DS3800NPSU
如前几章所述，标准规定了保护装置的大断开时间，以避免人们接触带电部件的病理生理影响。对于间接接触保护，应验证断路器在低于标准规定的大时间的时间内跳闸；通过比较待保护外露导电部分的小短路电流与标准规定时间对应的工作电流来进行验证。当相与受保护导体上远点处的保护导体之间短路时，出现小短路电流。对于小短路电流的计算，可以使用近似方法，假设：-由于短路电流引起的过热，导体电阻相对于20°C值可以增加50%；-电源电压降低80%视为短路电流的影响；-导线电抗仅适用于大于95 mm2的横截面。通过在保护装置和故障点之间应用欧姆定律，得出以下公式。公式符号和常数图例：-0.8是表示电压降低的系数；-1.5是表示阻力增加的系数；-Ur是相间的额定电压；-U0是相与地之间的额定电压；-S是相导体横截面；-SN是中性导体横截面；-SPE是保护导体横截面；-ρ是20°C时的导体电阻率；-L是电缆的长度；-是总相导体横截面（单相导体横截面S乘以n，并联导体数量）与保护导体横截面SPE之间的比率，假设它们由相同的导体材料制成；-是总中性导体横截面（单个中性导体横截面SN乘以n，并联导体数量）与保护导体横截面SPE之间的比率，假设它们由相同的导体材料制成；-k1是考虑到横截面大于95 mm2的电缆电抗的修正系数，可从下表获得：相导体横截面[mm2]120 150 185 240 300 k1 0.90 0.85 0.80 0.75 0.72

As described in the previous chapters, the Standards give indications about the maximum disconnecting time for the protective devices, in order to avoid pathophysiological effects for people touching live parts. For the protection against indirect contact, it shall be verified that the circuitbreaker trips within a time lower than the maximum time stated by the Standard; this verification is carried out by comparing the minimum short-circuit current of the exposed conductive part to be protected with the operating current corresponding to the time stated by the Standard. The minimum short-circuit current occurs when there is a short-circuit between the phase and the protective conductors at the farthest point on the protected conductor. For the calculation of the minimum short-circuit current, an approximate method can be used, assuming that: - a 50 % increasing of the conductors resistance, with respect to the 20 °C value, is accepted, due to the overheating caused by the short-circuit current; - a 80 % reduction of the supply voltage is considered as effect of the short-circuit current; - the conductor reactance is considered only for cross sections larger than 95 mm2 . The formula below is obtained by applying Ohm’s law between the protective device and the fault point. Legend of the symbols and constants of the formula: - 0.8 is the coefficient representing the reduction of the voltage; - 1.5 is the coefficient representing the increasing in the resistance; - Ur is the rated voltage between phases; - U0 is the rated voltage between phase and ground; - S is the phase conductor cross section; - SN is the neutral conductor cross section; - SPE is the protection conductor cross section; - ρ is the conductor resistivity at 20 °C; - L is the length of the cable; - is the ratio between the total phase conductor cross section (single phase conductor cross section S multiplied by n, number of conductors in parallel) and the protective conductor cross section SPE assuming they are made of the same conductor material; - is the ratio between the total neutral conductor cross section (single neutral conductor cross section SN multiplied by n, number of conductors in parallel) and the protective conductor cross section SPE assuming they are made of the same conductor material; - k1 is the correction factor which takes into account the reactance of cables with cross section larger than 95 mm2 , obtainable from the following table: Phase conductor cross section [mm2 ] 120 150 185 240 300 k1 0.90 0.85 0.80 0.75 0.72