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DS3800NCSA
如前幾章所述，標(biāo)準(zhǔn)規(guī)定了保護裝置的大斷開時間，以避免人們接觸帶電部件的病理生理影響。對于間接接觸保護，應(yīng)驗證斷路器在低于標(biāo)準(zhǔn)規(guī)定的大時間的時間內(nèi)跳閘；通過比較待保護外露導(dǎo)電部分的小短路電流與標(biāo)準(zhǔn)規(guī)定時間對應(yīng)的工作電流來進行驗證。當(dāng)相與受保護導(dǎo)體上遠點處的保護導(dǎo)體之間短路時，出現(xiàn)小短路電流。對于小短路電流的計算，可以使用近似方法，假設(shè)：-由于短路電流引起的過熱，導(dǎo)體電阻相對于20°C值可以增加50%；-電源電壓降低80%視為短路電流的影響；-導(dǎo)線電抗僅適用于大于95 mm2的橫截面。通過在保護裝置和故障點之間應(yīng)用歐姆定律，得出以下公式。公式符號和常數(shù)圖例：-0.8是表示電壓降低的系數(shù)；-1.5是表示阻力增加的系數(shù)；-Ur是相間的額定電壓；-U0是相與地之間的額定電壓；-S是相導(dǎo)體橫截面；-SN是中性導(dǎo)體橫截面；-SPE是保護導(dǎo)體橫截面；-ρ是20°C時的導(dǎo)體電阻率；-L是電纜的長度；-是總相導(dǎo)體橫截面（單相導(dǎo)體橫截面S乘以n，并聯(lián)導(dǎo)體數(shù)量）與保護導(dǎo)體橫截面SPE之間的比率，假設(shè)它們由相同的導(dǎo)體材料制成；-是總中性導(dǎo)體橫截面（單個中性導(dǎo)體橫截面SN乘以n，并聯(lián)導(dǎo)體數(shù)量）與保護導(dǎo)體橫截面SPE之間的比率，假設(shè)它們由相同的導(dǎo)體材料制成；-k1是考慮到橫截面大于95 mm2的電纜電抗的修正系數(shù)，可從下表獲得：相導(dǎo)體橫截面[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