请问电气方面的“二次原理图”怎么翻译？ 谢谢

secondary principle schematics
或
secondary principle diagrams

参考资料：
有使用说明书和必要的电路图和原理图。
The instrument must be accompanied by instructions manual with the necessary schematics and drawings.

如果可能，掌握各个设备串口的原理图是很有用的。
If possible it is often useful to have a schematic of the serial port of each device in a system.

给出硬件接口电路原理图以及主程序软件流程图。
Gives the principle diagram of the hardware interface circuit and the software flow chart of the main program.

就是单线图：single line diagram