Page 108 - Electrical Safety of Low Voltage Systems

Electric Currents Through the Human Body 91

Data from electrical accidents and experiments on animals cor-
roborate the fact that the risk of fibrillation is generally present
only for longitudinal currents, that is, for currents flowing upward/
downward through the person, for which the feet represent the pos-
itive/negative polarity. The most dangerous current path is upward
(i.e., feet at positive polarity), which is characterized by a threshold
of fibrillation about one-half of the downward current. For transverse
currents circulating, for example, from hand to hand, ventricular fib-
rillation may only occur with high current intensities.
Conventional body current–time curves, which describe the ef-
fects of d.c. currents on persons for a longitudinal upward current
path, have been elaborated 15 (Fig. 5.21).
In Zone 1 (0 up to 2 mA, curve a), there is generally no reaction.
In Zone 2 (2 mA up to curve b), no harmful physiological effects
will usually occur.

FIGURE 5.21 Conventional d.c. current–time curves and hazardous zones
(longitudinal upward current path).