Читать книгу Electrical Safety Engineering of Renewable Energy Systems - Rodolfo Araneo - Страница 17

R_Bi in Figure 1.3 represents the internal resistance of the human body, which depends on the chosen current pathway [16]. IEC 60479-1 expresses the resistance R_i of different segments of the body, ignoring the skin contribution at 50/60 Hz, as a percentage of the internal resistance of the human body related to the path hand-to-foot (Figure 1.5).

Figure 1.5 Internal partial impedances of the human body (no skin contribution).

For example, the partial impedance of the trunk (i.e., segment D-E) is only 1.3% of the total body impedance hand-to-foot, due to large amount of conductive fluids normally present in the trunk.

The resistance Ri of the segment are determined as Ri = ρl/S, where is the segment tissue mean resistivity, l the mean length of the segment, and S its mean cross-sectional area. The cross-sectional area of the body segment plays a crucial role in determining its resistance: fingers and joints, such as elbow and knee, have higher resistance values due to their relatively small cross-sectional areas, even though they are made of well-conductive tissues.

The resistance Ri of the segment are determined as Ri = ρl/S, where is the segment tissue mean resistivity, l the mean length of the segment, and S its mean cross-sectional area. The cross-sectional area of the body segment plays a crucial role in determining its resistance: fingers and joints, such as elbow and knee, have higher resistance values due to their relatively small cross-sectional areas, even though they are made 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.