Ladder diagrams (sometimes call as Ladder Logic) are specialized schematic commonly used to document industrial control logic systems. They are called "ladder" diagrams because they resemble a ladder, with two vertical rails (supply power) and as many "rungs" (horizontal lines) as there are control circuits to represent. The Ladder Logic reading is as below:
• Ladder Logic diagram are a type of electrical notation and symbol frequently used to illustrate how electromechanical switches and relays are interconnected.
• The two vertical lines are called "rails" and attach to opposite poles of a power supply, usually 120 volts AC. L1 designates the "hot" AC wire and L2 the "neutral" (grounded) conductor.
• Horizontal lines in a ladder diagram are called "rungs," each one representing a unique parallel circuit branch between the poles of the power supply.
• Typically, wires in control systems are marked with numbers and/or letters for identification. The rule is, all permanently connected (electrically common) points must bear the same label.
The illustration of Ladder diagram as shown on the diagram below:
The "L1" and "L2" designations refer to the two poles of a 120 VAC supply, unless otherwise noted. L1 is the "hot" conductor, and L2 is the grounded ("neutral") conductor.
Typically in industrial relay logic circuits, but not always, the operating voltage for the switch contacts and relay coils will be 120 volts AC. Lower voltage AC and even DC systems are sometimes built and documented according to "ladder" diagrams.