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High-voltage failsafe?

Discussion in 'General Electronics Discussion' started by dbooksta, May 13, 2013.

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  1. dbooksta

    dbooksta

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    May 10, 2013
    Is there a failsafe way to confirm that a high-voltage circuit could not store or develop enough amperage to become lethal?

    Assuming we're looking at circuits fed by regular 15A 120VAC, for example, if there are no capacitors larger than some size -- or no capacitors at all -- could we say it's "non-lethal?"

    Or suppose I tap into a circuit of unknown construction (e.g., I don't know ahead of time whether it is AC, DC, or what voltage) at only two conductive points at a time: Could I put a resistor or some other device in line with the two access points that would not attenuate voltage but would guarantee I couldn't experience a lethal discharge between them?
     
  2. GreenGiant

    GreenGiant

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    Feb 9, 2012
    what do you mean store/develop?

    Do you mean that once it is unplugged/disconnected is there a way to ensure that it will be non fatal?

    if its plugged in and there are connections inside exposed then there is no real safe way to do it, there will always be a potential for injury if you poke around in the circuit.
     
  3. dbooksta

    dbooksta

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    May 10, 2013
    I'm more interested in the question while connected to source power, although if there's a surefire way to drain all capacitors of a disconnected circuit I'd like to know that too.

    Let's assume we're doing this at a workbench, so we're not soaking wet, and we're not stabbing ourselves with conductors. In those circumstances straight 15A 120VAC current is not lethal. So the only way a circuit powered by household current can become lethal is to store more current and/or step-up the voltage, right?

    So my first question is, given full access to the circuit, whether we can define failsafe criteria. For example, if the circuit contains no capacitors it can't store power, and then the only question is what its peak voltage and amperage output is (and whether that could be lethal), right?
     
  4. alfa88

    alfa88

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    An example might be a 'bleeder resistor ' on the large capacitors of a power supply, say, 10k.
    When I worked for an electrified commuter rail system and the rail was powered down breakers had to be racked out, rail voltage tested and then shorted to ground. [email protected] can leave quite a mark.
     
  5. Mongrel Shark

    Mongrel Shark

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    Jun 6, 2012
    A grounded metal tub. filled with water, will discharge pretty much everything you throw in it :D

    Might not be so great if you want the discharged stuff to work again though.

    I use a long screwdriver, connected to ground pin in my power point, to discharge CRT tubes. You have to watch out for vaporized screwdriver tip though, among other dangers. A sheet of grounded metal mesh could be good, place circuit board on mesh, soldered side to the mesh, Should ground everything at once. No guarantee it wont kill parts or explode though.

    Do you have a particular project in mind? Or is this more of a general safety concern?
     
  6. duke37

    duke37

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    I think you are wrong to think that 120V AC is safe. If the current path is across the chest and if the hand clamps on the live component, then something will give.

    I had a friend who had a hedge trimmer (240V) and he touched the plug when the top had dropped off. His hand clamped on. He said that he was tired and decided to lie down and go to sleep. Luckily a neigbour saw him thrashing about and unplugged him. The difference between 240V and 120V can easily be made up by different connections.

    Electric fencers provide several kV but in a pulse so there is the opportunity to withdraw between pulses. The energy in the pulse of about 1J is normal, some go above this but they are vicious.
     
  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    In that case, definitely not.

    Who told you that? It certainly can be lethal. It depends on what part of the body it flows through, and how good a connection you make with it.
    To be honest, unless you're talking about voltages less than around 30V, there is always SOME danger, and unless you intend to play dice with human lives, you would be very foolish to take any kind of calculated risk as you seem to want to do.

    Wrong. 15 amps is enough current not only to kill you instantly, but in some circumstances, to burn you so badly that your family will need to use your dental records to identify you. At 115V it is difficult to get that much current to flow through a human body, but it can happen.

    The only sane approach you can take is to abandon this line of thought.
     
  8. dbooksta

    dbooksta

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    May 10, 2013
    I've done household electric for years, and pain is a good teacher but working on live wires I've accidentally made plenty of solid direct connections across body parts to grounds. Check my preconditions again: not soaking wet, no penetration of skin, and no increase in voltage. I might be able to create a superficial burn from 120V if I touched a very sharp conductor, but I've never gotten even a first degree burn. In the states we rarely bother to shut off 15A circuits when wiring because the risks and consequences are so low, and nothing teaches an apprentice the importance of due diligence around lethal power supplies like accidental shocks working on 15A wiring!
     
  9. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    So you think because it hasn't happened to you yet, that it can't happen?
    That's an irresponsible and foolhardy attitude.
    I hope you are not ever responsible for another person's safety.
     
  10. dbooksta

    dbooksta

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    May 10, 2013
    Yes, I want to do some hobby-level R&D on electrostatic precipitators. Since I'm having trouble finding cheap high-voltage low-amperage supplies I'm planning to improvise from old household electronics.
     
  11. GreenGiant

    GreenGiant

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    Feb 9, 2012
    In those cases you probably didnt latch on and it was a momentary shock, but if you have your hand around something that shocks you, you will latch, you WILL get burned, and you very possibly will die.

    The fact that you are dry would make it so that the voltage is more likely to burn you/melt skin and cause surface harm and eventually after breaking through the surface it will cause internal heart/brain problems, while wet will lead faster to internal issues, or broader surface burns.
     
  12. dbooksta

    dbooksta

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    May 10, 2013
    That's not my reasoning, it was an observation in support. My reasoning is that licensed electricians (at least in the U.S.) make a practice of working on hot 15A 120VAC lines, and let their apprentices do so knowing that they're going to make every mistake possible. (NB: They do not let apprentices do this with 30A or higher circuits where mistakes can burn quickly, and 100A+ service panels which can be instantly lethal are treated with extreme caution.)

    So I consider 15A 120VAC lines to be "failsafe" so long as the worker isn't wet and isn't working on conductors that could impale him.

    I'm looking for similar criteria and practices that would keep me from potentially killing or scarring myself while opening up and touching circuits fed by household current.
     
    Last edited: May 14, 2013
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Given that a couple of hundred mA can kill you extremely dead, I hardly think a 15A supply is failsafe.
     
  14. dbooksta

    dbooksta

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    May 10, 2013
    Yes, as few as 100 mA across heart muscle can cause fibrillation, but it takes a lot of voltage to get that much amperage through high-impedence skin and then conductive skeletal muscles -- more than straight household current can deliver.
     
  15. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    You cannot predict the unpredictable. Accidents do happen. Things will happen that you did not anticipate.

    When you are working with potentially deadly voltages and currents, any risk is not worth taking... unless you believe that the loss of a human life (or intense suffering and disability) has a particular value that can be offset against your estimate of the risk.

    I have made my position clear. Taking calculated risks with human lives, especially the lives of others, is not an acceptable or justifiable approach to safety. I will leave it at that.
     
  16. dbooksta

    dbooksta

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    May 10, 2013
    So are there any useful answers to my original questions?

    Are there any practices that can guarantee a person won't kill himself while working on household electronics?

    What are safe harbors for working on electronics? E.g.,
    1. If circuit is disconnected from power source and all capacitors are discharged
    2. If circuit is connected to line current but doesn't contain capacitors or transform voltage above 120V
    3. If all contact to circuit is made through at least 1 megohm resistors?
     
  17. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Take no precautions. It's perfectly safe. Electricity can't hurt you. A deadly shock will just make your hair grow back thicker and curlier.

    Is that the answer you want?

    You don't seem interested in the truth.
     
  18. dbooksta

    dbooksta

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    May 10, 2013
    I'm trying to home in on specific, correct, practical, and actionable guidelines.

    Obviously "the truth" is somewhere shy of "all electricity can kill you, so be just be careful, and if you don't want to be electrocuted then close yourself in a faraday cage and never go near anything that can carry a charge."
     
  19. duke37

    duke37

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    There are so many variables that it is impossible to be specific. You do not know what current your heart will take before it decides to take a rest.

    The idea that a 15A supply is safe is a delusion, it is no safer than a 100A or 1000A supply. The reason that novices are not allowed to work on high power circuits is probably that an error could be very expensive and the explosion could harm the supervisor.
     
  20. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    LOL Duke :)
     
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