Connecting mains line to floor - but no current flows? - Page 1
Forum section: electronics / beginners

author: VooDust
author_rank: Regular Contributor
timestamp: August 09, 2021, 08:03:12 am
content: If I touched the mains line (L), current would flow through my body and the floor back to the point where the electrical distribution would reference Neutral (N) to ground (PE) (or so I think).I assumed then, if I connected my ammeter from Line to my (hardwood) floor, it would read a current and the fault current fuse would trip (it's very sensitive, it trips all the time). But nothing happened.All I could read was a current of ~3uA. I think that's all capacitive current, because it changes whether the probe's cable lays on the floor or is suspended in air (we have floor heating).So why would I get zapped when touching the live wire, but hardly any current passes my multimeter when connecting the same two points?My multimeter surely would act as the path of least resistance (when set to measure AC current of course). I read that AC current begins to be noticeable above 1mA.I then suspected it may have to with the small surface area (pointy tips on the probe vs my feet area) but my further experiments have not resulted in a fruitful conclusion.I then realized that bringing random stuff "hot" by touching it with my ammeter probes falls into the category of "Stupid Shit", so I'm looking for enlightenment without putting my health or (god forbid) someone else's health at risk!P.S: I live in a 220V country, for what it's worth.

author: Ian.M
author_rank: Super Contributor
timestamp: August 09, 2021, 08:28:11 am
content: Dry clean hardwood is a fairly good insulator. Therefore when you touched your probe to the floor you did not complete a circuit and negligible current flowed.You were lucky - if the floor had been damp or if you had touched the probe to a crack between blocks containing conductive dirt, its likely that a large enough current would have flowed to blow your multimeter's expensive current range fuse (usually a specialist high voltage HRC part), or possibly even damage the meter, as there was nothing to limit the current other than the resistance of the floor.All it takes is one previously spilled coke, coffee or other watery spill that seeped down into a crack before it was fully cleaned up, the residue forming a conductive path, or a nail or screw hidden just below the floor surface for your misguided experiment to have a bad outcome, so *PLEASE* don't play 'Russian roulette' with the mains supply!Theoreticallyif you were standing on a good insulator, you could touch mains live without receiving a shock, and specially trained powerline workers do in fact work on live lines while standing on special safety insulating mats or platforms if there is no practical way to isolate the circuit they must work on. However they don't rely on a single insulator - they'll be using insulated tools, and also wearing insulating gauntlets and boots. Please do *NOT* attempt to copy them.

author: Alti
author_rank: Frequent Contributor
timestamp: August 09, 2021, 09:58:48 am
content: Quote from: VooDust on August 09, 2021, 08:03:12 amI am not sure what a "fault current fuse" is in CH but I think that is how you describe a residual current circuit breaker (RCCB) or one of its variants. I would not overestimate its reliability and if it "trips all the time" then this should indicate something is dodgy in there. Be warned, these devices are not designed to deal with dodgyness.

author: Just_another_Dave
author_rank: Regular Contributor
timestamp: August 09, 2021, 10:13:56 am
content: As others have stated your hardwood floor behaves as a resistor in series with the multimeter, limiting the current that circulates between the line and earth. That’s why they usually recommend you to use shoes with rubber soles when working with voltages above 50V, as they increase the resistance of that path reducing the risk of damage if you accidentally touch the circuit while it is connected. For higher voltages additional protections might be required.However, you should take into account that touching a 50Hz line with a voltage above 50V can cause permanent damage. Be careful with your experiments as you could easily get zapped if your circuit breaker fails

author: VooDust
author_rank: Regular Contributor
timestamp: August 09, 2021, 03:02:30 pm
content: Thanks for all the replies, much appreciated. "Trips all the time" was an overstatement, it actuates when it's supposed to, it's really OK.I understand that my floor is a large resistor in series. I just don't understand why the multimeter does hardly read any current (microamperes), yet when I insert my own body's resistance in series, I am 100% sure I will receive a violent shock (10mA+). But I'm not going to try of course.In a document published onwww.nih.orgI read that body tissue acts as a capacitor, such that part of the (damaging) shock current is of capacitive nature, maybe that's a reason for the discrepancies we're seeing.If current passes through the floor, that path would have a resistance measured in ohms, but my multimeter reads "open line" on any solid object that's not a conductor - yet I doubt that object would protect me from shock

author: amyk
author_rank: Super Contributor
timestamp: August 10, 2021, 12:19:37 am
content: Quote from: VooDust on August 09, 2021, 03:02:30 pmI am almost 100% sure you won't. Try touching the floor with the same area your body is in contact with (e.g. using conductive foil or similar), and measure the current that way. If that also results in almost no current, what makes you think your body would decrease the resistance further? Your body does not have negative resistance!Here's a must-watch video on the topic of electric shocks:(Obligatory disclaimer: I am not responsible if you injure yourself.)

author: IanB
author_rank: Super Contributor
timestamp: August 10, 2021, 12:55:32 am
content: Quote from: VooDust on August 09, 2021, 03:02:30 pmThis is not necessarily true, and it can be a cause of accidents. It is possible to touch live parts and not feel anything, and therefore not realize the danger. If you are insulated (for example, wearing rubber soled shoes, standing on a dry floor, not touching any earthed metal objects), then you may not even receive a tingle. Which means that if you do happen to make a current path through your body it can take you totally by surprise and shock you in more ways than one.

author: SG-1
author_rank: Regular Contributor
timestamp: August 10, 2021, 01:07:14 am
content: VooDust, I have felt this too many years ago. Even when well insulated it is very uncomfortable to make contact with a live wire. I believe this is calledsystem charging current. It requires a small amount of energy just to energize a system of conductors & keep them at at the source potential.Think of a house full of conductors, but no loads are turned on. A small amount of electric current (charging current) is required to keep these conductors at the supply voltage. You my friend are the house full of conductors. The supply voltage first tries to raise your potential to + 120Volts ac rms (this was in my case) then down to -120 volts ac rms. This happens 50 or 60 times per second.This would make an interesting experiment, using house wiring of course, not people!

author: VooDust
author_rank: Regular Contributor
timestamp: August 10, 2021, 06:26:52 am
content: Quote from: amyk on August 10, 2021, 12:19:37 amI did in fact try this before, with a metal tool box that was on the floor, current reached 0.1mA, however I wasn't sure if this was capacitive current because of the floor heating pipes.

author: Ian.M
author_rank: Super Contributor
timestamp: August 10, 2021, 07:12:02 am
content: "... floor heating pipes."Their presence probably increased your risk of electrocution by an order of magnitude.*DON'T* PLAY WITH THE AC MAINS SUPPLY!Working on live mains circuits should only be undertaken when there is no alternative, with proper safety training and with appropriate PPE, safety tools and test gear.

author: VooDust
author_rank: Regular Contributor
timestamp: August 10, 2021, 07:33:03 am
content: Thanks, I appreciate the warnings.It's important to understand and learn how an electrical system works. It's not enough to rely on either assumptions or measurements alone. Too many times accidents happen because what you measured is not what you got.Accident sources can be very insidious. For example, working on a 12V car battery battery would not be considered dangerous by some. But make accidental contact with a screwdriver and the sparks come flying.

author: Ian.M
author_rank: Super Contributor
timestamp: August 10, 2021, 07:36:24 am
content: Make accidental contact between a high current 12V feed from a car battery (e.g. to the alternator or starter motor) and chassis ground while wearing a ring and you can loose a finger!

author: radiolistener
author_rank: Super Contributor
timestamp: August 10, 2021, 07:57:02 am
content: Quote from: VooDust on August 09, 2021, 08:03:12 amYour experiments are very risky dangerous, because if your floor will have enough grounding, your DMM will be damaged and there is a high risk to be killed with electricity through floor.So, you're really lucky if you're still alive and wrote that question after such experiment

author: SteveyG
author_rank: Unknown
timestamp: August 10, 2021, 08:24:18 am
content: Your multimeter probe is very small. If the same test were performed with a metal sheet of similar area to your feet and in good contact with the wood the current would be higher. It's also important to note it doesn't take much current to be extremely uncomfortable.Don't repeat the tests however as there are many variables which impact the test. A wooden floor laid over concrete will behave very differently to a wooden suspended floor, as would day-to-day humidity changes etc. As you noted, capacitive coupling effects are also to be taken into account with AC as opposed to just the conductivity of the floor covering.

author: perieanuo
author_rank: Frequent Contributor
timestamp: August 10, 2021, 08:49:52 am
content: Quote from: VooDust on August 09, 2021, 08:03:12 amhi, if you want results, throw some water, smaller the resistance bigger the currentit's just a bad joke

author: Alti
author_rank: Frequent Contributor
timestamp: August 10, 2021, 02:11:52 pm
content: Quote from: VooDust on August 10, 2021, 07:33:03 amor calipers. Don't ask me how I know

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 11, 2021, 08:26:13 pm
content: To TS: it is much safer to use voltmeter (as ammeter) but not ammeter for such experiments. DMM voltmeter has at least 1MOhm (or 10 M) input resistance, so we may almost safely (but it is not safe too) touch a hot wire and a ground or our body.

author: VooDust
author_rank: Regular Contributor
timestamp: August 11, 2021, 08:43:15 pm
content: so curiosity got the better of me and I did what we all knew would do.to make it short, the floor did in fact insulate me and 0.1mA of current flowed through me, which is in line with the large surface area-experiment conducted earlier, where a metal tool box acted as a surrogate for my feet.I still can't tell if capacitive coupling played a role in any of this (probably negligible), but what matters is that a) I could put my initial observations in context with an electrical model and b) I was able to solidify my understanding of that model.Don't worry I approached the experiment in a reasonably safe manner, where the risk was adressed from multiple angles. (In a way, I believe my experiences in cold water diving taught me a fair bit on how to address and mitigate risk).Don't think I'll get complacent after this! Rather the opposite: Now I know that if I get a small tingle from a piece of equipment or cable, I won't think "well this thing has a minor defect, I mean it's powered by a 12V brick, so what gives?". Because that has happened before. But my reaction should be: "Shit, it could be that the carpet I'm standing on just saved my life and it could be that if I now touch something else, this might end badly".Oh yeah and it also puts in context the display of carelessness of a couple of my friends who work as certified electricians.Thanks to everyone who shared their knowledge on this (and, consequently, were right)