Both switches represented are normally- closed types. FIGURE 29—Fuses are used to prevent large overloads. This shows two common symbols for fuses.
Reading Electrical Schematic Diagrams 28 Some large breakers are activated by a magnetic release. They’re usually rated at high voltage and found in outdoor and indoor installations. If the current is high enough to cause a strong enough magnetic field, the magnetic force opens the breaker. Figure 30 displays the symbols for the breakers. Lightning Arrestors High-voltage surges caused by lightning may damage equip- ment unless a lightning arrestor is used on the installation. The lightning arrestor may be installed indoors or outdoors and must be properly grounded. An essential part of the lightning arrestor is the spark gap, which breaks up the volt- age surge. The arrestor allows the resulting current to bypass FIGURE 30—The type of application in which the circuit breaker is used determines which symbol is used. The general symbol is used primarily in single-phase panel box schematic diagrams.
Reading Electrical Schematic Diagrams 29 the equipment and flow to ground through a low-resistance path. Figure 31 contains the symbols for common lightning arrestors. Note that each symbol includes a space or “gap” between the two sides of the circuit. Relays The purpose of relays is to open and/or close sets of asso- ciated contacts. Figure 32 shows various relay symbols used in both electronic and electrical circuit schematics. The coil that controls the contacts is usually shown as a circle with a letter or letter designation in the center. Keep in mind that the coil and its associated contacts may not be, and in fact, are usually not in the same location in the circuit diagram. Often, the coil is in a lower-voltage control circuit, while its associated contacts are used to make or break a circuit in a higher-voltage circuit. However, the contacts will always be tagged to indicate which relay coil controls them. Resistors Resistors and rheostats are used to limit and control DC and AC circuits. As you know, resistors are devices that resist the flow of current. They vary greatly in size and shape from small components found in electronic circuits to large resis- tive-element heaters. A rheostat is a variable resistor that provides different resistance values according to connections of the contacts. Resistance devices are applied in heaters, toasters, stoves, ovens, motor starters, controllers, lamp dimmers, and other current-controlling devices. Figure 33 shows some symbols used to indicate these resistors in electrical schematic diagrams. Note that the symbol for the variable resistor (rheostat) includes a diagonal arrow across the symbol. Generally, this feature always designates some type of variable adjustment in a device.
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