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At what capacitance do HV capacitors become dangerous to human touch?

  1. Apr 30, 2013 #1
    HI, please do not get me wrong, I am trying to grasp and learn when HV capacitors get dangerous and I am confused right now. Your help is extremely appreciated. Caps discharge their energy very very quickly and said THERFORE to be dangerous, but a Van de graaf also discharge its energy very very quickly. For instance in theory to illustrate( my point):

    1- fully charged 100kv 25pF capacitor discharge is 250amp in 100 millionth of a second( 0.125J).
    2- fully charged 100kv 25pF V.D.G discharge is 250amp in 100 millionth of a second. (0.125J)

    So both have the same charge and capacitance, why is then a capacitor always said to be very dangerous? A VDG is a capacitor and also will discharge its entire charge in a very very short time?

    To make my point as clear as possible:
    Are fully charged 100kv 25pF VDG and a 100kv 25pF Capacitor equally "dangerous" to human touch? (they should be?)

    Thanks
    Best regards
     
  2. jcsd
  3. Apr 30, 2013 #2

    mfb

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    I would expect the same danger from both, but don't rely on that for experiments...
    The VDG as you described it is basically like a capacitor.
     
  4. Apr 30, 2013 #3


    Thanks
    so what energy at what duration is considered dangerous? I ask since all doorknobs, VDGs and capacitors all discharge huge energy in a very short time. It is hard to know "what magnitude of energy" at "what duration" that is dangerous. Certainly, an ordinary VDG isn't dangerous even though it can have 100s of amps in its discharges in a nano second. So when is a discharge dangerous, at what energy per time(our body resistance can be neglected at these high voltages), I read once that the danger starts in 0.002 A 1 millisecond pulse.. but I am not sure at all about that..
     
  5. Apr 30, 2013 #4

    mfb

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    Doorknobs don't discharge much energy, and they rarely have 100kV relative to you. As power grows with voltage squared, this reduces power a lot.

    I don't know numbers about dangerous fluxes.
     
  6. Apr 30, 2013 #5
    Your assumption that capacitors discharge their energy "very quickly" is not generally true.
    The time to discharge depends on the resistance in the circuit.
    How did you arrive at a time of " 100 millionth" of a second.
    The effective resistance in series with a Van de Graaf generator is probably of the order of megohms
     
  7. Apr 30, 2013 #6
    You seem to be confusing 'energy' and 'power'
     
  8. Apr 30, 2013 #7

    mfb

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    I am not.
    P=V*I=V^2/R (falling as the voltage goes down, of course)

    Energy, on the other hand, is given by 1/2 C V^2. This allows to calculate the time constant of the discharge as ##\tau = \frac{E}{P} = \frac{1}{2} C R##. With C=25pF and R=1MOhm (typical resistance of a human), this gives 25µs. Therefore, most of the discharge will happen in the 100 microseconds mentioned by leviterande.
     
  9. Apr 30, 2013 #8

    Yep, there is not a lot power in doorknobs I was just trying to make a point that anywhere there could be a discharge of very high current in minute durations :) and the lethal lmit therefore must be at what duration these occur.


    Thanks:) I see, do you maybe happen to know just approximately at what size the HV caps get dangerous? or who or where I should do know this?
     
    Last edited by a moderator: May 1, 2013
  10. May 1, 2013 #9
    energy vs danger

    It is generally and widely accepted that two caps with the same energy are equally dangerous. However, I am wondering this: Considering we are dealing high voltage caps and human touch with a specific resistance say maybe 1k ohm, is a certain Joule energy always equally dangerous and electrocuting regardless of capacitance ?

    let me illustrate with what I believe is considered a dangerous in capacitors, 10Jouls discharge:
    There is for instance around 10 Joules in 330V, 200uF capacitor. That is around maximum instantaneous starting current of 0.33A lasting about 0.2 seconds thru a 1kohm human. There is also 10J in a 1000,000V, 20pF Cap where the maximum instantaneous starting current through 1kohm human is 1000A lasting about(1/50,000,000 of a second). the difference between the above two caps however is that in the former cap, the duration of the 0.33A discharge is much longer than the duration of the 1000A in the latter, because of the equal joule energy.

    So as you get the picture and can see despite equal total 10J energy of the two caps, the discharges of the 2 caps are different in both duration and amount of current flow. One discharge has medium(lethal) current flow for a relatively long duration(0.2 seconds) while the other has an extremely high current flow(1000A) for an extremely short period of time at the beginning (1/50,000,000 of a second). (I know any cap discharge-power is actually a curve but you get the general picture of the discharge)

    Are the both caps therefore equally dangerous/electrocuting to the same 1khohm hand just because of the 10J and regardless of how the exact discharge-duration and power looks like?
    I hope you can see what I am trying to show and understand. All I really want to know is at what level the caps are dangerous..

    Thanks
    Best regards
    /levi
     
  11. May 1, 2013 #10
    Because the human body has resistance - you never see these maximum theoretical currents through the body - as compared to discharging a capacitor. Most safety standards are derived based on common hazards: large power caps, batteries and the power systems - in this realm the limits tend to be 50V ( for systems with power sources) - for limited energy systems the regulations can be very specific - nor non-existent. There are many variables -- Time, path through the body, current, AC vs DC, skin resistance - etc). So some VDGs will be OK for human touch(those sold for HS Physics) and some clearly not (those sold for Graduate Student Physics).

    The Wikipedia on Electric Shock this has some numbers - and a good write up.

    For components - like standard capacitors, there are not really any rules about how to use them safely. "Capacitors" sold for power factor correction for example, are not really a component - they are a product, and will follow certain rules - like having a bleeder resistor ( so the are not pure capacitors anymore) -if you can see the distinction.
     
  12. May 1, 2013 #11
    Windadct, yes, there are though only very broad general lines where safety standards lie, as you said large power systems are of course dangerous.

    Can anyone knowledgeable in this field if possible comment on my comparison example of the two 10J caps and whether if the two are equally dangerous or not?

    Thanks
     
  13. May 1, 2013 #12
    I found this :


    "Pulsed currents such as one might encounter with the discharge of a Van de Graaff generator or other charged capacitance present special considerations. One can endure currents that would otherwise be lethal if the duration is short enough. For pulses of less than a few seconds duration, the relevant quantity is the square of the current integrated over the time of the pulse. Values of I²t greater than about 0.01 A² sec can cause electrocution for a typical adult. For a reasonably low body resistance of 2000 ohms, this translates into an energy of about 10 joules. Severe shocks can occur at levels 10-100 times lower and can startle one into an accident, since it is natural to jerk away from such a shock. "

    So can we say that 10J is at least close to a danger limit ?
     
  14. Dec 17, 2013 #13
    I cannot tell you what is "dangerous" but I can tell you what is considered safe. Up to 15kV 45 micro-Coulombs is considered safe. Above 15kV 350mJ is considered safe.

    This is according to the IEC61010-1 2nd edition, electrical safety for measurement, control and lab equipment.

    Those values are for capacitors you could be subject to during normal operation, assuming you start poking fingers into ventilation slots etc.

    For a "single fault condition" larger charges are acceptable which means they are still "safe" but with less safety margin. You're not allowed to be subject to these charges unless the device is broken somehow.

    No values are given for single fault condition but acceptable capacitance for 200V is ca 8.5uF, 1kV is ca 680nF, 10kV ca 18nF and 40kV 2nF. You can plot it from there on logarithmic scale.

    The curves actually would cross before 100kV but 350mJ should be valid afterwards.

    Unless you're actually trying to kill someone, it's more important to know what is safe.
     
  15. Feb 9, 2015 #14
    In a recent search of the VDG, the article (in Wikepedia) stated that this type of static discharge generator has a steady or virtually unchanging (and low) current source while voltage can increase tremendously. It's my understanding that this is due to infinite resistance?
    My question how is this condition created by the VDG so that someone isn't harmed by the high voltage generated?
     
  16. Feb 9, 2015 #15
    1. It is not infinite, but very large resistance (GΩ range) of air between VDG electrode and ground. Once the voltages, and electric fields, become high enough, surrounding air become ionised and partial discharge called corona occurs (the effective resistance drops down to MΩ range). If the grounded conductive object gets close enough, a self-sustained discharge (spark) forms (resistance Ω-kΩ range) and high current in a fraction of μs discharges VDG capacity.
    2. People in demo usually standing on insulated platform while in contact with VDG, effective resistance is high, currents small.

    BTW, what means dangerous? Up to 1 J of electrostatic energy is simply annoying. About 10 J is very painful to take, and 50-100 J is considered potentialy lethal?
     
  17. Oct 13, 2016 #16
    The minimum current a human can feel depends on the current type (AC or DC) as well as frequency for AC.
    A person can feel at least 1 mA (rms) of AC at 60 Hz, while at least 5 mA for DC.
    At around 10 milliamperes, AC current passing through the arm of a 68-kilogram (150 lb) human can cause powerful muscle contractions; the victim is unable to voluntarily control muscles and cannot release an electrified object. This is known as the "let go threshold" and is a criterion for shock hazard in electrical regulations.
    The current may, if it is high enough and is delivered at sufficient voltage, cause tissue damage or fibrillation which can cause cardiac arrest; more than 30 mA of AC (rms, 60 Hz) or 300 – 500 mA of DC at high voltage can cause fibrillation.
    A sustained electric shock from AC at 120 V, 60 Hz is an especially dangerous source of ventricular fibrillation because it usually exceeds the let-go threshold, while not delivering enough initial energy to propel the person away from the source. However, the potential seriousness of the shock depends on paths through the body that the currents take.
    If the voltage is less than 200 V, then the human skin, more precisely the stratum corneum, is the main contributor to the impedance of the body in the case of a macroshock—the passing of current between two contact points on the skin. The characteristics of the skin are non-linear however. If the voltage is above 450–600 V, then dielectric breakdown of the skin occurs. The protection offered by the skin is lowered by perspiration, and this is accelerated if electricity causes muscles to contract above the let-go threshold for a sustained period of time.
    If an electrical circuit is established by electrodes introduced in the body, bypassing the skin, then the potential for lethality is much higher if a circuit through the heart is established.

    (source: wikipedia)
     
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