b） 应通过以下方式提供间接接触保护：
-FELV设备外露导电部分的连接
连接至一次电路保护导体的电路，前提是
后者受一项防止直接
联系
-将FELV电路的带电导体连接到一次电路的保护导体，前提是自动断开
电源用作保护措施；
c） FELV系统的插头不得进入其他系统的插座
电压系统和其他电压系统的插头不得
进入FELV系统的插座。
图1显示了SELV、PELV和FELV系统的主要特征。
图1:SELV、PELV、FELV系统
448电气设备| ABB
4保护人类
1SDC01035F0001
L1
L2
L3级
N
Ik
Ik
RB RA
4.4 TT系统
TT系统中的接地故障涉及图1所示的电路：
图1:TT系统中的接地故障
故障电流流经变压器的二次绕组
线路导体、故障电阻、保护导体和接地
电极电阻（用户工厂的RA和中性点的RB）。
根据IEC 60364-4规定，保护装置必须与接地装置协调，以快速断开电源
如果接触电压达到对人体有害的值。

b) protection against indirect contact shall be provided by: - connection of the exposed conductive parts of the equipment of the FELV circuit to the protective conductor of the primary circuit, provided that the latter is subject to one of the measures of protection against direct contact; - connection of a live conductor of the FELV circuit to the protective conductor of the primary circuit provided that an automatic disconnection of the supply is applied as measure of protection; c) plugs of FELV systems shall not be able to enter socket-outlets of other voltage systems, and plugs of other voltage systems shall not be able to enter socket-outlets of FELV systems. Figure 1 shows the main features of SELV, PELV and FELV systems. Figure 1: SELV, PELV, FELV systems 448 Electrical devices | ABB 4 Protection of human beings 1SDC010035F0001 L1 L2 L3 N Ik Ik RB RA 4.4 TT System An earth fault in a TT system involves the circuit represented in Figure 1: Figure 1: Earth fault in TT system The fault current flows through the secondary winding of the transformer, the line conductor, the fault resistance, the protective conductor, and the earth electrode resistances (RA, of the user’s plant, and RB, of the neutral). According to IEC 60364-4 prescriptions, the protective devices must be coordinated with the earthing arrangement in order to rapidly disconnect the supply if the touch voltage reaches harmful values for the human body.