Page 110 - Electrical Safety of Low Voltage Systems

Electric Currents Through the Human Body 93

we consider curves c3 in d.c. and c3 in a.c. for any time below 600 ms,
the threshold value that triggers the ventricular fibrillation is lower in
d.c. than in a.c.

FAQs
Q. Are the body current values expressed in Eq. (5.3) safe?
A. Equation (5.3), based on statistical extrapolations, gives values that pro-
voke ventricular fibrillation “only” in 0.5% of the population examined. As
said in Chap. 3, this is the acceptable risk as set by IEEE standard 80, well
knowing that we might not belong to the lucky 99.5% of the sample!
Q. What is the difference between the human body impedance Z B and the
internal resistance R i ?
A. Z B is the complex of resistive and capacitive components, which includes
the resistance and the capacitive reactance of the skin. R i is the resistive core
component of the human body impedance, which coincides with Z B at low
frequencies (i.e., 50/60 Hz) and for voltages exceeding 200 V. R i is basically
due to the resistances of the limbs, as the trunk is highly conductive.
Q. If the body current is alternating, why do we mark the directions “in” and
“out” in Fig. 5.13?
A. We are here taking into consideration alternating body current (50/60 Hz),
which changes direction 100/120 times in a second. Therefore, in Fig. 5.13, the
directions “in” and “out” alternate themselves at the same rate. Convention-
ally, though, we deem the direction “in” as the point of touch with live parts.

Endnotes

1. Their resistivity approximately equals 0.6 and 2 · m, respectively.
2. A depolarizing stimulus, that is an electric impulse, must have the opposite
potential of the cell.
3. The all-or-none law is no longer true if applied to functional units, such as the
heart. In fact, a unit’s global response is proportional to the stimulus.
4. As a curiosity, the discussion on the presumed higher danger of the a.c. current
versus d.c. initiated at the end of 1800 between Thomas Alva Edison, inventor
of the d.c. utilization and transmission, and George Westinghouse, supporter
of the a.c. system. The debate was animated by the New York State officials’
efforts in finding a more humane means to carry out capital punishments than
hanging. Eventually, New York’s decision for the electric chair in alternating
current, implicitly suggested its greater hazard to humans (W. Long, S. Nilsson,
“HVDC Transmission: Yesterday and Today,” IEEE Power and Energy Magazine,
Vol. 5, No. 2, March/April 2007).
5. A muscle cell contracts with a force that nearly equals 9.8 mN.
6. As previously said, the action potential can be elicited by an equal strength
stimulus only if this occurs after the refractory period.