YT212001-AN
通过使用公式（1），可以确定导体小截面S，前提是一般导体从已知初始温度到特定终温度进行绝热加热（如果故障在5秒内消除，则适用）：其中：•S是横截面【mm2】；•I是阻抗可忽略的故障的预期故障电流值（r.m.s），该故障可流经保护装置【a】；•t为自动断开保护装置的运行时间【s】；k可使用表2÷7进行评估，或根据公式（2）进行计算：其中：•Qc是导体材料在20°C下的体积热容【J/°Cmm3】；•B是导体在0°C时电阻率温度系数的倒数[°C]；•ρ20是导体材料在20°C下的电阻率[Ω毫米]；•θi导体初始温度[°C]；•θf导体的终温度[℃]。表1显示了上述参数的值。表1：不同材料的参数值材料B Qcρ20[℃][J/℃][Ω毫米]铜234.5 3.45⋅10-3 17.241⋅10-6 226铝228 2.5⋅10-3 28.264⋅10-6 148导线230 1.45⋅10-3 214⋅10-6 41钢202 3.8⋅10-3 138⋅10-6 78

By using the formula (1), it is possible to determine the conductor minimum section S, in the hypothesis that the generic conductor is submitted to an adiabatic heating from a known initial temperature up to a specific final temperature (applicable if the fault is removed in less than 5 s): where: • S is the cross section [mm2 ]; • I is the value (r.m.s) of prospective fault current for a fault of negligible impedance, which can flow through the protective device [A]; • t is the operating time of the protective device for automatic disconnection [s]; k can be evaluated using the tables 2÷7 or calculated according to the formula (2): where: • Qc is the volumetric heat capacity of conductor material [J/°Cmm3 ] at 20 °C; • B is the reciprocal of temperature coefficient of resistivity at 0 °C for the conductor [°C]; • ρ20 is the electrical resistivity of conductor material at 20 °C [Ωmm]; • θi initial temperature of conductor [°C]; • θf final temperature of conductor [°C]. Table 1 shows the values of the parameters described above. Table 1: Value of parameters for different materials Material B Qc ρ20 [°C] [J/°Cmm3 ] [Ωmm] Copper 234.5 3.45⋅10-3 17.241⋅10-6 226 Aluminium 228 2.5⋅10-3 28.264⋅10-6 148 Lead 230 1.45⋅10-3 214⋅10-6 41 Steel 202 3.8⋅10-3 138⋅10-6 78