Page 107 - Electrical Safety of Low Voltage Systems

90 Chapter Five

FIGURE 5.20
Schematic
representation of
the data of the
example.

R Hand-Hand medium area = 2.2k (the hand-to-hand body resistance for me-
B
dium contact area is given in the example).
R Hand-Trunk medium area = 0.5R Hand-Hand medium area = 1.1k
B B
The total hand-to-foot body impedance, in the conditions required by the
problem, is

R B Hand-Foot = R B Hand-Trunk medium area + R Trunk-Foot large area
B
= 1.1k + 0.382 k = 1.482 k .
The total hands-to-feet body impedance is
R Hands-Feet = 0.5R Hand-Foot = 0.741 k .
B B
The body current I B is
V ST 200
I B = Hands-Feet = = 114 mA.
R B + R BG 1741

5.10 Effects of Direct Currents
Direct currents have come into the spotlight in the recent years due
to the increased development of renewable sources of energy (e.g.,
photovoltaic cells, wind turbines, etc.), which generate d.c. currents.
By the term direct current, international standards intend a constant
current to which may be superimposed a sinusoidal ripple, whose
r.m.s value does not exceeds 10% of the d.c. current itself. In this case,
the expression ripple-free current is used.
Direct current is generally less dangerous, since the thresholds
of let-go, and of ventricular fibrillation, for contacts longer than the
cardiac cycle, are significantly higher than for a.c. current.