Page 102 - Electrical Safety of Low Voltage Systems

Electric Currents Through the Human Body 85

FIGURE 5.13
Impedance of the
human body.

capacitance C s . In parallel to C s , we consider the resistance R s offered
by the skin pores, which are small conductive elements. In series to
the skin impedance Z s , there is the internal resistance R i of the body
(Fig. 5.13).
At 50/60 Hz, the capacitive reactance is extremely high (i.e., the
capacitors of Fig. 5.13 are open circuits) and we can consider the hu-
man body as a purely resistive element (i.e., Z B ≈ R B ). In addition,
for touch voltages increasingly greater than 200 V, the skin ruptures
and no longer plays any insulating role from live parts. In this case,
R B ≈ R i .
R i essentially depends on the current path and is mainly concen-
trated in the lower and upper limbs, as they have a small cross section
with respect to the rest of the body. The trunk resistance, in fact, is
much lower due to its larger section and the presence of conductive
fluids in it. If we consider the trunk as a “short circuit” and suppose
arms and legs of equal resistance, we obtain a “ ” quadripole for
modeling the resistance of the human body (Fig. 5.14).

FIGURE 5.14
Equivalent “ ”
quadripole for the
body resistance
R B (= R i )ofthe
human body.