Page 94 - Electrical Safety of Low Voltage Systems

Electric Currents Through the Human Body 77

Another important result that we can infer from the previous para-
graph is that the direct current (i.e., frequency equals zero) is generally
less dangerous than the alternating current. The accommodation phe-
nomenon earlier described as occurring due to the prolonged stimu-
lus caused by the d.c. current, causes the cell excitability threshold to
increase. 4

5.4 Physiological Response to Electrical Currents
The physiological response of the body to electric current depends on
its magnitude and duration.
The threshold of reaction is the minimum value that causes invol-
untary muscle contraction. Normally, at 50/60 Hz, the most common
power frequencies, a conventional value for threshold of reaction is
assumed to be 0.5 mA (r.m.s), independently of the contact time (2 mA
for direct currents). This value, in reality, varies with the conditions
of contact (i.e., dry, wet, contact pressure, etc.), the area of the body in
touch with the live part, and the physiology of the individual.
The threshold of let-go is the maximum value of touch current that
allows the subject to voluntarily be able to release his/her grasp on
the energized part. This threshold depends on the contact area, the
shape of the live part, as well as on the physiology of the person. The
conventional threshold of let-go is assumed to be 10 mA (r.m.s) for
adult males.
Current can cause momentary or permanent pathologies to the
body, such as paralysis (tetanization), extensive burns, inability to
breathe, unconsciousness, ventricular fibrillation, and cardiac arrest.
In the following sections, we will examine the most important ones.

5.4.1 Tetanization
As previously stated, the right combination of strength and duration
of a stimulus can produce an action potential, which, by propagat-
ing along nerves, can “order” muscle fibers to mechanically contract. 5
After the contraction, the muscle slowly returns to the resting state,
unless, before the end of the contraction cycle, another effective stim-
6
ulus has elicited a subsequent action potential. In this case, the orig-
inal mechanical contraction, not expired yet, is re-initiated. If subse-
quent action potentials occur, like a “burst,” the resulting contraction
of the muscle is maintained as a constant global spasm (Fig. 5.7). This
7
phenomenon is called tetanization. The injured “can’t let go” of the
energized part an individual is in contact with, and the paralysis of
the respiratory muscles, can induce asphyxiation, with subsequent
oxygen deprivation resulting in death or irreparable brain damages.
When the burst stops the muscle slowly expands towards the resting
state.