Page 106 - Electrical Safety of Low Voltage Systems

Electric Currents Through the Human Body 89

FIGURE 5.19 Time–voltage safety curve in ordinary locations.

p
of V ST becomes the permissible prospective touch voltage V human
ST
beings can withstand for the maximum amount of time t, without
suffering ventricular fibrillation.
By solving Eq. (5.5) for all the values of V ST and Z 2H-2F in Table 5.2,
B
we can build the time–voltage safety curve in standard conditions, as
indicatively shown in Fig. 5.19.
It is apparent that as the touch voltage increases, the maximum
permissible contact time decreases. The time–voltage safety curve is the
“damage” curve for human beings: any combination of time and volt-
age above this curve is dangerous.

Example 5.2 Calculate the body current I B due to a touch with a metal part
energized at V ST = 200 V, in the case of dry conditions; current path hands-
to-feet with medium surface area of contact for hands (order of magnitude
3
2
10 mm , R Hand-Hand medium area = 2.2k ); large surface area of contact for feet;
B
R BG = 1000 . Figure 5.20 schematically represents the data of the example.
Hand-Hand large area
Solution R (=2R l )=1.275k (fromFig.5.15incorrespondence
B
to 200 V).
Hand-Foot large area Hand-Hand large area
R B = 0.8R B = 1.02 k (the hand-to-foot body
Hand-Hand large area
resistance is obtained by reducing by 20% the value R ).
B
Hand-Trunk large area Hand-Hand large area
R = 0.5R = 0.637 k (as said, the trunk
B B
has negligible resistance).
Trunk-Foot large area Hand-Foot large area Hand-Trunk large area
R = R − R
B B B
Hand-Hand large area Hand-Hand large area
= 0.8R − 0.5R
B B
Hand-Hand large area
= 0.3R B = 0.382 k .