Question About How Getting Electrocuted Works

[bostonman]

Recently I was talking to someone who told me years ago touched a dishwasher in a restaurant with one hand and something else with the other, and got electrocuted (probably a bad ground). She also claims she was stuck and couldn't release her hands.

I understand the concept of touching the hot of an outlet with one hand, and touching something such as Earth ground with the other which would cause current to go through your body.

Now, if I hold hot and nothing else, my body potential is 120V AC. I can hold that wire all day until I touch neutral or Earth ground, or someone touching Earth ground (or neutral) comes up and touches me; then we both get zapped.

The things I began wondering is: over the years I've heard of people who touch their refrigerators and get a bad shock (probably because Earth ground was broken or they used a three to two prong adapter). If their bodies are not touching neutral or Earth ground, why would they get a shock?

Maybe my interpretation of the story is wrong and they don't actually get shocked by their refrigerator, but I was curious: Can a person touch any voltage and not get electrocuted providing they are not touching neutral/ground?

Also, why would she get stuck to the dishwasher? If voltage/current crosses zero every 8.3ms, then at some point you can pull off it (whereas DC remains constant and you can't pull away).

Ignoring voltage would arc and/or go through the thin insulation on my shoes, could I touch 1,000,000 V and not feel a shock or would I get a serious shock due to my body being at a different potential, but I wouldn't get electrocuted?

[Andy Chee]

Also, why would she get stuck to the dishwasher? If voltage/current crosses zero every 8.3ms, then at some point you can pull off it (whereas DC remains constant and you can't pull away).

Human muscles do not have spring return like a standard electromechanical relay.  Human muscles are more like 2-coil latching relays, with a spring action on both sides.  The zero crossing merely provides a more amenable scenario for hand release, it does not guarantee it.  If you pulse relay current On and Off in the same coil, a latching relay will stay on, just like your arm muscles.  You need to apply current to the other coil to flip the relay, in human muscle terms, you need to electrocute the other muscle.

A strong will and awareness is still required to send the necessary, and large enough, brain-to-muscle motor control signal to override the convulsions from mains electricity stimulus.

Meanwhile, does this help?

[IanB]

You only get electrocuted once 

[moffy]

AFAIK 120VAC causes one to hold on, 240VAC will tend to throw a person, and the fall from say a ladder is an added complication. But I have not personally tested this, but can confirm that some low current 10kV pulses almost threw me out of a jet's doorway once.

[Berni]

It doesn't take much miliamps to get a shock, so even if you have a very weak connection to earth you can still get a shock(due to the high voltages involved, kiloohms are enough). But yes if you really are well insulated with the correct shoes and not touching anything then you can't get a shock by only touching live.

Yes DC is more likely to make you grab on and not be able to let go, but AC can do it too because 50/60 Hz is too fast for muscles and nerves to react. Go ahead and try wiggling your finger 60 times per second.

The polarity also doesn't make a difference, current flow in either direction can trigger nerves. The reason you grab on is because the muscles that handle closing fingers are much bigger and stronger than the muscles that extend fingers, so as both of these muscle groups contract the stronger closing muscles win and overpower the others, so making you close your hand together.

There is nothing about 120V making you grab and 220V throwing you. Both can do either. It it just that 220V can give you a stronger shock and i suppose that does make it more likely for other muscles in your body to contract and throw you off balance. Most injuries with electricity are actually physical damage of hitting something or falling, not the actual electricity. This is particularly dangerous when working on ladders since a good jolt can easily make you jump off a ladder. That is unless you weer unlucky enough to get stuck grabbing onto what is shocking you, that can definitely cook you to death, but this is rare as usually your arm will jump and move away.

[edpalmer42]

Ignoring voltage would arc and/or go through the thin insulation on my shoes, could I touch 1,000,000 V and not feel a shock or would I get a serious shock due to my body being at a different potential, but I wouldn't get electrocuted?

Sure.  People work on live high voltage power lines all the time.  There's lots of Youtube videos showing it.  Sometimes they're working from bucket trucks, sometimes helicopters!  There are various rules & procedures for different situations.  I've seen conductive gloves to protect against burns from the arcs and shorting sticks to safely raise the worker and the helicopter to the same potential as the line.  It's the same idea as a bird sitting on a high voltage line.

[BradC]

shorting sticks to safely raise the worker and the helicopter to the same potential as the line.

Now I'm intrigued as to the average capacitance of a helicopter

[IanB]

Now I'm intrigued as to the average capacitance of a helicopter

If you watch the videos, there is a very long arc produced when a helicopter approaches a high voltage power line. The arc continues until the shorting stick makes contact with the conductor.

It is of interest to consider the highest voltage power line that birds will sit on. While birds don't care at all about 11 kV lines, I don't think I have ever seen a bird sitting on a 400 kV line. I have often wondered where the threshold is before the capacitance of the bird becomes an issue?

[Whales]

If their bodies are not touching neutral or Earth ground, why would they get a shock?

They are touching ground.  Flooring, shoes and socks are not consistent between brands and environmental conditions.

[MathWizard]

If I remember correctly, they say to treat from hand, through your chest, to other hand, as if it's only 500 or 1000 ohms, depending on your skin moisture, saltiness, humility/etc. And then there's all the paths back to the neutral and ground wires, through the floors, and walls, etc.

I once got a mild current going through me when working on a 240V, 2-phase heater. And the current was going through a screw driver, or up my hand/arm somehow, and exiting through my knee into the floor, and then back to some wire.

This is a long video but there's some good stuff in there, I need a refresher.

[TimFox]

You only get electrocuted once 

Dictionaries vary, but "electrocution" can involve death or serious injury due to electric shock.

[TomKatt]

You only get electrocuted once 

Build a man a fire and he'll be warm for a day.

Set a man on fire and...

[pcprogrammer]

Don't forget about static electricity. It can give you quite a shock. Touching a large body of steel, like a refrigerator that is at low potential and you being charged up to some high static voltage can produce a nice spark when you touch the refrigerator or a car for that matter. The times I got a shock when touching a car door is high, especially in winter. It won't kill you, but can hurt.

Shock from mains supply is a different story of course, and for some it turned out to be lethal. It all depends on the path of the current, the duration of it, etc. I myself have had plenty of 220 - 240V shocks. My first one was when I was a kid and stuck a piece of metal in an extension cord. Nothing happened until my brother stuck a wire in the other hole and touched me with it. That was my first experience with 220V. Yes back in the day it was 220V. Still here though.

But don't go sticking your fingers into sockets to see what it feels like. I can assure you it is no pleasure.

[ajb]

AFAIK 120VAC causes one to hold on, 240VAC will tend to throw a person, and the fall from say a ladder is an added complication. But I have not personally tested this, but can confirm that some low current 10kV pulses almost threw me out of a jet's doorway once.

 

I don't see how there would be a voltage-dependent effect like that.  It just depends on the circumstances, in terms of which muscle groups are affected and what happens when those muscles contract, it's not like <120V affects these muscles and >120V affects those muscles.  I know of one case where someone had one hand on the frame of a (grounded) cabinet and, in a moment of pure "I just wasn't thinking", put another hand on a live 480VAC terminal.  He ended up staying like that for some seconds until his legs finally gave out and his body weight pulled him off of the terminal/cabinet, causing serious lacerations to his hands in the process.  I could imagine if the back of his hand touched the terminal instead, it could have caused a convulsion that would have knocked him away instead -- or if the current path had been through the legs, he might have jumped across the room instead.  People also tend to reflexively pull themselves away from things that hurt/startle them, which is also involuntary but doesn't really have anything to do with electricity, but could be confused as such in the circumstances.  Of course if arc flash is involved, or there is sufficient power delivered into the poor person's body due to a high enough voltage and available current, that could physically knock someone back anyway -- but at that point you're probably less worried about direct electrical effects on the body and more worried about burns and other physical trauma, at least in the immediate term.   

[bostonman]

Elaborating a bit on my initial message, besides the story of the girl who got stuck on a dishwasher and rack, I also promote safety to others and want to be accurate in my explanations; hence why I asked this question.

I've seen those helicopter videos (I think it was a documentary and/or Modern Marvels) about how they need to use a rod.

Does capacitance come into play in getting shocked? Static electricity is the build up of charge on a body, say walking across carpeting. The human would build up a charge as a result. The moment the person comes close to an object with a different charge, usually as their finger approaches a light switch, an arc jumps. Is this technically capacitance affecting the transfer of charge?

Or as a hypothetical example: if a single non-insulated wire (exposed copper) carrying 10kV (or some ridiculously higher voltage) was connected between two poles spaced miles apart and miles high (i.e. eliminating any sort of ground being in close proximity - or for purposes of discussion, let's assume ground is no where near), and I could "fly" like a bird to land on the wire. Would I get a a 10kV jolt in my body that would kill me due to a difference in voltage potential (my body is at "0V" and the power line is at 10kV) or would I just sit on the wire and not feel anything? Obviously if I began sliding down the wire, as I got closer to the next pole, the voltage would arc out my body and then I'd get electrocuted.

Muscles contracting and not allowing the person to let go makes sense. The statement I usually tell people is that you stand to get killed by a standard 12V car battery than an AC outlet because the DC will fuse you to the terminals whereas AC you can pull away. I guess while it's theoretically true you can pull away every 8.3ms with AC, technically the muscles will not allow it.

Until a few days ago, I thought a Tesla car powered the motors off 48V DC batteries. I began calculating the amperage based on just 200HP - if 1HP = 746W, that would be approx. 3100A.

This didn't seem correct to me, but reading showed the batteries actually put out over 300V which would reduce the actual current. This still seems crazy to me as 300+ V would be quite dangerous to have running through a car.

[moffy]

This is an example of a human body model, might help: https://incompliancemag.com/human-body-model-and-electrostatic-discharge-esd-tests/

[Someone]

AFAIK 120VAC causes one to hold on, 240VAC will tend to throw a person, and the fall from say a ladder is an added complication. But I have not personally tested this, but can confirm that some low current 10kV pulses almost threw me out of a jet's doorway once.

I don't see how there would be a voltage-dependent effect like that.

You should consult the literature then! The current passing through a person is not even close to simple resistance.

From IEC 60479-1 with regard to a hand-foot conduction path:

Attention is drawn to the fact that at UT=200V the touch current IT is four times as high as for 100V. If the duration of current flow is longer than approximately 0,2s, ventricular fibrillation would occur with a high probability.

[Geoff-AU]

This still seems crazy to me as 300+ V would be quite dangerous to have running through a car.

Wait till you hear about how combustion engines do it - running highly flammable liquid around everywhere 

[Andy Chee]

Until a few days ago, I thought a Tesla car powered the motors off 48V DC batteries. I began calculating the amperage based on just 200HP - if 1HP = 746W, that would be approx. 3100A.

This didn't seem correct to me, but reading showed the batteries actually put out over 300V which would reduce the actual current. This still seems crazy to me as 300+ V would be quite dangerous to have running through a car.

Tesla Roadster has been around since 2008.  Nissan Leaf has been around since 2010.  There's no excuse to have such outdated knowledge about EV technology!

[MarkF]

I was shocked once when I was a teenager by 120V @ 60 Hz from our school bus that we fixed into a camper.  I griped a rod and could not let go at first.  You could fell the 60 Hz pulsing through me. 

On another occasion many years later, I got shocked by 120V @ 400 Hz.  It was a much different experience.  More like a strong hard bite, like being stung by the end of a wipe.  I was not griping anything that time.  Just brushed against a stud in a disconnect box.  I remember the pain lasted much long (a few hours).

Those are two of my memorable experiments with electricity.  It gives you a healthy respect.

[bostonman]

Tesla Roadster has been around since 2008.  Nissan Leaf has been around since 2010.  There's no excuse to have such outdated knowledge about EV technology!

How is this comment contributing to the topic in this thread?

It's a thread about safety, not how many 9V batteries are in a Tesla.

[Andy Chee]

Tesla Roadster has been around since 2008.  Nissan Leaf has been around since 2010.  There's no excuse to have such outdated knowledge about EV technology!

How is this comment contributing to the topic in this thread?

It's a thread about safety, not how many 9V batteries are in a Tesla.

a) the Tesla cells are 3.7V, not 9V

b) Tesla battery packs (and Nissan, BMW, and other EV battery packs) demonstrate how high voltage +300VDC can be made safe.  I encourage you to examine the engineering that goes into EV electrical safety.  The electrical safety principles apply to fixed household wiring just as equally, albeit the safety regulatory codes will be different.

[IanB]

It's a thread about safety, not how many 9V batteries are in a Tesla.

Deaths from electricity are remarkably rare. I have known plenty of people of my personal acquaintance (far too many) who have died from cancer. I have known people who have died from heart attacks and strokes.

I have not heard of one person who has died from electrocution, even in my widest social circle.

[Andy Chee]

Deaths from electricity are remarkably rare. I have known plenty of people of my personal acquaintance (far too many) who have died from cancer. I have known people who have died from heart attacks and strokes.

I have not heard of one person who has died from electrocution, even in my widest social circle.

A few years ago in Australia, a botched government program to install roofing heat insulation, resulted in two people killed via electrocution.  Basically untrained labour were attaching the foil insulation using metal staples, and they stapled the foil through live wiring.

https://www.theguardian.com/world/2014/may/19/foil-insulation-recipe-disaster-royal-commission

And whilst not a death, a faulty earth in social housing caused a water faucet to become live, causing a girl brain damage.

https://www.abc.net.au/news/2019-09-27/denishar-woods-garden-tap-electrocution-report-released/11553734

[Bud]

If you think it is easy to electrocute a person, watch this (the history of the electric chair)

https://youtu.be/cpYU8oa8g_k?si=quY1aXShJ1_fvChZ