work).You should be able to use the oscilloscope without the fear of accidentally grounding a circuit that is not grounded and damaging YOU or the scope or the circuit.Any equipment with a metal case is a potential hazard (if a fault causes live mains to touch the case). Fully plastic enclosed equipment is safe(ish).Just think carefully what you are going to do to the circuit you are measuring with the scope before you start (it is a good habit to form).Try not to perform measurments on mains electricity while you have no ground.Stay safe and happy

author: Electro Detective
author_rank: Super Contributor
timestamp: August 09, 2019, 10:46:58 am
content: Without knowing how the local utility company implement their Earthing system, sinking a wire into soil, concrete or plumbing fixtures may or may not achieve much,and or score floating voltage drama.Maybe the dude next door is playing with electricity too after watching some backyardesque Youtubesperhaps going for it with coat hangers and leaky dumpster grade arc welder that escaped its overdue rendezvous with the metal scrap yardAFAIK+EMMV: in a proper Earth/ground system, a UPS in battery backup mode will 'usually' supply 60 volts on both live pins to give 120 volts,and NO EARTH/ground, unless it sources it from the UPS chassis and or output third pin out of the wall socket.i.e. even with a UPS in backup mode via battery, you still have a non-Earth referenced floating live electrical system that can still bite you, hopefully via 'inverter to output transformer'but not with the full force of a power station feeding a street transformer(but chances are still good for an unplanned demise from the Earth, and consequent burial in it..)How about a 120v isolation transformer with two 60v secondaries with a center tap between the two acting as the third pin ground-earth reference,for a 'better than nothing, hey might even work..' portable 'ghetto Earth'?NOTE: I will delete the above tip if it's not the 'balanced transformer' set up I was thinking of.Otherwise, lose the shorts and flip flops,and enter Float City with gloves, glasses, long sleeves, rubber soles..and someone nearby with a fully charged phone

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 09, 2019, 10:56:59 am
content: Quote from: Electro Detective on August 09, 2019, 10:46:58 amelectric shock happens when there is current path from contact back to the source. in the case of inside the building, it means the building is acting as conductor to ground and back to wherever the current came from. if no return path, this is just open circuit and contact will float along with the electric potential that is being contacted. another food for thought...

author: taydin
author_rank: Frequent Contributor
timestamp: August 09, 2019, 11:08:16 am
content: If that building is heated with a central hot water heater and metal pipes, you can use the metal pipe as a makeshift ground. It won't be a high quality ground, but still better than no ground at all.

author: Mr. Scram
author_rank: Super Contributor
timestamp: August 09, 2019, 12:13:02 pm
content: Quote from: taydin on August 09, 2019, 11:08:16 amPeople already mentioned the issue with that. You don't know who else in the building might put tension on those pipes, potentially exposing yourself to new and unseen dangers.

author: Bud
author_rank: Super Contributor
timestamp: August 09, 2019, 12:52:57 pm
content: You said the building is currently being renovated. Maybe you could ask one of the electricians guys when you see them how the existing system is grounded. Or play a dummy and tell the landlord you are worried about your washing machine not being grounded and let them give you an answer.

author: Jeroen3
author_rank: Super Contributor
timestamp: August 09, 2019, 02:13:14 pm
content: Quote from: taydin on August 09, 2019, 11:08:16 amNo. Many modern piping is not metal anymore. Plus if the pipe suddenly fails to be a proper ground the pipe could be live. You shower would be live. Your faucet would be live.If you own the building and have control over everybody working there, and there are RCD's on everything... then perhaps... but 21st floor means probably not owning the building.Anyway... I dealt with this.Make sure you have an RCD/GFCI, and make sure your DUT power supply is referenced to the scope potential before connecting the DUT. Ian talked about this.The hazard is in connecting two floating things, you will get equalization current, this can vaporize LQFP packages depending on the capacitance behind it.The slight problem though is the stingy to severely uncomfortable feeling if you are somehow a bit grounded and you touch your equipment receiving all Y-cap leakage current of your entire bench with equipment.

author: Fergo
author_rank: Newbie
timestamp: August 09, 2019, 04:07:02 pm
content: Thank you!Quote from: Jeroen3 on August 09, 2019, 02:13:14 pmNo RCDs, only regular breakerslike this onein my apartment's switchbox (not sure if that's the name, but is the place where you have all the circuit breakers and stuff... English is not my first language). RCDs only became mandatory here in 1997 and my building is from early 70s.Replacing the regular power plug of an extension block with something link this can provide a safer work enviroment?https://www.amazon.com/OAONAN-Replacement-Interrupter-Protection-Conditioner/dp/B073PRH8H3/ref=sr_1_3?keywords=OAONAN+GFCI+Replacement+Plug+Assembly+2-Prongs&qid=1565318806&s=gateway&sr=8-3Thanks!KR,Fergo

author: Electro Detective
author_rank: Super Contributor
timestamp: August 10, 2019, 09:17:05 am
content: RCD/GFCI needs an Earth/ground reference to work,otherwise an imbalance between the Live 1 and Live 2 conductors (Active and Neutral) during a fault won't do anything after passing through the internal sensing coilThey are not magic, they need a solid three wire system AND be tested with an external RCD/GFCI tester to ensure they will work.The TEST button on RCDs tests the RCD internals, it does not guarantee a trip on a real incoming fault,nor do most users test it regularly (if at all) anyway.Don't learn it the hard wayI would suggest to have a local MCB breaker and or glass fuse in a plastic CAT friendly box (not the meow kind) to power your gear, so it trips before the main breaker,and maybe saves you and or the gearespecially if the RCD isn't in the mood to play 'save the mancave dude"

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 10, 2019, 10:32:09 am
content: equipotential as Ian.M posted is the term, i googled "localized ground" and got disappointed. attached is probably okaish for lab that needs "localized virtual ground". a warning beeper may help to safety when the virtual ground raised to alarming level, this should make your lab a partial faraday cage. ymmv.

author: bjbb
author_rank: Contributor
timestamp: August 10, 2019, 03:10:07 pm
content: There is too much bad information in this thread. And unless you are able to provide solutions for TT, IT, and TN distribution systems, do not say anything. Perhaps Dave should find an engineer from an accredited body to explain this stuff in one of his interview videos.North American jurisdictions all have something to similar to article 250 of the NEC (NFPA70) for grounding/PE interface requirements. In Europe, reference your harmonized national standard per IEC60364, and BS7671 for the UK. Read the appropriate reference now.

author: Electro Detective
author_rank: Super Contributor
timestamp: August 11, 2019, 12:34:34 am
content: Whatever the local distribution 'system' is, I'd be very hesitant to assume anything electrical has been done right in any 'built to a price' multi-storey buildingwith 'hurry up/anything goes' and or 'make it up as you go...' snafu Earthing,much less any real 'R' testing to see if RCD and circuit breakers will work on a fault on ALL floors.i.e. if you wanna play with electronics involving AC, better get a leg up on safe electrics that power itfirst

author: Kalin
author_rank: Regular Contributor
timestamp: August 13, 2019, 04:15:48 pm
content: I am not an engineer but I am an electrician. Some people have weird ideas in this thread. An RCD or GFCI is a very good idea since if current that flows out through L1 and doesn't return on L2 wil cause it to trip. This is the danger with having no group d at the out let since you can become a path to ground just by touching the metal enclosure of the device. The danger in not having a grounding means at your outlet is that you could have damage or a fault in your device or dut and energize something that shouldn't be, if supplied from a grounded outlet this would cause your breaker or fuse to trip and deenergize the circuit. Since you don't have that the next best thing is to detect when current is flowing outside of the intended circuit and deenergize at that point. This is what an RCD/GFCI will do.Sent from my SM-G965W using Tapatalk

author: Fergo
author_rank: Newbie
timestamp: August 13, 2019, 07:55:25 pm
content: Quote from: Kalin on August 13, 2019, 04:15:48 pmThanks!But from what I've read in other replies, you do need a ground wire at least in the distribution box for the RCD do work. I don't have thatOne way or another. I couldn't be happier. The scope arrived and I've been playing and getting used to it almost non stop on my free time:It's so cool (and yes, the Arduino is connected to my laptop running on batteries). Definitely worth it's price, considering I managed to get it new with almost 50% off.KR,Fergo

author: Electro Detective
author_rank: Super Contributor
timestamp: August 14, 2019, 09:44:16 am
content: Quote from: Kalin on August 13, 2019, 04:15:48 pmOP Fergo states in the post that there is no Earthing,better to not blow money on a RCD that won't work (although the TEST button might)and buy up on PPE instead,and a fast fuse and MCB combo.Personally, in that situation, I would prefer two unearthed lines coming from a properly fused isolation transformer humming nearby,rather than two unearthed lines coming directly from the street transformer via power station,and relying on an aged sticky mechanism 16 to 30 amp breaker to trip at the mains board = good luck and happy barbeque!

author: Ian.M
author_rank: Super Contributor
timestamp: August 14, 2019, 12:48:09 pm
content: No ground at outlets does *NOT* mean the building is free of grounds. Its likely that any metal conduit and pipes go to ground, however their ground resistance is almost certainly too high to safely trip a breaker, and the integrity of any ground they might appear to provide is suspect.A RCB (GFCI) requires a ground referenced supply to operate, but it is extremely unlikely that the supply isn't ground referenced. In fact its virtually certain the neutral is grounded at the supply company's neighbourhood distribution transformer. The only non-ground referenced (floating) supplies you are ever likely to encounter are from isolation transformers or portable generators etc. operated without a ground rod. RCDs don't do anything useful if fed from a floating supply.Using a RCD (GFCI) does therefore increase safety in the O.P's situation as it will trip on any hazardous leakage to ground, even if the ground path is too high resistance to trip the existing circuit breaker or fuse.

author: kamtar
author_rank: Regular Contributor
timestamp: August 14, 2019, 02:49:15 pm
content: Quote from: Fergo on August 08, 2019, 08:50:35 pmjust a little bit of off-topic: most notebooks aren't grounded unless you connect an external monitor or something like that. There is only some minor leakage current as with every switching power supply.

author: Ian.M
author_rank: Super Contributor
timestamp: August 14, 2019, 03:50:26 pm
content: That depends on the notebook or rather its PSU 'brick'. If its got a two pin mains connector you can be certain its *NOT* grounded. However most OEM notebook PSUs with a three pin connector have the output 0V grounded.I say OEM, because there is a flourishing trade in fake replacement PSUs assembled in the far east from a clone of the OEM case moulding and whatever PSU board salvaged from eWaste, of near enough the same output voltage they can find to cram in there and those often omit any connection from the mains connector ground pin. Also beware of fake power leads with three pin connectors on both ends but only wired with two wire cable.Anyway, its easy enough to check - with the PSU unplugged, check continuity from the ground pin to the contacts of the DC output connector. If they aren't easy to probe, plug the DC connector in and check continuity from the ground pin of the mains connector (still unplugged) to the shell of a USB socket or similar on the notebook.

author: Electro Detective
author_rank: Super Contributor
timestamp: August 14, 2019, 11:14:02 pm
content: As stated earlier, ignoring the onboard TEST button for a moment, unless an RCD can be verified with an external tester to simulate a fault,and the RCD/GFCI performs reliably with consistency as per it's time/current specs (and not just a couple of 'lucky' trips)then OP Fergo is wasting money on a false sense of security buying into an RCD in a situation where it may/might/won't/ work, or all three togetherAs it is, even with an RCD in place in a properly grounded system, if I'm playing 'risky business' with electronicalsthere is my own portable fast trip 10>15ma RCD, or RCD/MCB combo close by,so Murphy can stay on holidays and not bother me with cheap travel offers to hot placesIt may be two RCDs to reset if a larger fault trips the main RCD too, but hey, still better value than post zap hospital therapyor zapping the relatives with a funeral billFWIW, you only have to spend maybe one weekend to assemble all the generic bits needed get the safety thing right ONCE,to save you and your gear MANY MANY times.It's actually fun doing it, testing the lot just to see and know what the outcomes and limitations are,and realize after a few tripsand sort of controlled BANGS!or pending snafus that it's not so predictable as you thought to make a system idiot proof.Cheap fast blow inline fuses, even no name knockoff/fakes? (pre-test a couple first!) rated just above the expected current draw,will save you LOTS of money on blunders that are guaranteed to come knocking.I've got an assortment from 100ma up to 10amps, and the best $20 investment I spent years ago.If it saves even one cheap multimeter or project or repair, you just got $20 back and the restAn isolation transformer (or three..) with switchable modes of earth/ground isolation is a great tool, but not for those unfamiliar with those modes.Still, a proper wired 'medical' style isolation transformer is a good bet for 'some' isolation and interference/noise from the mains,even though the earth/ground and neutral output are bonded internally.I use those whenever I can as they will only pump out whatever power their limit is, and some expensive types will regulate and current limit on an overload or dead short.But all that stuff is sort of useless unless you've simple PPE eyewear/face shield, gloves and rubber soles,A bucket of sand, fire blanket and fire extinguisher wouldn't hurt either and won't break your pocketI keep all that stuff in one flaky carton box, which saves on looking around for the bits, and doubles as a traveller tooStay safe and best of luck to OP with the new scope

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 15, 2019, 01:21:51 am
content: Quote from: Electro Detective on August 10, 2019, 09:17:05 amno...Quote from: Electro Detective on August 10, 2019, 09:17:05 amyes if you buy one hung low brand...Quote from: Electro Detective on August 14, 2019, 11:14:02 pmhave you encountered a properly brand working and installed RCD (2 wires LN in LN out), that wont trip during imbalance? regardless of ground line existence or not? afaik it will trip even if current lost in the air..read under the "Purpose and operation"https://en.wikipedia.org/wiki/Residual-current_deviceQuotethe only time you get a shock when current through you from live back to neutral, so no imbalance inside RCD. hence dont use both hand..

author: forrestc
author_rank: Unknown
timestamp: August 15, 2019, 05:21:32 am
content: Quote from: Mechatrommer on August 15, 2019, 01:21:51 amI'll second this.In the USA in particular it is perfectly acceptable to replace a 2 wire outlet with a 3 wire GFCI. The principle here is that a 3 wire GFCI provides more protection than a 2 wire outlet would provide.The only protection you lose in the US system by doing this is not providing a path for leakage currents/shorts to find their way to ground. With a good ground, a loose wire touching the grounded case will result in a trip of the circuit breaker or GFCI. Without the ground, the path will be potentially through a human to the ground, a path which will cause the GFCI to trip.I don't know what the OP's national electrical system looks like, so I can't be 100% sure this is the same in their country. But I can attest that a GFCI absolutely does not require a ground to work.

author: Ian.M
author_rank: Super Contributor
timestamp: August 15, 2019, 06:42:22 am
content: Quote from: forrestc on August 15, 2019, 05:21:32 amThere is a small error in your statement: A GFCI does *NOT* require a ground at its point of installation, but as I said before, it does require its supply to be ground referenced.Nearly all utility company mains supplies in the world are ground referenced.Exception: some parts of Norway and Finland, where due to low ground conductivity (typically due to massive igneous rock), older homes may be on an IT system (i.e. floating) supply.A GFCI wont work on a fully floating supply as it works by using a balanced current transformer to detect any imbalance between the current in the two (or more) live supply conductors to determine if there is a significant leakage current to ground, and, considering a two conductor GFCI directly fed from an isolating transformer secondary that is not connected to ground, unless the secondary itself is faulty, there is no way there can be an imbalance as the current in the wires to the two ends of the winding must be equal and opposite, no matter what fault is introduced on the load side of the GFCI, so it will never trip on any load fault.A GFCI's built-in test button introduces an imbalance by connecting a resistor diagonally across the GFCI terminals, from one conductor on the supply side to the opposite one on the load side, so the test resistor current only flows in one conductor passing through the GFCI. That will trip just that GFCI even if its fed from a floating supply that its otherwise incapable of tripping on. If the test button introduced a resistor to ground, it could potentially trip all GFCIs upstream of it and which one(s) actually tripped would depend on their relative speed and trip time vs current curves.

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 15, 2019, 09:15:22 am
content: Quote from: Ian.M on August 15, 2019, 06:42:22 ambut if one touch either wire downstream of the floating supply, the current will leak to wherever the person is standing to, as long as it is not returned to the other line, it will create current imbalance in the GFCI.

author: Electro Detective
author_rank: Super Contributor
timestamp: August 15, 2019, 11:40:08 am
content: Believe what you will, flog the math and have faith,but if an RCD/GFCI does not respond reliably and repeatedly to an external simulated fault test, and at 180 degrees,good luck.Then there's fault tripping and SMPS noise issues to contend with.If the power system in OPs case is suss, playing with non isolated input oscilloscope and other single ended test gear,he/she needs to be 100% sure a failsafe system in place worksi.e. you rely on an RCD/GFCI to bail you out, it works for a while (apparently...) and one day it either fails to work, or was never going to work,and or the system earthing develops issues over time, or some electricity maintenance friday helper twat that works for beer disconnects it,and the RCD faither may end up in the ground, with the 'unfortunate event' passed off as 'misfortune' or 'bad luck'An RCD is still better than nothing,but nothing with extreme caution is good too, especially when one solo RCD won't work when you need it toFWIW, even the best RCD will just sit there and look onwhile the owner cooks on a live chassis with no working earth ground

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 15, 2019, 01:59:50 pm
content: how about having an RCD, and be extremely careful? well i can think one safest foolproof way, be in a cave away from electricity. my wife is wiser to stay away from all this shit. remember, all this taboo is already there since the OP dwell that apartment, regardless if he buy a scope or not. unearthed refrigerator, unearthed tv, unearthed hair drier unearthed water heater unearthed everything, or the unreliable RCD thereof.

author: Jeroen3
author_rank: Super Contributor
timestamp: August 15, 2019, 02:29:56 pm
content: Quote from: Mechatrommer on August 15, 2019, 09:15:22 amNo significant current will flow aside from the capacitive leakage in the transformer.Quote from: Ian.M on August 15, 2019, 06:42:22 amConsidering some RCD's run amps of current trough their test resistor when their rating is only 30mA, yes, that would be a problem.

author: ArthurDent
author_rank: Super Contributor
timestamp: August 15, 2019, 05:09:04 pm
content: There is only one correct solution to solving this problem and that is to use a proper ground (earth) from the building’s A.C. power source that meets the current local code. Where this doesn’t appear to be immediately possible, anything done will be a kludge to one degree or another. What I would consider doing is constructing your own 3 wire A.C. supply completely isolated from the building’s mains supply and require no changes or additional connections to the building’s A.C. supply. Solar would work but a complete solar system with storage and conversion is totally out of the question and impractical.Here is a possible solution for you to discuss and critique that is a little different than some of the other suggestions above. What I think might work, if you’re just going to power your own little lab, is to buy an isolation transformer with enough capacity to power everything you could connect to the mains supply and then wire the secondary as you would a properly grounded 3 wire A.C. line from the power company. At the transformer secondary one lead would be the high side line and the other secondary lead, right at the transformer, would be the low and also have a ground or PE wire which would be a non-current carrying conductor, just like a properly grounded 3 wire A.C. supply. These wires would be treated just like a regular 3 wire A.C. line with a correctly rated breaker, GFCI/RCD, and outlets on your bench. Because there will be some capacitance coupling you might want to consider three about 330K ohm resistors in series from the core and the shield between the primary and secondary windings to the low primary lead to drain any small leakage - see drawing.This should work o.k. especially if your lab is located where there are no earth referenced conductors coming in from the outside like phone lines, tv cable, or water pipes. The idea is to create what amounts to a space station or clean room type environment that is galvanically isolated from the outside so there is no possible accidental connection to outside earth yet you still have a local 3 wire system for your bench that will allow safety circuits to be used normally.

author: Fergo
author_rank: Newbie
timestamp: August 15, 2019, 07:55:44 pm
content: Quote from: forrestc on August 15, 2019, 05:21:32 amHere's is a schematic of a distribution box in accordance to our code (or as the author said so...), which is mandatory since 1997. My building is from the 70s, so it's nothing like this:In this case, it's a 2-phase system (220V). The red device is an SPD. The big one in the middle is an RCD/GFCI. The other ones are regular breakers (2-phase input).This is an English speaking forum, but if eventually someone is interested, here is our current building code for low voltage installations:http://universidadeniltonlins.com.br/wp-content/uploads/2019/04/NBR-5410.pdf

author: Jeroen3
author_rank: Super Contributor
timestamp: August 16, 2019, 07:33:44 am
content: Does a GFCI also interrupt on broken ground or neutral? Since you all have a ground wire connected to it.I've never seen a RCD in a european store with a ground wire or terminal on it.Is that an extra feature required by code? Since broken ground or neutral is very rare here with all underground wiring.

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 16, 2019, 08:43:02 am
content: it should as per wiki i linked and from logic thinking of how it is constructed, any current imbalance will create non zero magnetic field and open the elecromagnetic coil circuit inside it, it alone doesnt required ground to work, ground wiring is imho only for double fold safety for current return path safely to earth in case wiring fault downstream such as live wire broken inside appliance and touching exterior body. with earth wiring, current flow to ground, gfci tripped, if no earth wiring, no current flow, gfci not tripped, the exterior body just float on mains potential waiting for somebody to touch to complete circuit for current to flow back to earth. without earth wiring, human body is the only mean for current to flow, but if there is earth wiring, both will share the current, either way, as long as there is current lost (non zero net) inside gfci, it will tripped, such as when a person touch bare live line (no appliances needed), i accidentally tripped gfci this way several time, never it tripped due to appliances fault (live touching metallic body) because i think products design are robust enough or i got lucky to not have shabby one hung low items in the house. not sure why is this difficult to be understood that it has to be explained several times. otoh, broken wiring is another matter, gfci should not be blamed for that.ps: why gnd wire is connected to neutral uptream gfci? to create low impedance path to ground localized to that house/area in case neutral line got raised in the middle by lightning, short on the pole or whatever (i've seen corona short somewhere, real magnificent!). by right neutral line is earthed back in the dist transformer, if not, the contractor who installed it should be blamed (at least in our country). if someone installed ground wire to neutral downstream the gcfi, thats why gfci probably not tripped in case electric shock, the current will flow from human to earth back to gfci downstream back to gfci, no current difference, net is zero, gfci will not tripped.

author: Jeroen3
author_rank: Super Contributor
timestamp: August 16, 2019, 10:00:32 am
content: Your GFCI are dual throw and short downstream to ground? I know this is how it can work on high voltage, but for low voltage is a bit weird...

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 16, 2019, 11:59:29 am
content: Quote from: Jeroen3 on August 16, 2019, 10:00:32 amno! dont do that i already explained that just beforeit should be upstream ie before the GFCI to your lab. if you look atreply #21, following mains supply from right to left, earth wiring is connected to neutral before the GFCI, ie after the isolator (the big CB, high current trip for all distribution smaller CBs, leftmost white component in Fergo diagramreply #39) but i dont check inside of my switchbox, whether it should be after or before isolator. i think its better after the isolator since everything (LNE) will be disconnected when isolator is tripped. what is certain, is NE connection should be before GFCI (RCCB). someone may check their certified switchbox to see at which point E and N is connected together. GFCI only for mains supply 110-220V, for non lethal low voltage you dont need any of that.

author: Jeroen3
author_rank: Super Contributor
timestamp: August 16, 2019, 02:04:05 pm
content: I asked why an GFCI's has a ground terminal. Not why the distribution network has a neutral-earth bond. That is obvious.I recently developed an automated tester for IEC 61009/62423 RCD's, and a ground terminal on an GFCI is not something I have seen before.

author: Zero999
author_rank: Super Contributor
timestamp: August 16, 2019, 03:45:33 pm
content: Yes, an RCD is a good idea.If it's not possible to fix the wiring in the building, then your best bet is to use an RCD and connect all metal objects (metal water, heating pipes etc.) and conductors which are supposed to be earthed together with equipotential bonding to ensure it all stays at the same voltage.

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 16, 2019, 04:07:18 pm
content: the my GFCI only 2 input 2 output. live and neutral input, live and neutral output (single phase).ground and neutral tied somewhere inside switch box, that is another story from GFCI.edit: ok you made me work. the neutral line is not connected to my ground, i was talking out of an arse just now. but neutral to ground is only 1.7Vrms

author: Electro Detective
author_rank: Super Contributor
timestamp: August 16, 2019, 11:37:54 pm
content: It won't matter to OP Fergo who is right, wrong, or somewhere in between,if he/she gets zapped, takes out equipment, any suggested wiring hacks affect other building dwellers,or all three scenarios, making for REAL news on the idiot box (TV)There is no guaranteed 'protection magic' buying and installing an RCD/GFCI, unless tested under real world conditions,and tested regularly. I've replaced and tested/pre-tested enough of them to have a bit of a clue..They can fault out in too many ways and display no symptoms of failure or pending failure,unless tested accordingly to verify that the trip current and trip times are up to spec and repeatable,and notes taken including ambient temperature.Every six months or 12 months is a good bet, and if the RCD/GFCI is suss, then swap it out with a pre-tested QUALITY unitmade by ThreeHungLow, rebadged for some former decent manufacturer.AFAIK the main marketing idea is that if the 'victim to be' touches something metallic that's connected to the earth/ground bonded neutral, and the active/live wire together,the RCD will protestand no longer supply electricity,saving the already battling household an unwanted funeral bill,and the hassle of flogging the NLR hobby gear on Ebay to get some money back to eat.FWIW RCD/GFCI can work nicely on the output of an isolation transformer, and not affect electrical codes, insurance policies etc..if done correctly, according to where you are, be it 120v, 240v and what the earthing arrangement isIf not done correctly, RCD will either not work or get cooked internally on the first TEST button flick, and never work again

author: pcmad
author_rank: Regular Contributor
timestamp: September 02, 2019, 08:16:42 pm
content: does your flat on the out side have access to a lighing conductor if so that goes to ground or are your water pipes metal they should also be grounded