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Basis for 50V limit for de-energized work

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    Basis for 50V limit for de-energized work

    So, I've been quizzed to answer this question. What is the basis for the 50V (30V) limit for de-energized work, and where do you find it written?

    I am ex-Navy, and the question is from a former Navy Nuke. If you ever read any safety manual, the highest safe voltage is 50V for shore based facilities, and 30V for sea-based facilities (i.e. ships). It is listed in OSHA, but never explained.

    As an ex-navy sailor, I know the basis is based on 0.1A being deadly, and the lowest possible resistance for the human body is 500 ohms (300 ohms onboard ships), and ohms law gives you 50V. This is found in NSTM 300 app. H. I remember this vividly as it was a very popular level of knowledge question in the Navy.

    As you can tell, this question still haunts me in the private sector, but I can't find the answer anywhere in OSHA and NFPA 70E, it is just implied. Has anybody run across this answer?

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    Quote Originally Posted by Kalbi_Rob View Post
    So, I've been quizzed to answer this question. What is the basis for the 50V (30V) limit for de-energized work, and where do you find it written?

    I am ex-Navy, and the question is from a former Navy Nuke. If you ever read any safety manual, the highest safe voltage is 50V for shore based facilities, and 30V for sea-based facilities (i.e. ships). It is listed in OSHA, but never explained.

    As an ex-navy sailor, I know the basis is based on 0.1A being deadly, and the lowest possible resistance for the human body is 500 ohms (300 ohms onboard ships), and ohms law gives you 50V. This is found in NSTM 300 app. H. I remember this vividly as it was a very popular level of knowledge question in the Navy.

    As you can tell, this question still haunts me in the private sector, but I can't find the answer anywhere in OSHA and NFPA 70E, it is just implied. Has anybody run across this answer?
    You are right, this rule is taught all over the industry but I have no clue who came up with it. I'm going to ask around, curious to see what you find out.

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    NSTM 300 App G

    For those interested in where I found the requirement in the NSTM 300 appendix G:

    300-g.3.2 BODY RESISTANCE. At the outset of any consideration of safety from electric shock, it is important
    to recognize that the resistance of the human body cannot be relied upon to prevent a fatal shock from 115
    volt or even lower voltage circuits. When the skin is dry, it has a high resistance where it makes contact with the
    electrodes through which current enters and leaves the body. The resistance may be high enough in this case to
    protect a person from fatal shock even if one hand touches a bare conductor on one side of a 115volt line while
    the other hand (or a foot) touches a bare conductor on the other side of the line. This is an exceptional case.
    Onboard a ship, it is far more likely that the skin will be wet with perspiration or salt water. The contact resistance falls when the skin is wet, and the body resistance, measured from electrode to electrode, is low. Tests made
    by the National Institute of Standards and Technology show that the resistance of the human body may be as low
    as 500 ohms under unfavorable conditions. In warm and moist Marine environments such as are encountered on
    naval vessels, body resistance as low as 300 ohms could be experienced. If 0.1 ampere is enough to cause death,
    and if the body resistance can be as low as 300 ohms, it follows immediately that circuits above 30 volts can be
    fatal. All circuits, even if of only a few volts, are potentially dangerous in that they may give rise to currents that
    are immediately fatal, or that keep a person from letting go and ultimately cause death if they are not rescued by
    their shipmates, or that cause a person to jump and perhaps fall under conditions that will cause serious injury.
    The resistance of the body itself cannot be relied upon to provide protection from shock.

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    Quote Originally Posted by Kalbi_Rob View Post
    For those interested in where I found the requirement in the NSTM 300 appendix G:

    300-g.3.2 BODY RESISTANCE. At the outset of any consideration of safety from electric shock, it is important
    to recognize that the resistance of the human body cannot be relied upon to prevent a fatal shock from 115
    volt or even lower voltage circuits. When the skin is dry, it has a high resistance where it makes contact with the
    electrodes through which current enters and leaves the body. The resistance may be high enough in this case to
    protect a person from fatal shock even if one hand touches a bare conductor on one side of a 115volt line while
    the other hand (or a foot) touches a bare conductor on the other side of the line. This is an exceptional case.
    Onboard a ship, it is far more likely that the skin will be wet with perspiration or salt water. The contact resistance falls when the skin is wet, and the body resistance, measured from electrode to electrode, is low. Tests made
    by the National Institute of Standards and Technology show that the resistance of the human body may be as low
    as 500 ohms under unfavorable conditions. In warm and moist Marine environments such as are encountered on
    naval vessels, body resistance as low as 300 ohms could be experienced. If 0.1 ampere is enough to cause death,
    and if the body resistance can be as low as 300 ohms, it follows immediately that circuits above 30 volts can be
    fatal. All circuits, even if of only a few volts, are potentially dangerous in that they may give rise to currents that
    are immediately fatal, or that keep a person from letting go and ultimately cause death if they are not rescued by
    their shipmates, or that cause a person to jump and perhaps fall under conditions that will cause serious injury.
    The resistance of the body itself cannot be relied upon to provide protection from shock.
    In addition to this, I recently found this page regarding the matter.

    https://testguy.net/content/191-NFPA...ries-Explained

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    Quote Originally Posted by rofo42 View Post
    In addition to this, I recently found this page regarding the matter.

    https://testguy.net/content/191-NFPA...ries-Explained
    Thanks, I'm curious where they obtained the information for that article. As I stated, I've only seen it officially documented in the NSTM 300. I'm sure back in the 70's this information was common throughout OSHA, and as updates have been pushed, they probably just let it fade.

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    OSHA
    1910.333(a)(1)
    "Deenergized parts." Live parts to which an employee may be exposed shall be deenergized before the employee works on or near them, unless the employer can demonstrate that deenergizing introduces additional or increased hazards or is infeasible due to equipment design or operational limitations. Live parts that operate at less than 50 volts to ground need not be deenergized if there will be no increased exposure to electrical burns or to explosion due to electric arcs.

    formal interpretation
    https://www.osha.gov/laws-regs/stand...ons/2015-09-04

    "However, OSHA considers all voltages of 50 volts or above to be hazardous. Electric current, not voltage, passing through the human body causes injury, and the amount of current passing through an object depends on the resistance of the object. As explained in Appendix C to 29 CFR 1910.269, theinternal resistance of the human body is 500 ohms, which is the minimum resistance of a worker with broken skin at the point of contact. The current through 500 ohms from a live part energized at 60 volts would be 120 milliamperes. This level of current, either ac or dc, is sufficient to cause serious injury.2"

    2 The International Electrotechnical Commission's publication on the Effects of current on human beings and livestock Part 1, General Aspects. Available at: http://www.iec.ch/.

    so its basically from IEC/TS 60479-1... I wanted to find you black and white but its a $280 doc



    from Appendix C to 29 CFR 1910.269
    https://www.osha.gov/laws-regs/regul...0/1910.269AppC

    "Employers may use the IEEE Std 1048-2003 equation to determine safe body current limits only if the employer protects workers from hazards associated with involuntary muscle reactions from electric shock (for example, the hazard to a worker from falling as a result of an electric shock). Moreover, the equation applies only when the duration of the electric shock is limited. If the precautions the employer takes, including those required by applicable standards, do not adequately protect employees from hazards associated with involuntary reactions from electric shock, a hazard exists if the induced voltage is sufficient to pass a current of 1 milliampere through a 500-ohm resistor. (The 500-ohm resistor represents the resistance of an employee. The 1-milliampere current is the threshold of perception.) Finally, if the employer protects employees from injury due to involuntary reactions from electric shock, but the duration of the electric shock is unlimited (that is, when the fault current at the work location will be insufficient to trip the devices protecting the circuit), a hazard exists if the resultant current would be more than 6 milliamperes (the recognized let-go threshold for workers 4)."

    "4 Electric current passing through the body has varying effects depending on the amount of the current. At the let-go threshold, the current overrides a person's control over his or her muscles. At that level, an employee grasping an object will not be able to let go of the object. The let-go threshold varies from person to person; however, the recognized value for workers is 6 milliamperes"

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