In order to ensure a reliable and safe operation of equipment, the need for the service of many auxiliary electrical equipment for the combination can achieve a plurality of electrical components of a control function, called the control circuit or the secondary circuit two. These devices should have the following functions:

1, automatic control functions: high voltage and large current switch equipment volume is very large, generally by the operating system to control the switching on and off, especially when the equipment failure, to cut off the circuit automatically switches to a set of automatic control of electrical equipment operation, automatic control of power supply equipment;

2, protection function: electrical equipment and line fault occurs in the operation process, the current (or voltage) will exceed the line equipment and allow the work scope and limit, which requires a set of detection of the fault signal and the automatic adjustment of the equipment and lines (disconnect, switch) protection equipment;

3, monitoring function: electricity is invisible to the eye, a device is electrified or power, can not distinguish from the appearance, it is needed to set up all kinds of audio signals, such as lighting and sound, electrical monitoring on a device;

4, measurement: light and sound signals can only qualitatively indicate the working status of the equipment (electrical or power), if you want to know the quantitative work of electrical equipment, but also need to have a variety of instrumentation equipment, all kinds of measuring circuits, such as voltage, current, frequency and power size etc..