Typically, wires in control systems are marked with numbers and/or letters for identification.Horizontal lines in a ladder diagram are called "rungs," each one representing a unique parallel circuit branch between the poles of the power supply.L 1 designates the "hot" AC wire and L 2 the "neutral" (grounded) conductor. The two vertical lines are called "rails" and attach to opposite poles of a power supply, usually 120 volts AC.Ladder diagrams (sometimes called "ladder logic") are a type of electrical notation and symbology frequently used to illustrate how electromechanical switches and relays are interconnected.For this reason, the load(s) must always be located nearest the grounded power conductor in the ladder diagram. It is much safer to have a system that blows a fuse in the event of a ground fault than to have a system that uncontrollably energizes lamps, relays, or solenoids in the event of the same fault. This time the accidental grounding of wire #1 will force power to the lamp while the switch will have no effect. However, consider what would happen to the circuit with the same fault (wire #1 coming in contact with ground), except this time we'll swap the positions of switch and fuse (L 2 is still grounded): If the switch were to close, there would be a short-circuit, immediately blowing the fuse. With both sides of the lamp connected to ground, the lamp will be "shorted out" and unable to receive power to light up. Our circuit would now function like this: Suppose that wire #1 were to accidently come in contact with ground, the insulation of that wire having been rubbed off so that the bare conductor came in contact with grounded, metal conduit. This is no accident or coincidence rather, it is a purposeful element of good design practice. Here, the lamp (load) is located on the right-hand side of the rung, and so is the ground connection for the power source. While it doesn't matter electrically where the relay coil is located within the rung, it does matter which end of the ladder's power supply is grounded, for reliable operation. In ladder diagrams, the load device (lamp, relay coil, solenoid coil, etc.) is almost always drawn at the right-hand side of the rung. Take this circuit section, for example, with wire #25 as a single, electrically continuous point threading to many different devices: What matters is that any one, electrically continuous point in a control circuit possesses the same wire number. Of course, it is preferable to maintain consistent wire colors, but this is not always practical. Wire numbers do not change at any junction or node, even if wire size, color, or length changes going into or out of a connection point. Each conductor has its own unique wire number for the control system that its used in. These wire numbers make assembly and maintenance very easy. Wires leading to the lamp would be labeled "1" and "L 2," respectively. Wires leading to the switch would be labeled "L 1" and "1," respectively. In the real world, that wire would be labeled with that number, using heat-shrink or adhesive tags, wherever it was convenient to identify. Note the number "1" on the wire between the switch and the lamp. So long as the switch contacts and relay coils are all adequately rated, it really doesn't matter what level of voltage is chosen for the system to operate with. Lower voltage AC and even DC systems are sometimes built and documented according to "ladder" diagrams: Typically in industrial relay logic circuits, but not always, the operating voltage for the switch contacts and relay coils will be 120 volts AC. In reality, the circuit looks something like this: #HOW TO TEST A FORWARD AND REVERSE MOTOR CIRCUIT GENERATOR#The actual transformer or generator supplying power to this circuit is omitted for simplicity. These designations have nothing to do with inductors, just to make things confusing. L 1 is the "hot" conductor, and L 2 is the grounded ("neutral") conductor. The "L 1" and "L 2" designations refer to the two poles of a 120 VAC supply, unless otherwise noted. If we wanted to draw a simple ladder diagram showing a lamp that is controlled by a hand switch, it would look like this: 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. Ladder diagrams are specialized schematics commonly used to document industrial control logic systems. Lessons In Electric Circuits - Volume IV Chapter 6 LADDER LOGIC Lessons In Electric Circuits - Volume IV (Digital) - Chapter 6
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