Читать книгу Electrical Safety Engineering of Renewable Energy Systems - Rodolfo Araneo - Страница 16

The electrical impedance Z_B of the human body is capacitive in nature [14] due to the capacitance C_s of the skin (Figure 1.3); therefore, it depends on the frequency of the applied touch voltage.R_Bi is the internal body resistance; R_cs represents the resistance of the skin at the surface area of contact, which takes into account the presence of the pores, which are small conductive elements; R_cs is strongly variable with environmental and physiological conditions (e.g., sweaty hands).

Figure 1.3 Impedances of the human body.

The model of Figure 1.3 has been validated on cadavers by analyzing the current response to a d.c. voltage V [15] (Figure 1.4).

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

When the d.c. touch voltage occurs, the capacitances C_s are not charged, and become short-circuits during the initial transient, bypassing the contact resistances R_cs; therefore, the ratio of the touch voltage V to the current peak equals R_Bi. After the transient expires, the capacitances of the skin become an open circuit, and the current reaches the steady-state value of V /(R_Bi +2R_cs ), 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, C_s “fails” and short-circuits the contact resistance R_cs, reducing the resistance of the human body to R_Bi: 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 Z_B ≈R_Bi+2R_cs.