Question about voltage, lethal current, skin resistance

Discussion in 'General Electronics Chat' started by kevin0228ca, Jun 7, 2015.

  1. kevin0228ca

    Thread Starter Member

    Jun 5, 2015
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    I am new to electrical engineering and have some basic questions.

    We know current is responsible for fatality.
    100 - 200 mA is lethal, while >10mA could damage.
    Suppose we have a 12V battery circuit loop with a 100 ohm resistor, current is 12/100 = 120 mA
    but when we put ourself in loop we wont get shocked, right?

    Is that because that our skin resistance will be take into account in that situation?
    so current would be 12V / (100 + whatever skin resistance) = less than 120 mA?

    If that is the case, how do we know a circuit or current source is safe to touch?
    because current will decrease when we touch?
    and how do we ensure a stable < 10 mA current source regardless of human skin resistance?

    Please can anyone answer these questions.
    Thank you.
     
  2. takao21203

    Distinguished Member

    Apr 28, 2012
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    upto 100 volts normally are harmless but limits are 48v/60v

    there is always worst case:

    wet concrete, barefoot, left hand + locking grip after touch.
    That could kill you at 100 volts.

    There are some rules:

    Avoid wet equipment, concrete floors by all means
    Never use the left hand
    Use a neon lamp checker, if it lights up the voltage is dangerous

    Or as for OP: If you cant apply common sense no circuit is safe to touch, best, dont deal with electricity.
    Only circuits approved for educational + safety tested are good for you in this case.

    The NEMA book and so on are just a guideline, by no means exhaustive.

    There isnt really such a thing such as 10mA supply no matter what, human touch can be close to 1 M Ohms and as low as 1 K Ohm under some conditions.
     
  3. wayneh

    Expert

    Sep 9, 2010
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    I think you've got it. Even wet skin will add enough resistance, 1000Ω or more, to the circuit that very little current flows. And a current that could be trouble for your heart isn't as dangerous if it's routed elsewhere. For instance, touching a fresh 9V battery to your tongue produces a strong sensation but is more unpleasant than dangerous.

    As they say about guns, treat them as if they are loaded. If there is the slightest doubt, proceed with caution. Other than home wiring, I rarely work on anything above 12V. I know I can directly grab the poles of a 12V source (eg. a car battery) without worry of a shock. I don't use my tongue! So I know I can relax around my <12V projects as long as there is not something in the circuit converting it to a higher voltage.
     
    Last edited: Jun 7, 2015
  4. wayneh

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    Sep 9, 2010
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    It's worth noting that even µA currents can have a profound biological effect over time (minutes), despite being far below a shock hazard.
     
  5. kevin0228ca

    Thread Starter Member

    Jun 5, 2015
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    thank you for your answers.

    so skin will add resistance to a circuit therefore current decreases.
    then supposedly how do we know 100 mA is the level of lethal current?
    I mean, suppose we know a source or circuit is 100 mA flowing, and we touch and get burned, so we say 100 mA is lethal.
    But when we touch, 100 mA is suppose to decrease, right? so maybe is 80 mA?

    Another question is about current flow route, suppose input is at my right finger, and I have a return ground at my right forearm.
    Then route would be from my finger out my forearm, so in theory would be harmless or some current will still flow away from ground in direction of my shoulder for example?
     
  6. WBahn

    Moderator

    Mar 31, 2012
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    You've got a number of misconceptions and near misconceptions, which is fine since you are new to this and trying to learn it.

    The amount of current that it takes through various paths through the body to produce negative effects is well studied both in controlled lab experiments on humans and animals and also through accident investigation.

    One of the first things to get out of your head is that there is some rigid threshold, such as the 100 mA you've mentioned numerous times, in which less than that is okay and more than that is fatal. Not only is each situation different in how much effective resistance there is that will limit the current, but also each person's tolerance is different, not only from everyone else but also over time. So the same current through the heart that might be very unpleasant today might be fatal tomorrow. The ranges you read about are aggregate values that are biased toward the safe end.

    Current flowing in your right finger and out your right forearm probably would not be fatal, but could be very far from harmless. Enough current could cause muscle contractions (which could cause secondary injuries from what you hit, but let's ignore that), possibly severe and damaging, it could cause nerve damage, possibly permanent, it could cause burns, possibly severe, it could cause your arm to literally explode.

    The current flowing through a circuit is almost unrelated to the current that will flow through you if you get tangled with that circuit. When I was at NIST we had wall-mounted bus bars that carried up to 4000 A to the sample and you could walk up and touch them and all that happen was that you injected a lot of noise into the measurement -- you didn't feel a thing because the voltage difference between the terminals was measured in millivolts. But there are circuits that are voltage with high impedance loads that normally draw a few milliamps but if put your hands across the load they will absolutely light you up.
     
  7. kevin0228ca

    Thread Starter Member

    Jun 5, 2015
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    Thank you for your answer.

    So even if we say current is value to look when determining fatality, voltage should also be considered because of varying human skin resistance.
    So for example 10V, 100A circuit could be harmless because skin resistance is at least 1000 ohm.
    But 100V, 1mA circuit could be harmful or harmless depending varying skin resistance


    I have a question about products on market that use electro stimulation of human then, TENS, EMS, tDCS, etc.
    they provide a constant current of say maybe 0 - 10 mA in order to be harmless.
    but how do they deal with varying skin resistance?
    if skin resistance ranges 1K - 100 K ohm, then 10 mA would require product to provide 10V - 1000V?
    if really 1000V, how is that harmless since our skin resistance could decrease to 1000 ohm?
     
    Last edited: Jun 8, 2015
  8. DickCappels

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    Aug 21, 2008
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    Not sure about the others, but tDCS stimulators usually if not always use a constant current source that adjusts the voltage between the electrodes to maintain a constant current in spite of changing circuit (head) resistance.
     
  9. WBahn

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    Mar 31, 2012
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    If the circuit takes 1mA from a 100V supply then the potential for harm is there. But if the power supply is only capable of outputting 1mA, then there isn't. Do you see the difference?

    A constant current source will change it's output voltage so as to maintain the current. So if the skin resistance drops (or any other change) the output voltage will drop accordingly. In circuits such as you describe there are also going to be limitations on the maximum current that can be delivered under any conditions, including short circuit operation.
     
  10. john*michael

    Member

    Sep 18, 2014
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    All of the remarks are pertinent. If you are looking for a guideline, per IEC 61010, non-hazardous voltages, designated SELV for "Safety Extra Low Voltage" is designated to be 60 VDC for direct current and 30 VRMS/42.4VPK for AC. These voltages are allowed on connectors for printers, mouses, etc that have pins that could possibly be touched by a user.
     
  11. dl324

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    Mar 30, 2015
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    Hi Kevin,

    Your take away should be that electricity should be treated with respect. A "safe" voltage/current can become unsafe under the right conditions. 5V is generally safe, but 5V @ 100A can cause a very bad burn (or worse).

    I knew a guy who could handle 120VAC without getting hurt, but most of us wouldn't enjoy the experience.

    I have heard of cases where high voltage power lines fell on cars and some people were killed trying to leave their vehicle and some survived.
     
  12. john*michael

    Member

    Sep 18, 2014
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    You handle 12 volts any time you work with jumper cables. Most thermostats and old-style doorbells run 24VAC. Old analog telephone lines (probably before your time) had 48 VDC on the lines at all times. If you are working with 12 volts, there is really nothing to worry about.
     
  13. wayneh

    Expert

    Sep 9, 2010
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    I built a couple of those. The constant (low) current device uses a 9V battery and is limited by that. It tries to maintain a signal at, say, 60µA but cannot do so if it requires more than 9V to drive the current. It goes to the rail and the current is dictated by the overall circuit resistance. If it was an audio amp, you might say it was clipping. With the tiny current and low voltage, the user perceives nothing until about 20 minutes have passed.

    The TENS device also runs off the 9V battery, but uses a transformer to boost the voltage to over 100V. The user can definitely feel this pulse and is able to control the frequency and intensity to suit themselves.
     
  14. dl324

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    Mar 30, 2015
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    Ever see a car battery shorted? I'm always cautious when working on my vehicles; especially after I had the negative battery cable clamp melt part of the battery case in my truck. Clamp went bad (high resistance) and literally melted into the battery when I tried to start the vehicle...
     
  15. john*michael

    Member

    Sep 18, 2014
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    The clamp conducts better than you, thankfully. Car batteries can provide 100's of amps when shorted, which requires a high current, low resistance path. This can cause heating as you described, but the voltage level itself does not present a direct hazard to people. Absolutely keep track of where you lay down those jumper cables.
     
  16. dl324

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    There are usually exceptions; the devil is in the details, or the current path as WBahn mentioned. Pacemakers operate as low as 1.5V. A malfunctioning pacemaker could kill at low voltage and low current...
     
  17. WBahn

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    Mar 31, 2012
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    If you pierce the skin resistance, say by sticking a needle into each wrist, then a 9V battery can deliver a lethal current. But that is an unusual exception and not something that most people should dictate their behavior by. I'm quite comfortable getting tangled with both sides of a 12V car battery, but very careful not to let a low resistance path tangle with it.
     
  18. takao21203

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    Apr 28, 2012
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    10mA or so will make you serious trouble and may do damage that doesnt show immediately.

    Batteries dont kill anyone thats a known fact or the sale would be restricted.

    In general there is no specific reason to touch electric electrodes or cables its abnormal situation. Thats all you need to know.

    You can try this:

    Twist bell wires together, connect to mains and cut off sharp so just the two cores have a point contact on the top. Then touch between them with one finger.

    You will get the idea. Just dont touch it at all.
     
  19. WBahn

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    Mar 31, 2012
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    Really? Ever hear of a B battery?

    I've seen discussions of zinc-ion batteries that are 180V. I haven't been able to track down any specifics, but I doubt that their sale would be restricted just because of the voltage and consequent ability to push lethal currents.
     
  20. dl324

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    Mar 30, 2015
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    Back when I was a teenager, socket sets came in metal boxes. I was careless and let the box touch both terminals on the battery. Put a hole in it, but I still have it...

    Had a friend who didn't have a tie down on his battery and it managed to get shorted against the hood on a bumpy road long enough/enough times to explode. Put a big dent in his hood...

    They're generally safe, but I always treat them with respect and make sure I know where I'm putting my tools and follow the proper order for installing/removing terminals.
     
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