No shock from grounded 500VA isolation transformer (Solved) - Page 1
Forum section: electronics / beginners

author: DeLorean
author_rank: Newbie
timestamp: April 19, 2022, 07:46:49 am
content: Hi,for a while now, i'm breaking my head on something that maybe has a simple answer,but i have searched the net, and ask many people that know much more from Electricity then me,and nobody can give me the answer.So i hope that somebody here can give the answer.Before i get reactions about grounding a isolation transformer, i'm aware that this will bypassthe safety feature of a isolation transformer, but this grounding is for testing purpose only.Here in Belgium we have the TN-S net, the Neutral that comes to my house is bounded to earth in the substation,and the earthwire here in house is connected to a copper rod in the ground.Further there is no connection in the breaker panel between this Neutral and earthwire, and if somewherein the house both touch eachother, the 30mA RCD trips, so the grounding is working.Now my question :When i use a 500VA isolation transformer, connect the primary to the 230V mains grid,then the output on the secundairy has no neutral anymore, but 2 floating phases (lets call them L1 and L2 from now).When i connect L1 to the earthwire of a wall outlet, the isolation transformer is grounded.But when i touch L2, that carries 230V and is now earth referenced, i don't feel nothing at all.When i place a DMM in µA range in series with one lead in my hand, and the other lead touching L2, i measure a current of 11µA,in voltage range, i measure 90V in high impedance mode, and arround 4V in Low impedance mode (300K on my DMM).I can light up a incandescent light bulb or a 1200Watt hairdryer between L1 and L2, and between L2 and earthwire, so the transformereis capable of delivering enough current.When i do a body capacitance measuring, with one lead in my hand, and the other lead touch the earthpin of a wall outlet,i measure 150pF. Standing on a floor with tiles and rubber sole shoes.When i use my 30V linear bench powersupply, holding the negative in one hand, and the positive in my other hand,i measure arround 100µA current. So my body resistance is at 30V DC arround 300K.What is the difference between touching the phase of the mains grid, that carries 230V and is earth referenced,and getting a zap, and touching L2 of the isolation transformer that also carries 230V is referenced to the same earth,where i don't feel anything?So what am i'm missing here ?Thanks in advanceGreetings from BelgiumDeLorean

author: capt bullshot
author_rank: Super Contributor
timestamp: April 19, 2022, 08:03:53 am
content: There's nothing wrong with your observation.measuring 11uA matches the 150pF, that is the current that flows through your body at 230V / 50Hz and the conditions you've described.I doesn't make any difference if you touch the line phase directly or the 230V output of the earthed isolation transformer.I expect you'd feel the same nothing if you touch the phase directly (No I don't recommend you doing this).If you got zapped by touching the 230V line, I suppose there must have been different conditions than with your isolation transformer experiment, maybe you were wearing other shoes, or your skin was more sweaty, ...

author: CaptDon
author_rank: Super Contributor
timestamp: April 19, 2022, 01:15:17 pm
content: Wow, TN-S, you guys really do have 230vac referenced to earth!!! That is a sure killer if you screw up, unless protected with ground fault type gear. Here in the states the 240vac system is made from 120-N-120 and so the worst shock to earth is only 120vac not 230/240. When I worked at the hospital in the O.R.'s the systems were 120vac center-tapped with something like 2.2 megaohms from the center tap to earth and a LEM module to measure offset and current attempting to flow to earth. In the U.S. the N.U.E.C. specifies that the 'earth prong' of the three prong plug be oriented at the bottom of the outlet. In our operating rooms the plugs were oriented with earth prong on the top side. This was in case the very fine catheterwires would accidentally fall down on an outlet it would likely hit the ground and be physically prevented from falling farther to the L1/L2 prongs.Don't go grabbing live conductors!! There is no reset button for electrocution victims.

author: CaptDon
author_rank: Super Contributor
timestamp: April 19, 2022, 01:29:17 pm
content: Ouch!!! Now I have been a fool, The N.U.E.C. apparently DOES NOT specify orientation of the 3 prong outlet!!! And to think I took a 4 year electrician journeyman course and we were always taught "The ground pin is on the lower side so it would be the last thing to lose contact if the plug were coming loose from the wall, and also in horizontal mountings the ground pin would be to the left with neutral on the upper side for safety reasons." Somehow we students came to believe this was specified in the code book. I see now that it isn't!!! I don't recall ever looking it up in school, we just assumed it was hidden somewhere in the code and always installed outlets 'ground down'. Geez, never to late to learn I guess, and never make assumptions without verification. I have been humbled by the U.S. code book, which I thought I knew cover to cover??

author: themadhippy
author_rank: Super Contributor
timestamp: April 19, 2022, 01:38:14 pm
content: Quotesounds more like a TT system,earthed earth.Not an earth neutral separate

author: rob77
author_rank: Super Contributor
timestamp: April 19, 2022, 06:57:07 pm
content: Quote from: DeLorean on April 19, 2022, 07:46:49 amyou don't feel anything because your're floating... if any of your body would be touching the ground while another part of your body touching the the phase.. only then you would be zapped.the whole point of isolation transformer is to make the phase floating... so when you touch touch the floating phase while your body is grounded it won't zap you. and that's why you never ground the isolation transformer's output because you're literally eliminating it's purpose by grounding.

author: BrokenYugo
author_rank: Super Contributor
timestamp: April 19, 2022, 07:04:51 pm
content: 1. You're REALLY playing with fire here, please continue this curiosity without involving your body.2. That said, what kind of floor were you on? Wearing what shoes? Leaning against anything with a good path to earth? As rob77 and capt bullshot point out, you were probably floating and only loosely capacitively coupled to earth, so no path for enough current to flow to give a tangible shock or trip a RCD.

author: ebastler
author_rank: Super Contributor
timestamp: April 19, 2022, 07:50:03 pm
content: Quote from: DeLorean on April 19, 2022, 07:46:49 amThis might be what got you confused. Your body resistance from hand to hand is irrelevant for the effect of the 230V which you are wondering about -- since you didnothold the 230V phase in one hand and the ground in the other hand. (And no, please don't try that...)After all, you just touched the 230V phase with one hand, and your connection to ground was via your feet -- presumably while wearing shoes? In that situation, the resistance of your connection to ground will be dominated by the resistance of your shoes and the floor, assuming that both are well-isolating.

author: vav
author_rank: Contributor
timestamp: April 20, 2022, 10:33:07 am
content: Depending on what you write, you do not have a TN-S system, but a TT system. Here in Slovakia we have it so that the supply to the house is TN-C and on TN-S it changes in the house when the PEN is divided into PE and N. If you measured such a small current during the experiment, it was because because you did not have good contact with the ground. Do not continue with these experiments. It can hurt you.

author: DeLorean
author_rank: Newbie
timestamp: April 20, 2022, 12:55:18 pm
content: Hi,Thank you everybody for the quick replies.It seems that i didn't had notifications enabled.I was wrong by mentioning about the TN-S system, it's indeed the TT system.The neutral that comes to the house, is centertapped at the substation, and connected to ground over there,and here at home, the earthing is done by a local copper rod in the ground.Regarding touching the Line (Phase) wire from the mains , that is something i definitely feel enough to get a good zap,but from the output isolation transformer, i feel nothing, no tinkle , nothing.If i hold a 3Watt G9 230V LED, touching one lead, and the other lead to the grounded output of the isolation transformer,this LED light up very very dim.I'm now gonna read every comment and will respond to further questions.ThanksGrtzDeLorean

author: IanB
author_rank: Super Contributor
timestamp: April 20, 2022, 01:18:53 pm
content: Quote from: DeLorean on April 20, 2022, 12:55:18 pmI have more than once touched the live/line wire of 240 V mains in the UK and felt nothing. It all depends on your path to ground at the time. In most cases the insulation resistance of shoes/carpet/clothing is enough to prevent current from flowing. (This is not advice, and don't try this!)Therefore, if I touched only the ungrounded pole of the isolation transformer without completing a circuit I would not expect to feel anything. However, if I also completed a path to ground or the other pole of the transformer then I would be concerned.

author: DeLorean
author_rank: Newbie
timestamp: April 20, 2022, 03:01:53 pm
content: Quote from: capt bullshot on April 19, 2022, 08:03:53 amI have touched in my life many times accidentally the 230V mains, and believe me, even with rubber sole shoes, dry hands and standing on wood floor, you get a decent shock.That why i'm surprised that i don't feel anything when i touch the 230V output of the grounded isolation transformer.That's the reason also of this topic, i do not want to kill myself, but i like to know, where the difference is between both power sources.Quote from: CaptDon on April 19, 2022, 01:15:17 pmPrior before mid 80's (1986 i think) residential electric installations here in Belgium needed one RCD of 300mA for the entire installation.From 1986 and above, there are 2 RCD's needed, one RCD of 300mA for the entire installation, and a 30mA RCD for the wet roomslike basement, garage, laundryroom etc....Here at my home, my installation is from 1975, and i have one RCD of 30mA for the entire house, and if wanted, i can usea portable 10mA RCD when working on electrical devices.About grabbing live wires, when i first touched the grounded isolation transformer, i was not directly, but with a 1M resistor between me and the live wire.After that, i kept lowering the resistor value untill i reached 10K and still didn't feel anything.That was the moment that i quickly tapped the live wire without resistor for the first time.Quote from: IanB on April 20, 2022, 01:18:53 pmYou are right, when the isolation transformer is not grounded, touching only 1 pole, won't do anything (the main purpose of a isolation transformer).But in theory, if you ground 1 pole of the isolation transformer, and you touch the second pole, you will get a zap.But it seems that in my case, even then this is safe.My only concerns about this is, and only in case of IF, that if i'm working on a device that have a hidden Line to chassis fault , that i can make the final pathbetween both poles of the isolation transformer if i touch the chassis and the second pole accidentally.Then i'm getting the full 230V through my body, while if get a tinkle by only touching one pole, i get a warning so that i can't touch both poles at once.Like a mentioned before, this grounding of the secundairy side of the isolation transformer, is for testing purposes onlyand not ment to be kept connected all the time.GrtzDeLorean

author: DeLorean
author_rank: Newbie
timestamp: April 20, 2022, 03:12:17 pm
content: Quote from: CaptDon on April 19, 2022, 01:29:17 pmAlmost all wall outlets are here in Belgium installed with the ground pin on the upper side, because are 90° angled 3 prong cables are designed with the ground on top,so that the cable is point to the floor, and doesn't stress the wall outlets when pulling on the cable.Besides the earth connection, are mains L and N are not polarised. Device cables without earth connection can be plugged in both ways in are wall outlets.GrtzDeLorean

author: capt bullshot
author_rank: Super Contributor
timestamp: April 20, 2022, 03:16:13 pm
content: Quote from: DeLorean on April 20, 2022, 03:01:53 pmBeen there, done that too (accidentally touching live 230V wires, at various conditions, receiving shocks of various strength).Note, there's quite a difference if you receive the electrical shock unexpectedly or you're mentally prepared to feel "something".I'd never touch a 230V live wire on purpose, but I've seen people demonstrating they could arbitrarily touch 230V live wires and at least stated: "no, I don't feel the electricity".From a technical and theoretical view, both the live output of the grounded isolation transformer and the input live wire should be at the same potential in respect to earth. So I'd expect the same experience of receiving shocks of the same strength, no matter which of both you touch within an otherwise unchanged setup. Still: No I don't encourage you to touch the live wire (though IMO it's the only way to confirm the theory of both nodes being equal in shock potential).

author: IanB
author_rank: Super Contributor
timestamp: April 20, 2022, 03:23:20 pm
content: Quote from: DeLorean on April 20, 2022, 03:01:53 pmNo, that's not what I said. If you ground one pole of the isolation transformer and if I then touch the other pole while standing indoors on a dry floor, I will not get a zap.Similarly, if I touch the line conductor of the mains supply while standing indoors on a dry floor, I will also not get a zap. I have done this before (accidentally) more than once in my life, although I do not recommend doing it intentionally. There is no reason to take such risks.Quote from: DeLorean on April 20, 2022, 03:01:53 pmThis is not consistent with my experience. I have done this more than once and have not received a shock. I do not know why your experience is different.

author: IanB
author_rank: Super Contributor
timestamp: April 20, 2022, 03:35:39 pm
content: To add further information to the previous post, you generally need a current of 1 mA or greater through your body to feel the effects. To obtain a current of 1 mA from line to ground at 230 V you would need a path resistance of less than about 230 kΩ. The resistance of dry materials like wood, carpet or concrete is generally much higher than this, being in the tens or hundreds of megaohms range. The capacitance of the human body is also not enough to couple a current of 1 mA by capacitive coupling.

author: themadhippy
author_rank: Super Contributor
timestamp: April 20, 2022, 03:49:20 pm
content: QuoteBut was that proper 240v to earth ,not the girly 240v american version thats really only 120v to earth?

author: IanB
author_rank: Super Contributor
timestamp: April 20, 2022, 04:00:20 pm
content: Quote from: themadhippy on April 20, 2022, 03:49:20 pmIt was the UK mains, touching the red or brown wire.From higher up the thread:Quote from: IanB on April 20, 2022, 01:18:53 pm

author: DeLorean
author_rank: Newbie
timestamp: April 20, 2022, 04:41:54 pm
content: Hi everyone,The puzzle is completed.After reading the comments about touching the 230V mains without getting a zap, i did a final test.I used in decending rank a 480K , 300K, 220K, 100K, 47K and finally no resistor for touching the live wire of the mains (not the isolation transformer),and without no resistor, i don't feel anything and measure also the same 11µA and arround 90V between the live wire and myself.Exact the same measurings as with the isolation transformer.So it seems, the previous times that i got a zap, i must have touched Live and earth, or Live and Neutral at the same time.And because there is only a current of 11µA when touching the Live wire of the mains, a RCD won't trip.As in mather a fact, the RCD here has never tripped when i got a zap in the past, the only times that my 30mA RCD tripped,was when the Neutral or Live have accidentally touched the earthwire.So now i can focus on the "touch protection" that i have in mind, to prevent getting zapped, when accidentally touching both poles of the isolation transformer.I have already done some testing with a BC547 and BD139 in Darlington as a wireless detector.The goal is to let this detector only trip when touching 1 pole of the isolation transformer,and not when a light switch is flipt in the house, or a device like a refrigerator turns on or off.During this test, i have used a 3mm white LED , but the final version will have a relay that interruptsthe voltage to the contactor coil, that then shuts off the isolation transformers output.Here is a video about this basic testing on breadboard :https://youtu.be/ETUVi_Y-cmQBig thank you for everyone that contributed to this topic.GrtzDeLorean