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Introduction: An electrical system or an electrical device is usually composed of various basic parts, components, and assemblies. In order to represent these basic parts, components, and assemblies in electrical diagrams or other technical documents, various graphical symbols must be used, and some textual symbols and project codes must also be marked to distinguish the different functions, states, and characteristics of these devices and lines.Textual symbols usually consist of basic textual symbols, auxiliary textual symbols, and numbers. They are used to provide letter codes for the types of electrical equipment, devices, and components, as well as functional letter codes.1. Basic Textual Symbols
Basic textual symbols can be divided into single-letter symbols and double-letter symbols. (1) Single-letter symbols: Single-letter symbols categorize various electrical equipment, devices, and components into 23 major categories, each represented by a dedicated letter symbol. For example, “R” represents the resistance category, and “Q” represents the 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 for use; the letter “J” is also not adopted. Table 1-6 Common Single-letter Symbols for Electrical Equipment (2) Double-letter symbols: Double-letter symbols consist of a single-letter symbol representing the category and another letter. Their combination form is: the single-letter symbol is in front, and the other letter is behind. Double-letter symbols can express the names of electrical equipment, devices, and components in more detail and specifically. The other letter in the double-letter symbol is usually the first letter of the English name of the equipment, device, and component, or commonly used abbreviations, or conventionally used letters. For example, “G” represents the synchronous generator in English, so the double-letter symbol for the synchronous generator is “GS”. Common double-letter symbols used in electrical diagrams are shown in Table 1-7.2. Auxiliary Textual Symbols
Auxiliary textual symbols are used to represent the function, status, and characteristics of electrical equipment, devices, and components, as well as circuits. For example, “ACC” represents acceleration, and “BRK” represents braking. Auxiliary textual symbols can also be placed after the single-letter symbol to form a double-letter symbol, such as “SP” representing pressure sensor. When the auxiliary textual symbol consists of more than two letters, only the first letter is allowed for combination to simplify the textual symbol, such as “MS” representing synchronous motor. Auxiliary textual symbols can also be used alone, such as “OFF” representing disconnection, and “DC” representing direct current. Auxiliary textual symbols generally cannot exceed three letters. Common auxiliary textual symbols used in electrical diagrams are shown in Table 1-8.3. Combination of Textual Symbols The combination of textual symbols generally takes the form: basic symbol + auxiliary symbol + numeric sequence. For example, the first motor has the textual symbol M1; the first contactor has the textual symbol KM1.4. Special Purpose Textual Symbols
In electrical diagrams, some special-purpose terminal blocks, wires, etc., usually use some dedicated textual symbols. For example, three-phase AC system power is represented by “L1, L2, L3”, and the devices in the three-phase AC system are represented by “U, V, W”.
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