Relay is an electrical control device. When the input quantity (also known as the excitation quantity) changes to the specified requirements, the control quantity will undergo a predetermined step change in the electrical output circuit. It has an interactive relationship between the controlling system (also called the input loop) and the controlled system (also called the output loop). Usually used in automatic control circuits, it is actually an "automatic switch", which uses a small current to control the operation of a large current. Therefore, it plays the role of conversion circuit, automatic adjustment and safety protection in the circuit.

There are three basic forms of relay contacts:

1. When the moving (normally open) coil is not energized, the two contacts are disconnected, and the two contacts are closed when the coil is energized. Indicated by the phonetic prefix "H" for the ligature and the letter "M" in the permanent relay selection number.
2. The two contacts of the dynamic (normally closed) coil are closed when the coil is not energized, and the two contacts are disconnected after the coil is energized. Indicated by the hyphenated Pinyin prefix "D" and the letter "B" in the permanent relay selection number.
3. Conversion Type This is a contact group type. This contact group has three contacts in total, namely, the middle is the moving contact, and the upper and lower are the static contact. When the coil is not energized, one of the moving and stationary contacts is opened and the other is closed. After the coil is energized, the moving contact acts to make the original disconnection become closed, and the original closed state becomes the open state, reaching the conversion state. Such contact groups are called changeover contacts. It is represented by the pinyin prefix "z" of the word "zhuan", and there is no letter in the selection number of the permanent relay to indicate the conversion type.