Page 104 - Electrical Safety of Low Voltage Systems

Electric Currents Through the Human Body 87

FIGURE 5.16
Hand-to-hand
current path.

In the first case, in fact, the current is limited by the series of the
twoarms’resistances R Hand-Hand = 2R l ,whileinthelattercase,thetwo
B
legs are in parallel; therefore, R Hand-Feet = 1.5R l , and hence, R Hand-Feet
B B
results to be 75% of R Hand-Hand .
B
Some measurements would indicate that the impedance hand-
to-foot R H-F , for large areas of contact, is 10% to 30% lower than
B
R Hand-Hand . Hence, an average reductive factor of 0.8 may be applied
B
to R Hand-Hand to calculate the hand-to-foot body resistance.
B
The worst case, assumed in the electrical design, is a person touch-
ing a live part with both hands (Fig. 5.18). In this case, R B Hands-Feet = R l ,
as all the limbs are in parallel. R Hands-Feet is 50% of R Hand-Hand .
B B
In relation to the above worst-case scenario, the values for the total
body impedance for large areas of contact are detailed in Table 5.2 as
a function of the prospective touch voltage and the percentages of
population.
p
5.9 Permissible Prospective Touch Voltage V
ST
If we assume in series to the human body a resistance-to-ground
R BG of 1000 in standard conditions (see Chap. 4), Table 5.2 allows

FIGURE 5.17
Hand-to-feet current
path.