Page 157 - Electrical Safety of Low Voltage Systems

140 Chapter Seven

FIGURE 7.25 Prospective step voltage V SS and step voltage V S .

7.11.1 A Comparison Between the Dangerousness of Touch
and Step Voltages
In high and medium voltage, the standard contact is assumed to be
applied between a person’s hand and both feet. With regard to the
touch voltage, therefore, the body resistance is composed of the series
of the upper limb’s resistance with the parallel of the resistance of two
lower limbs (i.e., total resistance equals 1.5R l ).
With reference to the step voltage, instead, the body resistance
is the series of the resistances of the lower limbs (i.e., total resis-
tance equals 2R l ). As a result, step voltages of the same magnitude
as touch voltages are less dangerous, as the larger body resistance
limits the magnitude of the current through the person, benefiting the
safety.
In addition and as already substantiated in Chap. 5, the probabil-
ity of ventricular fibrillation depends not only on the current’s inten-
sity and its duration but also on the pathway through the person’s
body.
As per Table 5.1, the heart-current factor F for the left-foot-to-
right-foot current path equals 0.04. Therefore, as per Eq. (5.4):

I LF-RF = I LH-2F (7.14)
F = 25I LH-2F
F
Thus, a left-foot-to-right-foot current, as caused by step voltages, min-
imally involves the cardiac region and, therefore, must be 25 times
larger than the left-hand-to-feet reference current in order to have