Author Topic: why'd i get shocked by wall outlet?  (Read 2381 times)

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Offline willbanks

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why'd i get shocked by wall outlet?
« on: July 18, 2017, 04:12:46 pm »
a month ago i accidentally touched live and neutral exposed wire and shocked myself. it didn't hurt that much but it was enough to scare the crap outta me.
i got a new multimeter and decided to measure the resistance between my thumbs. 10M ohms!
using ohms law there should be about .000012 amps running through me.
sometimes my body is 5M ohms, which is still very little amps.
if the amperage was so little, why did i feel the electricity?
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Offline Richard Crowley

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Re: why'd i get shocked by wall outlet?
« Reply #1 on: July 18, 2017, 04:24:31 pm »
How much voltage did your new DMM use to measure your skin "resistance"?
The 120 volts from the mains wall outlet was able to punch through that semi-insulating layer of dead cells on your epidermis. An ordinary DMM can't do that with a low battery voltage. 

Try measuring your "resistance" with a megger.  No, I was only kidding. Don't do that unless you want another shock. Or if you need to re-start your heart. But an AED would be a better equipment to use than a megger.

There is that perpetual argument about whether "voltage kills" or "current kills". But my view is that it takes voltage to punch through your epidermis (unless you have an electrode already inside). But it takes only a few mA (or uA) to de-synch or stop your heart.  When they want to make electrical connection with your body, they use those electrode pads with some kind of liquid that gets through the dead epidermis cells.
 

Offline suicidaleggroll

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Re: why'd i get shocked by wall outlet?
« Reply #2 on: July 18, 2017, 04:26:55 pm »
In addition to what Richard Crowley said, skin resistance also varies dramatically with moisture.  Lick your fingers before making the measurement and it should drop much lower, into the tens of kiloohms most likely.  120 Vrms is 170 Vpeak for a sine wave, over 10k that's 17mA.  It only takes ~10mA to stop your heart.
 
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Offline NANDBlog

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Re: why'd i get shocked by wall outlet?
« Reply #3 on: July 18, 2017, 05:24:10 pm »
And also, we are able to sense very low currents. Your nervous system is working with millivolts of difference, and you can move your arm with that. Also, pain receptors are easily triggered with that small current.
 

Offline willbanks

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Re: why'd i get shocked by wall outlet?
« Reply #4 on: July 18, 2017, 06:50:57 pm »
it runs off 2 1.5V batteries (klein mm600)
when i measured resistors and a 9V battery it was accurate enough.
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Offline deflicted

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Re: why'd i get shocked by wall outlet?
« Reply #5 on: July 18, 2017, 07:56:29 pm »
When they want to make electrical connection with your body, they use those electrode pads with some kind of liquid that gets through the dead epidermis cells.

I don't know about defibrillator devices, but when I had an EKG last year, they shaved the area first, then had some abrasive pads that they twisted on to get through the top layer of skin, followed by the gel stuff, and finally the sticky pads. When you go home, you have all these red spots where the skin has been scraped away (not to mention shaved). Fun times. I highly recommend it. Not.  :--
 

Offline RGB255_0_0

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Re: why'd i get shocked by wall outlet?
« Reply #6 on: July 18, 2017, 08:08:57 pm »
When they want to make electrical connection with your body, they use those electrode pads with some kind of liquid that gets through the dead epidermis cells.

I don't know about defibrillator devices, but when I had an EKG last year, they shaved the area first, then had some abrasive pads that they twisted on to get through the top layer of skin, followed by the gel stuff, and finally the sticky pads. When you go home, you have all these red spots where the skin has been scraped away (not to mention shaved). Fun times. I highly recommend it. Not.  :--
On the bright side, they didn't wax it off.
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Online IanB

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Re: why'd i get shocked by wall outlet?
« Reply #7 on: July 18, 2017, 08:12:48 pm »
i got a new multimeter and decided to measure the resistance between my thumbs. 10M ohms!

When you measure the resistance of your body with a meter, either grip the probes tightly, or dig the points into your skin. You should then get a much more realistic measure of a few hundred kilohms.
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Offline MK14

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Re: why'd i get shocked by wall outlet?
« Reply #8 on: July 18, 2017, 08:15:41 pm »
it runs off 2 1.5V batteries (klein mm600)
when i measured resistors and a 9V battery it was accurate enough.

Please be very careful with higher voltages, such as mains voltage. You can easily accidentally kill or seriously injure, yourself and/or others.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2763825/

Quote
The body has resistance to current flow. More than 99% of the body's resistance to electric current flow is at the skin. Resistance is measured in ohms. A calloused, dry hand may have more than 100,000 Ohms because of a thick outer layer of dead cells in the stratum corneum. The internal body resistance is about 300 Ohms, being related to the wet, relatively salty tissues beneath the skin. The skin resistance can be effectively bypassed if there is skin breakdown from high voltage, a cut, a deep abrasion, or immersion in water
 
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Online Gyro

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Re: why'd i get shocked by wall outlet?
« Reply #9 on: July 18, 2017, 08:31:49 pm »
When they want to make electrical connection with your body, they use those electrode pads with some kind of liquid that gets through the dead epidermis cells.

I don't know about defibrillator devices, but when I had an EKG last year, they shaved the area first, then had some abrasive pads that they twisted on to get through the top layer of skin, followed by the gel stuff, and finally the sticky pads. When you go home, you have all these red spots where the skin has been scraped away (not to mention shaved). Fun times. I highly recommend it. Not.  :--

You want to try a 3 day EEG when there are loads of them glued all over your head, then you'd have something to complain about.  :P

Luckily I didn't have much hair to start with!


P.S. They use Silver Chloride (and sintered Silver electrodes) to get decent low level signal skin contact with EEGs. It gets very itchy after a while.
« Last Edit: July 18, 2017, 08:36:15 pm by Gyro »
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Offline ebastler

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Re: why'd i get shocked by wall outlet?
« Reply #10 on: July 18, 2017, 09:20:40 pm »
it runs off 2 1.5V batteries (klein mm600)
when i measured resistors and a 9V battery it was accurate enough.

The point of the prior posts was not that your multimeter would be inaccurate or otherwise bad quality. The point is that your body, when contacting it via the skin, does not behave like a simple resistor where I = U/R with a constant resistance R.

Rather, the resistance will change with the voltage you apply. (I will also change with the way you make contact between the measurement electrodes and your skin, but that's a slightly different matter.) As explained by Richard, the higher voltage of a mains outlet will "burn through" the poorly conducting layer of dry/dead skin cells, while the low measurement voltage of your multimeter won't. So, your multimeter measures accurately. But the conditions while measuring with the multimeter are different than when you touch mains voltage -- your body actually becomes more conductive (lower resistance) when you apply the high voltage.

Edit: Ohm's law corrected -- ouch, shouldn't post that late in the day...
« Last Edit: July 19, 2017, 04:05:19 am by ebastler »
 

Offline NexusKoolaid

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Re: why'd i get shocked by wall outlet?
« Reply #11 on: July 19, 2017, 12:05:49 am »
it runs off 2 1.5V batteries (klein mm600)
when i measured resistors and a 9V battery it was accurate enough.
The point of the prior posts was not that your multimeter would be inaccurate or otherwise bad quality. The point is that your body, when contacting it via the skin, does not behave like a simple resistor where I = R*U with a constant resistance R.

Rather, the resistance will change with the voltage you apply. (I will also change with the way you make contact between the measurement electrodes and your skin, but that's a slightly different matter.) As explained by Richard, the higher voltage of a mains outlet will "burn through" the poorly conducting layer of dry/dead skin cells, while the low measurement voltage of your multimeter won't. So, your multimeter measures accurately. But the conditions while measuring with the multimeter are different than when you touch mains voltage -- your body actually becomes more conductive (lower resistance) when you apply the high voltage.

@willbanks this video helped me understand what's going on.  It seeks to address a slightly different question than you are asking, but nicely addresses what eblaster, Richard Crowley and others are saying.
 

Offline vealmike

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Re: why'd i get shocked by wall outlet?
« Reply #12 on: July 19, 2017, 02:40:22 pm »
^^ Wot he said. ^^
Volts jolts, it's mills that kills.
 

Offline dorin

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Re: why'd i get shocked by wall outlet?
« Reply #13 on: July 19, 2017, 04:35:14 pm »


As the video explains the human impedance can drop down to 1k Ohm when considering high voltage safety. It is not just due to the breaking of outer skin layer it's also about the electrical channels that create inside your whole body after a certain current threshold. It is an avalanche process similar to a lightning strike, because as the channel begins to form the impedance lowers, current increases which in turn strengthens the channel and further lowers the impedance.

Now, to get even more precise, the damage done to your body is caused by a combination of instantaneous dissipated energy density (the amount of energy per volume of tissue) and steady state current density. And different organs have different sensitivity factors of these 2 densities. The density matters because if the same amount of current passes through a large cross sectional area, each cell needs too carry less electrons. That's why you also feel the static discharge from a sharp point fairly painful at the point of touch, because the current density there is much higher than the rest of the body where the current starts dispersing through multiple channels.

You can usually stick to basic guidelines and be on the safe side even if you are not qualified, but if you want to accurately assess whether or not something is dangerous, there is no universal way because there are too many practical details and it requires a deep understanding of electricity and biology to cover all possible cases/exceptions.
 

Offline ebastler

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Re: why'd i get shocked by wall outlet?
« Reply #14 on: July 19, 2017, 05:33:18 pm »
^^ Wot he said. ^^
Volts jolts, it's mills that kills.

I think that video could be a bit clearer and more consistent...

"It's wrong to say that the Volts don't kill you".
"Actually you can be exposed to many kV of static electricity and be fine, so the Volts don't seem to hurt."
"8 Amps don't necessarily hurt you either."
"Actually it's the total energy, not the Volts or the Amps."
"Look, here is a table of danger levels depending on Amps."
"Amps are proportional to Volts, so in a way the Volts do kill you."

I could follow along by interpreting "what he must have meant", but if this video does not manage to thoroughly confuse a beginner, I would be surprised. Nice animations though!
 

Offline willbanks

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Re: why'd i get shocked by wall outlet?
« Reply #15 on: July 19, 2017, 06:38:01 pm »
ah! yeah it got a reading of around 200k ohms!
that makes a lot more sense.
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Offline stj

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Re: why'd i get shocked by wall outlet?
« Reply #16 on: July 19, 2017, 07:11:27 pm »
actually, it's not the electricity that kills you usually, it's the thing you hit your head on when your thrown back!!
 

Offline deflicted

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Re: why'd i get shocked by wall outlet?
« Reply #17 on: July 19, 2017, 07:30:23 pm »
actually, it's not the electricity that kills you usually, it's the thing you hit your head on when your thrown back!!

I can definitely see how that would be the case, at least with household mains AC. I've known several electricians who routinely test whether a wire is live by touching it with the back of their finger (so that if it is live, the resulting muscle contraction pulls their finger away from the wire instead of causing them to grip the wire). Apparently, for them, using a proper measuring device and proper protective gear is for wusses, and besides it takes too long.

I'm guessing that high voltage (e.g., places where arc flash explosions are a serious concern, etc) is a whole different story entirely, both in terms of how much you can get away with before proper gear becomes an absolute necessity, and the nature and frequency of causes of death.
« Last Edit: July 19, 2017, 07:34:18 pm by deflicted »
 


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