ABB 塑壳断路器附件，外置中性线电流互感器 CT EXT NEUTRAL 10A XT2；
电流互感器的作用是可以把数值较大的一次电流通过一定的变比转换为数值较小的二次电流，用来进行保护、测量等用途。如变比为400/5的电流互感器，可以把实际为400A的电流转变为5A的电流。
The role of the current transformer is to convert a larger value of a current through a certain variable ratio to a smaller two times of current, used for protection, measurement and other uses. For example, a current transformer with a ratio of 400/5 can transform the actual 400A current to 5A current.
电流互感器原理是依据电磁感应原理的。电流互感器是由闭合的铁心和绕组组成。它的一次侧绕组匝数很少，串在需要测量的电流的线路中，因此它经常有线路的全部电流流过，二次侧绕组匝数比较多，串接在测量仪表和保护回路中，电流互感器在工作时，它的二次侧回路始终是闭合的，因此测量仪表和保护回路串联线圈的阻抗很小，电流互感器的工作状态接近短路。电流互感器是把一次侧大电流转换成二次侧小电流来使用 ，二次侧不可开路。
The principle of current transformer is based on the principle of electromagnetic induction. The current transformer is made up of closed core and winding. It has a small number of one side winding turns in a line that needs to be measured, so it often flows through all the current of the line, and the number of the two side winding turns is more. The series is connected in the measuring instrument and the protection loop. The current transformer is closed at the end of the two side circuit when the current transformer is working, so the measuring instrument and the protection are guaranteed. The impedance of the series coil of the protection loop is very small, and the working state of the current transformer is close to the short-circuit. The current transformer is used to convert the primary side current to two side small current, and the two side can not be opened.