Page 105 - Electrical Safety of Low Voltage Systems

88 Chapter Five

FIGURE 5.18 Both
hands-to-both feet
current path.

p
the definition of the permissible prospective touch voltage V ST through
Eq. (5.5):

V ST = I B Z Hands-Feet + R BG (5.5)
B
We can solve Eq. (5.5) for the body current I B , assuming for
Z Hands-Feet the values of body resistance not exceeded by 95% of the
B
population at any chosen value of V ST . In correspondence to the ob-
tained I B , we can read on curve c1 in Fig. 5.11 the maximum time t that
does not cause ventricular fibrillation. As a result, the chosen value

Z 2H-2F (Ω)
B
Percentage of Population
5% 50% 95%
V ST
25 875 1625 3050
50 687 1250 2300
75 562 1000 1800
100 495 862 1562
125 450 775 1337
150 425 700 1175
175 412 662 1087
200 400 637 1025
225 387 612 950
400 350 475 637
500 312 425 575
700 287 387 525
1000 287 387 525
TABLE 5.2 Statistical Values of the Human Body Impedance Z 2H-2F for Both
B
Hands-to-Both Feet Current Path