Rodolfo Araneo

Electrical Safety Engineering of Renewable Energy Systems


Скачать книгу

of well-conductive tissues.

ZB (Ω) RB (Ω)
Touch Voltage (V) 5% 50% 95% 5% 50% 95%
25 1750 3250 6100 2100 3875 7275
50 1375 2500 4600 1600 2900 5325
150 850 1400 2350 875 1475 2475
200 800 1275 2050 800 1275 2050
225 775 1225 1900 775 1225 1900
400 700 950 1275 700 950 1275
500 625 850 1150 625 850 1150
1000 575 775 1050 575 775 1050

      Table 1.2 shows ZB and RB in the population percentile; for instance, for a touch voltage of 50 V, 95% of the population has an impedance of 4,600 Ω or less.

      The body resistance for direct current (i.e., f = 0) is higher than the body impedance for alternating currents (i.e., f = 50/60 Hz) for touch voltages up to approximately 200 V, thanks to the blocking effect of the capacitances of the skin (i.e., they are open circuits at steady state); for a.c. contacts, the capacitances Cs are instead in parallel to the contact resistances Rcs.

      For durations of current flow longer than 0.1 s, the skin will rupture, and ZB approaches RB.

      The total body impedance ZB depends on the area of contact with the energized part. Surface areas of contact are defined as large, medium, and small, with order of magnitude respectively of 100 cm2, 10 cm2, and 1 cm2, and characterized by dry, water-wet, and saltwater-wet conditions.

      It can be observed that ZB increases with polynomial law when the area of contact decreases. For a given area of contact, no appreciable differences in ZB are present in dry and water-wet conditions for a touch voltage of 125 V.

      The effect on ZB of the surface area of contact increases when the touch voltage decreases; this is because touch voltages exceeding 200 V may rupture the capacitance of the skin and short-circuit the contact resistance.

       the touch voltage;

       the supply frequency;

       the duration of the current flow;

       the conditions of wetness of the skin and surface area of contact;

       the general environment.

      1.4 Thermal Shock

      The amount of heat generated in the tissue depends on the current density and the tissue conductivity. However, the conductivity is in turn determined by the heat generated in the tissue; since the tissue ionic conductivity increases with the increasing temperature, a further intensification in current density and temperature occurs. Thus, the thermal injury is determined as the result of a feedback mechanism; however, the conductivity change is not considered in burn models due to the complexity of the resulting nonlinear equations.

      w delivered by a current i during the time t to a homogeneous volume of biological tissue of length l, cross-sectional area and ionic conductivity σ (Eq.