Page 109 - Electrical Safety of Low Voltage Systems

92 Chapter Five

In Zone 3 (curve b and above), no organic damage will usually
occur; by increasing current magnitude and exposure time
reversible disturbances of formation and conduction of im-
pulses in the heart may occur.
In Zone 4 (above curve c1), ventricular fibrillation is likely to
occur with a probability that increases with magnitude and
duration of the current; pathophysiological effects, such as
burns, may occur in addition to those of Zone 3. Curves c1–
c2 enclose the area characterized by the probability of 5% of
ventricular fibrillation, while in the area c2–c3, this probability
increases up to 50%; beyond curve c3 this probability exceeds
50%.

In order to compare the fibrillation curves of alternating and direct
currents, we can place them in a single chart (Fig. 5.22).
We note that for contact times below certain values, the direct
current is more dangerous than the alternating one. For instance, if

FIGURE 5.22 Comparison between a.c. and d.c. fibrillation curves.