Electrical Safety Engineering of Renewable Energy Systems. Rodolfo Araneo

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Electrical Safety Engineering of Renewable Energy Systems - Rodolfo Araneo

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target="_blank" rel="nofollow" href="#fb3_img_img_b0df4db0-1bbf-582e-8fa6-717ec5e611b5.png" alt="F equals I subscript r e f end subscript over I subscript h"/> (1.1)

Path Heart-Current Factor F
IEC Simulated
Left hand–feet 1 1
Hands–feet 1 0.85
Left hand–right hand 0.4 0.75
Right hand–feet 0.8 0.88
Hands–seat 0.7 0.84
Left foot–right foot 0.04 0.01
Left hand–right foot 1.00 1.00
Left hand–left foot 1.00 1.00
Right hand–right foot 0.88
Right hand–left foot 0.8 0.89

      The larger is the heart-current factor; the more dangerous is the current pathway through the body.

      As an example, a current of 225 mA hand-to-hand has the same likelihood of producing ventricular fibrillation as a current of 90 mA left hand-to-both feet. Therefore, the hand-to-hand pathway is less hazardous than the left hand-to-both feet.

      The comparison between the two sets of heart-current factors shows that the IEC may underestimate the magnitude of F for pathways involving the right hand, probably due to the extrapolation of the results of measurements on animals to humans.

      The same heart current factors are also applicable to d.c. currents.

      1.3 The Electrical Impedance of the Human Body

      Figure 1.4 Current response of human body to d.c. voltage.

      When the d.c. touch voltage occurs, the capacitances Cs are not charged, and become short-circuits during the initial transient, bypassing the contact resistances Rcs; therefore, the ratio of the touch voltage V to the current peak equals RBi. After the transient expires, the capacitances of the skin become an open circuit, and the current reaches the steady-state value of V /(RBi +2Rcs ), where the denominator is the total body resistance.

      The impedance of the skin is the primary barrier against the flow of the body current, providing that the voltage is not high enough to puncture it (i.e., below 200 V), and the skin is not wet. Voltages greater than 200 V exceed the dielectric strength of the skin, Cs “fails” and short-circuits the contact resistance Rcs, reducing the resistance of the human body to RBi: the body current can cause greater damages to the internal organs.

      In addition, at 50/60 Hz, the capacitive reactance of the skin is practically an open circuit, and ZB ≈RBi+2Rcs.

      1.3.1 The Internal Resistance of the Human Body

      Figure 1.5 Internal partial impedances of the human body (no skin contribution).

      For example, the partial impedance of the trunk (i.e., segment D-E) is only 1.3% of the total body impedance hand-to-foot, due to large amount of conductive fluids normally present in the trunk.

      The resistance Ri of the segment are determined as Ri = ρl/S, where is the segment tissue mean resistivity, l the mean length of the segment, and S its mean cross-sectional area. The cross-sectional area of the body segment plays a crucial role in determining its resistance: fingers and joints, such as elbow and

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