Current limiting body resistance

It is on the basis of a body resistance of 500 ohms and a current of 100 mA that the hand-to-hand maximum voltage limit of 50 volts (root-mean-square alternating voltage) has been established and used in the international literature, e.g. 1EC60364. The corresponding hand-to-hand direct voltage... 

The most important current limiting resistance of the human body is the dry skin. This may be impaired be high-field electrical breakdown, skin moisturizing, or a skin wound. Skin breakdown may occur at less than 10 V AC 50/60 Hz from electro-osmotic breakdown (Grimnes, 1983b). 

From the threshold current levels, the corresponding voltages are found by estimating the minimum current limiting resistance. These worst case minima are found by assuming no protective action from the skin at all, only from the volume resistance of the living parts of the body. The levels are summarized in Table 10.2. 

To use this equation to set safe voltage limits in an equipotentiai zone around the worker, the employer will need to assume a value for the resistance of the worker s body. IEEE Std 1048-2003 states that total body resistance is usually taken as 1000 LI for determining. .. body current limits. However, employers should be aware that the impedance of a worker s body can be substantially less than that value. For instance, IEEE Std 1048-2003 reports a minimum hand-to-hand resistance of 610 ohms and an internal body resistance of 500 ohms. The internal resistance of the body better represents the minimum resistance of a worker s body when the skin resistance drops near zero, which occurs, for example, when there are breaks in the worker s skin, for instance, from cuts or from blisters formed as a result of the current from an electric shock, or when the worker is wet at the points of contact. 

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... 

Limiting to a safe level the amount of current that can be supplied into a load representative of the human body. In general this means that the current should be limited to a maximum of 5 mA. As an example, this limitation is commonly applied to the probes of insulation resistance test units which may be energised in excess of 1000 V but which can be... 

Pyrometer pl- ra-mo-tor n [ISV] (1796) (1) Instrument for measuring temperatures beyond the upper limit of the usual liquid thermometer. They may operate on the differential expansion of two metallic strips joined together, the measurement of changes of resistance, and the measurement of current flowing through two joined pieces of metal. In addition, radiation pyrometers are based on the measurement of heat radiated from a hot body, and optical pyrometers on the measurement of the intensity of light emitted from a hot body. (2) An Infrared Pyrometer. 

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