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Trying to understand electric shocks...

  1. Aug 29, 2016 #1
    Hi all. I've studied some physics, electronics and electrical engineering in my time but still can't quite get an answer to a very basic question about mains electric shocks...

    I am referenced to ground. The live (hot) in my country is at 230V rms to neutral, which is in turn referenced to ground and therefore to me. I get that. If I hold a ground rod and touch a live wire with a piddling 1mm of plastic insulation, all is well. If I (hypothetically) touch the bare live wire and nothing else, I get a shock even though I am insulated from ground by a good 20mm of rubber boot sole. Why? 230V (or even 330V peak) isn't enough to drive current through my boot soles, is it? Nothing would happen if I put live and neutral across the boot sole on its own.

    So, is the shock in the second scenario to do with some sort of capacitive coupling? Am I being charged to +330V then -330V 50 times a second? Surely not, as otherwise isolation transformers wouldn't work.

    I'm sure I'm missing something here, and hope this isn't a stupid question.
     
  2. jcsd
  3. Aug 29, 2016 #2

    davenn

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    you shouldn't get a shock as there is no complete circuit

    Dave
     
  4. Aug 29, 2016 #3

    berkeman

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    Dave is correct. You will not get a shock through your insulating boot. Why would you?
     
  5. Aug 29, 2016 #4
    Thanks for the replies. I have had a shock in that situation; not anything that was serious enough to cause muscle contraction but more an unpleasant 'buzz'. From what you both say the answer must be that there was some minimal current through my boots due to a crack or water path.

    That said, is there no significant capacitive effect in minor shocks? If I were to hang from a live overhead line would I feel nothing, even though I'm a bag of salt water?
     
  6. Aug 29, 2016 #5

    berkeman

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    At 60Hz, parasitic capacitance is negligible.
     
  7. Aug 29, 2016 #6
    Many thanks.
     
  8. Aug 30, 2016 #7

    Averagesupernova

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    That's what I thought. Otherwise the question would not have been posed this way. A human is able feel much much less current than what becomes obvious by sizzling, crackling, etc. that would be tell-tale signs of current going from one end of the soles of your shoes to the other in the example you gave in your first post.
     
  9. Aug 30, 2016 #8

    NascentOxygen

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    In this strictly hypothetical situation, there will be a brief pulse of current, and if the timing is such that contact is made at the AC cycle's peak then you may feel this in muscles where your finger contacts the live wire (because that's where skin current density is greatest). An applicable electrical model is a 300V step applied to a series RC circuit, 1.5kΩ & 100pF, though your capacitance would be greater than this if you were standing near an earth plane, e.g., beside a fridge.

    I don't know how sensitive our muscles are to a momentary current spike, so can't say whether this will even be felt. At worst, it might be like a pin prick to your finger.
     
    Last edited: Aug 30, 2016
  10. Aug 30, 2016 #9
    Ah, so there might be some parasitic (self?) capacitance effect. I could have built up some charge that destabilised some sensory neurons or neuromuscular junctions enough for them to depolarise or 'fire'. There are lots of variables to how easy it is to fire a nerve ending - not only magnitude but speed of voltage change, for example. A bit like those old-fashioned chain-flush toilets - you have to pull the chain all the way down but also with enough speed to make it flush...

    The shock was a long time ago and I really can't remember the condition of my boots that day. I was only 16 or so, and wondered for a long time how any current could have flowed through me with the thick insulation I was standing on.

    Thanks again to all - you've cleared up a niggling question I've had for a long time.
     
  11. Aug 30, 2016 #10

    jim hardy

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    Overhead line at what voltage?

    Electrical model of the human body is 100 picofarads in series with 1500 ohms of resistance, connected to the rest of the distant universe..
    .https://www.esda.org/assets/Uploads/documents/FundamentalsPart5.pdf

    As you said if you hold the wire you'll be charged to +/- 230√2 volts at line frequency
    Impedance of 100 pf at 60hz is 1/(2πf X 100 X10-12) = 26.5 megohms

    230 volts will push through 26.5 megohms 230/26.5E6 = 8.67 microamps which you probably can't feel.

    But that 100 picofarads is to a distant 'ground'. If your feet are 20mm from a conductive floor, their area makes a little more capacitance.
    My feet are roughly 10 inches long by 3½ inches wide , so each has area of 35 square inches = 0.022 square meters, and your boot thickness was 20mm = 0.02 meter
    so each foot had capacitance to that floor of ε0Area/Distance = 8.854×10−12 X .022 / 0.02 = only 9.7 picofarad more(for each foot),
    making perhaps ~120 picofarads total , = 22.1 megohms, through which 230 volts can push 10.4 microamps.
    The threshold of feeling electric current is around a half milliamp, and a very few hundred microamps matches my own experience,
    suggesting that when you felt that shock there was somehow some leakage current through your boot or around it.
    http://www.highvoltageconnection.com/articles/ElectricShockQuestions.htm
    upload_2016-8-30_16-44-49.png

    You are correct that small current flows to bring your body to same potential as the line
    that's dramatically demonstrated on high voltage lines

    watch around 0::56 here


    that's why you don't see birds on wires more than about 50kv, it tickles their feet.

    old jim
     
    Last edited: Aug 31, 2016
  12. Aug 30, 2016 #11

    NascentOxygen

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    That is an entirely separate issue---having acquired a static charge which discharges to any conductive body you come into contact with. This static charge, which can reach many thousands of volts, will jump across to a house electrical wire whether that wire carries 300V or 0V at the moment of contact. It is a good point you raise: that you may have acquired a static charge in low humidity conditions, and when this discharged as you touched a wire you felt that sudden current and mistakenly attributed it to the voltage on a live wire when in reality it was simply a static discharge from your body.

    In dry winter conditions, in supermarket aisles in particular, I routinely build up a charge sufficient to jump through ⅓" of air to any metal body, there's a visible blue spark, an audible snap, and I feel a sharp pain in my knuckle. Reaching for a metal can is a shocking experience! ⚡⚡
     
  13. Aug 30, 2016 #12
    Jim Hardy - that bit about the current flowing in to bring me to the potential of the line is very interesting, but, as your calculations suggest, couldn't really account for the shock.( The only slight difference here is that our line frequency is 50 Hz, but that's nothing.)

    NascentOxygen - we've all experienced the cheap-carpet shock! But what is this 'dry winter' you speak of? I live in Scotland.

    I went to my neighbour's the other day to help fix his 10 kW shower. He's a fussy bugger and wanted me to remove my boots before coming in. That is, he expected me to work on the live shower in my socks, standing in the shower tray. Some people...
     
  14. Aug 31, 2016 #13
    One more question, this time about the video Jim posted and the wand the man uses that arcs to the helicopter. If it's bringing the heli to the same potential as the line, what potential is that? Wouldn't the line be alternating between +X kV and -XkV, so the final potential would depend entirely on when in the cycle he breaks the arc, ie arbitrary?
     
  15. Aug 31, 2016 #14

    jim hardy

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    Correct.
    I don't think he said. Judging from the length of the insulators i'd guess it's either 240 or 500 kilovolts AC..
    Ahh, you mean as the helicopter draws away....
    Yes, they could carry quite a charge .
    I've never been on one of those crews so i don't know how they handle that. Seems upon landing there might be a discharge.
    In winter when it's extremely dry i get a shock when i step out of my car and it discharges though me. The car accumulates a static charge by its movement , just as we do by walking across a carpet. Aircraft do too.

    Were i this guy:
    upload_2016-8-31_8-46-4.png
    when landing i'd raise my feet so the helicopter discharges through its landing gear not mine.

    Military built an active device to bring helicopters to zero charge before landing, try a search on Dynasciences D0-3 Charge Controller
    http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=AD0463268

    Caveat - i'm no expert. I've noticed the static discharge wires on trailing edges of airplane wings, so did a search to see what helicopters used and search engine found that old report.

    old jim
     
  16. Aug 31, 2016 #15
    Thanks again, Jim. I didn't actually mean 'what specific potential', although the man does mention 500 kV. I thought the wand was to equalise the potential of the heli with the line, but then the line is alternating between + and - 500 kV, so the potential would never stay at a steady value.

    What I assume is happening is that the wand allows charge to flow to and from the aircraft's self-capacitance. As it gets closer I notice the technician touches the wand to the platform so that when he bridges the gap with his body, charge won't flow through him.

    Is this correct? I also thought that if the wand was held near the line indefinitely, then the arc would continue indefinitely. If this is true, then the wand is not so much to equalise the potential of line and aircraft but rather to 'short the plates' of the line-heli capacitor so that the technician doesn't have to.

    Feel free to tell me I am talking rubbish.
     
  17. Aug 31, 2016 #16

    jim hardy

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    No, your words paint in my alleged mind an accurate picture . I hope it's the same picture that's in your mind.

    The heli has capacitance to the distant universe. The wand provides a path for current to flow into and out of that capacitance on alternate half cycles. It's sure safer to use the wand than his body parts.

    The suits they wear for that work have conductive fibers woven into the cloth from which they're made.
    That way the suit forms a "Faraday Cage" around the lineman , keeping him comfortably free of current. A "Shield" if you will.
    Theory of shielding is you want capacitively coupled currents to flow in the shield, not in what it's shielding in this case a lineman.

    old jim
     
  18. Sep 4, 2016 #17

    Tom.G

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    Once, when I was commissioning a new industrial installation, just as I was leaving for the day the plant manager wanted we to check one of the motor contactors. It looked like a screw connection was loose on the contactor. The only tool I had with me was one of those 'universal', do-everything devices; knife, pliers, a couple screwdrivers, etc. I was wearing boots with synthetic soles that were pretty much waterproof and standing on a concrete floor. I started to tighten the 480V power connection. As I tightened my grip on the uninsulated 'screwdriver' I could feel the tingling and muscle contractions about 2/3 of the way up my forearm. That fixed the problem; and also removed that approach from my bucket list of thing to do before I die!

    So, yes @Guineafowl, very likely capacitive coupling. But whatever it is, it is potentially lethal.

    (It's a dangerous World out there!)
     
  19. Sep 4, 2016 #18

    jim hardy

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    You knowingly touched 480 barehanded ? At the end of the day when we're tired and wanting to go home we're apt to make bad choices, aren't we ?

    I've touched 480 by accident but always with one hand behind my back . One develops the habit of never getting in a posture where you'd be in a "cant let go" situation.

    I'm glad you lived to tell that tale. Had there been a metal splinter or an office staple embedded in your shoe sole .........

    here's a "safety meeting presentation" of a 480 volt arcflash. (it's staged using a dummy)
     
  20. Sep 4, 2016 #19

    davenn

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    yeah really silly thing to do .... I read his statement in disbelief :rolleyes:


    D
     
  21. Sep 4, 2016 #20

    Tom.G

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    Yup, no arguement with there.
    My only (semi) defense is I normally work with plastic handled screwdrivers and just went ahead and automatically did what needed doing.
    ...really silly thing to do...
    (and it is definitely off my bucket list.:oops: o:))
     
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