Page 103 - Electrical Safety of Low Voltage Systems

86 Chapter Five

FIGURE 5.15 Statistical values of the human body impedance for hand-to-
hand current path.

As anticipated in Chap. 4, IEC standards consider the human body
impedance as a nonlinear function of the prospective touch voltage
V ST . 12 IEEE standard, 13 instead, considers the human body resistance
as a constant value of 1000 for hand-to-hand, hand-to-feet, and foot-
to-foot paths, independent of the voltage.
In Fig. 5.15, statistical values of hand-to-hand impedance are re-
ported, in the case of large surface areas of contact (i.e., order of mag-
2
4
nitude 10 mm ) and in dry conditions; 5%, 50%, and 95% of the pop-
ulation does not exceed these statistical values indicated by the curves
in Fig. 5.15, 14 respectively.
In Fig. 5.15, note the conservative use of the source voltage V ST
instead of the touch voltage V T . If safety is achieved for values of V ST ,
it will surely be achieved for the lesser values of V T , since V T < V ST .

5.8 Current Paths
Different current paths through the body cause different hazards to
persons. The hand-to-hand path in Fig. 5.16 is less dangerous than the
hand-to-feet path in Fig. 5.17.