Electrical Safety Engineering of Renewable Energy Systems. Rodolfo Araneo
of well-conductive tissues.
The total body impedance for a given current path is obtained by adding the resistances Ri of the body segments for that path and the impedances of the skin at the surface areas of contact.
To underscore the role of the skin as the primary barrier against the flow of the body current, the US NIOSH3 states that “under dry conditions, the resistance offered by the human body may be as high as 100,000 Ω. Wet or broken skin may drop the body’s resistance to 1,000 Ω”.
IEC/TS 60479-1 affirms the variability of the body impedance ZB and body resistance RB related to the touch voltage, both a.c. (50/60 Hz) and d.c., and provides impedance values for the hand-to-hand pathway, in the case of dry skin and large contact areas (i.e., order of magnitude 100 cm2), herein shown in Table 1.2.
Table 1.2 Body impedances and resistances for a current path hand-to-hand
| 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.
In Figure 1.6, values of impedances not exceeded by the 95% of population, for a current path hand-to-hand and for a 125 V touch voltage (a.c. 50/60 Hz), are shown as a function of the surface areas of contact.
Figure 1.6 Body impedances at 1250 V for a path hand-to-hand vs. the area of contact.
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.
In sum, ZB is different from person to person and is dependent on several factors, including [17] but not limited to:
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 current i passing through the human body during the contact time t with an energized part produces a physiological damage due to the generation and transfer of heat, per the Joule effect, to the biological tissues. An electric burn is defined as the burning of the skin or of an organ caused by the flow of an electric current along its surface or through it. Electric burn injuries account for 4% to 6% of all admissions to burn-care facilities [18].
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.