在描述此类处方之前，了解不同的电路很有用
上述标准中描述的类型；特别是在工厂中
电路可分为：
•终电路：通常为设备供电的电路（例如
吸气器、桥式起重机等）
•配电电路：为配电板供电的电路
连接其他终电路。
在TT系统中，通过以下方式实现对间接接触的正确保护：
在自动断开电路时，必须尊重其中一个
以下条件（符合IEC 60364-4）：
通过剩余电流装置进行保护
假设50V为极限电压（标准环境），以实现保护
对于通过剩余电流装置进行的间接接触，有必要
满足以下条件：
RA·I∆n≤ 50V然后：RA≤ 50V
我
∆n
1.
接地电极的电阻与保护导体的电阻串联，如果
与电阻RA比较；因此，在公式中可以考虑
仅限用户工厂接地电极的电阻。
哪里：
RA是接地电极和保护电极的总电阻（单位：欧姆）
外露导电部件1的导体
;
我
∆n是剩余电流断路器的额定剩余工作电流。
ABB |电气设备449
4保护人类
4.4 TT系统
50V<U0
≤120V
s
130V<U0
≤230V
s
230V<U0
≤400V
s
U0
>400V
s
系统a.c.d.c.a.c.d.c.a.c.d.c.a.c.d.c。
TT 0.3注1 0.2 0.4 0.07 0.2 0.04 0.1
Uo是标称交流或直流线对地电压。
从上面可以看出，接地电阻RA的值
通过使用剩余电流断路器和
灵敏度不同，因为上述分母处的电流量
上述关系不同。事实上，通过使用剩余电流装置
具有30mA灵敏度，接地电阻值低于
表1：终电路的大断开时间不超过32A
关于断开时间，该标准区分了两种可能性；
•额定电流不超过32A的终电路：在这种情况下，有必要
上述条件的时间如表1所示（数值
指明显高于额定剩余电流的故障电流
剩余电流断路器通常为5·I∆n
)满足；
•额定电流超过32A的配电电路或终电路：
在这种情况下，有必要在一段时间内满足上述条件
不超过1秒（常规时间）。

Before describing such prescriptions, it is useful to know the different circuit types described in the above mentioned Standard; in particular, in a plant, the circuits can be divided into: • final circuit: it is a circuit which usually supplies equipment (for example an aspirator, a bridge crane, etc.) • distribution circuit: it is a circuit which supplies a distribution board to which other final circuits are connected. In a TT system, to achieve a correct protection against indirect contact through the automatic disconnection of the circuit, it is necessary to respect one of the following conditions (in compliance with IEC 60364-4): Protection by means of residual current devices By assuming 50V as limit voltage (standard environments), to achieve protection against indirect contact by means of residual current devices it is necessary to satisfy the following condition: RA · I∆n ≤ 50V then: RA ≤ 50V I ∆n 1 The resistance of the earth electrode is in series with that of the protective conductor, which is negligible if compared with the resistance RA; as a consequence, in the formula it is possible to take into consideration only the resistance of the earth electrode of the user’s plant. where: RA is the total resistance (in ohm) of the earth electrode and of the protective conductor of the exposed-conductive-parts1 ; I ∆n is the rated residual operating current of the residual current circuitbreaker. ABB | Electrical devices 449 4 Protection of human beings 4.4 TT System 50V400V s System a.c. d.c. a.c. d.c. a.c. d.c. a.c. d.c. TT 0.3 Note 1 0.2 0.4 0.07 0.2 0.04 0.1 Uo is the nominal a.c. or d.c. line to earth voltage. From the above, it is evident that the value of the resistance RA of the earthing arrangement results to be different by using residual current circuit-breakers with different sensitivity, since the current quantity at the denominator in the above mentioned relationship is different. In fact, by using a residual current device with 30mA sensitivity, an earthing resistance value lower than Table 1: Maximum disconnection times for final circuits not exceeding 32A As regards the disconnection times, the Standard distinguishes two possibilities; • final circuits with rated currents not exceeding 32A: in this case it is necessary that the above mentioned condition with the times shown in Table 1 (values referred to fault currents significantly higher than the rated residual current of the residual current circuit-breakers typically 5·I∆n ) is fulfilled; • distribution circuit or final circuit with rated currents exceeding 32A: in this case it is necessary that the above mentioned condition is fulfilled with a time not exceeding 1 s (conventional time).