Author Topic: Ungrounded Mains Electric Shock - Possible?  (Read 5081 times)

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Offline Mechatrommer

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Ungrounded Mains Electric Shock - Possible?
« on: October 19, 2011, 11:10:22 am »
remembering back how few times i got electric shock, and the concept is the current will flow to ground thru our body, surface effect? thats why maybe i still can breath oxigen, it doesnt go thru my heart?. the worst shock will be when we are wet. we cannot get electric shock while hanging to live line feet off ground right? so makes me wonder how about our friends in very dry place such as Arizona where electrostatic can build up by just thinking, there must be very high impedance between their feet and the ground. so maybe they dont get electric shock while touching "HOME" mains live wire? or just feel a slight massaging sensation?
http://www.massager-machines-and-more.com/tens-unit-pulse-massager-452.asp
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Offline Psi

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Re: Ungrounded Mains Electric Shock - Possible?
« Reply #1 on: October 19, 2011, 12:08:48 pm »
It doesn't always flow through your body to ground.
It flows towards the other side of the circuit (the neutral connection). It just so happens that often the easiest way to get there is through the ground. Since the ground is connected to the earth wire and the earth wire is connected to the neutral wire at the fuse/breaker box.

The connection from your body through the ground and back to neutral can vary quite a bit in terms of resistance, so sometimes a mains shock can be quite mild and other times very serious/painful.

If you grabbed the phase wire in one hand and the neutral wire in the other hand it will flow directly from one hand to the other.
In that situation the best path to neutral is to the hand holding it rather than to ground.
In any case, one wire in each hand is not going to end well for the person, since their heart is right in the path between the two and there's nothing else in the circuit except them.

Same thing if you touch a live wire and happen to also be touching the metal case of an earthed appliance (since earth is connected to neutral)

Note: earthing systems are a bit different between countries, i'm only going on how it works in NZ
« Last Edit: October 19, 2011, 12:24:15 pm by Psi »
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Offline buxtronix

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Re: Ungrounded Mains Electric Shock - Possible?
« Reply #2 on: October 19, 2011, 12:16:45 pm »
Strictly speaking, if you are not grounded and not "completing the circuit", it is safe to touch the active conductor. But you never know what paths you forgot to check for..

Electric shocks are generally survivable if the current happens to go the right way (ie not through your heart...or your heart is a tough one...or it gets it at the right part of your pulse cycle, etc...). Perhaps wetness on the skin can conduct some away from your internals...

Whilst hanging off the ground, you'd need to be touching both live and neutral/ground to get a shock. that's why birds can safely sit on power lines.

Humidity or lack of it doesnt have much effect - you can get a shock from just a slight bit of sweat between your skin and the surface.

Overall, there are all sorts of factors in determining the effects of a shock. You are best to assume the worst could happen, and avoid getting a shock in any circumstance.

 

Offline Chet T16

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Re: Ungrounded Mains Electric Shock - Possible?
« Reply #3 on: October 19, 2011, 12:30:51 pm »
Isn't it something like 6mA across the heart will kill you? Trying to remember what i heard on mythbusters!
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Offline Wartex

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Re: Ungrounded Mains Electric Shock - Possible?
« Reply #4 on: October 19, 2011, 01:31:56 pm »
If you hand on a 100-500 Kv powerline with hands spread apart on the same wire, you will die because you will form a shunt and there will be enough current to kill you.

Also just jumping off the pylon onto the high voltage line will kill you while it's charging you to the same potential.

With AC you don't need to "complete the circuit" ground to feel the shock, your body acts as a capacitor (around 100 pF) and you can feel the shock even if you stand in rubber boots and voltage is high enough.
 

Offline IanB

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Re: Ungrounded Mains Electric Shock - Possible?
« Reply #5 on: October 19, 2011, 04:02:41 pm »
If you hand on a 100-500 Kv powerline with hands spread apart on the same wire, you will die because you will form a shunt and there will be enough current to kill you.
Uh, no. A moment's thought and a quick calculation will tell you this isn't going to be the case.

Suppose there was a 100 V potential difference across a 3 ft length of conductor (100 V being nominally enough to kill you if you touch two points 3 ft apart). With a potential gradient of 100 V/ft, you would have a voltage drop of about 200 kV over a 1 mile length of power line, 2 MV over a ten mile length. Clearly this would be entirely hopeless from a design point of view and the potential gradient along a power line in reality is much less than that.

Regarding capacitance effects, maybe that could be painful. The corona effect might be interesting too. However I have seen birds sitting on a 400 kV power line without seeming to be concerned, so I'm not too sure what would happen with a person.
« Last Edit: October 20, 2011, 02:59:30 am by IanB »
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Offline IanB

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Re: Ungrounded Mains Electric Shock - Possible?
« Reply #6 on: October 19, 2011, 04:08:35 pm »
maybe they dont get electric shock while touching "HOME" mains live wire? or just feel a slight massaging sensation?
When I was young and foolish, I once twisted the stranded ends of a 240 V mains live wire with my bare fingers while kneeling on a concrete floor (I forgot the other end was plugged in). I did not even feel a slight tingling. I only remembered the wire was live when I accidentally shorted live to neutral and blew the fuse.

I don't recommend anybody try this of course, but it does mean you need a fairly good conductive path to ground before you will get a serious shock.
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Offline Mechatrommer

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Re: Ungrounded Mains Electric Shock - Possible?
« Reply #7 on: October 19, 2011, 07:23:57 pm »
It doesn't always flow through your body to ground.
It flows towards the other side of the circuit (the neutral connection). It just so happens that often the easiest way to get there is through the ground. Since the ground is connected to the earth wire and the earth wire is connected to the neutral wire at the fuse/breaker box.
sorry my post is not very clear. when i say ground, i mean floor, or where you step at, and assuming you keep one hand inside your pocket.

So Ian, you did that. it must be very dry where you worked it. I only experienced that with my (lower voltage) welding machine. if i really dry, i can put the welding rod with bare hand, the other hand touching the ground cable, but when i get wet, i got slight shock, so now i used glove to put the rod, no matter wet or dry.

and i caps the "HOME" (110-240V), not KV stuff. KVolt can cook you pretty good smell. i dont dare even getting near it.
« Last Edit: October 19, 2011, 07:30:10 pm by Mechatrommer »
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Offline IanB

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Re: Ungrounded Mains Electric Shock - Possible?
« Reply #8 on: October 19, 2011, 08:01:20 pm »
So Ian, you did that. it must be very dry where you worked it.
It was an unheated outdoor garage, so probably slightly damp. On another occasion in the same building a live wire feeding a light switch was touching a damp brick wall where the insulation had worn through from rubbing against the bricks. I noticed my hand would tingle any time I touched the wall near the switch  :)
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Offline ciccio

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Re: Ungrounded Mains Electric Shock - Possible?
« Reply #9 on: October 19, 2011, 08:41:09 pm »
I want to annoy you with an old story:
Many years ago I worked for a Company who manufactured live sound systems. It was a small company, and sometimes, when no service technician was available, I had to service the equipment that our customers (bands and pop groups) carried in (or at least I had to listener to the customer's lamentations).
There was this gentleman,  the "factotum" of a big folk group (they were very famous, and played about 300 gigs a year), who had to drive the bus, unload and install the sound system,  reload it into the bus and drive again.
It was a little drunk most of the times (or at least that was my impression), and he told me that the bigger problem, apart multicore microphone cables punched by the ladies' stiletto heels, was to find the electrical power for the sound and light system when they had a show in a small town fair.
At those times the Italian power distribution system was a real mess, with many small towns still powered by 110 V dual phase,  other (most) with 220 single phase, and the junction boxes where he had to screw-in his wires were unlabeled, and many carried 380 V three-phase without neutral. So he usually opened the box and touched an uninsulated wire end with his palm.
By the "strength" of the shock he could tell the voltage, and after one or two shocks he could realize how to get reliable 220 V.  He had a big callus in his palm...
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Offline Mechatrommer

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Re: Ungrounded Mains Electric Shock - Possible?
« Reply #10 on: October 19, 2011, 11:39:15 pm »
By the "strength" of the shock he could tell the voltage, and after one or two shocks he could realize how to get reliable 220 V.  He had a big callus in his palm...
maybe we can call him "Flukea" man!
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Offline Psi

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Re: Ungrounded Mains Electric Shock - Possible?
« Reply #11 on: October 20, 2011, 12:43:33 am »
It doesn't always flow through your body to ground.
It flows towards the other side of the circuit (the neutral connection). It just so happens that often the easiest way to get there is through the ground. Since the ground is connected to the earth wire and the earth wire is connected to the neutral wire at the fuse/breaker box.
sorry my post is not very clear. when i say ground, i mean floor, or where you step at, and assuming you keep one hand inside your pocket.

Yep, although, the floor is part of the house which is sitting on the dirt/ground, so it all forms part of the circuit.
Since the house is mostly made of material that aren't very good conductors it acts like a resistor to neutral reducing the severity of a shock most of the time.

Of course quite often there are grounded metal appliances all around the house. So the current path from your body may not have to go all the way to the physical dirt under the house to get to back to neutral, most of the time it will probably do something like track along the moisture in the carpet until it gets to the nearest grounded object that the carpet is in contact with.
It's all speculation though, there are so many variables and ways the current could get to neutral.
« Last Edit: October 20, 2011, 12:49:26 am by Psi »
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Offline Mechatrommer

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Re: Ungrounded Mains Electric Shock - Possible?
« Reply #12 on: October 20, 2011, 02:56:50 am »
It's all speculation though, there are so many variables and ways the current could get to neutral.
no its not that complex. its just v=ir, only condition from place to place will differ, the dryness and wetness etc. as other member stated from experience it is possible not to get the shock if the current cannot find its way thru our body (or our body forms very hi impedance to ground). but good replies guys at least i gain some knowledge esp on high voltage gradient in conductor (the shunt stuff explained above). but make me thinking how our body can become a shunt (if really the conductor is in hi gradient, hi-V/ft)? our body by itself is a higher Z material compared to the mains line which is made of efficient conductor, current will go most to the least resistance right?
« Last Edit: October 20, 2011, 02:59:50 am by Mechatrommer »
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Offline Psi

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Re: Ungrounded Mains Electric Shock - Possible?
« Reply #13 on: October 20, 2011, 03:05:39 am »
It's all speculation though, there are so many variables and ways the current could get to neutral.
no its not that complex. its just v=ir, only condition from place to place will differ, the dryness and wetness etc. as other member stated from experience it is possible not to get the shock if the current cannot find its way thru our body (or our body forms very hi impedance to ground). but good replies guys at least i gain some knowledge esp on high voltage gradient in conductor (the shunt stuff explained above). but make me thinking how our body can become a shunt (if really the conductor is in hi gradient, hi-V/ft)? our body by itself is a higher Z material compared to the mains line which is made of efficient conductor, current will go most to the least resistance right?

yeah, its all v=ir but the environment is quite complex.
Almost an infinite number of paths back to neutral all with their own resistance and all in parallel with each other.
 
« Last Edit: October 20, 2011, 03:07:13 am by Psi »
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Offline IanB

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Re: Ungrounded Mains Electric Shock - Possible?
« Reply #14 on: October 20, 2011, 03:06:56 am »
at least i gain some knowledge esp on high voltage gradient in conductor (the shunt stuff explained above)
As long as you don't actually try to touch such a high voltage power line  ;)

With the power levels available in transmission and distribution lines you don't actually die from electric shock, you die from incineration. Sometimes it is slow and painful as the electricity makes a path through the wet insides of your body where the conductivity is lower, so it char broils your internal organs while leaving your skin intact.  :o
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