In circuit diagrams, a relay coil is represented by a rectangular symbol. If a relay possesses two coils, two adjacent rectangles are drawn side-by-side. Additionally, the textual symbol for a relay-typically the letter "J"-is inscribed either inside or alongside the rectangle(s). There are two methods for representing relay contacts: one involves drawing them directly adjacent to the rectangle representing the coil; this method offers a more intuitive visual representation. The other method, dictated by the specific requirements of the circuit connections, involves drawing individual contacts separately within their respective control circuits. In this latter approach, the contacts and the coil belonging to the same relay are typically marked with identical textual symbols, and the contact groups are assigned numerical identifiers to distinguish them from one another.
Relay contacts generally fall into three basic categories:
(1) Normally Open (NO) Type (H-type): When the coil is de-energized, the two contacts remain open (disconnected); when the coil is energized, the two contacts close (connect). This type is designated by the letter "H," derived from the initial phonetic consonant of the Chinese word for "close" (*He*).
(2) Normally Closed (NC) Type (D-type): When the coil is de-energized, the two contacts remain closed (connected); when the coil is energized, the two contacts open (disconnect). This type is designated by the letter "D," derived from the initial phonetic consonant of the Chinese word for "open/break" (*Duan*).
(3) Changeover Type (Z-type): This constitutes a contact group configuration. Such a group consists of three contacts: a central moving contact flanked by two fixed contacts-one positioned above and one below. When the coil is de-energized, the moving contact is disconnected from one fixed contact while remaining connected to the other. When the coil is energized, the moving contact shifts position, causing the previously open connection to close and the previously closed connection to open, thereby achieving a switching or changeover function. Such a contact group is referred to as a changeover contact. It is designated by the letter "Z," derived from the initial phonetic consonant of the Chinese word for "change/switch" (*Zhuan*).
