218 Print Reading for HVACR
Copyright Goodheart-Willcox Co., Inc.
Summar y
■ A switch is used to open or close a connection in an electrical
circuit. It has two states: open and closed.
■ The operation of a switch is identifi ed by its number of poles and
throws. The number of poles indicates how many power lines are
controlled, while the throw indicates how many possible current
paths there are.
■ A set of contacts is the part of a switch that closes to complete an
electrical circuit. Sets of contacts are identifi ed as normally open
(NO) or normally closed (NC).
■ A relay is an electrical switch controlled by an external electrical
signal.
■ A solenoid is an electromagnetic device that converts current to
mechanical movement by producing a magnetic fi eld.
■ Wiring diagrams serve as electrical road maps for electric current to
follow.
■ Electrical symbols and abbreviations are not uniform throughout
the industry, so diagram legends must be referred to when reading
wiring diagrams.
■ Be sure to always follow the manufacturer’s literature when wiring,
or evaluating the wiring, of a system.
■ Pictorial diagrams often provide a visual representation of the actual
appearance of system and circuit components.
■ Schematic diagrams provide circuit details, including all wires,
wiring connections, and color-coding.
■ The ladder, or line, diagram is especially useful for system
troubleshooting.
■ The system’s sequence of operations can often be determined by
evaluating the system’s wiring diagrams.
■ Wiring diagrams often include both line and low-voltage wiring
circuits. Some systems utilize both line voltage and low-voltage
control devices.
■ Controls devices, when wired in series with the load, must be able to
withstand the current draw of the load.
Copyright Goodheart-Willcox Co., Inc.
Summar y
■ A switch is used to open or close a connection in an electrical
circuit. It has two states: open and closed.
■ The operation of a switch is identifi ed by its number of poles and
throws. The number of poles indicates how many power lines are
controlled, while the throw indicates how many possible current
paths there are.
■ A set of contacts is the part of a switch that closes to complete an
electrical circuit. Sets of contacts are identifi ed as normally open
(NO) or normally closed (NC).
■ A relay is an electrical switch controlled by an external electrical
signal.
■ A solenoid is an electromagnetic device that converts current to
mechanical movement by producing a magnetic fi eld.
■ Wiring diagrams serve as electrical road maps for electric current to
follow.
■ Electrical symbols and abbreviations are not uniform throughout
the industry, so diagram legends must be referred to when reading
wiring diagrams.
■ Be sure to always follow the manufacturer’s literature when wiring,
or evaluating the wiring, of a system.
■ Pictorial diagrams often provide a visual representation of the actual
appearance of system and circuit components.
■ Schematic diagrams provide circuit details, including all wires,
wiring connections, and color-coding.
■ The ladder, or line, diagram is especially useful for system
troubleshooting.
■ The system’s sequence of operations can often be determined by
evaluating the system’s wiring diagrams.
■ Wiring diagrams often include both line and low-voltage wiring
circuits. Some systems utilize both line voltage and low-voltage
control devices.
■ Controls devices, when wired in series with the load, must be able to
withstand the current draw of the load.