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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of all admissions to burn-care facilities [18].

      The amount of heat generated in the tissue depends on the current density and the tissue conductivity. However, the conductivity is in turn determined by the heat generated in the tissue; since the tissue ionic conductivity increases with the increasing temperature, a further intensification in current density and temperature occurs. Thus, the thermal injury is determined as the result of a feedback mechanism; however, the conductivity change is not considered in burn models due to the complexity of the resulting nonlinear equations.

      w delivered by a current i during the time t to a homogeneous volume of biological tissue of length l, cross-sectional area and ionic conductivity σ (Eq. 1.2), it is conservatively assumed an adiabatic process. Such a process calls for no heat removal into neighboring tissues by blood flow or by conduction and/or convection into the air, but it is presumed that all the heat stays within the tissue.

Tissue σ (S/m)
Blood 7.00E-1
Bone 8.07E-2
Cartilage 1.71E-1
Fat 4.04E-2
Heart 8.27E-2
Kidney 8.92E-2
Muscle 2.33E-1
Nerve 2.74E-2
Skin (dry) 2.00E-4
Skin (wet) 4.27E-4

      c is the volume-specific heat capacity, defined as the heat necessary to increase the temperature of a unit volume of a substance by 1˚C.

      The temperature rise ΔT is given by Eq. 1.4

      Equation 1.4 shows that the temperature rise ΔT depends on the square of the current density J and on the duration t of the current circulation.

      The skin has the lowest conductivity among the biological tissues, and the current density is higher at the point of contacts on the body (referred to as entry and exit sites); therefore, for a given current, the highest temperature rise is achieved on the skin, which therefore suffers the greatest level of damage.

      Thermal shock can also be caused by the heat released by electric arcs, which are accompanied by the vaporization of metal to form a superheated toxic gas.

      1.5 Heated Surfaces of Electrical Equipment and Contact Burn Injuries

      Burns can also be triggered by unintentional contact with hot surfaces of electrical equipment, which may be readily accessible during normal operations (e.g., the surface of a PV module) [20, 21].

      Most apparatus and appliances in industrial, commercial, and residential environments are thermally insulated unless the insulation would prevent their functions (e.g., the bottom surface of a flatiron). However, superficial temperatures of insulated equipment may still be high enough to cause burns from contact with readily accessible parts. The severity of such burns will depend on the thermal resistivity of the material of the touchable surface, and the pressure and duration of the contact.

      An effective protection against burns can be established based on the acceptable contact period and on the level of acceptable injury.

Age Group Exposure time (s)
Adults 0.5–1
age < 2 years 15
2 years < age < 6 years 4
6 years < age < 14 years 2
Elderly persons

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