which causes schock current or voltage?

Discussion in 'General Electronics Chat' started by madhu, Dec 9, 2003.

  1. madhu

    Thread Starter New Member

    Dec 9, 2003
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    can any one explain it clearly why schock results when we touch electricity?
    which causes schock current or voltage?

    Madhu
     
  2. Battousai

    Senior Member

    Nov 14, 2003
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    Current flows only when there is a votlage difference across something. An electrical outlet has three ports, one at 120V, one at 10V, and the other at ground (US). If you touch the 120V port, (don't do this :rolleyes: ), you'll feel a current run through you since your feet are touching the floor which is at a much lower potential than the electrical outlet- creating a voltage difference across your body.

    I believe the resistance of your body is on the order of megaohms.
     
  3. captkilowatt

    New Member

    Dec 18, 2003
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    In response to your question about electric shock, it is voltage that causes the current to flow through your body. What you feel is the electron flow (current)
    disturbing your neuro-muscular system. The amount of current flowing is determined by first the resistance of you body(For most people from hand to hand is about 300 to 500k ohms with dry ,unbroked skin. You can test this with most digital volt-ohm meters), the next factor is the amount of voltage, which is synonymous with the pressure pushing the electrons. The lower the resistance
    the more current will flow and the higher the voltage, the higher the current.
    I think most text books say it takes only 2mA for 1s to stop a human hart!!

    captkilowatt
     
  4. Dave

    Retired Moderator

    Nov 17, 2003
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    2mA is a little low. This what I've been tought:

    From 0 to 0.5 mA - no sensation

    1 mA - threshold of perception

    From 1 to 3mA - weak sensation

    From 3 to 10 mA - painful sensation

    10 mA - threshold of muscular contraction in the arms

    30 mA - threshold of respiratory paralysis

    75 mA to 100 mA - threshold of cardiac fibrillation (probability 0.5%)

    250 mA - cardiac fibrillation with 99.5% probability (for an exposure time of 5 seconds)

    4A - threshold of cardiac paralysis (sudden stoppage of the heart)

    5A - burning of organic tissues

    The magnitude of the applied voltage necessary to produce dangerous current values depends on the resistance of the body, where body resistance varies between wide limits. Between hand and foot, for example, assuming good electrical contact, the resistance is about 500 Ohms , excluding skin resistance. The skin resistance varies from about 1000 Ohms /cm^2 for wet skin to about 3 x 10^5 Ohms /cm^2 for dry skin.
     
  5. lightingman

    Senior Member

    Apr 19, 2007
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    Yes Dave... You are spot on.

    That is why the trip curent for a standard RCD is 30mA.
    50mA through the heart is enough to kill you under some conditons.
    For a workshop bench mains supply, it is allways a good idea to use an isolating transformer or an RCD with a trip curent of 10mA.

    Great forum by the way... Well done to all..

    Damiel.
     
  6. cumesoftware

    Senior Member

    Apr 27, 2007
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    Normally, the voltage has all the weight when having a shock. Some people think the effects are due to current instead of voltage. But if we think in a car battery, it can give 600A, more than enought to kill suposely, but nobody gets killed by touching both terminals of such battery.
    Then you ask: "Getting a shock throught a high value resistor gives less damage! How do you explain that?"
    You should think that the human body also has a resistance (a high one indeed). When you get a shock throught a resistor, you feel less effects because your body resistance in series with that resistor acts like a voltage divider. Thats why the higher the resistor value is, the less voltage you will get, and the less of a shock you will feel.
    Moreover, remind that you can be killed when touching the terminals of a 230V outlet.
     
  7. recca02

    Senior Member

    Apr 2, 2007
    1,211
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    i believe the battery is a voltage source not a current source.

    secondly the current is determined by the resistance in the circuit.

    consider a hypothetical case of wire with no electrons.

    in this case the resistance of wire can be said to infinity. if
    this happens to be your body u wont get a shock no matter how hi the
    voltage is because there is no charge/sec flow.
     
  8. cumesoftware

    Senior Member

    Apr 27, 2007
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    You are right. The battery is a voltage source. When I'm saying that a battery can give 600A, I say it can give 600A without lowering its voltage (i.e. mantaining the voltage of 13.5V constant). Remember that the battery it is not an ideal voltage source. Its voltage can drop when overloaded.

    Also, I a certain sense it' is the current that is responsible for an electrical shock. But it is the voltage that motivates the electron flow (and that is the question here). So, no voltage, no current.
     
  9. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    The "load" dictates the current flow. Remember I = E/R?

    Measure the resistance between your two hands by pressing the ohmmeter leads between your thumb and forefinger.

    Wet your thumbs and forefingers and repeat the test.

    It's been stated that in humid enviroments, the body resistance can be as low as 300 ohms, hence there has always been a recommendation to observe safety precautions when measuring potentials 30 volts and greater. Why? Because 30/300 is 100 mA. At 100 mA you are on the threshold of cardiac fibrillation.

    So, if your body resistance dropped to 120 ohms, that 12 V 600AH battery would be capable of supplying 100 mA to your body for a very, very long time [6000 hours]. I hope your friends would come looking for you.

    The a$$ you save may be your own. You don't want to be the low resistance load.
     
  10. gootee

    Senior Member

    Apr 24, 2007
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    That's all very close to what was in a book about electronic components and measurements, by Roberge, that I had for my first EE lab course, "way back when" (circa 1975).

    While we're at it, perhaps it should also be mentioned that the 0.1 to 0.2 Ampere range (and a little higher, too, as your data indicated), referred to as "the death zone", by Roberge, is actually a lot more dangerous than some of the higher currents, because heart fibrillation will not usually stop, EVEN IF your body is disconnected from the electricity's source, or the source was only on briefly, and you will still, then, almost certainly die (at least without almost-immediate defibrillation, which is very unlikely to be available).

    However, at higher currents, the heart muscle may be "merely" clamped, and might restart on its own, if/when your body is disconnected from the electricity's source, or the source turns off, or runs out of charge, or whatever.

    Of course, if your body is conducting REALLY-high levels of current, and your internal organs, etc, begin to burn or be seriously damaged, the result could be at least as bad as fibrillation, to put it mildly.

    Sort of coincidentally, my Dad had a high-current experience, during WWII, when he picked up a live electric line that a storm had downed from a pole. Someone found him, within about 60 seconds he guessed, and knocked the line out of his hand with a piece of wood. By then, one of his combat boots was on fire, and its heel section was mostly gone. His heel had a nasty burn on the bottom. And his foot and ankle were burned by the boot. But he didn't have any other serious injuries, that he knew of, except for some relatively minor burns along his arm and leg. It was in The Phillipines, and just after a storm. So it was probably humid. And he was probably sweaty. I wonder if maybe his skin carried the brunt of the current, saving his internal organs.

    - Tom Gootee

    http://www.fullnet.com/~tomg/index.html

    -
     
  11. Dave

    Retired Moderator

    Nov 17, 2003
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    It is some 3 and a half years since I posted that information up, but this was the information provided to me as part of an electrical safety course during my degree (which means it possibly was adapted from the Roberge book).

    Dave
     
  12. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
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    Ok. Lets do the test. 1900K is my resistance when dry (sometimes I can't read it).

    Now, it I wet my fingers my resistance drops to 330K. That is not sufficient to be electrocuted by a car battery. The current here would be 0.036mA. Then again, is the voltage that motivates the current flow.
    In these conditions, I would be surely death by touching a 230V AC outlet (peak voltage is 325V, 220V is the RMS value). But the RMS current would be only 0.6mA, not enough to kill. I think the current theory doesn't apply very well to all situations.
     
  13. recca02

    Senior Member

    Apr 2, 2007
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    i understand what u are trying to say.
    but i wud stand by current as the sole reason for shock.
    b'coz the sensation of shock itself is due to the current flow
    your arguement is valid one but this is due to the fact that here current is the result of the voltage,actually not voltage but voltage difference.
    this is the cause effect relationship that is the culprit here.
    but if u consider a current source the scenario changes altogether.
    one more thing is that ,as already discussed by mr dave and mr gooty
    shock magnitude is defined in terms of current.
    but then again depending on circumstances, since we generally
    deal in with voltage sources the warnings are written in voltage terms
    to give an idea abt the severeness of the shock.
    one thing i wud like to bing forth here is that without flow of current no shock will be experienced irrespective of voltage or voltage difference.
     
  14. cumesoftware

    Senior Member

    Apr 27, 2007
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    If you consider a current source you have to consider that a current source is a almost infinite voltage source in series with a limiting resistor (that limits the current). Although, I can see your point.
     
  15. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    I agree you wouldn't be affected by that 12 volt battery. The humid enviroments that expressed the 300 ohm body resistance would be those who work at sea, surrounded by salt water.

    I don't advocate this, but I've seen people test standard 9V batteries using their tongue. A good battery will invoke an involuntary response [the perception of current]. That involuntary movement could be about 10 mA. That would make the tongue about 900 or so ohms. I've never known anyone to actually measure the current and probably wouldn't want to know anyone who did such a measurement.

    I asked for that simple illustration on how quickly your body resistance can be reduced. Your resistance dropped to about 16 percent of it's original value of about 2 megaohms. Imagine what would happen if you measured yourself just after a long, hot shower or after being in a hot tub or steam room for an hour or so.

    If your body resistance reached 300 ohms, that 12 Volt battery would deliver 40 mA ... which in Dave's chart would produce at least the threshold of respiratory paralysis.
     
  16. cumesoftware

    Senior Member

    Apr 27, 2007
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    300 Ohm is very low for human body resistance. Are you sure about this value?
     
  17. recca02

    Senior Member

    Apr 2, 2007
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    how ,were these values arrived at?

    were they tested on rats or Frankenstein :) (how much voltage wud wake him up)?

    were they calculated by the use of some theory?
     
  18. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Yes I'm sure of the 300 ohm value.

    From Volume 1 of the NEETS series ... under electrical safety

     
  19. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    recca,

    There were plenty of studies during the "electro-shock" therapy treatments that were popular 50+ years ago. I would tend to belive what they observed. Not to mention the studies done by the Germans in 1930.
     
  20. thingmaker3

    Retired Moderator

    May 16, 2005
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    Human bodies are not resistors. Human bodies are "active components." Resistance of live meat changes as a result of current flow.

    Different combinations of voltage and frequency will travel through bodies in different ways. High frequencies tend to stay on the skin, so the circus side-show performers can amaze you with lightning from their fingertips. High voltages at 60 Hz tend to run through the muscle and nerves, cooking one from the inside out. (I've seen some nasty pictures of the aftermath.)

    Different parts of the body can withstand different ammounts of current. The skull and brain can withstand much more current than the chest cavity. This is why EST didn't kill the patients.

    Electric shock can also cause problems in heart rythm, causing problems or death several hours after exposure. This is why all persons should have an EEG after being shocked.

    50V can be fatal under certain conditions. (Look up NIOSH statistics if you don't believe me.)

    As to the effects of electric shock being caused more by current or by voltage: Ohm's law applies universally to ciruits, biological or not.
     
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