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Thread: Current through body on an ungrounded system

  1. #11
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    Quote Originally Posted by don_resqcapt19 View Post
    There will be no continuous current but there will be current flow at the time of the fault. I would expect that the capacitive coupling current can be more than enough to be fatal in some cases. I have seen it pull in my wiggy, a 20 to 30 mA load, a number of times. That is enough current to kill..
    O.k. so it looks like on an ungrounded system the current through the capacitance coupling is still enough to kill. The current runs through the body and returns on the capacitance coupling of the other two phases.

    Did you simply connect your wiggy between one of the phases and ground?

    Quote Originally Posted by don_resqcapt19 View Post
    The trip point of GFCIs is set at ~5mA to protect people. Your current is 100 times that level..
    Is 500ohms a typical value that is used for the human body for such rough calculations? I thought I have seen this number used somewhere before?

    So using this resistnace value the maximum current that will flow through the body at 277V will be about .554A? I have heard others state that 100's of amps would flow through body which I never chose to believe.

    Quote Originally Posted by don_resqcapt19 View Post
    It is the voltage at the point of contact that drives the current through the person. If the voltage is the same the current is the same. The point at which you contact the circuit does not change this. There is no need to look at the impedance of the system for this. It doesn't matter.
    Does the impedance of the system then only come into play for a bolted fault condition? For such faults with a fault resistance there is no need to consider upstream impedance? Therefore shock would be that same at 480V MCC bus as it would at a 480V lighting circuit?

  2. #12
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    Quote Originally Posted by mull982 View Post
    O.k. so it looks like on an ungrounded system the current through the capacitance coupling is still enough to kill. The current runs through the body and returns on the capacitance coupling of the other two phases.

    Did you simply connect your wiggy between one of the phases and ground?
    Yes the wiggy was between an ungrounded conductor and non-current carrying metal parts.

    Is 500ohms a typical value that is used for the human body for such rough calculations? I thought I have seen this number used somewhere before?

    So using this resistnace value the maximum current that will flow through the body at 277V will be about .554A? I have heard others state that 100's of amps would flow through body which I never chose to believe.
    I am not sure what resistance is normally used for these types of calculations. The current will be limited by the impedance of the path and the available voltage. I think it would be almost impossible to get 100s of amps to flow through a person.

    Does the impedance of the system then only come into play for a bolted fault condition? For such faults with a fault resistance there is no need to consider upstream impedance? Therefore shock would be that same at 480V MCC bus as it would at a 480V lighting circuit?
    Yes the impedance only comes into play when there is enough current flowing that it limits the amount of current that can flow. With a person as the load in the circuit you only look at the voltage across the person and the impedance of the path through the person. The impedance of the system itself does not enter into this.
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  3. #13
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    Quote Originally Posted by don_resqcapt19 View Post
    I think it would be almost impossible to get 100s of amps to flow through a person.
    I think we would have fire long before that. EEK

  4. #14
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    Quote Originally Posted by don_resqcapt19 View Post
    I think it would be almost impossible to get 100s of amps to flow through a person.
    Isn't that about what it took to wake up Herr Frankenstein ? If I remember correctly he only smoked a little bit. :confused:
    - Resistance is Futile ..... (if less than < 1 ohm) -

  5. #15
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    Body Resistance

    A quick cruise through Wikipedia gives a wide range of resistances for the human body. Hand-to-hand with dry skin can be as high as 100K ohms, wet skin as low as 1K ohms. Dielectric breakdown at high (~450) volts can drive resistance as low as 500 ohms.

  6. #16
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    I would think then that with such little amount of current flowing through the body (less than 1/2A) this would not be enough to trip a breaker on a 480V system. The only way a breaker would trip is if it had ground fault protection settings, and typically on a 480V system these are such greater then 1/2A so most likely no OCPD would clear the circuit?

  7. #17
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    Resistance of the Body

    IEEE 80, "IEEE Guide for Safety in AC Substation Grounding" lists the internal resistance of the body as 300 ohms with skin resistance ranging from 500 to 5,000 ohms or more. Most of our resistance is in the skin, once that is punctured, the resistance drops.

    The same reference quotes Dalziel's work where he passed current through student "volunteers" whose hands and feet were wet with saltwater. He calculated body resistances of 2330 ohms for hand-to-hand and 1130 ohms hand-to-foot.

    The subjects experienced "no let go" at currents of 6-18 mA. At 60 mA ventricular fibrillation occurred. These current values vary person to person and are affected by their size and the duration of shock.

    Using 50 mA as lethal threshold, 277V/.05 A = 5,540 ohms. Your body + contact resistance has to be 5 kohms or greater to survive at 277V.

    Some other safety standards use 1500 ohms as the body resistance when investigating shock hazards on ungrounded systems, like the hot working zone of DC bus in chlorine and aluminum potlines. (See NEC Art 668). Connect a 1.5kohm resistor across the voltmeter and measure for shock hazards. If you read over a 100V there is a hazard.
    Bob Wilson

  8. #18
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    Quote Originally Posted by rcwilson View Post
    Some other safety standards use 1500 ohms as the body resistance when investigating shock hazards on ungrounded systems, like the hot working zone of DC bus in chlorine and aluminum potlines. (See NEC Art 668). Connect a 1.5kohm resistor across the voltmeter and measure for shock hazards. If you read over a 100V there is a hazard.
    Using V=I*R, it looks like the threshold would be more like 75V.

  9. #19
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    Quote Originally Posted by gadfly56 View Post
    Using V=I*R, it looks like the threshold would be more like 75V.
    Actually it is 50V, based on a 500 Ohm body resistance and a 100mA threthhold of fibrilation, which is almost always fatal.

    That is why you see 50V referenced all over OSHA standards.

  10. #20
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    Quote Originally Posted by zog View Post
    Actually it is 50V, based on a 500 Ohm body resistance and a 100mA threthhold of fibrilation, which is almost always fatal.

    That is why you see 50V referenced all over OSHA standards.
    Correct, Zog.

    50 V is a better threshold.

    I was typing from memory on what we did in aluminum plants 20 years ago.
    Bob Wilson

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