Physiological effects of electricity-AC is more dangerous

Discussion in 'Feedback and Suggestions' started by RKEM, May 1, 2008.

  1. RKEM

    Thread Starter New Member

    May 1, 2008
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    In the section Physiological effects of electricity it is stated that DC is "more likely to "freeze" a victim in a shock scenario" but the Ohm's Law (again) section has a chart which seems to contradict this statement by showing that it is hard to let go of a wire with only 16ma of AC at 60HZ vs. 76ma DC. If you look at high frequency 10khz the threshold is about equal between AC and DC, but 60Hz is what people would most likely be exposed to.

    Merck states that 60HZ AC is "3 to 5 times more dangerous than DC of the same voltage and amperage. Low-frequency AC produces extended muscle contraction (tetany), which may freeze the hand to the current's source, prolonging exposure. DC is most likely to cause a single convulsive contraction, which often forces the victim away from the current's source."
    <http://www.merck.com/mmpe/sec21/ch316/ch316b.html>
     
  2. Dcrunkilton

    E-book Co-ordinator

    Jul 31, 2004
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    RKEM,

    After, reading the reference, I agree with what you say eventhough it conflicts with what I though was the case. I will work on revising the text and place the reference you provide in the Bibliography. Thanks for your input.

    Dennis
     
  3. studiot

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    Nov 9, 2007
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    I would appreciate a link to the section on the physiological effects as I can't find it.
     
  4. Unregistered

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    <http://www.allaboutcircuits.com/vol_1/chpt_3/2.html>
     
  5. studiot

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    Thanks for that.
     
  6. mrmeval

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    Jun 30, 2006
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    I can state that 15,734 Hz at 32,000 volts pulsed DC is rather easy to let go of.

    You let go of it no matter what damage your body will take in the process. Even if that means ripping your hand out of the TV you're working on.

    Some bastard stuck a pin through the high voltage cable in a hotel TV. It would build up and arc repeatedly but the interval was a few seconds. That is until some part of you came near or into contact with the pin.

    I've also come into full contact with a cut cable, once.
     
  7. Dcrunkilton

    E-book Co-ordinator

    Jul 31, 2004
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    This is how I propose to corret the text:


    remove:
    Direct current (DC), because it moves with continuous motion through a conductor, has the tendency to induce muscular tetanus quite readily. Alternating current (AC), because it alternately reverses direction of motion, provides brief moments of opportunity for an afflicted muscle to relax between alternations. Thus, from the concern of becoming "froze on the circuit," DC is more dangerous than AC.

    REplace with direct quotation of Merk reference:
    How AC affects the body depends largely on frequency. Low-frequency (50- to 60-Hz) AC is used in US (60 Hz) and European (50 Hz) households; it can be more dangerous than high-frequency AC and is 3 to 5 times more dangerous than DC of the same voltage and amperage. Low-frequency AC produces extended muscle contraction (tetany), which may freeze the hand to the current's source, prolonging exposure. DC is most likely to cause a single convulsive contraction, which often forces the victim away from the current's source.


    Dennis
     
  8. studiot

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    Nov 9, 2007
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    I think your original statement is closer to the truth.

    Since muscles can only pull, they cannot push, muscles operate in pairs in the body. One for each opposing direction. Opposing muscles are called extensor and relaxor muscles.

    An important point is that for most extensor/relaxor pairs one is dominant (much more powerful). The muscles that allow you to grasp something are much more powerful than those which cause you to let go.

    Normal operation of the muscles is under the control of the central nervous system, which prevents opposing muscles conflicting. The control is exercised by selective electrical current impulses.

    If a part of the body comes into contact with a live conductor current will flow locally through the body, depending upon the nature of the voltage and the intimacy of contact. 'Locally' depends upon the return paths through the body and may extend to part or all the body. For example say contact is initiated through a hand it depends what the other hand is holding and whether the subject is wearing insulating soled shoes.

    This current affects all muscles equally in its path and may be sufficient to outweigh the normal impulses. In particular both relaxor and extensor muscles will be equally stimulated. Since the grab muscles are much stronger they will tend to maintain the contact.

    There are two types of contact which can profoundly affect the outcome of any contact. Glancing contact and grasping contact.

    If the locality of the current does not extend over the whole body the subject may be able to use other muscles (eg the shoulder) to force disconnection and literally drag the hand away. This is, of course, easier with a glancing contact.


    British Standards specify a 'standard' test finger for checking that no one can reach live parts through any opening in equipment cases.
     
  9. jpanhalt

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    Jan 18, 2008
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    I just searched the NLM and Archives of Pathology on electric shock. The on-line indexes have no information on the relative risks associated with AC and DC shock. It is said that AC is more prone to cause heart fibrillation while DC can stop the heart. Either way, the heart is not working. Of course defibrillators use modified DC and it may be easier to restart a stopped heart than one that is in fibrillation, but that gets into therapeutic interventions and opens a lot of other issues.

    Many people are aware that the electric chair used AC current, but the selection of AC over DC was greatly influenced by the political power of Edison over Westinghouse/Tesla, not efficacy. The unfortunate thing about the Merck reference is that it doesn't cite any references.

    Thus, I would recommend taking a course similar to what studiot recommends. Describe the freeze effects of DC vs. the jolting of AC, but leave discussion of which is more dangerous out.

    Here is a quote from Wikipedia on electric shock:

    "The comparison between the dangers of alternating current and direct current has been a subject of debate ever since the War of Currents in the 1880s. DC tends to cause continuous muscular contractions that make the victim hold on to a live conductor, thereby increasing the risk of deep tissue burns. On the other hand, mains-frequency AC tends to interfere more with the heart's electrical pacemaker, leading to an increased risk of fibrillation. AC at higher frequencies holds a different mixture of hazards, such as RF burns and the possibility of tissue damage with no immediate sensation of pain. Generally, higher frequency AC current tends to run along the skin rather than penetrating and touching vital organs such as the heart. While there will be severe burn damage at higher voltages, it is normally not fatal.

    It is sometimes suggested that human lethality is most common with alternating current at 100–250 volts, [emphasis added]however death has occurred from supplies as low as 32 volts and supplies at over 250 volts frequently cause fatalities."

    Comment: I think that is a good description. The one thing I would modify is the italicized statement. It needs to be put into context. Namely, the fact that most deaths from electric shock are caused by AC may be more related to the prevalence of potentially lethal AC supplies than the inherent risk of AC vs. DC of the same voltage and current.


    John
     
  10. Dcrunkilton

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    Jul 31, 2004
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    studiot,

    I was only proposing to replace the one paragraph above (not the entire section) with the new paragraph. The remainder of the text in the section would remain unchaged.

    Dennis
     
  11. studiot

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    Nov 9, 2007
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    It may be the glancing contact I mentioned, it may also be the fact that the 32KV DC has very limited current capability. The normal threshold of fatality is considered to be 50milliamps. At 32KV this implies a 1.6 kilowatt DC supply!

    It may simply be that most supplies of any capacity are the AC mains. Thus most serious electric shocks come from here. It is well known that people have little chance of surviving contact with heavy duty DC supplies like electrifed train pickup rails.

    So I agree with John, discuss the physiology and effects but don't specifically assess and compare the risks.
     
  12. RKEM

    Thread Starter New Member

    May 1, 2008
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    Although it is somewhat counter-intuitive the research suggest the opposite effects of AC vs. DC

    This section of Wikipedia does not cite any references. For all we know the author used allaboutcircuits as his reference, some of the wording is similiar.

    Here is another article that could be cited:
    http://www.emedicine.com/MED/topic2810.htm

    This article references: Fish R. Electric shock, Part I: Physics and pathophysiology. J Emerg Med. May-Jun 1993;11(3):309-12.
    Which I don't have free access to but is probably the original source of the claims about AC being more dangerous than DC.

    Based on the data and research, I agree with
    Dcrunkilton's revision of the physiological effects of AC and DC. Like some of the others here, I'm not sure about the value of discussing the relative danger of AC vs. DC. Unless you are in a controlled lab environment there are some many other variables involved in assesing the danger or risk of electrical injury, that a blanket statment that AC is more dangerous than DC may give people a false sense of security while working around DC. On the other hand I don't like the idea of "protecting" people from the truth to keep them safe.
     
  13. studiot

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    Perhaps you should read your reference critically, for it contradicts itself,
    something we all hope this discussion will prevent All About Circuits doing.

     
  14. jpanhalt

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    I read the full articles by Dr. Fish in his three-part series on emergency management of electrical shock. Nowhere does he compare AC and DC shock. It is a trivial point, but he also refers to AC shock as being a "jolt."

    Apparently, my original thesis that effects of electric shock should be described, but comparative risk should not be was missed. There is a paucity of documented information on relative effects of AC and DC available on-line. Fish's reports were based entirely on contact with AC electric service in the USA. Another article by Peng and Shikui (Forensic Science International 76(1995)115-119) reported pathology from 7 instances of low-voltage (<80V) electrocution and serious injury. The subjects were all welders; six died after being subjected to DC shock. The one survivor was found unconscious, but survived from an AC shock. That proves nothing, and the authors did not comment on any differences between AC and DC shock. The lowest lethal voltage observed in this study was 47V DC (open-circuit).

    I did find an abstract of a book chapter that unfortunately is not on-line, but sounds interesting, particularly since it cites three references that might have results from original studies. The chapter is: Effects of Electrical Current in the Human Body by DC Winburn (no joke). The book is Practical Electrical Safety, Marcel Dekker, Inc. New York, pages5-16, 1988. Both AC and DC can cause the limb to freeze and be unable to release. The author refers to "let go currents," i.e., the maximum current at which the subject could voluntarily release a grip on the conductor, and gives the following values: for AC, males= 15.9 mA; females= 10.5 mA; for DC, males=76.1 mA; females=50.7 mA. That result, while interesting, does not show relative risk of death or serious injury.

    There are hundreds of articles on homicide and suicide by electricity. It can kill, regardless of whether it is AC or DC. That should be the message. There are also lots of articles about the physiology, pathology, clinical signs, and emergency care for victims electrical shock, but this is not All About Medicine.

    John
     
  15. theamber

    Active Member

    Jun 13, 2008
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    I think it depends on the scenario. At high currents there are both equaly lethal. At low currents they can stop the heart equally. Ther are too many factors to decide. Like depending on the amount of sodium in your body, sweet, humidity, heart conditions, body weight and so on.
    For a personal experience I have been shocked at 220 AC and 640 DC. The 640 DC was much worst I felt I could not separate myself from the chassis of a tube type microwave generator it felt like 2 to 3 seconds I even had time to call for help finally I pulled away really hard. Part of my skin got stuck to the chassis It burned a black hole inside my right thumb that lasted years to desapear and it did change my fingerprint for life. I also felt my arm numb for several hours, by the way (but I do not recomend to change your fingerprint this way). The sensation of a high volt DC shock is the worst thing I have ever felt and I am 6.04 and 235 pounds. The AC shocks were like a disgusting tickling sensations that soon goes away. Hope we just learn to use a lot of care when working on live circuits no matter what.
     
  16. m4yh3m

    Senior Member

    Apr 28, 2004
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    or you could just tell people to slap everything they're about to work on with the top of their hand instead of trying to grab it :p that way if there IS current, any risk of hand contraction that would keep you holding onto the wire would be zero. One of the first two rules of electricity my teacher in vocational school taught us waaaay back when.

    #1. Use only your right hand as much as possible.
    #2. Never grab, use the back of your hand to test.

    Reason for #1: If you grab an energized circuit with your left hand, the current will go up your arm, near your heart, and down to your shoes. If you grab it with both hands, the current willl flow across your arms through your heart. So when possible, right hand only -- unless you know you can safely work with both hands.

    #2 was explained above.

    Thankfully I followed them when I went to move a tv board that someone left plugged in on a table. The muscles in my lower arm started tingling and then throbbing bad. Hurt like a sonofabiscuit.
     
  17. theamber

    Active Member

    Jun 13, 2008
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    You are right but usually those are the last things on your mind when troubleshooting. Actually I even forget that the equipment or a particular section are energized sometimes as happened to you in your post that is mayor problem not knowing is energized.
    I heard about the left hand dangers because of close proximity with the heart, but about the back of the hand I never heard it nor understand why will you touch a circuit with the back of the hand if you know is going to shock you?. But I understand that if you grab your muscles may contract and will be harder to separate from the equipment but what can you do with the back of the hand, hold something??? If it is energized the best is not to touch any conducting part.(period)
     
  18. m4yh3m

    Senior Member

    Apr 28, 2004
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    A family friend of ours used to perform electrical maintenance in an office building. He was on a ladder up in the ceiling tiles. He grabbed onto conduit that, for some reason, was live. The electricity caused the muscles in his hand to contract and kept him holding onto the conduit. He had to knock himself off the ladder, and in the process landed on a desk and broke a couple of ribs and knocked himself out.

    After that happened, I took the good teachers advise and never "grab" for anything without checking it for current. Don't care what anyone else says :p I used to be a game technician, and had to reach under a portion of the machine to fix some oddity, but the machine was still turned on. As I reached in, something bit me. Someone did not properly insulate the cables to the lights on the game (they ran standard 120/60). I don't trust anybody and their "say-so" until I do my own check to ensure everything is safe.

    That's why I will forever follow those two rules.
     
  19. Unregistered

    Guest

    It does not contradict itself. The statement about DC vs. AC was _at the same voltage/current_. The statement that DC is high voltage was a description of power transmission systems.
     
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