Copyright Goodheart-Willcox Co., Inc. 384 Motors and Electric Control Systems There are two basic types of electrical diagrams: lad- der diagrams and pictorial diagrams. In general, ladder diagrams are more useful for troubleshooting electri- cal circuits, while pictorial diagrams are used to locate devices within the unit and properly wire the unit. 17.2.1 Ladder Diagrams Ladder diagrams clearly show the parallel paths through which electricity can travel through the circuit. Each electrical component is represented by a symbol. Lines between the components represent wiring. Ladder dia- grams are primarily used for understanding the sequence of operation for the circuit. Ladder diagrams generally contain four primary com- ponents, Figure 17-12: • Rails are the two vertical lines at the sides of the diagram. The rails represent the voltage being supplied to the circuit. The supplied voltage is often 24 V for low-voltage control circuits, 120 V or 240 V for single-phase systems, and 208 V, 277 V, or 480 V for three-phase systems. This supplied voltage is often labeled at the top of a ladder diagram. • Rungs are horizontal lines that connect the rails. Each rung represents one parallel path in the circuit. Rungs typically contain switching devices and a load device. • Switching devices are generally located on the left side of a rung, and a rung can include multiple switching devices. These devices control the flow of electricity through the circuit. Examples of switching devices include thermostats, pressure switches, temperature switches, and contacts. See Figure 17-13. • Load devices are the devices that use or consume the electrical power being provided by the circuit. Generally, each rung includes only one load and the load is shown as the right-most symbol on the rung. A continuously operating load would be shown as the only item on its rung. Examples of load devices include motors, coils, solenoids, lights, pumps, and heaters. Some loads, such as coils and solenoids, close or open contacts when energized. These contacts are shown on a different rung of the ladder diagram. Electrical Wiring Diagram Names This chapter presents two basic types of electrical wiring diagrams: ladder diagrams and pictorial diagrams. In the field, manufacturers may provide wiring diagrams that vary from those described here. In addition, HVACR technicians may use different names for various diagrams, such as schematics or wiring diagrams. The fundamentals of reading these diagrams applies to all types of electrical diagrams, regardless of what they are called. Pro Tip In any electrical circuit, a completed circuit is required to flow current from the source of power in to power out. If any switching device on a rung is open, the load device is de-energized. When all switching devices on the rung are closed, the circuit is completed and the load device is energized. See Figure 17-14. Ladder diagrams showing both the high-voltage power circuit and the low-voltage control circuit are com- mon. See Figure 17-15. These diagrams show high-voltage relay and contactor contacts along with the low-voltage coils that control the contacts. 17.2.2 Pictorial Diagrams Pictorial diagrams are similar to a picture of the actual wiring diagram of a unit. They show components in place in relationship to how the manufacturer built the unit. Unlike ladder diagrams, pictorial diagrams specify wir- ing color and terminal locations on devices. Pictorial dia- grams are useful for physically locating terminals and for tracing wires from terminal to terminal. See Figure 17-16. Pictorial diagrams show the relationship of compo- nents to each other. In some cases, the position of compo- nents in a pictorial diagram indicate the position of the components in the equipment. For example, the pictorial Goodheart-Willcox Publisher Figure 17-12. Ladder diagrams show all of the switching devices in a load device’s circuit path. This presentation is useful for troubleshooting and testing the circuit. Rails Rungs Switching devices Load devices