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Electrocution. What kills?

Discussion in 'General Electronics Discussion' started by (*steve*), Dec 19, 2012.

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  1. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The fact that it's AC doesn't make it dangerous.
  2. quantumtangles


    Dec 19, 2012

    It depends on the power in Watts. If a kitchen oven is under load (turned up high for example), touching the wrong cables would probably be fatal because of the higher current draw x standard voltage. Slightly less dangerous if not under load...but still eye widening.

    In any event, I built a bridge rectifier from 4 x N5406 Diodes. It rectified splendidly, pumping out no AC at all. For reasons buried somewhere in the datasheet...15 volts DC popped out the other side. (Note to self...get a new tuxedo for the ceremony in Stockholm)

    But the amp hummed ominously and then (horror) began to crackle.

    I might have to think before taking the next step (check the datasheets).

    I toyed with the idea of posting a photo of the bridge rectifier online, but decided not to. If machines take over the planet, it could warrant summary execution (crimes against components) :eek:
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    No, that won't matter a bit.

    It's the current that flows through YOU that makes it dangerous.

    At low voltages it's simply very unlikely to happen. At higher voltages it's much easier.
  4. quantumtangles


    Dec 19, 2012
    I am not a circuit component

    I agree. Remove the human from the circuit and the risk of electrocution is greatly diminished.:D

    But I get your point. The greatest danger is when high power cables cause thermal damage that breaks the skin and get inside the body cavity (which is largely fluid). Terminal.
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    That is always the intent :)

    But no you don't. Power doesn't mean much art all.

    The impedance of the voltage source matters greatly. Whilst that means it is capable of delivering more power, it does not follow that it does in normal use.

    The current required to kill you is relatively small, and it can be very easily carried on quite fine wires.

    Your explanation would be better if you replaced the term "power" in your post with "voltage"
  6. quantumtangles


    Dec 19, 2012
    Static electricity is not dangerous, despite extremely high voltage, because it is only in contact with human beings for pico-seconds.

    I have always understood that power in watts is what matters. Volts x Amps = Watts
    Both are needed to perform work, whether that be the work of rotating an electric motor or the work of electrocution.

    I know so called 'low voltage' can be dangerous in certain situations, but this makes sense when you consider that human voltage characteristics are between -0.04 volts and +0.07 volts.

    You still think I don't get it and you may be right. What algorithm have I failed to grasp? I am on the verge of understanding what you mean.
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Static electricity can kill. Have you ever heard of being struck by lightning? (you don't even need to be struck by it. Animals are regularly killed just by being near a strike so that current flows up one leg and down the other. People, being bipedal (and not having our heart in our groins), are less often killed this way.

    A current path through your heart is often the killer. Burns to your arms, legs, etc, are not going to kill you (or at least not immediately).

    High frequencies are also less dangerous due to the skin effect (note that the term skin here does not refer directly to your skin).

    A very brief pulse will be inherently high frequency. As such it's not going to be so problematic.

    Once current flowing through your heart reaches a certain value, your hear will lose rhythm (fibrillation). At higher levels it will stop completely.

    It's the current that does it, and the voltage required is not very high (since conductivity is relatively good.

    However, we are covered by a relatively good insulator (our skin). It takes a higher voltage to break through that barrier. Also important is the surface area (so wet feet are an issue in some cases) because it reduces the resistance.

    You will note that if people have certain sensors on their skin, or if they are being operated on, electrical safety standards are VERY high. This is because even a low voltage can kill them.

    At moderate frequencies, the difference between AC and DC is not significant. Both will kill you.

    However, what is ultimately important is that current can be delivered across your heart. For that, the voltage source needs a low enough impedance to supply the current (20 to 100 mA) for long enough to overcome certain inductive effects, across points that allow the current to pass through your heart.

    This can come from relatively low power circuits, but not low voltage or low current ones. It doesn't matter that you might overload a circuit to kill yourself!

    Thinking that only high power circuits are dangerous can get you killed.

    Consider the size of wire that can carry a current of 50mA without burning out?
  8. quantumtangles


    Dec 19, 2012
    Interesting post. Consider me to have been reprogrammed :D
  9. Relayer


    Dec 15, 2012
    I have to say that microwave ovens are a proven killer, even when disconnected from the mains after several weeks.
    The high voltage capacitor in them can store a charge for quite some time. The bleed resistor incorporated in them are around 9+ Meg ohm and can take ages for the cap to discharge normally. These caps can be rated from about 1.5kv to 3kv, from memory. Been ages since I've worked on one.
    I have heard of many technicians being killed by microwave ovens, some have done so carelessly.
    In one case the tech used a pair of insulated heavy duty pliers to pry off one of the connections to the magnetron while the unit was being cycled. Little did he know that a crack had formed in the insulation of his pliers. The last time he did it cost him his life. A spark arched over from the crack into his hand, killing him instantly.
    The reason for doing this type of test is to tell whether or not the magnetron was in working condition. By disconnecting during a heating cycle, the hum of the microwave will change if the magnetron is working.
    Common sense approach was to take note of the hum prior to testing, then turning the power off at the mains, but leaving the mains plug connected for a ground point. Then using a long insulated screwdriver inserted into the positive side of the HV capacitor and shorting the shaft of the screwdriver to the metal of the microwave case.
    Then disconnecting one connection to the magnetron, keeping it away from any exposed metal parts, then cycling the oven to see if the hum pitch had changed.
    I never liked working on these units and did as little with them as possible.
    Relayer :D
  10. donkey


    Feb 26, 2011

    during a soccer (football) match lightning struck..... pardon the pun here but this is shocking.
  11. GreenGiant


    Feb 9, 2012
    its amusing to me how no matter how much you explain it to some people they wont understand it.
    Working with battery cells (Li-ion) at full charge they have a voltage of 3.6-4.2V not enough to pass enough current to do any damage. But people still freak out when I touch both tabs, and are super hesitant about it.
    I mean even if we have a malfunction the max voltage our testers output is 5 volts, we have one that will do 12 but that's it.

    Yes they are high POWER cells (can deliver over a kW of instantaneous power) but they are low VOLTAGE cells so you wont get hurt because the bodies impedance is too high.

    Oh well
  12. npomeroy


    Dec 9, 2012
    As mentioned earlier, it takes a few tens of milliamps passing through the heart. If the duration is more than about three quarters of a heartbeat the heart may stop. It most readily passes through the heart if the circuit passes from one hand to the other, or from a hand (or upper trunk) to the feet. The greatest resistance is at the skin, which varies greatly. My understanding is that under "ideal" conditions (wet skin) anything over 50 V can be enough to provide damaging current through the heart. (Ohm's law) When the current path is not through the heart the body can survive huge currents but with resulting burns.
  13. donkey


    Feb 26, 2011
    you could do it at 9volts given the right circumstances. like if you applied it to the heart directly.
    its current that stops a heart not volts, try not to confuse the 2.
    having said this a defibrillator does this exact thing. alot of people don't realise this. alsoa secondary thing from electrocution is putting the heart into fibrillation. what a defib does is like rebooting the heartby sending a current across it. this stops the heart for bout a second, then when it comes back it SHOULD (not always though hence multiple shocks) go back into normal rythym. anyway thats of the topic a little.
    if your heart is shocked (and yes this is the technical term, going into shock) a heap of things happen.
    TRUE electrocution iswhenenough current passes over theheart to stop it. an electric shock can send the body intoshock andthen you end up dying from other things. eitherway just be very cautious around electricity.
  14. cjdelphi


    Oct 26, 2011
    What people are missing out here is the word "Luck",

    Electricity will flow where there is the least amount of resistance in your body, eg, I once touched an almost fully charged 340v capacitor, I felt the spark penetrate my skin on my right hand then almost a fraction of a second later through my LEFT hand i felt arc jump from my finger to whatever i was touching, ground.

    it fascinated me and freaked me out,.

    Because there was potentially >250v DC in that cap, my skin does not have enough resistance to prevent myself being used as an electrical wire, unlike a wire which is nice and conductive from start to end, humans in general, however are not, so the current just flows wherver it happens to go until it gets to earth/ground taking the least resistive path,.

    I got lucky, if any of that current (not volts, be 5ma from 1000v or 5m from 3v) had of passed through my heart....... it could have stopped it beating and i'd have gone into cardiac arrest.. why did it not? luck.

    take a multimeter, set it to measure resistance, it can use up to 9v dig the probes deep enough into your skin on both hands where the lest resistance is, you then have current running freely from one arm to the other, you could in theory die if you're unlucky.
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