Among the three pins of a DC socket, there is usually one positive (+), one negative (-), and the third pin is the ground wire. The main reasons for this design are as follows:

1. safety: Grounding wire can effectively prevent the risk of electric shock caused by equipment failure or short circuit. By grounding the metal casing of the device, the current can be directed to the ground in case of accidental leakage, thereby reducing the risk of electric shock.

2. stability: Grounding can reduce electromagnetic interference and provide a more stable power supply, especially in some sensitive electronic devices. Good grounding can effectively reduce noise and improve equipment performance.

3. Signal reference: In some applications, the grounding wire serves as a reference point for signals, helping to improve the reliability and accuracy of signal transmission, especially in audio and communication equipment.

Therefore, the design of three leads can effectively enhance the safety and stability of the system, ensuring the normal operation of the equipment.