Trying to understand electric shocks....

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In summary: Dave is correct. You will not get a shock through your insulating boot. Why would you?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?At 60Hz, parasitic capacitance is negligible.Many thanks. Dave's answer of "In this strictly hypothetical situation
  • #1
Guineafowl
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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.
 
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  • #2
Guineafowl said:
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?

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

Dave
 
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  • #3
Guineafowl said:
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.
Dave is correct. You will not get a shock through your insulating boot. Why would you?
 
  • #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?
 
  • #5
Guineafowl said:
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?
At 60Hz, parasitic capacitance is negligible.
 
  • #6
Many thanks.
 
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  • #7
Guineafowl said:
Thanks for the replies. I have had a shock in that situation...

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.
 
  • #8
Guineafowl said:
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?
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.
 
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  • #9
NascentOxygen said:
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. 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 earthed 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.
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.
 
  • #10
Guineafowl said:
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?

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
 
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  • #11
Guineafowl said:
I could have built up some charge that destabilised some sensory neurons or neuromuscular junctions enough for them to depolarise or 'fire'.
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! ⚡⚡
 
  • #12
Jim Hardy - that bit about the current flowing into 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...
 
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  • #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?
 
  • #14
Guineafowl said:
Jim Hardy - that bit about the current flowing into 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.)

Correct.
Guineafowl said:
If it's bringing the heli to the same potential as the line, what potential is that?
I don't think he said. Judging from the length of the insulators i'd guess it's either 240 or 500 kilovolts AC..
Guineafowl said:
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?

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
PDF Url : AD0463268 (<<< this link takes you to a description of it - jh)

Report Date : 26 Apr 1965

Pagination or Media Count : 28

Abstract : Helicopters in flight or hover accumulate a change of static electricity which, when discharged, could cause harm to personnel or ignite fuel. The project was established to study the charge and discharge characteristics of static electricity on the UH-2A helicopter and to evaluate the preproduction Dynasciences model DO-3 discharger system. Flight tests were conducted during summer conditions when the available charging current and helicopter static voltages were low. The maximum charging current measured on the UH-2A helicopter was one microampere. The discharger system reduced a 20- kilovolt helicopter static voltage to a level between 0 to -800 volts in 0.5 second. The predicted maximum charging current for the UH-2A helicopter is 30 microamperes. The Dynasciences model DO-3 discharger system provides an effective means to discharge static electricity from the UH-2A helicopter. Recommendations are made for redesign to adapt the discharger system to the UH- 2A helicopter.

Descriptors : *HELICOPTERS, *STATIC DISCHARGERS, FLIGHT TESTING, PERFORMANCE(ENGINEERING), STATIC ELECTRICITY, VOLTAGE

Subject Categories : Helicopters
Electrical and Electronic Equipment

Distribution Statement : APPROVED FOR PUBLIC RELEASE
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
 
  • #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.
 
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  • #16
Guineafowl said:
Feel free to tell me I am talking rubbish.
No, your words paint in my alleged mind an accurate picture . I hope it's the same picture that's in your mind.

Guineafowl said:
rather to 'short the plates' of the line-heli capacitor so that the technician doesn't have to.

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
 
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  • #17
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!)
 
  • #18
Tom.G said:
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.

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)
 
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  • #19
jim hardy said:
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 ?

yeah really silly thing to do ... I read his statement in disbelief :rolleyes:D
 
  • #20
davenn said:
really silly thing to do

jim hardy said:
At the end of the day when we're tired and wanting to go home...
Yup, no argument 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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  • #21
So... Those mains tester screwdrivers (with the neon and resistor) - do they work by capacitive coupling too? The neon stays lit even if you jump in the air while holding it, so I assume this is the case.
 
  • #22
jim hardy said:
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.
,

At those voltages, corona discharge is a major factor. The transmission line, and ultimately the helicopter become surrounded by plasma. The physics of that is influenced by the radius of curvature of little parts, pieces, and screws of the objects. For example those static dissipators have pointy ends. Both positive and negative plasmas can be generated simultaneously from different points on the surfaces, and those plasmas are free to move thus making currents in the air. It is far too nonlinear and 3D distributed to reason out with R L C analogies.
 
  • #23
anorlunda said:
,

At those voltages, corona discharge is a major factor. The transmission line, and ultimately the helicopter become surrounded by plasma. The physics of that is influenced by the radius of curvature of little parts, pieces, and screws of the objects. For example those static dissipators have pointy ends. Both positive and negative plasmas can be generated simultaneously from different points on the surfaces, and those plasmas are free to move thus making currents in the air. It is far too nonlinear and 3D distributed to reason out with R L C analogies.
OK, so the plasma surrounding the line extends the distance it can arc. But doesn't the wand, with its small radius of curvature and hence high E-field strength start the arc earlier and allow the heli to become safely 'one' with the line and short out any chance of random arcing between the line and aircraft through the plasma? Once the wand touches the line, isn't the heli now charging and discharging through the wand's lead, a low impedance, rather than trying to arc across the lineman? I sort of feel that you're both right here, but this is well beyond my knowledge level.
 
  • #24
Curious about your boots - are they electrically isolating boots as in actually made for electrical isolation - or just work boots? Not all of the rubber sole material is a great insulator. And if you happened to have ESD protective shoes - these actually conduct!
 
  • #25
Windadct said:
Curious about your boots - are they electrically isolating boots as in actually made for electrical isolation - or just work boots? Not all of the rubber sole material is a great insulator. And if you happened to have ESD protective shoes - these actually conduct!
Just normal work boots
 
  • #26
I would not rely on them to be good insulators. That is why they make shoes / boots explicitly for http://www.lehighsafetyshoes.com/by-feature/safety/electrical-hazard/. Certainly could have enough leakage current for you to feel it.
 
  • #27
Windadct said:
I would not rely on them to be good insulators. That is why they make shoes / boots explicitly for http://www.lehighsafetyshoes.com/by-feature/safety/electrical-hazard/. Certainly could have enough leakage current for you to feel it.
I wouldn't argue with you there, but I refer back to my initial post, where I contrasted the thin PVC that insulates cables, which prevents a shock, with the order-of magnitude thicker rubber of my boots, which permitted a shock. That's how we got on to the whole thing about capacitance, then high-voltage linemen, then corona discharge. It's all been very interesting.
 
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What causes electric shocks?

Electric shocks are caused by the flow of electric current through the body. This can happen when a person comes into contact with a source of electricity, such as an outlet or a lightning bolt.

Why do electric shocks hurt?

Electric shocks can hurt because the electric current disrupts the normal functioning of the body's nervous system. This can cause muscle contractions, pain, and in severe cases, burns or other injuries.

Can electric shocks be dangerous?

Yes, electric shocks can be dangerous, especially if they are high voltage or last for a prolonged period of time. They can cause burns, heart problems, and even death in extreme cases. It is important to seek medical attention if you experience an electric shock.

How can I prevent electric shocks?

To prevent electric shocks, it is important to practice electrical safety. This includes using electrical devices and outlets properly, keeping them away from water, and avoiding touching electrical sources with wet hands. It is also important to have your home's wiring and electrical systems regularly inspected by a professional.

What should I do if someone gets an electric shock?

If someone gets an electric shock, it is important to turn off the source of electricity if possible and call for medical help immediately. Do not touch the person while they are still in contact with the source of electricity, and do not attempt to move them unless it is necessary for their safety.

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