An electrical system or a piece of electrical equipment is usually composed of various basic components, parts, and assemblies. In order to represent these basic components, parts, and assemblies on electrical diagrams or other technical documents, in addition to using various graphic symbols, it is also necessary to mark some text symbols and project codes to distinguish the different functions, states, and characteristics of these devices and lines.
Letter codes typically consist of basic letter codes, auxiliary letter codes, and numbers. They are used to classify electrical equipment, devices, and components using letter codes for their types and functions.
1. Basic Symbols
Basic writing symbols can be divided into two types: single-letter symbols and double-letter symbols.
(1) Single-letter symbols: Single-letter symbols are English letters that divide various electrical equipment, devices and components into 23 categories. Each category is represented by a special letter symbol, such as “R” for resistors and “Q” for switching devices in power circuits, as shown in Table 1-6-. Among them, “I” and “O” are easily confused with the Arabic numerals “1” and “0”, and are not allowed to be used. The letter “J” is also not used.
Table 1-6 Commonly Used Single-Letter Symbols for Electrical Equipment
(2) Two-letter symbols: Two-letter symbols consist of a single-letter symbol representing a type from Table 1-7 and another letter. The combination is as follows: the single-letter symbol first, followed by the other letter. Two-letter symbols can express the names of electrical equipment, devices, and components in more detail and with greater specificity. The other letter in the two-letter symbol is usually the first letter of the English name of the equipment, device, or component, or a commonly used abbreviation, or a letter that is conventionally used. For example, "G" is the English name for a synchronous generator, so the two-letter symbol for a synchronous generator is "GS".
The commonly used two-letter symbols in electrical diagrams are shown in Table 1-7.
Table 1-7 Commonly Used Two-Letter Symbols in Electrical Diagrams
2. Auxiliary text symbols
Auxiliary letter symbols are used to represent the function, status, and characteristics of electrical equipment, devices, components, and circuits. For example, "ACC" indicates acceleration, and "BRK" indicates braking. Auxiliary letter symbols can also be placed after the single-letter symbol indicating the type to form a two-letter symbol, such as "SP" for pressure sensor. If an auxiliary letter symbol consists of more than two letters, to simplify the symbol, only the first letter is allowed to be used in the combination, such as "MS" for synchronous motor. Auxiliary letter symbols can also be used alone, such as "OFF" for disconnection and "DC" for direct current. Auxiliary letter symbols generally cannot exceed three letters.
Commonly used auxiliary text symbols in electrical diagrams are shown in Table 1-8.
Table 1-8 Commonly Used Auxiliary Text Symbols in Electrical Diagrams
3. Combinations of written symbols
The combination of written symbols is generally: basic symbols + auxiliary symbols + numerical sequence numbers.
For example, the first electric motor is symbolized as M1; the first contactor is symbolized as KM1.
4. Special Purpose Symbols
In electrical diagrams, special-purpose terminals, wires, etc., are usually represented by special symbols. For example, the power supply of a three-phase AC system is represented by "L1, L2, L3", and the equipment in a three-phase AC system is represented by "U, V, W".
